DUAL DRILLING CO /DE/
DEFM14A, 1996-05-10
DRILLING OIL & GAS WELLS
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<PAGE>
                            SCHEDULE 14A INFORMATION
 
                  Proxy Statement Pursuant to Section 14(a) of
            the Securities Exchange Act of 1934 (Amendment No.    )
 
    Filed by the Registrant /X/
    Filed by a Party other than the Registrant / /
 
    Check the appropriate box:
    / /  Preliminary Proxy Statement
    / /  Confidential, for Use of the Commission Only (as permitted by Rule
         14a-6(e)(2))
    /X/  Definitive Proxy Statement
    / /  Definitive Additional Materials
    / /  Soliciting  Material  Pursuant  to  Section  240.14a-11(c)  or  Section
         240.14a-12
 
                                      DUAL DRILLING COMPANY
- --------------------------------------------------------------------------------
                (Name of Registrant as Specified In Its Charter)
 
- --------------------------------------------------------------------------------
    (Name of Person(s) Filing Proxy Statement, if other than the Registrant)
 
Payment of Filing Fee (Check the appropriate box):
 
/ /  $125 per  Exchange Act  Rules 0-11(c)(1)(ii),  14a-6(i)(1), 14a-6(i)(2)  or
     Item 22(a)(2) of Schedule 14A.
/ /  $500  per  each party  to  the controversy  pursuant  to Exchange  Act Rule
     14a-6(i)(3).
/ /  Fee  computed  on   table  below   per  Exchange   Act  Rules   14a-6(i)(4)
     and 0-11.
     1) Title of each class of securities to which transaction applies:
        ------------------------------------------------------------------------
     2) Aggregate number of securities to which transaction applies:
        ------------------------------------------------------------------------
     3) Per unit price or other underlying value of transaction computed
        pursuant to Exchange Act Rule 0-11 (Set forth the amount on which the
        filing fee is calculated and state how it was determined):
        ------------------------------------------------------------------------
     4) Proposed maximum aggregate value of transaction:
        ------------------------------------------------------------------------
     5) Total fee paid:
        ------------------------------------------------------------------------
/X/  Fee paid previously with preliminary materials.
/ /  Check box if any part of the fee is offset as provided by Exchange Act Rule
     0-11(a)(2)  and identify the  filing for which the  offsetting fee was paid
     previously. Identify the previous filing by registration statement  number,
     or the Form or Schedule and the date of its filing.
     1) Amount Previously Paid:
        ------------------------------------------------------------------------
     2) Form, Schedule or Registration Statement No.:
        ------------------------------------------------------------------------
     3) Filing Party:
        ------------------------------------------------------------------------
     4) Date Filed:
        ------------------------------------------------------------------------
<PAGE>
                                  [DUAL LOGO]
 
                             DUAL DRILLING COMPANY
 
                          5956 SHERRY LANE, SUITE 1500
                              DALLAS, TEXAS 75225
 
                                  May 14, 1996
 
To the Stockholders of DUAL DRILLING COMPANY:
 
    Enclosed are a Notice of Special Meeting of Stockholders, a Prospectus/Proxy
Statement,  and  a Proxy  for a  Special Meeting  of Stockholders  (the "Special
Meeting") of DUAL DRILLING COMPANY ("DUAL") to be held on June 12, 1996 at 10:00
a.m., Central time, at  Park City Club, 5956  Sherry Lane., Suite 1700,  Dallas,
Texas 75225.
 
    At  the Special Meeting you will be asked to consider and vote on a proposal
to approve  and  adopt a  Merger  Agreement  pursuant to  which  a  wholly-owned
subsidiary of ENSCO International Incorporated ("ENSCO") will be merged with and
into  DUAL, as a result  of which DUAL will  become a wholly-owned subsidiary of
ENSCO. The terms  of the  Merger Agreement provide  that holders  of the  common
stock  of DUAL will receive for each share  of DUAL common stock owned as of the
effective  time  of  the  merger  0.625  of  a  share  of  ENSCO  common  stock.
Additionally,  you will  be asked to  approve the DUAL  Special Performance Unit
Plan. Details of the  matters to be  considered at the  Special Meeting are  set
forth  in  the accompanying  Prospectus/Proxy  Statement which  you  should read
carefully.
 
    After careful consideration, the Board of Directors of DUAL has approved the
Merger Agreement and recommends that all stockholders vote for its approval. The
Board of Directors  of DUAL has  received a written  opinion from its  financial
advisor,  Simmons & Company International, dated as  of March 21, 1996, that the
consideration to  be  received  by  DUAL stockholders  pursuant  to  the  Merger
Agreement  is fair to the DUAL stockholders from a financial point of view as of
that date.
 
    All stockholders are invited  to attend the Special  Meeting in person.  The
affirmative vote of a majority of the outstanding shares of common stock of DUAL
will  be necessary for  approval and adoption  of the Merger  Agreement, and the
affirmative vote of a majority of the total number of shares of common stock  of
DUAL  represented and entitled to vote at the Special Meeting or any adjournment
thereof will  be necessary  for approval  of the  adoption of  the DUAL  Special
Performance Unit Plan.
 
    IN ORDER THAT YOUR SHARES MAY BE REPRESENTED AT THE SPECIAL MEETING, YOU ARE
URGED  TO PROMPTLY COMPLETE, SIGN, DATE AND RETURN THE ACCOMPANYING PROXY IN THE
ENCLOSED ENVELOPE, WHETHER OR NOT YOU PLAN TO ATTEND THE SPECIAL MEETING. If you
attend the Special Meeting in  person you may, if  you wish, vote personally  on
all  matters  brought before  the Special  Meeting even  if you  have previously
returned your Proxy.
 
                                          Sincerely,
 
                                               [LOGO]
                                          DAVID W. SKARKE
                                          CHAIRMAN OF THE BOARD
<PAGE>
                             DUAL DRILLING COMPANY
 
                          5956 SHERRY LANE, SUITE 1500
                              DALLAS, TEXAS 75225
                              TEL: (214) 373-6200
 
                            ------------------------
 
                   NOTICE OF SPECIAL MEETING OF STOCKHOLDERS
                          TO BE HELD ON JUNE 12, 1996
                             ---------------------
 
    A Special Meeting of Stockholders  (the "Special Meeting") of DUAL  DRILLING
COMPANY, a Delaware corporation ("DUAL"), will be held on June 12, 1996 at 10:00
a.m.,  Central time, at Park  City Club, 5956 Sherry  Lane., Suite 1700, Dallas,
Texas 75225, for the following purposes:
 
    (i) To consider and vote upon a  proposal to approve and adopt an  Agreement
       and Plan of Merger, dated as of March 21, 1996 (restated, as amended, the
       "Merger  Agreement") among  ENSCO International  Incorporated, a Delaware
       corporation ("ENSCO"), DDC  Acquisition Company,  a Delaware  corporation
       and  a wholly-owned subsidiary of  ENSCO ("Merger Subsidiary"), and DUAL,
       pursuant to  which, among  other things  (a) Merger  Subsidiary would  be
       merged  with and  into DUAL,  as a  result of  which DUAL  would become a
       wholly-owned subsidiary of ENSCO and  (b) each stockholder of DUAL  would
       receive  for each share of DUAL's common  stock owned as of the effective
       time of the Merger 0.625 of a share of ENSCO common stock, par value $.10
       per share, as described in the accompanying Prospectus/Proxy Statement. A
       copy  of  the  Merger  Agreement  is  attached  to  the  Prospectus/Proxy
       Statement as Appendix A.
 
    (ii)  To  consider  and vote  on  a  proposal to  approve  the  DUAL Special
       Performance Unit Plan.
 
    (iii) To  transact such  other  business as  may  properly come  before  the
       Special Meeting or any adjournment(s) or postponement(s) thereof.
 
    Stockholders  of record of DUAL's  common stock at the  close of business on
May 1,  1996,  will  be entitled  to  vote  as set  forth  in  the  accompanying
Prospectus/Proxy  Statement at the Special  Meeting and any adjournment thereof.
Only holders of record of  DUAL's common stock at the  close of business on  the
record  date are entitled to  notice of, and to vote  at, the Special Meeting. A
complete list of stockholders  entitled to vote at  the Special Meeting will  be
available  for  examination by  any Company  stockholder  at DUAL's  office, for
purposes pertaining to the Special Meeting,  during normal business hours for  a
period of ten days prior to the Special Meeting.
 
    You are cordially invited and urged to attend the Special Meeting in person.
Whether  or not  you plan  to attend, please  complete, sign,  date and promptly
return the enclosed Proxy  in the enclosed  self-addressed, stamped envelope.  A
stockholder  of DUAL  who has given  a proxy may  revoke such proxy  at any time
prior to its exercise at the Special Meeting.
 
    THE BOARD OF DIRECTORS OF DUAL HAS APPROVED AND ADOPTED THE MERGER AGREEMENT
AND HAS ADOPTED THE DUAL SPECIAL  PERFORMANCE UNIT PLAN. THE BOARD OF  DIRECTORS
OF  DUAL RECOMMENDS THAT YOU  VOTE IN FAVOR OF THE  APPROVAL AND ADOPTION OF THE
MERGER AGREEMENT AND  APPROVAL OF  THE DUAL  SPECIAL PERFORMANCE  UNIT PLAN.  IN
ORDER  TO ASSURE  YOUR REPRESENTATION AT  THE SPECIAL  MEETING, PLEASE COMPLETE,
SIGN, DATE AND MAIL PROMPTLY THE ENCLOSED PROXY, WHICH IS BEING SOLICITED BY THE
BOARD OF  DIRECTORS OF  DUAL, WHETHER  OR NOT  YOU PLAN  TO ATTEND  THE  SPECIAL
MEETING. YOUR COOPERATION IS APPRECIATED.
 
                                          By order of the Board of Directors,
 
                                          DUAL DRILLING COMPANY
 
                                               [LOGO]
                                          ROBERT F. CHRONE
                                          SECRETARY
 
Dallas, Texas
May 14, 1996
<PAGE>
PROSPECTUS/PROXY STATEMENT
 
<TABLE>
<S>                                             <C>
                       [LOGO]                                    [DUAL LOGO]
       ENSCO INTERNATIONAL INCORPORATED                     DUAL DRILLING COMPANY
               4,651,474 SHARES                        SPECIAL MEETING OF STOCKHOLDERS
               OF COMMON STOCK,                                   TO BE HELD
           PAR VALUE $.10 PER SHARE                             JUNE 12, 1996
</TABLE>
 
    This  Prospectus/Proxy Statement (the "Prospectus/Proxy Statement") is being
furnished to the stockholders of  DUAL DRILLING COMPANY, a Delaware  corporation
("DUAL"),  in  connection  with the  solicitation  of  proxies by  the  Board of
Directors of DUAL for  use at a  special meeting of stockholders  of DUAL to  be
held  on  June 12,  1996,  and any  adjournments  or postponements  thereof (the
"Special Meeting").
 
    At the Special Meeting,  holders of shares of  common stock, par value  $.01
per  share, of DUAL ("DUAL Common Stock") will consider and vote upon a proposal
to approve and adopt an Agreement and Plan of Merger dated as of March 21,  1996
(restated,  as  amended,  the  "Merger  Agreement"),  among  ENSCO International
Incorporated, a  Delaware  corporation  ("ENSCO"), DDC  Acquisition  Company,  a
Delaware   corporation  and  a   wholly  owned  subsidiary   of  ENSCO  ("Merger
Subsidiary"), and DUAL, pursuant to which  DUAL will merge with and into  Merger
Subsidiary  (the  "Merger"). As  a  result of  the  Merger, DUAL  will  become a
wholly-owned subsidiary of ENSCO and each share of DUAL Common Stock outstanding
at the effective  time of the  Merger (the "Effective  Time") will be  converted
into  0.625 of a share of  common stock of ENSCO, par  value $.10 per share (the
"ENSCO Common  Stock"). A  copy of  the  Merger Agreement  is attached  to  this
Prospectus/Proxy   Statement  as  Appendix  A  and  is  incorporated  herein  by
reference. The stockholders of DUAL also will consider and vote upon a  proposal
to  approve the DUAL  Special Performance Unit  Plan and consider  and vote upon
such other business as may come before the Special Meeting. Consummation of  the
Merger is not subject to or conditioned upon approval of the proposal to approve
the DUAL Special Performance Unit Plan.
 
    This  Prospectus/Proxy  Statement also  constitutes  the prospectus  for the
offering of shares  of ENSCO  Common Stock  to be issued  in the  Merger to  the
holders  of DUAL Common Stock. ENSCO has  filed a Registration Statement on Form
S-4 (together with  any amendments thereto,  the "Registration Statement")  with
the  Securities  and  Exchange  Commission  (the  "Commission")  of  which  this
Prospectus/Proxy Statement is a part. All information concerning DUAL  contained
or  incorporated  by  reference  in  this  Prospectus/Proxy  Statement  has been
furnished  by  DUAL,   and  all  information   concerning  ENSCO  contained   or
incorporated  by reference in this Prospectus/Proxy Statement has been furnished
by ENSCO.
 
    SEE "RISK FACTORS"  ON PAGE  17 FOR  INFORMATION THAT  SHOULD BE  CONSIDERED
REGARDING THE SECURITIES OFFERED HEREBY.
 
    This  Prospectus/Proxy Statement and the  accompanying proxy are first being
mailed to DUAL stockholders on or about May 14, 1996.
                            ------------------------
 
THE SECURITIES TO BE ISSUED IN THE MERGER HAVE NOT BEEN APPROVED OR DISAPPROVED
 BY THE SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION
   NOR HAS THE SECURITIES AND EXCHANGE COMMISSION
     OR ANY  STATE  SECURITIES  COMMISSION  PASSED  UPON  THE  ACCURACY  OR
      ADEQUACY  OF  THIS PROSPECTUS/PROXY  STATEMENT.  ANY REPRESENTATION
                           TO THE CONTRARY IS A CRIMINAL OFFENSE.
 
                            ------------------------
 
          The date of this Prospectus/Proxy Statement is May 14, 1996.
<PAGE>
                             AVAILABLE INFORMATION
 
    ENSCO  and DUAL  are each subject  to the informational  requirements of the
Securities Exchange  Act  of 1934,  as  amended  (the "Exchange  Act"),  and  in
accordance  therewith  file reports,  proxy statements  (if required)  and other
information with  the  Commission.  The  reports,  proxy  statements  and  other
information  filed by  ENSCO or  DUAL with the  Commission may  be inspected and
copied at  the  public reference  facilities  maintained by  the  Commission  at
Judiciary  Plaza, Room 1024, 450 Fifth Street, N.W., Washington, D.C. 20549, and
should be  available at  the  Commission's Regional  Offices  at 7  World  Trade
Center,  13th Floor, New  York, New York 10048,  and Northwestern Atrium Center,
500 West Madison  Street, Suite 1400,  Chicago, Illinois 60661.  Copies of  such
material  can also  be obtained  at prescribed  rates from  the Public Reference
Section of the Commission at 450 Fifth Street, N.W., Washington, D.C. 20549. The
shares of ENSCO Common Stock are listed on the New York Stock Exchange  ("NYSE")
and  the shares of DUAL  Common Stock are quoted  on the Nasdaq National Market,
and certain of ENSCO's and DUAL's reports, proxy materials and other information
may be available for inspection at the  offices of the New York Stock  Exchange,
Inc.,  20 Broad Street, New  York, New York 10005 or  The Nasdaq Stock Market at
1735 K Street, N.W., Washington, D.C. 20006, respectively.
 
    ENSCO has filed  with the  Commission the Registration  Statement under  the
Securities  Act of 1933, as amended (the  "Securities Act"), with respect to the
shares of  ENSCO  Common  Stock  to  be issued  pursuant  to  the  Merger.  This
Prospectus/Proxy Statement does not contain all the information set forth in the
Registration  Statement and the Appendices thereto,  certain parts of which were
omitted as  permitted by  the  rules and  regulations  of the  Commission.  Such
additional information may be obtained from the Commission's principal office in
Washington, D.C.
 
                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE
 
    The following documents filed with the Commission by ENSCO (File No. 1-8097)
are incorporated by reference in this Prospectus/Proxy Statement:
 
    1.   Annual Report on Form 10-K, as amended, for the year ended December 31,
       1995 (the "ENSCO 1995 Form 10-K");
 
    2.  The description of ENSCO's Preferred Share Purchase Rights contained  in
       its  Registration  Statement on  Form 8-A  filed  with the  Commission on
       February 23, 1995;
 
    3.  The  description of  ENSCO Common  Stock contained  in its  Registration
       Statement  on Form 8-B,  filed with the Commission  on November 12, 1987,
       and its Registration Statement on Form 8-A, filed with the Commission  on
       February  3, 1981,  as amended  by Form 8,  filed with  the Commission on
       August 22, 1985;
 
    4.  Current Report on Form 8-K of January 25, 1996;
 
    5.  Current Report on Form 8-K of March 21, 1996; and
 
    6.  Quarterly Report on Form 10-Q for the three months ended March 31, 1996.
 
The following documents filed with the Commission by DUAL (File No. 0-11758) are
incorporated by reference in this Prospectus/Proxy Statement:
 
    1.   DUAL's Annual  Report on  Form 10-K,  as amended,  for the  year  ended
       December 31, 1995 (the "DUAL 1995 Form 10-K");
 
    2.    The description  of DUAL  Common Stock  contained in  its Registration
       Statement on Form S-1 filed with the Commission on June 16, 1993;
 
    3.  Current Report on Form 8-K of January 25, 1996;
 
    4.  Current Report on Form 8-K of March 21, 1996; and
 
                                       2
<PAGE>
    5.  Quarterly Report on Form 10-Q for the three months ended March 31, 1996.
 
    All documents filed by ENSCO or DUAL pursuant to Section 13(a), 13(c), 14 or
15(d) of  the Exchange  Act  subsequent to  the  date of  this  Prospectus/Proxy
Statement  and prior to  the date of the  Special Meeting shall  be deemed to be
incorporated by reference  herein from the  date of filing  such documents.  Any
statement  contained  herein  or in  a  document  incorporated or  deemed  to be
incorporated by reference herein  shall be deemed to  be modified or  superseded
for  purposes of this Prospectus/Proxy Statement  to the extent that a statement
contained herein (or  in any  subsequently filed document  which also  is or  is
deemed  to  be incorporated  by reference  herein)  modifies or  supersedes such
statement. Any such  statement so modified  or superseded shall  not be  deemed,
except   as  so   modified  or  superseded,   to  constitute  a   part  of  this
Prospectus/Proxy Statement.
 
    THIS PROSPECTUS/PROXY STATEMENT  INCORPORATES DOCUMENTS  BY REFERENCE  WHICH
ARE  NOT PRESENTED  HEREIN OR  DELIVERED HEREWITH.  THESE DOCUMENTS  (OTHER THAN
APPENDICES TO SUCH DOCUMENTS THAT ARE NOT SPECIFICALLY INCORPORATED BY REFERENCE
HEREIN) ARE AVAILABLE WITHOUT CHARGE UPON REQUEST FROM, IN THE CASE OF DOCUMENTS
RELATING TO ENSCO, ATTN.: CORPORATE SECRETARY, AT 2700 FOUNTAIN PLACE, 1445 ROSS
AVENUE, DALLAS, TEXAS 75202-2792,  (214) 922-1500 AND IN  THE CASE OF  DOCUMENTS
RELATING  TO DUAL, ATTN.: CORPORATE SECRETARY,  AT 5956 SHERRY LANE, SUITE 1500,
DALLAS, TEXAS 75225, (214) 373-6200. TO ENSURE TIMELY DELIVERY OF THE DOCUMENTS,
ANY REQUEST SHOULD BE MADE BY JUNE 5, 1996.
 
    No  person  is  authorized   to  give  any  information   or  to  make   any
representation  not  contained  in  this Prospectus/Proxy  Statement  or  in the
documents incorporated herein by reference  in connection with the  solicitation
and  the  offering  made hereby  and,  if  given or  made,  such  information or
representation should not be relied upon  as having been authorized by ENSCO  or
DUAL.  This Prospectus/ Proxy Statement does not constitute an offer to sell, or
a solicitation  of  an  offer  to  purchase,  the  securities  offered  by  this
Prospectus/Proxy  Statement, or the solicitation of  a proxy from any person, in
any jurisdiction in which it is unlawful to make such offer, solicitation of  an
offer  or  proxy solicitation.  Neither  the delivery  of  this Prospectus/Proxy
Statement  nor   any   distribution   of  the   securities   made   under   this
Prospectus/Proxy Statement shall, under any circumstances, create an implication
that  there has not  been any change in  the affairs of ENSCO  or DUAL since the
date of this Prospectus/Proxy Statement other than as set forth in the documents
incorporated herein by reference.
 
                                       3
<PAGE>
                               TABLE OF CONTENTS
<TABLE>
<CAPTION>
                                                    PAGE
                                                    -----
<S>                                              <C>
AVAILABLE INFORMATION..........................           2
INCORPORATION OF CERTAIN DOCUMENTS BY
 REFERENCE.....................................           2
SUMMARY........................................           5
  Risk Factors.................................           5
  The Companies................................           5
  The Special Meeting..........................           5
  The Merger and Certain Provisions of the
   Merger Agreement............................           6
  Principal Stockholder Agreement..............          10
  Comparisons of Stockholders' Rights..........          11
  Comparative Market Prices and Dividends......          11
  Selected Historical Financial Data...........          12
  Selected Unaudited Combined Condensed Pro
   Forma Financial Data........................          14
  Proposal to Approve the DUAL Special
   Performance Unit Plan.......................          15
RISK FACTORS...................................          17
  Industry Conditions and Competition..........          17
  Environmental Matters........................          17
  Limitations on Ownership by Non-U.S.
   Citizens....................................          18
  Operational Risks and Insurance..............          18
  Government Regulation........................          18
  International Operations.....................          19
  Certain Anti-takeover Effects................          19
  Interests of Certain Persons; Possible
   Conflicts of Interests of Certain Persons in
   the Merger..................................          19
  No Dividends.................................          19
THE SPECIAL MEETING............................          19
  General......................................          19
  Purposes of the Special Meeting..............          20
  Vote Required................................          20
  Voting of Proxies............................          20
  Revocability of Proxies......................          20
  Solicitation of Proxies......................          20
  Record Date; Shares Entitled to Vote.........          21
  Quorum.......................................          21
THE MERGER.....................................          21
  General......................................          21
  Background of the Merger.....................          21
  DUAL's Reasons for the Merger; Recommendation
   of DUAL's Board of Directors................          24
  ENSCO's Reasons for the Merger...............          25
  Effective Time...............................          26
  Terms of the Merger..........................          26
  Fairness Opinion.............................          26
  Certain United States Federal Income Tax
   Consequences................................          32
  Regulatory Approvals.........................          33
  Accounting Treatment.........................          34
  NYSE Listing.................................          34
  Interests of Certain Persons; Possible
   Conflicts of Interest.......................          34
  Restrictions on Resales by Affiliates........          37
  Management...................................          37
  Effect of Merger on Certain Debt of DUAL.....          37
CERTAIN PROVISIONS OF THE MERGER AGREEMENT.....          38
  Exchange Procedures..........................          38
  Certain Representations and Warranties.......          40
  Conduct of Business Pending the Merger.......          41
  No Solicitation of Transactions..............          41
 
<CAPTION>
                                                    PAGE
                                                    -----
<S>                                              <C>
  Conditions to the Consummation of the
   Merger......................................          42
  Termination..................................          43
  Alternative Proposal Fee; Expenses;
   Damages.....................................          43
  Amendment and Waiver.........................          44
  Effect of the Merger on Stock Options........          44
  Effect of the Merger on Certain Benefit
   Plans.......................................          45
PRINCIPAL STOCKHOLDER AGREEMENT................          46
  Voting and Proxy.............................          46
  Abstention from Voting.......................          47
  Covenants and Representations................          47
  Registration of Merger Shares................          47
  No Solicitation..............................          48
THE COMPANIES..................................          48
  ENSCO International Incorporated.............          48
  DUAL DRILLING COMPANY........................          49
  Merger Subsidiary............................          49
PRINCIPAL STOCKHOLDERS.........................          50
UNAUDITED COMBINED CONDENSED PRO FORMA
 FINANCIAL INFORMATION.........................          51
  Notes to Unaudited Combined Condensed Pro
   Forma Financial Statements..................          55
  Pro Forma Adjustments to Historical Combined
   Condensed Balance Sheet.....................          55
  Pro Forma Adjustments to Historical Combined
   Condensed Statements of Operations..........          55
COMPARISONS OF STOCKHOLDERS'
 RIGHTS........................................          56
  Capital Stock................................          56
  Special Meetings of Stockholders.............          56
  Number of Directors..........................          57
  Classification of Board......................          57
  Removal of Directors.........................          57
  Anti-Takeover Provisions.....................          57
  Indemnification and Limitation of Monetary
   Liabilities.................................          57
  Dissolution..................................          58
  Preemptive Rights............................          58
  Payment of Dividends.........................          59
  Restrictions on Alien Ownership..............          59
  Dissenting Stockholders' Rights..............          59
  Stockholders' Lists and Inspection of Books
   and Records.................................          59
PROPOSAL TO APPROVE THE ADOPTION OF THE DUAL
 SPECIAL PERFORMANCE UNIT PLAN.................          59
NEW PLAN BENEFITS..............................          60
COMPENSATION OF DUAL'S EXECUTIVE OFFICERS......          62
  Option Cancellations.........................          63
  Option Grants................................          65
  Aggregated Exercises of Options/SARs and
   Fiscal Year-End Option/SAR Value Table......          65
  Supplemental Executive Retirement Plan.......          65
LEGAL MATTERS..................................          66
EXPERTS........................................          66
STOCKHOLDER PROPOSALS..........................          66
INDEX -- Defined Terms.........................          67
APPENDICES
  A -- Merger Agreement........................         A-1
  B -- Fairness Opinion........................         B-1
  C -- DUAL Special Performance Unit Plan......         C-1
  D -- Principal Stockholder Agreement.........         D-1
</TABLE>
 
                                       4
<PAGE>
                                    SUMMARY
 
    THE  FOLLOWING IS  A SUMMARY OF  CERTAIN INFORMATION  CONTAINED ELSEWHERE OR
INCORPORATED BY  REFERENCE  IN  THIS PROSPECTUS/PROXY  STATEMENT.  IT  DOES  NOT
PURPORT  TO  BE COMPLETE  AND IS  QUALIFIED IN  ITS ENTIRETY  BY THE  FULL TEXT,
INCLUDING THE APPENDICES ATTACHED HERETO. CERTAIN CAPITALIZED TERMS USED IN THIS
SUMMARY  ARE  DEFINED   ELSEWHERE  IN  THIS   PROSPECTUS/PROXY  STATEMENT.   THE
INFORMATION  CONTAINED IN THIS PROSPECTUS/PROXY  STATEMENT WITH RESPECT TO ENSCO
AND ITS AFFILIATES HAS BEEN PROVIDED BY ENSCO, AND THE INFORMATION WITH  RESPECT
TO DUAL AND ITS AFFILIATES HAS BEEN PROVIDED BY DUAL.
 
RISK FACTORS
 
    STOCKHOLDERS OF DUAL SHOULD CAREFULLY EVALUATE CERTAIN RISK FACTORS RELATING
TO THE COMBINED COMPANY AFTER THE MERGER. SEE "RISK FACTORS."
 
THE COMPANIES
 
    ENSCO.   ENSCO is  an international offshore  contract drilling company that
also provides marine transportation services in the U.S. Gulf of Mexico. ENSCO's
complement of  offshore drilling  rigs  includes 24  jackup  rigs and  10  barge
drilling  rigs, and ENSCO's marine transportation  fleet consists of 37 vessels.
ENSCO's operations are integral to the exploration, development, and  production
of oil and gas.
 
    ENSCO  was formed as a  Texas corporation in 1975  and was reincorporated in
Delaware in 1987.  At ENSCO's  Annual Meeting of  Stockholders held  on May  23,
1995,  the stockholders of ENSCO approved the change in ENSCO's name from Energy
Service Company,  Inc. to  ENSCO International  Incorporated. ENSCO's  principal
office  is  located at  2700  Fountain Place,  1445  Ross Avenue,  Dallas, Texas
75202-2792, and its telephone number is (214) 922-1500.
 
    DUAL.  DUAL is  a domestic and  international offshore drilling  contractor.
DUAL was formed in 1951 as a domestic onshore drilling contractor. In 1975, DUAL
entered  the  offshore drilling  business and  built a  fleet of  deep drilling,
self-contained platform rigs. During 1981, DUAL entered the international sector
of the offshore drilling business  when it began to  acquire a fleet of  premium
independent-leg  cantilever jackup rigs. Since that  time, DUAL has expanded its
offshore fleet  by  purchasing  additional  platform and  jackup  rigs,  and  by
bareboat  chartering  jackup  rigs. DUAL  now  operates  a fleet  of  20 premium
offshore  rigs  represented   by  10   self-contained  platform   rigs  and   10
independent-leg  cantilever jackup  rigs. DUAL's  principal executive  office is
located at 5956 Sherry Lane, Suite 1500, Dallas, Texas 75225, and its  telephone
number is (214) 373-6200.
 
    MERGER  SUBSIDIARY.  Merger Subsidiary was formed on March 5, 1996, by ENSCO
solely for the purpose of  effecting the Merger. It  has no material assets  and
has  not engaged  in any  activities except in  connection with  the Merger. Its
principal executive office is located at 2700 Fountain Place, 1445 Ross  Avenue,
Dallas, Texas 75202-2792, and its telephone number is (214) 922-1500.
 
THE SPECIAL MEETING
 
    TIME,  DATE AND PLACE.  The Special Meeting  is scheduled to be held on June
12, 1996 at 10:00 a.m., Central time, at Park City Club, 5956 Sherry Lane, Suite
1700, Dallas, Texas 75225.
 
    PURPOSES OF THE SPECIAL MEETING.  The purposes of the Special Meeting are to
(i) consider and vote upon a proposal to approve and adopt the Merger Agreement,
(ii) consider and vote upon a  proposal to approve the DUAL Special  Performance
Unit  Plan (the  "Unit Plan"),  and (iii)  transact such  other business  as may
properly  be  brought  before  the  Special  Meeting  or  any  adjournments   or
postponements thereof.
 
    VOTE  REQUIRED.  The  affirmative vote of  the holders of  a majority of all
outstanding shares of  DUAL Common Stock  is required to  approve and adopt  the
Merger  Agreement.  As  a  result,  abstentions,  failures  to  vote  and broker
non-votes will have the same effect as votes against the Merger Agreement  since
they  are  not votes  for approval.  The affirmative  vote of  the holders  of a
majority of the shares of  DUAL Common Stock present  and voting at the  Special
Meeting is required to approve
 
                                       5
<PAGE>
the  Unit Plan. Abstentions from voting on the Unit Plan will be included in the
voting tally and  will have the  same effect  as a vote  against that  proposal.
Because  broker non-votes are not considered  "shares present" with respect to a
matter requiring the affirmative vote of a majority of shares present in  person
or by proxy at the Special Meeting, broker non-votes will not affect the outcome
with respect to the proposal to approve the Unit Plan. Pursuant to the Principal
Stockholder  Agreement (as hereafter defined),  the Principal Stockholder Shares
(as hereafter  defined) (representing  approximately  59.6% of  the  outstanding
shares of DUAL Common Stock) will be present and voted at the Special Meeting in
favor  of the Merger Agreement and the Unit Plan. Therefore, no additional votes
from the stockholders of DUAL are  necessary to approve the Merger Agreement  or
the Unit Plan. See "Principal Stockholder Agreement."
 
    VOTING  OF PROXIES.   Shares  of DUAL  Common Stock  represented by properly
executed proxies received at or prior to the Special Meeting, and which have not
thereafter been properly revoked as described below, will be voted in accordance
with the instructions indicated therein. If no instructions are indicated,  such
proxies  will be voted  FOR approval and  adoption of the  Merger Agreement, FOR
approval of the Unit Plan, and in the  discretion of the proxy holder as to  any
other matter that may properly come before the Special Meeting. See "The Special
Meeting -- Voting of Proxies."
 
    REVOCABILITY  OF PROXIES.   A  DUAL stockholder  who has  given a  proxy may
revoke such proxy at any  time prior to its exercise  at the Special Meeting  by
any  manner permitted by law, including  (i) giving written notice of revocation
by mail or  facsimile to  DUAL prior  to the  Special Meeting  or (ii)  properly
submitting  to DUAL by mail  or facsimile a duly  executed proxy bearing a later
date. Submissions  to  DUAL should  be  made  to DUAL  DRILLING  COMPANY,  Attn:
Corporate  Secretary,  at 5956  Sherry Lane,  Suite  1500, Dallas,  Texas 75225,
facsimile number  (214) 373-0533.  Any  beneficial owner  whose shares  of  DUAL
Common  Stock are registered in  the name of a  broker, dealer, commercial bank,
trust company or other nominee and who wishes to change such stockholder's  vote
should  contact  such registered  holder promptly  and instruct  such registered
holder on such beneficial owner's behalf. There is no guarantee that there  will
be  sufficient time prior  to the Special  Meeting for the  registered holder to
deliver a revocation upon instruction by the beneficial owner. See "The  Special
Meeting -- Revocability of Proxies."
 
    RECORD  DATE; SHARES ENTITLED TO VOTE.  The close of business on May 1, 1996
has been fixed as the record date (the "Record Date") for determining holders of
shares of DUAL Common  Stock entitled to  notice of and to  vote at the  Special
Meeting.  As of  the Record  Date, 15,765,713 shares  of DUAL  Common Stock were
outstanding and held of record by 28 holders. The DUAL Common Stock is the  only
class  of  capital stock  of DUAL  issued and  outstanding. Each  stockholder of
record as of the close of business on the Record Date is entitled at the Special
Meeting to one vote for each share  of DUAL Common Stock held. See "The  Special
Meeting -- Record Date; Shares Entitled to Vote."
 
    QUORUM.   The presence, in person or by  proxy, of the holders of a majority
of the outstanding shares of DUAL Common  Stock entitled to vote at the  Special
Meeting  is necessary to constitute a quorum  for the transaction of business at
the Special  Meeting.  Pursuant  to the  Principal  Stockholder  Agreement,  the
Principal Stockholder Shares will be present and voted at the Special Meeting in
favor  of  the Merger  Agreement  and the  Unit  Plan. Therefore,  no additional
stockholders are necessary to be present at the Special Meeting to constitute  a
quorum.   See  "The  Special  Meeting  --  Quorum"  and  "Principal  Stockholder
Agreement."
 
THE MERGER AND CERTAIN PROVISIONS OF THE MERGER AGREEMENT
 
    GENERAL.  If all stockholder approvals are obtained and all other conditions
to the Merger are satisfied (or  waived, if permissible), then at the  Effective
Time,  Merger Subsidiary will be merged with and  into DUAL, with DUAL to be the
surviving corporation (the  "Surviving Corporation")  and automatically  renamed
DUAL Holding Company. In the Merger, each share of DUAL Common Stock outstanding
immediately  prior to the Effective Time (other than shares of DUAL Common Stock
 
                                       6
<PAGE>
owned by DUAL, ENSCO  or a subsidiary of  either of them ("Cancelable  Shares"))
will  be converted  into 0.625 of  a share  of ENSCO Common  Stock. See "Certain
Provisions of the Merger Agreement -- Exchange Procedures."
 
    RECOMMENDATION OF THE BOARD OF DIRECTORS OF DUAL.  DUAL'S BOARD OF DIRECTORS
HAS UNANIMOUSLY DETERMINED THAT THE TERMS OF THE MERGER AGREEMENT AND THE MERGER
OFFER THE BEST VALUE REASONABLY AVAILABLE TO, AND ARE IN THE BEST INTERESTS  OF,
DUAL  AND  ITS STOCKHOLDERS.  ACCORDINGLY, THE  BOARD OF  DIRECTORS OF  DUAL HAS
UNANIMOUSLY APPROVED THE MERGER AGREEMENT  AND RECOMMENDS THAT THE  STOCKHOLDERS
OF   DUAL  VOTE  FOR  APPROVAL  AND   ADOPTION  OF  THE  MERGER  AGREEMENT.  The
recommendation of the Board of Directors of DUAL is based on a number of factors
described in "The Merger -- Background of the Merger" and "The Merger --  DUAL's
Reasons for the Merger; Recommendation of DUAL's Board of Directors."
 
    EFFECTIVE  TIME.  The Effective Time will occur as soon as practicable after
the requisite approval  of the stockholders  of DUAL has  been obtained and  all
other  conditions have been satisfied or waived,  and in no event later than the
first business day  after all conditions  to the Merger  have been satisfied  or
waived,  unless the parties agree otherwise.  The Effective Time will occur upon
the filing of a Certificate of Merger  with the Secretary of State of the  State
of Delaware.
 
    FAIRNESS  OPINION.    On March  21,  1996, Simmons  &  Company International
("Simmons"), financial advisor to DUAL, delivered a written opinion to the Board
of Directors of DUAL that, as of that date, the consideration to be provided  to
DUAL  stockholders pursuant to  the Merger Agreement  is fair to  the holders of
shares of  DUAL Common  Stock  from a  financial point  of  view. The  Board  of
Directors  of DUAL has not  requested that Simmons update  its opinion. The full
text of the written opinion of Simmons, which sets forth the procedures followed
and the  factors  considered by  Simmons,  is attached  as  Appendix B  to  this
Prospectus/Proxy Statement. HOLDERS OF SHARES OF DUAL COMMON STOCK ARE URGED TO,
AND  SHOULD, READ  SUCH OPINION  IN ITS  ENTIRETY. See  "The Merger  -- Fairness
Opinion."
 
    CERTAIN UNITED  STATES  FEDERAL INCOME  TAX  CONSEQUENCES.   The  Merger  is
intended   to  qualify,  for  federal  income   tax  purposes,  as  a  "tax-free
reorganization" so that generally  no gain or loss  would be recognized by  DUAL
stockholders  who exchange  their DUAL Common  Stock solely for  shares of ENSCO
Common Stock. Holders of shares of DUAL Common Stock who receive cash in lieu of
fractional shares generally will be treated as if the fractional shares had been
issued and subsequently redeemed by ENSCO. Unless the redemption is found to  be
essentially  equivalent to a  dividend, each stockholder  of DUAL will recognize
gain or loss measured by the difference between such stockholder's basis in  the
fractional  share and the amount of cash received. DUAL has received the opinion
of Akin, Gump, Strauss, Hauer &  Feld, L.L.P., its counsel, (the "Tax  Opinion")
that the Merger should constitute a reorganization within the meaning of Section
368(a)  of the Internal Revenue Code of  1986, as amended (the "Code") and that,
generally, the  DUAL stockholders  should  not recognize  gain  or loss  on  the
exchange  of their  DUAL stock for  the stock  of ENSCO pursuant  to the Merger.
Depending upon uncertain factual  developments, it is  possible that the  Merger
might not be treated as a reorganization within the meaning of Section 368(a) of
the  Code, as a result of which each holder of shares of DUAL Common Stock would
recognize gain or loss in the amount  of the difference between the fair  market
value  of the  shares of  ENSCO Common Stock  and any  cash received  in lieu of
fractional shares, and such holder's adjusted tax basis in the DUAL Common Stock
exchanged  therefor.  In   particular,  tax-free   "reorganization"  status   is
dependent, in part, on stockholders of DUAL that receive a certain percentage of
the  ENSCO Common Stock  distributed in the reorganization  not having a present
plan or intention to dispose of such  stock. The conclusions in the Tax  Opinion
were  predicated on certain assumptions  and representations regarding the plans
or intentions of certain  stockholders of DUAL  (or the Principal  Stockholder),
including  the Principal Stockholder. The Principal Stockholder intends to sell,
for cash, a portion of  the ENSCO Common Stock  received in the Merger.  Because
the  number  of  shares  to  be sold  by  the  Principal  Stockholder  cannot be
determined at this time, there is  no assurance that the assumptions upon  which
the Tax Opinion is
 
                                       7
<PAGE>
in  part based  will be  correct, in which  event (i)  the Tax  Opinion would be
rendered inapplicable  and (ii)  the  Merger may  or may  not  be treated  as  a
tax-free "reorganization" depending on all the facts and circumstances. See "The
Merger -- Certain United States Federal Income Tax Consequences."
 
    REGULATORY  APPROVALS.  ENSCO and DUAL are aware of no government regulatory
approvals remaining to be  obtained for consummation of  the Merger, other  than
compliance  with  notification  requirements  of  environmental  agencies,  with
federal securities laws, with state securities or "Blue Sky" laws, and with  the
U.S.   Maritime  Administration.  The  Merger   was  subject  to  the  premerger
notification provisions of the  Hart-Scott-Rodino Antitrust Improvements Act  of
1976,  as amended (the "HSR Act"). Filings were  made under the HSR Act by ENSCO
and DUAL  with  the Federal  Trade  Commission  and the  Department  of  Justice
Antitrust  Division.  On February  28, 1996,  early  termination of  the waiting
period under the  HSR Act  was granted, thus  permitting the  Merger to  proceed
pursuant  to  the  HSR  Act,  subject  to  stockholder  approval  and  the other
conditions described herein. ENSCO, DUAL, and Merger Subsidiary have each agreed
to use their  best efforts to  obtain any additional  regulatory approvals.  See
"The Merger -- Regulatory Approvals."
 
    ACCOUNTING TREATMENT.  ENSCO intends to account for the Merger as a purchase
of  DUAL in accordance  with generally accepted  accounting principles ("GAAP").
See "The Merger -- Accounting Treatment".
 
    NYSE LISTING.  The shares of ENSCO  Common Stock to be issued in  connection
with  the Merger  will be listed  on the NYSE.  Evidence from the  NYSE that the
shares of ENSCO Common Stock to be issued in connection with the Merger will  be
listed  on the NYSE immediately  following the Effective Time  is a condition to
consummation of the Merger. See "Certain  Provisions of the Merger Agreement  --
Conditions to the Consummation of the Merger."
 
    INTERESTS   OF  CERTAIN  PERSONS   IN  THE  MERGER.     In  considering  the
recommendation of the  Board of  Directors of DUAL  with respect  to the  Merger
Agreement  and  the transactions  contemplated  thereby, stockholders  should be
aware that  certain members  of  the management  of DUAL  and  of its  Board  of
Directors  have certain  interests in  the Merger  that are  in addition  to the
interests of stockholders of DUAL  generally including, without limitation,  the
rights  to  receive  certain  severance  and  other  employee  benefits  and  to
indemnification. See  "The  Merger --  Interests  of Certain  Persons;  Possible
Conflicts of Interest" and "-- Management."
 
    MANAGEMENT.   After the Merger, the officers and directors of ENSCO will not
change. Pursuant to the Merger Agreement,  upon the consummation of the  Merger,
the  officers  and  directors  of Merger  Subsidiary  immediately  prior  to the
Effective Time  will be  the initial  officers and  directors of  the  Surviving
Corporation. See "The Merger -- Management."
 
    PROCEDURES  FOR EXCHANGE OF CERTIFICATES.  As soon as reasonably practicable
after  the  Effective  Time,  a  letter  of  transmittal  and  instructions  for
surrendering  stock certificates  will be mailed  to each holder  of DUAL Common
Stock for use in  exchanging such holder's  stock certificates for  certificates
evidencing  shares of ENSCO Common  Stock and cash in  lieu of fractional shares
and any dividends or other distributions to  which such holder is entitled as  a
result  of  the  Merger. See  "Certain  Provisions  of the  Merger  Agreement --
Exchange Procedures."
 
    REPRESENTATIONS AND  WARRANTIES.    The Merger  Agreement  contains  various
representations  and warranties of  ENSCO, the Merger  Subsidiary, and DUAL. See
"Certain Provisions  of  the Merger  Agreement  -- Certain  Representations  and
Warranties."
 
    CONDUCT  OF  BUSINESS  PENDING THE  MERGER;  NO SOLICITATIONS.    The Merger
Agreement restricts the ability  of DUAL and ENSCO  to take certain actions  and
enter  into certain transactions pending the  Merger. See "Certain Provisions of
the Merger  Agreement --  Conduct of  Business Pending  the Merger"  and "--  No
Solicitation of Transactions."
 
    CONDITIONS  TO THE  CONSUMMATION OF THE  MERGER.  The  obligations of ENSCO,
Merger Subsidiary,  and  DUAL  to  consummate the  Merger  are  subject  to  the
satisfaction or, where legally permissible,
 
                                       8
<PAGE>
waiver  of various conditions, including, among others: (i) the effectiveness of
the Registration Statement  and the  absence of  any stop  order suspending  the
effectiveness   thereof;  (ii)  approval   of  the  Merger   Agreement  and  the
transactions contemplated thereunder by the  holders of the requisite number  of
shares  of DUAL Common Stock;  (iii) the absence of  any order, executive order,
stay, decree, judgment, injunction, statute,  rule, or regulation issued by  any
United States (federal, state, or local) or foreign government, or governmental,
regulatory,  or  administrative authority,  agency,  or commission  or  court of
competent jurisdiction  prohibiting consummation  of the  Merger or  making  the
Merger  illegal; and (iv) evidence from the NYSE that the shares of ENSCO Common
Stock to be issued  to the holders of  DUAL Common Stock in  the Merger will  be
listed  on  the  NYSE immediately  following  the Effective  Time.  See "Certain
Provisions of the  Merger Agreement  -- Conditions  to the  Consummation of  the
Merger."
 
    TERMINATION.   The Merger Agreement  may be terminated at  any time prior to
the Effective Time by mutual consent  of ENSCO, Merger Subsidiary, and DUAL,  or
by  either ENSCO or DUAL  if, subject to certain  limitations: (i) the Effective
Time has not occurred on or before  July 31, 1996; (ii) there exists any  order,
executive  order, stay, decree, judgment or  injunction (each, an "Order") which
is final and nonappealable preventing the consummation of the Merger; (iii)  the
other  party has breached any representation, warranty, covenant or agreement in
the Merger  Agreement such  that the  related closing  conditions would  not  be
satisfied  (unless  the breach  is curable  by the  breaching party  through the
exercise of its best efforts  and it continues to  exercise its best efforts  to
cure the breach); or (iv) by mutual written consent of ENSCO, Merger Subsidiary,
and DUAL. In addition, ENSCO may terminate the Merger Agreement, if (i) a tender
offer  or exchange  offer for 50%  or more  of the outstanding  shares of DUAL's
capital stock is commenced and the Board of Directors of DUAL fails to recommend
against the holders  of DUAL tendering  their shares into  such tender offer  or
exchange  offer, or (ii) the stockholders of  DUAL fail to approve and adopt the
Merger Agreement  and the  transactions  contemplated thereunder.  See  "Certain
Provisions of the Merger Agreement -- Termination."
 
    ALTERNATIVE  PROPOSAL FEE; EXPENSES.  In  connection with the termination of
the Merger  Agreement, upon  the occurrence  of certain  events, DUAL  would  be
required  to pay to ENSCO  a fee (the "Alternative  Proposal Fee") of $5 million
(which amount is  inclusive of  all expenses incurred  by ENSCO  related to  the
Merger).  Upon the occurrence of certain other events, DUAL would be required to
pay to ENSCO its expenses only. See "Certain Provisions of the Merger  Agreement
- -- Alternative Proposal Fee; Expenses; Damages."
 
    DISSENTING  STOCKHOLDERS' RIGHTS.   Holders of  DUAL Common  Stock and ENSCO
Common Stock  are not  entitled to  dissenters' rights  in connection  with  the
Merger.
 
    TREATMENT  OF STOCK OPTIONS.  Pursuant  to the Merger Agreement, all options
to purchase shares of stock in DUAL ("DUAL Options") outstanding under the  DUAL
DRILLING  COMPANY  Non-Employee Director  Stock  Option Plan  (the "Non-Employee
Director Plan") and the DUAL DRILLING COMPANY 1993 Long-Term Incentive Plan (the
"1993 Plan"), each as amended, will be surrendered and canceled and/or redeemed.
Within two days after the Effective Time, the Surviving Corporation will deliver
to the holders of each such DUAL Option a number of whole shares of ENSCO Common
Stock equal to the excess,  if any, of 0.625 over  a fraction, the numerator  of
which is the exercise price under such DUAL Option, and the denominator of which
is  the average of the closing prices of  ENSCO Common Stock on the NYSE for the
five business days  preceding the Effective  Time, multiplied by  the number  of
shares  covered  by such  DUAL  Option. See  "Certain  Provisions of  the Merger
Agreement -- Effect of the Merger on Stock Options."
 
    TREATMENT OF BENEFIT PLANS.  ENSCO and DUAL have agreed to continue  certain
of  DUAL's employee  benefit plans following  the Effective  Time. Certain other
such plans will be terminated and/  or modified. See "Certain Provisions of  the
Merger Agreement -- Effect of the Merger on Certain Benefit Plans."
 
                                       9
<PAGE>
PRINCIPAL STOCKHOLDER AGREEMENT
 
    VOTING  AND PROXY.  ENSCO has entered into an agreement with Dual Invest ASA
(formerly  Mosvold  Shipping  AS),  a  Norwegian  corporation  (the   "Principal
Stockholder"),  which owns approximately 59.6% of the outstanding shares of DUAL
Common Stock (the  "Principal Stockholder  Shares"), dated March  21, 1996  (the
"Principal  Stockholder  Agreement"),  a  copy  of  which  is  attached  to this
Prospectus/Proxy Statement as Appendix D. Pursuant to the terms of the Principal
Stockholder Agreement, the  Principal Stockholder  has agreed to  vote, and  has
granted  a proxy to allow ENSCO to vote, the Principal Stockholder Shares (i) in
favor of the  Merger and the  Merger Agreement,  (ii) in favor  of adoption  and
approval   of  the   Unit  Plan,  and   (iii)  against  any   proposal  for  any
recapitalization, merger  (other  than the  Merger),  sale of  assets  or  other
business  combination between DUAL and any person or entity (other than ENSCO or
Merger Subsidiary) or  any other  action or  agreement that  ENSCO notifies  the
Principal Stockholder in writing before any vote would result in a breach of any
covenant,  representation or  warranty or any  other obligation  or agreement of
DUAL under the Merger Agreement or which  could result in any of the  conditions
to  the Merger Agreement not being fulfilled. ENSCO and Merger Subsidiary intend
that the Principal Stockholder Shares be so voted.
 
    ABSTENTION FROM VOTING.   Pursuant to  the Principal Stockholder  Agreement,
the  Principal Stockholder has agreed that until July 31, 1996, it will not vote
any of the  Principal Stockholder  Shares at  any annual,  special or  adjourned
meeting  of the stockholders of  DUAL, including the right  to sign its name (as
stockholder) to any consent, certificate or other document relating to DUAL that
the law of the State  of Delaware may permit or  require, (i) to approve of  the
adoption  of the Unit Plan,  effective August 21, 1995,  in any manner except as
contemplated by the Merger Agreement, or (ii) in any manner that is intended, or
could reasonably be  expected, to  impede, interfere with,  delay, postpone,  or
materially   adversely  affect  the  transactions  contemplated  by  the  Merger
Agreement. See "Principal Stockholder Agreement -- Abstention from Voting."
 
    COVENANTS AND  REPRESENTATIONS.    Pursuant  to  the  Principal  Stockholder
Agreement,  the Principal Stockholder  has agreed that it  has been advised that
(i) the offer,  sale and delivery  of the  ENSCO Common Stock  to the  Principal
Stockholder  pursuant to the  Merger may not be  registered under the Securities
Act, despite  ENSCO's  obligations to  use  commercially reasonable  efforts  to
effect  such registration;  (ii) if  the offer, sale  and delivery  of the ENSCO
Common Stock to the  Principal Stockholder pursuant to  the Merger has not  been
registered  under the Securities Act, then such shares may not be offered, sold,
pledged, hypothecated or  otherwise transferred  unless subsequently  registered
under  the Securities Act  or an exemption from  such registration is available;
and (iii) even if such sale and delivery to the Principal Stockholder of  shares
of  ENSCO Common Stock is registered under the Securities Act, to the extent the
Principal Stockholder  is considered  an "affiliate"  of DUAL  at the  time  the
Merger Agreement is submitted for a vote of the stockholders of DUAL, any public
offering  or sale  by the  Principal Stockholder of  its shares  of ENSCO Common
Stock will, under current  law, require (a) the  further registration under  the
Securities  Act of  such shares,  which ENSCO  is obligated  to use commercially
reasonable efforts to effect,  (b) compliance with Rule  145 promulgated by  the
Commission  under  the  Securities  Act,  or  (c)  the  availability  of another
exemption from  such registration  under the  Securities Act.  Furthermore,  the
Principal  Stockholder also  has agreed that  it understands  that stop transfer
instructions will be given to ENSCO's transfer agent with respect to the  shares
of  ENSCO Common Stock receivable by the Principal Stockholder and that a legend
will be  placed on  the certificates  for such  shares issued  to the  Principal
Stockholder,   to  the  extent  the   Principal  Stockholder  is  considered  an
"affiliate" of DUAL at the time the Merger Agreement is submitted for a vote  of
the  stockholders of DUAL. See "Principal Stockholder Agreement -- Covenants and
Representations."
 
    REGISTRATION OF  MERGER  SHARES.   Pursuant  to  the  Principal  Stockholder
Agreement, ENSCO has agreed to use all commercially reasonable efforts to effect
the registration under the Securities Act of the transfer to the stockholders of
the  Principal Stockholder of certain of the  shares of ENSCO Common Stock to be
received by the Principal Stockholder in exchange for the Principal  Stockholder
 
                                       10
<PAGE>
Shares  and  the  resale  of  shares of  ENSCO  Common  Stock  by  the Principal
Stockholder  or  one  of  its  significant  stockholders,  subject  to   certain
restrictions.  See "Principal  Stockholder Agreement  -- Registration  of Merger
Shares."
 
    NO SOLICITATION.    Pursuant to  the  Principal Stockholder  Agreement,  the
Principal Stockholder has agreed that until July 31, 1996, it will not negotiate
with  any person other than ENSCO with respect to the acquisition of DUAL or the
DUAL Common Stock owned by the Principal  Stockholder and it will not, and  will
not  permit any of its officers, directors, employees, agents or representatives
(including without limitation, investment bankers, attorneys and accountants) to
(i) initiate contact  with, (ii)  make, solicit  or encourage  any inquiries  or
proposals,  (iii) enter into  or participate in  any discussions or negotiations
with, (iv) disclose,  directly or  indirectly, any  information not  customarily
disclosed  concerning  the  business and  properties  of DUAL  or  the Principal
Stockholder's interest in DUAL  under the control  of the Principal  Stockholder
to,  or (v)  afford any access  to DUAL's  properties, books and  records in its
possession or under its  control to any person  in connection with any  possible
proposal   relating  to  (a)   the  disposition  of   DUAL's  or  the  Principal
Stockholder's businesses or  substantially all of  their respective assets,  (b)
the  acquisition  of  equity  or  debt  securities  of  DUAL  or  the  Principal
Stockholder, including equity or debt securities in DUAL owned by the  Principal
Stockholder,  or  (c) the  merger, share  exchange  or business  combination, or
similar  acquisition  transaction  of  or   involving  DUAL  or  the   Principal
Stockholder  with any person other than ENSCO. In addition, until July 31, 1996,
the Principal Stockholder  has agreed  to immediately notify  ENSCO orally,  and
subsequently  confirm  in  writing, all  the  relevant details  relating  to all
inquiries and  proposals which  it may  receive relating  to any  such  matters.
Furthermore,  until July 31, 1996, the  Principal Stockholder has agreed that it
will not, and will not permit any of its representatives, at any time, to  enter
into or participate in any discussions or negotiations regarding, or accept, any
proposal  for such a transaction  received by them from a  third party or that a
third party expresses a desire to communicate to it. See "Principal  Stockholder
Agreement -- No Solicitation."
 
COMPARISONS OF STOCKHOLDERS' RIGHTS.
 
    Upon   consummation  of  the  Merger,   stockholders  of  DUAL  will  become
stockholders of  ENSCO.  Various  differences  exist  between  their  rights  as
stockholders  of  DUAL  and  as  stockholders  of  ENSCO.  See  "Comparisons  of
Stockholders' Rights."
 
COMPARATIVE MARKET PRICES AND DIVIDENDS
 
    The following table sets forth, for the quarters indicated, the high and low
sales prices per share  of ENSCO Common  Stock and share  of DUAL Common  Stock.
ENSCO  Common Stock was listed on the  American Stock Exchange prior to December
20, 1995, and subsequent to such date  on the NYSE under the symbol "ESV."  DUAL
Common  Stock is quoted on  the Nasdaq National Market  under the symbol "DUAL."
Each of ENSCO's and DUAL's fiscal year ends on December 31. On January 24, 1996,
the last full trading day preceding the public announcement of the execution  of
the  letter of intent between  ENSCO and DUAL, the  last reported sale price per
share of ENSCO Common  Stock was $22  3/4 and the last  reported sale price  per
share  of DUAL Common  Stock was $13 1/16.  Based on such  DUAL and ENSCO market
prices, the market value of 0.625 of  a share of ENSCO Common Stock was  $14.22,
calculated  as of that  date. On May 8,  1996, the last  reported sale price per
share of ENSCO
 
                                       11
<PAGE>
Common Stock was $26, and the last reported sale price per share of DUAL  Common
Stock  was $15 3/4. Based on such DUAL and ENSCO market prices, the market value
of 0.625 of  a share of  ENSCO Common Stock  was $16.25, calculated  as of  that
date.
 
<TABLE>
<CAPTION>
                                                                          ENSCO                  DUAL
                                                                     COMMON STOCK (1)        COMMON STOCK
                                                                    ------------------    ------------------
                                                                     HIGH        LOW       HIGH        LOW
                                                                    -------    -------    -------    -------
<S>                                                                 <C>        <C>        <C>        <C>
CALENDAR YEAR 1994
  1st Quarter....................................................   $17        $12 1/2    $11 3/4    $ 8 1/2
  2nd Quarter....................................................    18 5/8     13 1/2     11 3/8      8 3/8
  3rd Quarter....................................................    19 1/4     14 5/8     12 5/8     10 1/2
  4th Quarter....................................................    15 1/2     10 3/4     13 5/8      8 1/4
CALENDAR YEAR 1995
  1st Quarter....................................................    14 3/8     11 1/4      9 1/8      7
  2nd Quarter....................................................    17 3/8     14         11 1/4      7 7/8
  3rd Quarter....................................................    19 1/2     14 1/4     10 3/4      9
  4th Quarter....................................................    23         16         11 5/8      9 1/4
CALENDAR YEAR 1996
  1st Quarter....................................................    29 1/8     20         18 1/4     11 1/4
  2nd Quarter (through May 8, 1996)..............................    31 3/8     25 3/8     19 3/4     14 1/4
</TABLE>
 
- ------------------------
(1) Prices  for shares  of ENSCO Common  Stock prior  to June 1,  1994 have been
    adjusted to  reflect  a  one  share for  four  shares  reverse  stock  split
    effective on that date.
 
    ENSCO  has never paid dividends to holders  of shares of ENSCO Common Stock.
DUAL has not paid dividends  to holders of DUAL  Common Stock since its  initial
public  offering in August 1993. ENSCO intends to retain all of its earnings for
use in its business and, therefore, does not anticipate paying cash dividends on
ENSCO Common Stock in the foreseeable future.
 
    HOLDERS OF DUAL  COMMON STOCK  ARE URGED  TO OBTAIN  CURRENT QUOTATIONS  FOR
SHARES OF ENSCO COMMON STOCK AND SHARES OF DUAL COMMON STOCK.
 
SELECTED HISTORICAL FINANCIAL DATA
 
    The  following tables  set forth selected  historical consolidated financial
data of ENSCO and  DUAL. The selected consolidated  financial data for the  five
years  ended December  31, 1995 for  ENSCO and  DUAL has been  obtained from the
consolidated financial statements of each  of ENSCO and DUAL, respectively.  The
consolidated  financial statements for  ENSCO were audited  by Deloitte & Touche
LLP, independent accountants, for the years 1991-1992, and Price Waterhouse LLP,
independent accountants, for the years  1993-1995. The financial statements  for
ENSCO's  Venezuela  operations were  audited by  Krygier, Montilla  & Asociados,
independent accountants,  for 1993.  The consolidated  financial statements  for
DUAL  were audited  by Price  Waterhouse LLP,  independent accountants,  for the
years 1991-1995.  This data  should be  read in  conjunction with  the  separate
Consolidated  Financial Statements and Notes thereto  of each of ENSCO and DUAL,
included in  the  ENSCO  1995  Form  10-K  and  in  the  DUAL  1995  Form  10-K,
respectively,   both   of  which   are   incorporated  by   reference   in  this
Prospectus/Proxy Statement.
 
                                       12
<PAGE>
                        ENSCO INTERNATIONAL INCORPORATED
                SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA
                    (IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
 
<TABLE>
<CAPTION>
                                                        THREE MONTHS
                                                      ENDED MARCH 31,                    YEAR ENDED DECEMBER 31,
                                                    --------------------  -----------------------------------------------------
                                                      1996       1995       1995       1994       1993     1992 (1)   1991 (1)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
<S>                                                 <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA (2)
  Operating revenues..............................  $  84,546  $  61,130  $ 279,114  $ 245,451  $ 227,410  $  84,271  $  76,221
  Operating expenses, excluding D&A...............     45,739     38,238    165,529    144,581    151,182     81,999     66,301
  Depreciation and amortization (D&A).............     16,374     13,546     58,390     51,798     41,181     12,539     13,030
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Operating income (loss).........................     22,433      9,346     55,195     49,072     35,047    (10,267)    (3,110)
  Other expense...................................     (2,549)    (1,299)    (7,856)    (8,751)    (6,696)    (8,028)    (2,025)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) from continuing operations before
   income taxes, minority interest and cumulative
   effect of accounting change....................     19,884      8,047     47,339     40,321     28,351    (18,295)    (5,135)
  Provision for income taxes......................     (4,767)       (39)    (3,397)    (3,759)    (5,942)    (2,007)    (4,109)
  Minority interest...............................       (427)      (602)    (2,179)    (2,984)    (6,932)    --         --
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) from continuing operations........     14,690      7,406     41,763     33,578     15,477    (20,302)    (9,244)
  Income (loss) from discontinued operations
   (2)............................................     --            216      6,296      3,593      3,556     (9,062)    (3,543)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) before cumulative effect of
   accounting change..............................     14,690      7,622     48,059     37,171     19,033    (29,364)   (12,787)
  Cumulative effect of accounting change, net of
   minority interest (3)..........................     --         --         --         --         (2,542)    --         --
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Net income (loss)...............................     14,690      7,622     48,059     37,171     16,491    (29,364)   (12,787)
  Preferred stock dividend requirements...........     --         --         --         (2,135)    (4,260)    (4,260)    (4,607)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) applicable to common stock........  $  14,690  $   7,622  $  48,059  $  35,036  $  12,231  $ (33,624) $ (17,394)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) per common share:
    Continuing operations.........................  $     .24  $     .12  $     .69  $     .55  $     .28  $    (.82) $    (.57)
    Discontinued operations.......................     --            .01        .10        .06        .09       (.30)      (.14)
    Cumulative effect of accounting change........     --         --         --         --           (.07)    --         --
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
    Income (loss) per common share................  $     .24  $     .13  $     .79  $     .61  $     .30  $   (1.12) $    (.71)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Weighted average common shares outstanding......     60,651     60,648     60,527     57,843     40,325     30,003     24,407
 
BALANCE SHEET DATA
  Working capital.................................  $  83,890  $  86,368  $  78,945  $ 129,172  $ 124,587  $  33,771  $  22,947
  Total assets....................................    824,620    776,469    821,451    773,090    689,254    272,397    295,722
  Long-term debt, net of current portion..........    150,518    148,967    159,201    162,466    125,983     23,628     31,437
  $1.50 preferred stock...........................     --         --         --         --         70,977     70,977     70,977
  Stockholders' equity (4)........................    546,837    488,803    531,249    487,950    383,925    142,512    163,990
</TABLE>
 
- ------------------------
(1)  Amounts  have  been  restated  for  adoption  of  Statement  of   Financial
     Accounting  Standards No.  109 "Accounting  for Income  Taxes." For further
     discussion of  this matter,  see Notes  1 and  11 to  ENSCO's  Consolidated
     Financial  Statements,  contained in  the ENSCO  1995  Form 10-K,  which is
     incorporated herein by reference.
 
(2)  In 1995 ENSCO sold  its technical services segment  and in 1993 ENSCO  sold
     its  supply segment. Prior years' results of the technical services segment
     and the supply  segment have been  reclassified as discontinued  operations
     for  comparative purposes. The 1995 results  include a gain of $5.2 million
     in connection with the sale of the technical services segment and the  1993
     results  include a gain of $2.1 million  in connection with the sale of the
     supply segment.  For further  discussion  of this  matter  see Note  16  to
     ENSCO's Consolidated Financial Statements, contained in the ENSCO 1995 Form
     10-K, which is incorporated herein by reference.
 
(3)  Effective  January 1, 1993, Penrod Holding Company, the outstanding capital
     stock of  which  was  fully  acquired by  ENSCO  in  August  1993,  adopted
     Statement of Financial Accounting Standards No. 106, "Employers' Accounting
     for Postretirement Benefits Other Than Pensions." For further discussion of
     this  matter,  see Note  10 to  ENSCO's Consolidated  Financial Statements,
     contained in the  ENSCO 1995  Form 10-K,  which is  incorporated herein  by
     reference.
 
(4)  ENSCO  has never paid cash dividends on  its common stock. ENSCO intends to
     retain all of its earnings for use in its business and, therefore, does not
     anticipate paying cash dividends on  ENSCO Common Stock in the  foreseeable
     future.
 
                                       13
<PAGE>
                             DUAL DRILLING COMPANY
 
                SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA
                    (IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
 
<TABLE>
<CAPTION>
                                                        THREE MONTHS
                                                      ENDED MARCH 31,                    YEAR ENDED DECEMBER 31,
                                                    --------------------  -----------------------------------------------------
                                                      1996       1995       1995       1994       1993     1992 (1)   1991 (1)
                                                    ---------  ---------  ---------  ---------  ---------  ---------  ---------
<S>                                                 <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA
  Drilling contract revenues......................  $  29,461  $  18,858  $  85,889  $ 104,265  $ 116,152  $  95,577  $  88,179
  Drilling contract expenses......................     18,589     14,605     60,229     78,571     77,671     74,540     64,525
  Depreciation and amortization...................      4,813      5,158     19,608     24,943     20,411     19,280     19,844
  General and administrative......................      2,040      1,914      7,563      8,979      7,481      5,854      9,184
  Interest expense, net...........................      3,107      3,116     12,305     11,083      8,482      8,831     11,197
  Gain on sale of assets, net.....................     --             21      5,127        994      3,951      6,947      3,524
  Income (loss) before income taxes and cumulative
   effect of accounting change....................      1,004     (5,967)    (8,353)   (17,153)     6,183     (6,134)   (12,126)
  Cumulative effect of accounting change (1)......     --         --         --         --         (4,865)    --         --
  Net income (loss)...............................        974     (6,023)    (9,238)   (17,196)    (1,786)       (62)    (8,327)
  Weighted average common shares outstanding (2)..     15,766     15,765     15,766     15,780     --         --         --
  Net income (loss) per common share(3)...........       0.06      (0.38)     (0.59)     (1.09)    --         --         --
 
BALANCE SHEET DATA
  Working capital (deficit).......................  $  55,423  $  21,333  $  55,612  $  26,748  $  36,552  $  (4,568) $ (30,380)
  Total assets....................................    303,760    311,808    303,762    322,621    283,607    233,729    240,384
  Long term debt, net of current portion..........    135,249    142,427    138,163    144,702     86,550     86,550     85,000
  Stockholders' equity (3)........................    141,121    143,361    140,146    149,384    165,691     83,090     65,867
</TABLE>
 
- ------------------------
(1)  DUAL  adopted SFAS No. 109, "Accounting for Income Taxes" effective January
     1, 1993. Prior  to the adoption  of SFAS  109, DUAL followed  SFAS No.  96,
     "Accounting for Income Taxes."
 
(2)  Weighted average common shares outstanding and earnings per share data have
     not been presented for the periods ending prior to December 31, 1994, since
     the  results of operations of DUAL  are not indicative of future operations
     due to DUAL's restructuring resulting  from the initial public offering  of
     DUAL Common Stock. For further discussion of this matter, see Note 2 to the
     Consolidated  Financial Statements  contained in  the DUAL  1995 Form 10-K,
     which is incorporated herein by reference.
 
(3)  DUAL has  not paid  dividends to  holders of  DUAL Common  Stock since  its
     initial public offering in August 1993.
 
SELECTED UNAUDITED COMBINED CONDENSED PRO FORMA FINANCIAL DATA
 
    The following selected unaudited combined condensed pro forma financial data
is presented for illustrative purposes only and is not necessarily indicative of
the  financial position  or results of  operation that would  actually have been
reported had the Merger been in effect during the periods presented or that  may
be  reported in the future. The  selected unaudited combined condensed pro forma
financial data  should  be  read  in conjunction  with  the  unaudited  combined
condensed pro forma financial statements, including the notes thereto, appearing
elsewhere  in this Prospectus/Proxy Statement. See "Unaudited Combined Condensed
Pro Forma Financial Information."
 
                                       14
<PAGE>
           ENSCO INTERNATIONAL INCORPORATED AND DUAL DRILLING COMPANY
                          SELECTED UNAUDITED COMBINED
                       CONDENSED PRO FORMA FINANCIAL DATA
                    (IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
 
<TABLE>
<CAPTION>
                                                                              ENSCO        DUAL        PRO FORMA
                                                                           HISTORICAL   HISTORICAL     COMBINED
                                                                           -----------  -----------  -------------
<S>                                                                        <C>          <C>          <C>
THREE MONTHS ENDED MARCH 31, 1996
STATEMENT OF OPERATIONS DATA
  Operating revenues.....................................................  $    84,546  $    29,461  $     114,007
  Income from continuing operations......................................       14,690          974         14,663
  Income from continuing operations per share of common stock............         0.24         0.06           0.21
  Current assets.........................................................      161,812       78,823        223,904
  Current liabilities....................................................       77,922       23,400        109,642
  Working capital........................................................       83,890       55,423        114,262
  Total assets...........................................................      824,620      303,760      1,243,918
  Long-term obligations..................................................      199,861      139,239        365,683
  Stockholders' equity...................................................      546,837      141,121        768,593
  Weighted average common shares outstanding.............................       60,651       15,766         70,789
COMPARATIVE PER SHARE DATA (1)
  Book value per common share as of March 31, 1996.......................         9.02         8.95          10.86
  Income per common share from continuing operations.....................         0.24         0.06           0.21
DUAL EQUIVALENT PER SHARE DATA (1)(2)
  Book value per common share as of March 31, 1996.......................                                     6.79
  Income per common share from continuing operations.....................                                     0.13
 
YEAR ENDED DECEMBER 31, 1995
STATEMENT OF OPERATIONS DATA
  Operating revenues.....................................................  $   279,114  $    85,889  $     370,130
  Income (loss) from continuing operations...............................       41,763       (9,238)        28,960
  Income (loss) from continuing operations per share of common stock.....         0.69        (0.59)          0.41
  Weighted average common shares outstanding.............................       60,527       15,766         70,665
COMPARATIVE PER SHARE DATA (1)
  Book value per common share as of December 31, 1995....................         8.78         8.89          10.66
  Income (loss) per common share from continuing operations..............         0.69        (0.59)          0.41
DUAL EQUIVALENT PER SHARE DATA (1)(2)
  Book value per common share as of December 31, 1995....................                                     6.66
  Income (loss) per common share from continuing operations..............                                     0.26
</TABLE>
 
- ------------------------
(1) ENSCO has never paid dividends to  holders of shares of ENSCO Common  Stock.
    DUAL  has  not paid  dividends to  holders  of DUAL  Common Stock  since its
    initial public offering in August 1993.  ENSCO intends to retain all of  its
    earnings  for use in its business and, therefore, does not anticipate paying
    cash dividends on ENSCO Common Stock in the foreseeable future.
 
(2) Based upon conversion of each share of ENSCO Common Stock into 1.6 shares of
    DUAL Common Stock.
 
                                       15
<PAGE>
PROPOSAL TO APPROVE THE DUAL SPECIAL PERFORMANCE UNIT PLAN
 
    Effective August 21, 1995, DUAL's Board of Directors, upon a  recommendation
of  its Compensation Committee, adopted the  Unit Plan and the performance goals
set forth therein,  subject to approval  by the stockholders  of DUAL. The  Unit
Plan  is designed to retain  and reward key executives  of DUAL by granting them
special cash  bonuses in  the event  of a  Sale Transaction  (as defined  below)
involving  DUAL during the term  of the Unit Plan. The  Unit Plan is intended to
provide the participants  in the  Unit Plan with  an incentive  to increase  the
value  of DUAL's business by  allowing them to participate  in a cash bonus pool
that is commensurate with the sale price of DUAL.
 
    The following summary does not purport to be complete and is subject in  all
respects to, and qualified by, the provisions of the Unit Plan, which appears as
Appendix  C to this Prospectus/Proxy Statement,  and the description of the Unit
Plan contained  at  "Proposal  to  Approve the  Adoption  of  the  DUAL  Special
Performance Unit Plan." The Unit Plan is intended to allow the award of benefits
that  qualify as  performance-based compensation  within the  meaning of Section
162(m) of the Code.
 
    The Unit Plan will be administered by a committee appointed by the Board  of
Directors  of DUAL. Receipt of benefits pursuant to the Unit Plan is conditioned
upon occurrence of a merger, consolidation,  or other reorganization of DUAL  in
which  the  outstanding DUAL  Common Stock  is converted  into or  exchanged for
securities of another issuer, cash, or  other property, or upon occurrence of  a
sale,  lease, or  exchange of  all or  substantially all  of the  assets of DUAL
(collectively, "Sale Transactions"). The bonus  pool to be distributed  pursuant
to  the Unit Plan is a cash amount  based on the consideration to be received in
such Sale Transaction  per share  of DUAL  Common Stock  (the "Equivalent  Share
Price").
 
    Pursuant  to the  Merger Agreement,  DUAL has agreed  to set  the bonus pool
associated with the Merger, together with  certain payments due to Mr. David  W.
Skarke, Chairman of the Board of DUAL, under his employment agreement with DUAL,
at  $2,000,000.  See  "The  Merger --  Interests  of  Certain  Persons; Possible
Conflicts of Interest -- Skarke Agreement."
 
    Unless the Unit Plan is required by  applicable law to be extended, it  will
terminate  on August  20, 1997,  and payments  under it  will only  be made with
respect to a Sale Transaction  which is effective, or  as to which a  definitive
binding  agreement is in  effect, on or before  that date. The  Unit Plan may be
amended by the Board of Directors  of DUAL, subject to any stockholder  approval
required by Section 162(m) of the Code or other applicable law.
 
    Approval  of the Unit Plan is not a condition to consummation of the Merger,
and approval of the Merger is not a condition to effectiveness of the Unit Plan.
 
    The proposal  to approve  the adoption  of the  Unit Plan  must receive  the
favorable  vote of a majority of the total number of shares of DUAL Common Stock
represented and  entitled to  vote at  the Special  Meeting or  any  adjournment
thereof  for approval. An affirmative vote by a stockholder shall also be deemed
to be approval  of the performance  goals under  the Unit Plan  for purposes  of
Section 162(m) of the Code. Pursuant to the Principal Stockholder Agreement, the
Principal   Stockholder   Shares  (representing   approximately  59.6%   of  the
outstanding shares of DUAL Common Stock) will be voted at the Special Meeting in
favor of the Unit Plan. Therefore, no additional stockholder votes are  required
to  approve the Unit Plan.  THE BOARD OF DIRECTORS OF  DUAL HAS ADOPTED THE UNIT
PLAN AND RECOMMENDS THAT DUAL STOCKHOLDERS VOTE FOR THE PROPOSAL TO APPROVE  THE
ADOPTION  OF THE UNIT  PLAN. See "Proposal  to Approve the  Adoption of the DUAL
Special Performance Unit Plan."
 
                                       16
<PAGE>
                                  RISK FACTORS
 
    DUAL STOCKHOLDERS SHOULD CAREFULLY EVALUATE ALL OF THE INFORMATION CONTAINED
AND  INCORPORATED  BY  REFERENCE  IN  THIS  PROSPECTUS/PROXY  STATEMENT  AND, IN
PARTICULAR, THE FOLLOWING:
 
INDUSTRY CONDITIONS AND COMPETITION
 
    Historically, the  offshore  contract  drilling  industry  has  been  highly
competitive and cyclical, with periods of high demand, short rig supply and high
day  rates followed  by periods  of low  demand, excess  rig supply  and low day
rates. The industry is characterized by high capital costs, long lead times  for
construction of new rigs and numerous competitors.
 
    ENSCO  conducts its business in  the U.S. Gulf of  Mexico, the North Sea and
Venezuela. Business activity levels for  ENSCO, and its corresponding  operating
results,  are significantly affected  by worldwide expenditures  for oil and gas
drilling, particularly  in the  U.S. Gulf  of  Mexico where  ENSCO has  a  large
concentration  of its  rigs and vessels.  Expenditures for oil  and gas drilling
activity fluctuate based upon many factors including world economic  conditions,
the  legislative  environment  in  the  U.S.  and  other  major  countries,  the
availability of drilling units, production levels, and other activities of  OPEC
and  other oil and gas producers and the impact that these and other events have
on the current and expected future pricing of oil and natural gas.
 
    For a number of  years, depressed oil  and gas prices  and an oversupply  of
drilling rigs have adversely affected the offshore drilling market. In addition,
ENSCO  has significant competition from many other offshore drilling contractors
in all of  the areas in  which it operates.  Activity levels for  most areas  in
which ENSCO operates, including the U.S. Gulf of Mexico, increased in the second
half  of 1995  and the first  part of 1996  due, in part,  to increased domestic
natural gas  prices. During  this  time ENSCO  has experienced  a  corresponding
increase  in day rates. ENSCO cannot predict  the extent to which current market
conditions will continue.
 
    There can be no  assurance that the  rig count or  drilling activity in  the
areas  in which ENSCO operates will not decline in 1996 or in other periods, nor
can there be  any assurance concerning  any adverse effect  resulting from  such
decrease in activity.
 
ENVIRONMENTAL MATTERS
 
    ENSCO  and DUAL  are subject to  numerous domestic  and foreign governmental
regulations controlling  the  discharge of  materials  into the  environment  or
otherwise  relating to the  protection of the  environment. Laws and regulations
specifically applicable to ENSCO's and  DUAL's business activities could  impose
significant  liability on them for damages, cleaning costs, and penalties in the
event of oil spills or similar discharges of pollutants into the environment  in
the  course  of the  companies'  operations, although,  to  date, such  laws and
regulations have  not had  a materially  adverse effect  on the  results of  the
companies'  operations, nor have  they experienced an  accident that has exposed
them to material liability  for discharges of  pollutants into the  environment.
Under  certain  circumstances,  environmental laws  and  regulations  may impose
"strict liability" and render a company liable for environmental damage  without
regard  to negligence or fault; such laws  and regulations could expose ENSCO or
DUAL to  liability  for  the conduct  of  or  conditions caused  by  others.  In
addition,  events of recent  years have heightened  environmental concerns about
the oil and gas industry generally. From time to time legislative proposals have
been introduced that  would materially  limit or prohibit  offshore drilling  in
certain  areas. To  date, no proposals  that would materially  limit or prohibit
drilling in certain areas  have been enacted  into law. If  laws are enacted  or
other  governmental action is taken that  restrict or prohibit offshore drilling
in ENSCO's  or DUAL's  areas  of operation  or impose  environmental  protection
requirements  that  materially  increase  the  costs  of  offshore  exploration,
development or production  of oil and  gas, they could  be materially  adversely
affected.
 
    The  United  States  Oil  Pollution  Act  of  1990  ("OPA'90")  and  similar
legislation enacted in  Texas, Louisiana  and other coastal  states address  oil
spill  prevention and control and significantly expand liability exposure across
all segments of the oil and  gas industry. OPA'90, such similar legislation  and
 
                                       17
<PAGE>
related regulations impose a variety of obligations on ENSCO and DUAL related to
the  prevention  of oil  spills and  liability for  damages resulting  from such
spills. OPA'90 imposes strict  and, with limited  exceptions, joint and  several
liability  upon each responsible  party for oil  removal costs and  a variety of
public and private damages. OPA'90 also imposes ongoing financial responsibility
requirements  on  a  responsible  party.  A  failure  to  comply  with   ongoing
requirements  or inadequate  cooperation in  a spill  may subject  a responsible
party, including in some cases ENSCO  or DUAL, to civil or criminal  enforcement
action.  Also, the  U.S. Minerals Management  Service is  required to promulgate
regulations to implement the financial responsibility requirements for  offshore
facilities. If implemented as written, the financial responsibility requirements
of  OPA'90  could have  the  effect of  significantly  increasing the  amount of
financial responsibility that oil and  gas operators must demonstrate to  comply
with  OPA'90. While  industry groups and  marine insurance  carriers are seeking
modification of  these requirements,  implementation  of these  requirements  in
their current form could adversely affect the ability of some customers of ENSCO
or DUAL to operate in U.S. waters, which could have a material adverse effect on
either ENSCO or DUAL.
 
LIMITATIONS ON OWNERSHIP BY NON-U.S. CITIZENS
 
    ENSCO,  as  the owner  of  United States  flag  vessels, is  subject  to the
Shipping Act, 1916, as  amended, which provides that  a controlling interest  in
ENSCO  may not be acquired by a non-U.S. citizen without the consent of the U.S.
Secretary  of  Transportation,  acting   through  the  United  States   Maritime
Administration  ("MARAD"). If a  non-U.S. citizen were  to acquire a controlling
interest in  ENSCO  without MARAD's  consent,  MARAD  would have  the  right  to
exercise  various remedies under  the Shipping Act,  1916, as amended, including
seizure of vessels, civil penalties and certain misdemeanor criminal penalties.
 
    Therefore, ownership and control of ENSCO Common Stock by non-U.S.  citizens
is limited by the terms of the ENSCO Certificate of Incorporation. Under certain
circumstances,  transfers of ENSCO Common Stock to non-U.S. citizens may be void
and certain ENSCO Common Stock owned  by non-U.S. citizens may not be  permitted
to  vote  or  receive dividends.  See  "Comparisons of  Stockholders'  Rights --
Restrictions on Alien Ownership."
 
OPERATIONAL RISKS AND INSURANCE
 
    ENSCO's and DUAL's operations  are subject to the  many hazards inherent  in
the  drilling business, including blowouts,  cratering, fires, reservoir damage,
loss of production, loss of well  control, collisions or groundings of  drilling
equipment,  and damage or loss from adverse  weather and seas, which could cause
substantial damage to the environment.  These hazards could also cause  personal
injury  and loss  of life,  suspend drilling  operations or  seriously damage or
destroy the property and  equipment involved and,  in addition to  environmental
damage,  could cause substantial damage  to producing formations and surrounding
areas. Their offshore drilling equipment also is subject to hazards inherent  in
marine  operations, such as capsizing, grounding, collision, damage from weather
or sea  conditions or  unsound location.  In addition,  they may  be subject  to
liability  for oil spills, reservoir damage and other accidents that could cause
substantial damages.
 
    ENSCO and DUAL  generally insure their  drilling rigs for  amounts not  less
than  the estimated  fair market value  thereof. ENSCO  also maintains liability
insurance coverage in amounts  and scope which  ENSCO's management believes  are
comparable  to the levels of coverage carried by other energy service companies.
To date, ENSCO has not  experienced difficulty in obtaining insurance  coverage.
While  ENSCO and DUAL  believe that their insurance  coverages are customary for
the energy service  industry, the occurrence  of a significant  event not  fully
insured  against  could have  a  material adverse  effect  on ENSCO's  or DUAL's
financial position.
 
GOVERNMENT REGULATION
 
    ENSCO's and DUAL's  business is  affected by political  developments and  by
federal,  state, local and foreign laws  and regulations that relate directly to
the oil and gas industry. Statutory provisions generally include requirements as
to well  spacing, waste  prevention, production  limitation, well  and  dredging
permits  and similar matters. The drilling industry is also affected by changing
tax laws, price
 
                                       18
<PAGE>
controls and  other  laws  affecting  the energy  business.  Drilling  rigs  and
operations are subject to federal, state, local and foreign laws and regulations
relating  to engineering, design, structural, safety, operational and inspection
standards. The  adoption  of laws  and  regulations curtailing  exploration  and
development  and drilling for  oil and gas for  economic, environmental or other
policy reasons would and have  adversely affected ENSCO's and DUAL's  operations
by   limiting  available  drilling  opportunities  for  their  customers  and/or
increasing the costs of such activities to ENSCO, DUAL, and their customers.
 
INTERNATIONAL OPERATIONS
 
    ENSCO's and  DUAL's  international  operations  are  subject  to  political,
economic,  and other uncertainties, such as  the risks of expropriation of their
equipment,  expropriation  of   a  customer's  property   or  drilling   rights,
repudiation  of contracts, adverse tax policies, general hazards associated with
international sovereignty over certain  areas in which  ENSCO and DUAL  operate,
and  fluctuations in  international economies.  To lessen  the risk  of possible
future adverse developments outside the United States, ENSCO, in some instances,
enters into  contracts  for indemnification  from  operators for  whom  drilling
services are being performed.
 
    ENSCO's  and  DUAL's international  operations face  the additional  risk of
fluctuating currency values and  exchange controls. Occasionally, the  countries
in  which  ENSCO or  DUAL operates  have enacted  exchange controls  to regulate
international currency  exchange. Historically,  ENSCO has  been able  to  limit
these  risks  by  obtaining  compensation in  United  States  dollars  or freely
convertible international  currency and,  to the  extent possible,  by  limiting
acceptance   of  blocked  currency  to   amounts  which  match  its  expenditure
requirements in local currencies.
 
    ENSCO has  significant  operations  in  Venezuela.  Venezuela  has  recently
encountered certain political and economic crises. To date, such crises have not
adversely affected ENSCO's business operations in that country.
 
CERTAIN ANTI-TAKEOVER EFFECTS
 
    ENSCO  has adopted a stockholder rights  plan, which may make an unsolicited
acquisition  of  ENSCO  more  difficult   or  expensive.  See  "Comparisons   of
Stockholders' Rights -- Anti-takeover Provisions."
 
INTERESTS OF CERTAIN PERSONS; POSSIBLE CONFLICTS OF INTERESTS OF CERTAIN PERSONS
IN THE MERGER
 
    Certain members of the management of DUAL and the Board of Directors of DUAL
have  interests in the Merger in addition  to their interests as stockholders of
DUAL. For instance,  certain such  persons will receive  severance and/or  other
employment  benefits under their employment agreements  or applicable plans as a
result of the Merger. See "The Merger -- Interests of Certain Persons;  Possible
Conflicts  of Interest."  Similarly, certain such  persons hold  options for the
purchase of DUAL Common Stock which will be extinguished in return for shares of
ENSCO Common Stock. See "Certain Provisions of the Merger Agreement -- Effect of
the Merger on Stock Options."
 
NO DIVIDENDS
 
    ENSCO has never paid dividends to  holders of shares of ENSCO Common  Stock.
DUAL  has not paid dividends  to holders of DUAL  Common Stock since its initial
public offering in August 1993. ENSCO intends to retain all of its earnings  for
use in its business and, therefore, does not anticipate paying cash dividends on
ENSCO Common Stock in the foreseeable future.
 
                              THE SPECIAL MEETING
 
GENERAL
 
    This  Prospectus/Proxy Statement is being  furnished to stockholders of DUAL
in connection with the solicitation of proxies by the Board of Directors of DUAL
for use at the Special Meeting of stockholders  to be held on June 12, 1996,  at
10:00  a.m., Central  time, at  Park City  Club, 5956  Sherry Lane,  Suite 1700,
Dallas, Texas 75225 and at any adjournments or postponements thereof.
 
                                       19
<PAGE>
    This Prospectus/Proxy Statement also constitutes the Prospectus with respect
to the shares of ENSCO Common Stock issuable in connection with the Merger.
 
PURPOSES OF THE SPECIAL MEETING
 
    At the Special Meeting, holders of shares of DUAL Common Stock will consider
and vote  upon  a proposal  to  approve and  adopt  the Merger  Agreement.  DUAL
stockholders  will also vote on a proposal to approve the Unit Plan and transact
such other business as may properly be brought before the Special Meeting or any
adjournments or  postponements  thereof. THE  BOARD  OF DIRECTORS  OF  DUAL  HAS
APPROVED  THE MERGER  AGREEMENT AND RECOMMENDS  THAT DUAL  STOCKHOLDERS VOTE FOR
APPROVAL AND ADOPTION OF THE MERGER AGREEMENT.
 
VOTE REQUIRED
 
    The affirmative vote of the holders of a majority of all outstanding  shares
of DUAL Common Stock is required to approve and adopt the Merger Agreement. As a
result,  abstentions, failures to  vote and broker non-votes  will have the same
effect as  votes against  the Merger  Agreement  since they  are not  votes  for
approval.  The affirmative vote  of the holders  of a majority  of the shares of
DUAL Common  Stock present  and voting  at the  Special Meeting  is required  to
approve the Unit Plan. Abstentions from voting on the Unit Plan will be included
in  the  voting tally  and will  have the  same  effect as  a vote  against that
proposal. Because  broker non-votes  are not  considered "shares  present"  with
respect  to a  matter requiring  the affirmative  vote of  a majority  of shares
present in person or by proxy at the Special Meeting, broker non-votes will  not
affect  the  outcome with  respect to  the  proposal to  approve the  Unit Plan.
Pursuant to  the  Principal  Stockholder Agreement,  the  Principal  Stockholder
Shares  (representing  approximately 59.6%  of  the outstanding  shares  of DUAL
Common Stock) will be present and voted  at the Special Meeting in favor of  the
Merger  Agreement and  the Unit  Plan. Therefore,  no additional  votes from the
stockholders of DUAL are necessary to  approve the Merger Agreement or the  Unit
Plan. See "Principal Stockholder Agreement."
 
VOTING OF PROXIES
 
    Shares  of  DUAL  Common  Stock  represented  by  properly  executed proxies
received at or prior to the Special Meeting, and which have not thereafter  been
properly  revoked  as described  below,  will be  voted  in accordance  with the
instructions indicated therein. If no  instructions are indicated, such  proxies
will be voted FOR approval and adoption of the Merger Agreement, FOR approval of
the  Unit Plan, and in the discretion of the proxy holder as to any other matter
that may properly come before the Special Meeting.
 
REVOCABILITY OF PROXIES
 
    A DUAL stockholder who has given a  proxy may revoke such proxy at any  time
prior  to its exercise  at the Special  Meeting by any  manner permitted by law,
including (i) giving written notice of  revocation by mail or facsimile to  DUAL
prior  to the  Special Meeting or  (ii) properly  submitting to DUAL  by mail or
facsimile a duly executed proxy bearing a later date. Submissions to DUAL should
be made to  DUAL DRILLING  COMPANY, Attn:  Corporate Secretary,  at 5956  Sherry
Lane,  Suite 1500, Dallas, Texas 75225, facsimile (214) 373-0533. Any beneficial
owner whose shares of DUAL Common Stock are registered in the name of a  broker,
dealer, commercial bank, trust company or other nominee and who wishes to change
such  stockholder's  vote should  contact  such registered  holder  promptly and
instruct such registered holder on such  beneficial owner's behalf. There is  no
guarantee  that there will be  sufficient time prior to  the Special Meeting for
the registered holder to deliver a revocation upon instruction by the beneficial
owner.
 
SOLICITATION OF PROXIES
 
    DUAL  will  bear  the  cost  of   the  solicitation  of  proxies  from   its
stockholders,  except  that  ENSCO  and  DUAL will  share  equally  the  cost of
printing, filing,  and certain  other  fees and  expenses associated  with  this
Prospectus/Proxy  Statement. In addition to the solicitation of proxies by mail,
directors, officers and employees of DUAL may solicit proxies from  stockholders
personally   or  by  telephone,  telegraph   or  facsimile  transmissions.  Such
directors, officers and employees will not be additionally
 
                                       20
<PAGE>
compensated for such solicitation but may be reimbursed for their  out-of-pocket
expenses  incurred in connection  therewith. Arrangements may  also be made with
banks, brokerage houses and other  custodians, nominees and fiduciaries for  the
forwarding  of solicitation  material to  the beneficial  owners of  DUAL Common
Stock held of  record by such  persons, and, upon  request, DUAL will  reimburse
such  custodians, nominees  and fiduciaries  for their  reasonable out-of-pocket
expenses in connection therewith. The Company also has employed D.F. King & Co.,
a proxy solicitation firm, to solicit proxies  from brokers and banks at a  cost
of approximately $3,000.
 
RECORD DATE; SHARES ENTITLED TO VOTE
 
    The  close of business on May 1, 1996  has been fixed as the Record Date for
determining holders of DUAL Common  Stock entitled to notice  of and to vote  at
the  Special Meeting. As  of the Record  Date, 15,765,713 shares  of DUAL Common
Stock were outstanding and held of record  by 28 holders. The DUAL Common  Stock
is  the  only  class of  capital  stock  of DUAL  issued  and  outstanding. Each
stockholder of record as of the close of business on the Record Date is entitled
at the Special Meeting to one vote for each share of the DUAL Common Stock held.
 
QUORUM
 
    The presence, in person  or by proxy,  of the holders of  a majority of  the
outstanding  shares of DUAL Common Stock entitled to vote at the Special Meeting
is necessary  to constitute  a quorum  for the  transaction of  business at  the
Special  Meeting. Pursuant to the Principal Stockholder Agreement, the Principal
Stockholder Shares will be present and voted at the Special Meeting in favor  of
the  Merger Agreement and  the Unit Plan.  Therefore, no additional stockholders
are necessary to be present at the  Special Meeting to constitute a quorum.  See
"Principal Stockholder Agreement."
 
                                   THE MERGER
 
GENERAL
 
    The  Merger Agreement provides for a  business combination between ENSCO and
DUAL in which a wholly-owned subsidiary of  ENSCO would be merged with and  into
DUAL and the holders of DUAL Common Stock would be issued shares of ENSCO Common
Stock. As a result of the Merger, DUAL would become a wholly-owned subsidiary of
ENSCO.  A  copy of  the Merger  Agreement is  attached to  this Prospectus/Proxy
Statement as Appendix A and is incorporated herein by reference.
 
BACKGROUND OF THE MERGER
 
    In July 1995, the Board of Directors of DUAL instructed Mr. David W. Skarke,
the Chairman of the Board of DUAL, and Simmons to prepare an analysis of  DUAL's
operating  and  financial history  and future  outlook,  asset base  and matters
relating to DUAL Common  Stock, including trading  volume, liquidity, float  and
value  relative to comparable  companies in the  offshore drilling industry, and
other factors  relevant to  determining  how to  enhance stockholder  value  and
improve  the liquidity of DUAL's Common Stock, and to distribute the analysis to
the Board prior to their next meeting on September 14, 1995.
 
    On September 6, 1995, each member of the Board of Directors of DUAL received
a copy  of  the  analysis prepared  by  Mr.  Skarke and  Simmons.  The  analysis
discussed  DUAL's  current  focus  and form  and  DUAL's  related  strengths and
weaknesses, including, among other  things, (i) a  review of DUAL's  performance
and  value  relative to  DUAL's competitors,  (ii) the  activity of  DUAL Common
Stock, including trading volume, liquidity and float, and (iii) the  performance
of  DUAL Common Stock relative to comparable companies in the drilling industry.
The analysis  also addressed  the positive  and negative  implications, and  the
expected  effect  on  stockholder  value,  if  DUAL  were  to  (i)  maintain its
independence, (ii)  pursue  a  sale  or  merger  strategy,  or  (iii)  pursue  a
divestiture  strategy. In  reviewing the  implications arising  out of remaining
independent, the analysis indicated the need to acquire additional rigs and  the
potential  need to recapitalize DUAL through a securities offering. The analysis
also considered  the possibility  of  selling DUAL  to,  or merging  DUAL  with,
another  party (a "Potential Sale or  Merger Transaction"), and addressed, among
other things, the effect such Potential Sale or
 
                                       21
<PAGE>
Merger Transaction would have on DUAL stockholder value arising from a (i)  sale
of  platform rigs  or jackup  rigs as  separate fleets,  (ii) Potential  Sale or
Merger Transaction pursuant to which DUAL would remain intact as a going concern
and potentially realize a premium for synergistic value, (iii) cash transaction,
(iv) stock transaction, or (v) transaction involving a combination of stock  and
cash.
 
    On  September 14,  1995, the Board  of Directors  of DUAL held  a meeting at
which it addressed the matters presented in the analysis prepared by Mr.  Skarke
and  Simmons.  In  reviewing  the various  strategic  alternatives  for pursuing
enhanced stockholder value,  the Board  of Directors  of DUAL  noted the  issues
which it felt would be associated with remaining independent. These issues would
include the desirability of acquiring additional quality rigs, very few of which
were then available for purchase. The Board also discussed the issues DUAL might
face  in pursuing a recapitalization due to DUAL's leverage position relative to
other companies in the industry. In its discussion of a Potential Sale or Merger
Transaction, the Board of Directors of DUAL noted the importance of  considering
all   potential  candidates  including,  among  others,  foreign  companies  and
companies operating outside the drilling industry ("Potential Candidates").  The
Board  of Directors of DUAL also addressed the importance of carefully assessing
the value of securities  that might be  received in a  Potential Sale or  Merger
Transaction. In the case of a Potential Sale or Merger Transaction involving the
exchange  of DUAL Common Stock, the Board  expressed the need to be sensitive to
possible restrictions  on the  sale of  the securities  to be  received by  DUAL
stockholders.  Based on the Board's discussion  and the analysis prepared by Mr.
Skarke and Simmons, the Board of Directors  of DUAL concluded that there was  an
opportunity  to enhance DUAL stockholder value  by implementing a sale or merger
strategy.
 
    The Board  of Directors  of DUAL  then discussed  the issue  of engaging  an
investment  banker  to assist  in the  development of  Potential Sale  or Merger
Transaction scenarios. The Board of Directors of DUAL was of the opinion that an
investment banker would add credibility, facilitate a merger or sale transaction
and assure that DUAL  stockholders would receive the  best price for their  DUAL
Common  Stock in  connection with such  merger or sale  transaction. Following a
discussion  relating  to  the  relative  strengths  and  weaknesses  of  several
investment  banking  firms, the  Board  of Directors  of  DUAL agreed  to engage
Simmons to (i)  identify Potential Candidates,  (ii) evaluate various  Potential
Sale  or Merger Transaction structures, (iii)  assist in structuring a Potential
Sale or Merger Transaction, and (iv) provide a fairness opinion. As a result  of
this  meeting,  Simmons presented  a  draft engagement  letter  to the  Board of
Directors of DUAL (the "Simmons Engagement Letter"), which was circulated among,
and reviewed by, the members of the  Board of Directors of DUAL and Akin,  Gump,
Strauss, Hauer & Feld, L.L.P., DUAL's counsel, prior to its execution.
 
    A  telephonic meeting of the Board of  Directors of DUAL was held on October
12, 1995, at  which it  addressed the Simmons  Engagement Letter.  The Board  of
Directors  of DUAL  discussed, among other  things, the  likelihood that Simmons
would prepare  a fairness  opinion if  a Potential  Sale or  Merger  Transaction
materialized,  and the  potential additional cost  to DUAL if  another firm were
asked to provide the fairness opinion.  Based upon these discussions, the  Board
of Directors of DUAL confirmed the engagement of Simmons and also confirmed that
the  fee arrangement with Simmons (the  "Simmons Transaction Fee") was inclusive
of (i) the fairness opinion, and (ii) the $50,000 fee previously paid to Simmons
for their strategic analysis. The Board of Directors of DUAL requested that  the
draft   of  the  Simmons   Engagement  Letter  be   modified  to  confirm  these
understandings.
 
    During the October 12, 1995 telephonic meeting, Mr. Skarke updated the Board
of Directors of DUAL on the status  of the Potential Sale or Merger  Transaction
and  his interaction with Simmons. The Board  of Directors of DUAL discussed the
information document being prepared  for the Potential  Candidates and the  need
for  strategic reassurance and  careful consideration of the  type and amount of
information related to DUAL to be included in such document.
 
    The Board of  Directors of DUAL  then asked a  representative of Simmons  to
join  it at the meeting. This representative commented on the types of Potential
Candidates, foreign buyer potential, market conditions and certain other  issues
related to the Potential Sale or Merger Transaction.
 
                                       22
<PAGE>
    On  November  2, 1995,  the Board  of  Directors of  DUAL held  a telephonic
meeting pursuant to  which it  authorized Simmons to  begin contacting  selected
Potential  Candidates for a Potential Sale  or Merger Transaction (the "Selected
Candidates"). The Selected Candidates  included many domestic and  international
offshore  drilling companies, including ENSCO, and certain companies outside the
offshore drilling industry.
 
    A regularly scheduled  meeting of  the Board of  Directors of  DUAL held  on
December  1, 1995, began with  a presentation from Simmons  on the status of the
Potential Sale  or  Merger  Transaction  and  expressions  of  interest  therein
received  from various Potential Candidates.  Certain representatives of Simmons
and Mr. Tharald Brovig, a director  of the Principal Stockholder, were asked  to
join the meeting. The Simmons representatives informed the Board of Directors of
DUAL  that preliminary  expressions of  interest in  a Potential  Sale or Merger
Transaction had  been  received  from six  major  offshore  drilling  companies,
including  ENSCO (the "Final Candidates"). Some of these expressions of interest
were based upon a stock-for stock exchange  with the potential for some cash  to
be  paid to  DUAL stockholders. The  preliminary values for  DUAL's common stock
arising out  of the  expressions  of interest  by  the Final  Candidates  ranged
between  $10 1/8 per share  to approximately $12 per  share. The Board discussed
the Final  Candidates'  expressions  of interest  in  detail,  which  discussion
included, among other things, the potential transactional structures relating to
each Final Candidate, and a comparison of the strengths and weaknesses of a sale
or merger with each Final Candidate.
 
    The  Board of Directors  of DUAL and  Simmons representatives next discussed
the strategy to  be implemented to  generally increase levels  of interest in  a
Potential  Sale or  Merger Transaction and  improve DUAL  stockholder value. The
Board of Directors of  DUAL concluded that  Simmons should continue  discussions
with  all Potential Candidates. The Board of  Directors of DUAL also agreed that
DUAL would prepare a data room  and management presentations for each  Potential
Candidate,  anticipating that the presentations and data room visits would begin
during the week of December 11, 1995.
 
    A special meeting of the Board of Directors of DUAL was held on January  17,
1996,  which meeting began with a review of the indications of interest received
from Potential  Candidates.  The  Board  of Directors  of  DUAL  identified  the
proposals  submitted  by  three  of the  Potential  Candidates,  including ENSCO
(collectively, the "Final Three  Candidates") as having  the potential to  offer
the  best  value  reasonably  available to  DUAL's  stockholders.  The  Board of
Directors of DUAL  then reviewed  the presentations made  by each  of the  Final
Three  Candidates on January  16, 1996, to  the Board of  Directors of DUAL, and
discussed their impressions relating  to the question  and answer sessions  with
each  of the Final Three Candidates which followed such presentations. The Board
of Directors of DUAL noted and  discussed alternatives to a merger  transaction,
and  concluded that  a merger  with any  of the  Final Three  Candidates offered
enhanced value to DUAL's stockholders. The Board of Directors of DUAL then again
reviewed the proposals submitted  by each of the  Final Three Candidates,  which
review  included, among other things, a review  of the potential structure for a
merger with each of the Three Candidates, the strengths and weaknesses of a sale
to or merger with each of the  Final Three Candidates, and the projected  effect
on  stockholder  value  arising from  a  merger  with each  of  the  Final Three
Candidates. The Board discussed  that the values offered  for DUAL by the  Final
Three  Candidates were substantially the same, and that the determination of the
preferred merger candidate would likely be dependent on other factors. The Board
noted that  each  of the  Final  Three  Candidates had  discussed  the  improved
operating results that would likely occur as a result of the combination of DUAL
with  such  company  due  to  operating  efficiencies  and  the  elimination  of
duplicative functions. ENSCO, however, also identified an additional significant
strategic benefit of a merger with it. ENSCO noted that a merger of DUAL with it
would not only result in  a larger, more efficient  company, but would have  the
additional  strategic benefit of significantly  increasing the number of premium
jackup rigs in  ENSCO's total fleet  and immediately expanding  its presence  in
attractive  international markets  such as  the Middle  East and  Southeast Asia
where DUAL was currently operating but ENSCO was not. The Board of Directors  of
DUAL discussed that these
 
                                       23
<PAGE>
additional  strategic factors were  particularly compelling and  felt they might
contribute to a greater improvement in  the stock value of the combined  company
than would a merger with either of the other two candidates.
 
    L.H.  Dick Robertson, the President and Chief Executive Officer of DUAL, was
then asked to render his opinion relating to each of the Final Three Candidates.
Mr. Robertson discussed his views on the relative strengths and weaknesses of  a
sale  of DUAL to, or merger of DUAL with, each of the Final Three Candidates. At
the conclusion of this discussion, Mr. Robertson informed the Board of Directors
of DUAL that because of his position as a senior executive of DUAL, he deemed it
advisable that he abstain from the vote  on which of the Final Three  Candidates
DUAL  should pursue in  connection with a Potential  Sale or Merger Transaction.
With Mr.  Robertson abstaining,  the  remaining five  members  of the  Board  of
Directors of DUAL voted in favor of further exploration of, and the beginning of
negotiations  with respect to, a sale or  merger with ENSCO, which they believed
offered the best opportunity  to maximize stockholder value  for the holders  of
DUAL Common Stock based on the stock value of the combined companies. Mr. Skarke
and  Simmons were directed to engage in  further discussions with ENSCO with the
objectives of (i)  improving ENSCO's offer  within certain specified  parameters
and  (ii) entering into a letter of intent  as soon as practicable. The Board of
Directors of DUAL also discussed that ENSCO had indicated that it would  require
a  fee of  $5,000,000, and Mr.  Skarke and  Simmons were authorized  to pursue a
letter of intent on that basis. The Board of Directors of DUAL noted that it had
been impressed  with  the  presentations  of  each  of  the  other  Final  Three
Candidates  and  recognized that  if  ENSCO were  to  be unwilling  to  pursue a
transaction with DUAL, a sale of DUAL to, or merger of DUAL with, either of  the
other Final Three Candidates would constitute attractive alternatives.
 
    Following the January 17, 1996 meeting of the Board of Directors of DUAL and
prior  to January 25,  1996, there were  several telephone conversations between
Simmons and  representatives of  ENSCO discussing  the structure  of a  proposed
transaction  involving DUAL and ENSCO, including the share exchange ratio. ENSCO
and Simmons each made several proposals  in respect of the share exchange  ratio
during  such period.  The final  share exchange ratio  was offered  by ENSCO and
after negotiations  between ENSCO  and  Simmons such  share exchange  ratio  was
submitted  to, and accepted by,  the Board of Directors  of DUAL at a telephonic
Board of Directors' meeting on January 24,  1996. On January 25, 1996, DUAL  and
ENSCO  entered into a letter of intent setting forth the framework governing the
terms of the proposed merger.  Pursuant to the letter  of intent DUAL and  ENSCO
agreed,  among other things, (i)  that each of the  holders of DUAL Common Stock
would receive for each share  of DUAL Common Stock  .625 shares of ENSCO  Common
Stock,  (ii) that all outstanding stock options  or other rights to acquire DUAL
Common Stock would terminate immediately prior  to the proposed merger, and  the
holders  thereof would be entitled to receive  from DUAL an amount in cash equal
to the difference between the fair market value of the DUAL Common Stock on  the
date  of termination and the exercise price  of the option or other right, (iii)
that the form of the proposed merger would be structured and agreed to by  ENSCO
and DUAL after review of tax, regulatory, contract consent and other appropriate
issues,  (iv)  to submit  a definitive  merger agreement  for approval  by their
respective boards of directors not later than March 31, 1996 and (v) to  certain
conditions precedent to the proposed merger.
 
    A special meeting of the Board of Directors of DUAL was held on February 21,
1996.  At this meeting,  the Board of  Directors of DUAL  discussed the proposed
merger, including certain  remaining outstanding business  issues regarding  the
proposed  merger and the status of the ongoing due diligence review by ENSCO. In
addition, at this meeting, representatives of Simmons presented to the Board  of
Directors  of DUAL an oral summary of the analysis Simmons undertook to form its
fairness opinion regarding the proposed merger and rendered the oral opinion  of
Simmons  that, as of that date, the  consideration to be received by the holders
of DUAL Common Stock in the proposed  merger was fair from a financial point  of
view to those holders. See "-- Fairness Opinion."
 
                                       24
<PAGE>
    A telephonic meeting of the Board of Directors of DUAL was held on March 12,
1996,  at which the Board  of Directors of DUAL  unanimously approved the Merger
and the execution of the Merger Agreement.
 
DUAL'S REASONS FOR THE MERGER; RECOMMENDATION OF DUAL'S BOARD OF DIRECTORS
 
    The Board of Directors of DUAL has unanimously determined that the terms  of
the  Merger Agreement and  the transactions contemplated  thereby offer the best
opportunity to maximize stockholder value for  the holders of DUAL Common  Stock
based  on the stock value  of the combined companies.  Accordingly, the Board of
Directors of DUAL has unanimously  approved the Merger Agreement and  recommends
approval thereof by the stockholders of DUAL. In reaching its determination, the
Board  of Directors of DUAL consulted with DUAL management, as well as its legal
counsel and  its  financial  advisors,  and  considered  a  number  of  factors,
including, without limitation, the following:
 
    (i) the  common  business strategies  and  complementary nature  of  the rig
        fleets of DUAL and ENSCO;
 
    (ii) the strong performance of the  ENSCO Common Stock, and the  opportunity
         for  future liquidity associated with the ENSCO Common Stock due to its
         high average trading volume  as compared to the  trading volume of  the
         DUAL Common Stock;
 
   (iii) the    opportunity   afforded   DUAL   stockholders   to   maintain   a
         publicly-traded equity investment  in a larger  combined enterprise  in
         the  offshore drilling  industry and to  participate in  the growth and
         appreciation of the business of the combined company;
 
   (iv) the opportunity afforded  DUAL stockholders  to share  in the  potential
        cost savings achieved through consolidation;
 
    (v) its  belief that the  future business prospects  of the combined company
        were strong  due to  the large  size  of the  rig fleet  and  geographic
        diversity of operations;
 
   (vi) the  balance sheet strength  of the combined  company relative to DUAL's
        stand-alone balance sheet;
 
   (vii) the exchange ratio  between ENSCO  Common Stock and  DUAL Common  Stock
         which  the Board believed  would result in enhanced  value for the DUAL
         stockholders;
 
  (viii) the expected tax-free nature of the Merger;
 
   (ix) enhanced growth  opportunities for  the combined  company from  expanded
        geographic presence and greater resources; and
 
    (x) recent  consolidation  in  and  the outlook  for  the  offshore drilling
        industry.
 
    Based on the opinion of Simmons to  the effect that, from a financial  point
of   view,  the  consideration  to  be  received  by  DUAL's  stockholders  upon
consummation of the Merger  is fair to such  stockholders, and on the  foregoing
matters  and such other matters as were  deemed relevant, the Board of Directors
of DUAL unanimously approved the Merger as being in the best interests of DUAL's
stockholders.
 
    The foregoing discussion of the information and factors considered and given
weight by the Board of Directors of DUAL is not intended to be exhaustive but is
believed to  include  the  material  factors the  Board  of  Directors  of  DUAL
considered.  In addition, in reaching the determination to approve and recommend
the Merger, considering  the wide  variety of factors  considered in  connection
with  its evaluation of the proposed Merger,  the Board of Directors of DUAL did
not find it practical to, and did  not, quantify or otherwise attempt to  assign
any  relative  or  specific weights  to  the foregoing  factors,  and individual
directors may have given different weights to different factors.
 
                                       25
<PAGE>
    THE BOARD OF DIRECTORS OF DUAL UNANIMOUSLY RECOMMENDS THAT THE  STOCKHOLDERS
OF  DUAL  VOTE  FOR  APPROVAL  AND ADOPTION  OF  THE  MERGER  AGREEMENT  AND, AS
CONTEMPLATED THEREBY, APPROVAL OF THE MERGER.
 
ENSCO'S REASONS FOR THE MERGER
 
    The Board of Directors  of ENSCO has  determined that the  Merger is in  the
best  interests of the stockholders  of ENSCO because it  is the best option for
achieving the strategic goals  of ENSCO, offering  the ENSCO/DUAL combination  a
broadened geographic presence, an increased fleet of offshore drilling rigs, and
potential  economies of scale. Accordingly, the Board of Directors believes that
the Merger  could allow  ENSCO to  compete more  effectively for  contracts  and
relationships  internationally.  In  reaching its  determination,  the  Board of
Directors considered certain factors, including the following:
 
    (i) the business  and  financial  prospects of  an  ENSCO/DUAL  combination,
        including  the size of the combined rig fleet, the potential operational
        benefits, and the outlook for the respective fleets;
 
    (ii) the economies  of  scale  which  could be  afforded  to  an  ENSCO/DUAL
         combination;
 
   (iii) the  geographic diversification  of a combined  ENSCO/DUAL drilling rig
         fleet, in accordance with ENSCO's objective of geographic expansion  of
         its offshore drilling rig fleet;
 
   (iv) the  balance sheet  strength of an  ENSCO/DUAL combination  and the debt
        levels relative to equity of the two companies;
 
    (v) the outlook of the offshore drilling industry internationally and in the
        geographic markets  serviced by  ENSCO and  DUAL, including  the  market
        trends and prospects for premium jackup rigs and self-contained platform
        rigs;
 
   (vi) the  structure of the Merger, the terms  of the Merger Agreement and the
        exchange ratio between ENSCO Common  Stock and DUAL Common Stock,  which
        were the result of arms'-length negotiations between ENSCO and DUAL; and
 
   (vii) recent  and historical market prices of  the ENSCO Common Stock and the
         DUAL Common Stock.
 
    In determining whether the Merger  was fair to and  in the best interest  of
stockholders  of ENSCO, the  Board of Directors of  ENSCO considered the factors
above as  a whole  and  did not  assign specific  or  relative weights  to  such
factors.
 
EFFECTIVE TIME
 
    If the Merger Agreement is approved and adopted by the requisite vote of the
stockholders  of DUAL and  the other conditions  to the Merger  are satisfied or
waived (if permissible), the Merger will be consummated and effected at the time
a Certificate of Merger  is filed with  the Secretary of State  of the State  of
Delaware  or at such later time as the  parties to the Merger Agreement agree to
in writing  and specify  in such  Certificate of  Merger. The  Merger  Agreement
provides  that ENSCO and DUAL will cause the Effective Time to occur as promptly
as practicable, but in no event later  than the first business day (unless  such
other  date is  agreed to  in writing  by the  parties to  the Merger Agreement)
following the satisfaction or, if permissible, waiver of all the conditions  set
forth  in the Merger Agreement. The Merger  Agreement may be terminated prior to
the Effective Time  by either ENSCO  or DUAL in  certain circumstances,  whether
before   or  after  approval  and  adoption  of  the  Merger  Agreement  by  the
stockholders of  DUAL.  See  "Certain  Provisions of  the  Merger  Agreement  --
Termination."
 
TERMS OF THE MERGER
 
    If  all required stockholder approvals are obtained and all other conditions
to the Merger  are satisfied or  waived, if permissible,  then at the  Effective
Time  Merger Subsidiary  will be merged  with and  into DUAL, which  will be the
Surviving Corporation and automatically renamed DUAL Holding
 
                                       26
<PAGE>
Company, and the Certificate of Incorporation and Bylaws of DUAL will be amended
automatically to conform with  those of Merger Subsidiary.  In the Merger,  each
share  of DUAL Common Stock outstanding  immediately prior to the Effective Time
(other than Cancelable Shares) will be converted into the right to receive 0.625
of a  share of  ENSCO  Common Stock.  The Merger  is  structured as  a  tax-free
reorganization  to the extent  DUAL stockholders receive  shares of ENSCO Common
Stock. Depending upon uncertain  factual developments, it  is possible that  the
Merger  might not be treated  as a reorganization within  the meaning of Section
368(a) of the Code, as  a result of which each  holder of shares of DUAL  Common
Stock  would recognize  gain in  the amount of  the difference  between the fair
market value  of  the shares  of  ENSCO Common  Stock  and/or cash  in  lieu  of
fractional  shares,  and  such  holder's  tax basis  in  the  DUAL  Common Stock
exchanged therefor. See "The Merger -- Certain United States Federal Income  Tax
Consequences."
 
FAIRNESS OPINION
 
    DUAL  retained  Simmons to  act as  its  financial advisor  and to  render a
fairness opinion in connection  with the Merger.  At a meeting  of the Board  of
Directors  of DUAL on February  21, 1996, Simmons rendered  an oral opinion that
the consideration to  be received by  the holders  of DUAL Common  Stock in  the
Merger  was fair from a  financial point of view  to such holders. Subsequently,
Simmons confirmed this opinion in writing as of March 21, 1996.
 
    The full text of Simmons' fairness opinion, dated March 21, 1996, which sets
forth the assumptions made, general procedures followed, matters considered  and
limits  on review undertaken, is attached as Appendix B to this Prospectus/Proxy
Statement. Simmons' opinion is directed only  to the fairness, from a  financial
point of view, of the consideration to be received by the holders of DUAL Common
Stock  and does  not constitute  a recommendation to  any holder  of DUAL Common
Stock  as  to  how  such  stockholder  should  vote  on  the  Merger  Agreement.
STOCKHOLDERS OF DUAL ARE URGED TO READ THE OPINION IN ITS ENTIRETY.
 
    In connection with rendering its opinion, Simmons has reviewed and analyzed,
among  other things, the following:  (i) the Letter of  Intent between ENSCO and
DUAL dated January  25, 1996,  (ii) the  Merger Agreement,  (iii) the  financial
statements  and other information concerning  DUAL, including the Annual Reports
on Form  10-K of  DUAL  for the  three years  ended  December 31,  1995,  DUAL's
prospectus  dated  August 5,  1993 relating  to the  initial public  offering of
6,250,000 shares of DUAL Common Stock, DUAL's prospectus dated January 20,  1994
relating  to the offering of $100,000,000 principal  amount of the 9 7/8% Senior
Subordinated Notes due 2004, and the Current  Reports on Form 8-K of DUAL  dated
January  25, 1996, August 5, 1995 and  June 5, 1995, (iv) certain other internal
information,  primarily  financial  in  nature,  concerning  the  business   and
operations  of DUAL  furnished by  DUAL for  purposes of  Simmons' analysis, (v)
certain publicly available  information concerning ENSCO,  including the  Annual
Reports  on Form 10-K of  ENSCO for the three years  ended December 31, 1995 and
the Current Reports on Form 8-K dated January 25, 1996, September 11, 1995,  May
23, 1995 and March 23, 1995, (vii) certain other internal information, primarily
financial  in nature, concerning the business  and operations of ENSCO furnished
by ENSCO for purposes  of Simmons' analysis,  (viii) certain publicly  available
information  concerning the  trading of,  and trading  market for,  ENSCO Common
Stock, (ix) certain publicly available information with respect to certain other
companies that  Simmons believes  to be  comparable  to DUAL  or ENSCO  and  the
trading  markets for  certain of such  other companies'  securities, (x) certain
publicly available information concerning the estimates of the future  operating
and  financial performance of DUAL, ENSCO  and the comparable companies prepared
by industry analysts unaffiliated  with either DUAL or  ENSCO, and (xi)  certain
publicly  available information concerning the nature and terms of certain other
transactions considered relevant to the  inquiry. Simmons also met with  certain
officers  and employees of  DUAL and ENSCO  to discuss the  foregoing as well as
other matters believed relevant to the  inquiry. In addition, Simmons made  such
other  analyses and examinations as it  considered appropriate in expressing its
opinion.
 
                                       27
<PAGE>
    In arriving at its opinion, Simmons has assumed and relied upon the accuracy
and  completeness of all of the financial and other information provided by DUAL
and ENSCO,  or publicly  available, including,  without limitation,  information
with  respect  to  asset  conditions,  tax  positions,  liability  reserves  and
insurance coverages, and has not attempted  independently to verify any of  such
information.  Simmons  has not  conducted a  physical inspection  of any  of the
assets, properties  or facilities  of DUAL  or ENSCO,  nor has  Simmons made  or
obtained  any  independent  evaluations  or appraisal  of  any  of  such assets,
properties or facilities. In  addition, Simmons has  discussed the prospects  of
DUAL  with certain  representatives of DUAL  and has been  provided with certain
financial projections of DUAL. Simmons has also discussed the prospects of ENSCO
with certain representatives of ENSCO.
 
    In conducting its analysis and  arriving at its opinion, Simmons  considered
such   financial  and  other   factors  as  it   deemed  appropriate  under  the
circumstances, including, among  others, the following:  (i) the historical  and
current financial position and results of operations of DUAL and ENSCO, (ii) the
business  prospects of DUAL  and ENSCO, (iii) the  historical and current market
for DUAL Common Stock, for ENSCO Common  Stock and for the equity securities  of
certain  other companies believed  to be comparable  to DUAL or  ENSCO, (iv) the
respective contributions  in terms  of various  financial measures  of DUAL  and
ENSCO  to the combined  company, and the  relative pro forma  ownership of ENSCO
after the Merger by the  current holders of DUAL  Common Stock and ENSCO  Common
Stock,  and (v) the nature  and terms of 18  other acquisition transactions that
Simmons believed to be relevant. Simmons  also took into account its  assessment
of  general  economic, market  and financial  conditions  and its  experience in
connection  with  similar  transactions  and  securities'  valuation  generally.
Simmons' opinion necessarily was based upon conditions as they existed and could
be  evaluated, and on the  information made available, at  the date thereof. The
opinion rendered by  Simmons does  not constitute an  opinion as  to the  future
value  of ENSCO  Common Stock upon  consummation of  the Merger or  the price at
which ENSCO Common Stock will trade at any time.
 
    IN CONNECTION  WITH A  PRESENTATION TO  THE BOARD  OF DIRECTORS  OF DUAL  ON
FEBRUARY  21, 1996,  SIMMONS ADVISED  THE BOARD  OF DIRECTORS  OF DUAL  THAT, IN
EVALUATING THE CONVERSION RATE TO BE USED IN THE MERGER, SIMMONS HAD PERFORMED A
VARIETY OF FINANCIAL ANALYSES WITH RESPECT TO DUAL AND ENSCO.
 
    CONVERSION RATE PROFILE.  Simmons performed an analysis of the ratio of  the
market  price of  DUAL Common Stock  to the  market price of  ENSCO Common Stock
during the period from August 1993 through January 1996. Simmons calculated  the
ratio  of the DUAL Common  Stock closing price for  each trading day during that
period to  the ENSCO  Common Stock  closing price  for each  day. This  analysis
implied  a conversion rate  ranging from a  high of 1.39  shares of ENSCO Common
Stock for each  share of  DUAL Common Stock  to a  low of 0.49  shares of  ENSCO
Common  Stock for each  share of DUAL  Common Stock, with  an average during the
period of 0.73 shares of ENSCO Common Stock for each share of DUAL Common Stock.
Simmons also calculated the average  ratio during the past  3, 6 and 12  months,
respectively, of the DUAL Common Stock closing price for each trading day to the
ENSCO  Common  Stock  closing  price  for  such  day.  This  analysis  implied a
conversion rate of 0.57, 0.58 and 0.59 of a share of ENSCO Common Stock for each
share of DUAL Common Stock, based on the last 3, 6 and 12 months,  respectively.
Simmons  also calculated  the ratio  of the DUAL  Common Stock  closing price on
January 24, 1996 ($13 1/16 per share) to the ENSCO Common Stock closing price on
such day ($22 3/4 per share). This  implied a conversion rate of 0.57 shares  of
ENSCO Common Stock for each share of DUAL Common Stock.
 
    PREMIUM  ANALYSIS.  Simmons calculated the premium to holders of DUAL Common
Stock of the "Implied Consideration" (obtained by multiplying the closing  price
for  ENSCO Common  Stock on February  20, 1996  by the Merger  exchange ratio of
0.625) to the closing price  for DUAL Common Stock on  January 24, 1996, on  the
dates  one month,  three months and  six months prior,  as well as  to the daily
average closing prices for DUAL Common Stock for the 10 days, 20 days, 30  days,
and  60 days prior, and to the daily average closing price for DUAL Common Stock
the 52 weeks prior and  to each of the 52-week  high and low closing prices  for
DUAL  Common  Stock. Based  upon the  closing  price for  ENSCO Common  Stock of
$26 3/8 on  February 20, 1996,  Simmons calculated premiums  to holders of  DUAL
Common  Stock  equal to  26.2% of  the closing  price for  DUAL Common  Stock of
$13 1/16 on
 
                                       28
<PAGE>
January 24, 1996; 49.9% of  the closing price for DUAL  Common Stock of $11  one
month  earlier; 73.5% of the closing price for DUAL Common Stock of $9 1/2 three
months earlier; 69.1% of the closing price  for DUAL Common Stock of $9 3/4  six
months  earlier; 29.2%  of the  10-day average  price for  DUAL Common  Stock of
$12.76; 34.1% of the 20-day average price for DUAL Common Stock of $12.29; 39.5%
of the 30-day average price for DUAL Common Stock of $11.82; 50.9% of the 60-day
average price for  DUAL Common  Stock of $10.92;  71.6% of  the 52-week  average
price  for DUAL Common Stock  of $9.61 and 26.2% and  125.4% of the 52-week high
and low closing  prices, respectively,  for DUAL Common  Stock of  $13 1/16  and
$7 5/16.
 
    Simmons  also analyzed average acquisition  premiums for acquisitions of 721
public  companies  in  the  years  1990  through  1995,  focusing  on  completed
transactions  in the  $50 to  $500 million  range. The  average premium  to last
closing price prior to announcement of such transactions during any year  ranged
from  a low  of 25.7%  to a  high of  32.4%, with  the weighted  average for 721
transactions being 29.7% compared to the premium for the Merger of 26.2%,  based
on  a closing price  of $13 1/16 for  DUAL Common Stock on  January 24, 1996 and
$26 3/8 for ENSCO Common Stock on February 20, 1996.
 
    RELATIVE CONTRIBUTION  ANALYSIS.    Simmons  analyzed  the  relative  equity
contributions  of DUAL and  ENSCO to, among  other things, the  combined last 12
months' ("LTM") earnings before depreciation, interest, and taxes ("EBDIT"), LTM
net income,  LTM  cash  flow  (defined  as  net  income  plus  depreciation  and
amortization),  projected 1996 EBDIT, projected  1996 net income, projected 1996
cash flow and market equity  value (based on January  24, 1996 share prices)  of
the two companies, assuming consummation of the Merger (without giving effect to
any transaction adjustments). Simmons calculated contributions to equity by DUAL
of  approximately 9% of combined LTM EBDIT; a not meaningful percent of combined
reported LTM net income;  4% of combined  LTM cash flow;  18% of projected  1996
EBDIT; 13% of projected 1996 net income; 18% of projected 1996 cash flow and 13%
of  market equity value. Simmons also  calculated the percentage of the combined
company's equity  that  would  be  held by  former  DUAL  stockholders  assuming
consummation  of the Merger as 14% (using the  conversion rate to be used in the
Merger)  and   compared  such   percentage  with   the  foregoing   contribution
percentages.
 
    DISCOUNTED  CASH FLOW ANALYSES.   Simmons used DUAL management's projections
to calculate projected future cash flows for DUAL through the year 2000. Simmons
discounted to present value the future free cash flows of DUAL and the  terminal
value  of six times year 2000 EBDIT  by applying discount rates ranging from 12%
to 15%. Based  on these calculations,  Simmons then derived  present values  per
share ranging from $13.36 to $15.79 for the DUAL Common Stock.
 
    PRO  FORMA MERGER ANALYSIS.  Simmons performed  an analysis of the effect of
the Merger on the earnings per share and cash flow per share of the ENSCO Common
Stock, which would be  the surviving stock after  the Merger, for the  projected
results  for the year ended December 31, 1996, which assumed that the Merger was
consummated on January  1, 1996. Such  analysis was based  on DUAL  management's
projections  and estimates for ENSCO  prepared by industry analysts unaffiliated
with either DUAL  or ENSCO. In  performing this analysis,  Simmons also  assumed
that  estimated  annual  consolidation cost  savings  of $8  million  were fully
realized as of January 1, 1996. Simmons also excluded one-time costs  associated
with  the Merger for the purposes of this analysis. This analysis indicated that
the pro  forma  impact  of the  Merger  was  non-dilutive to  the  earnings  and
accretive  to the cash  flow per share  of ENSCO Common  Stock in 1996. Simmons'
conclusion that the Merger would be  non-dilutive to earnings and cash flow  per
share  of ENSCO  Common Stock  was based  on a  financial model  of the combined
company developed by  Simmons and  using assumptions,  including the  following:
projected  consolidation cost  reductions, write-up of  asset values, increasing
day rates,  continued high  utilization of  offshore drilling  rigs,  increasing
daily  operating costs, interest rates on debt  and cash balances and income tax
rates.
 
                                       29
<PAGE>
    Simmons also analyzed the  balance sheet impact of  the Merger. At  December
31,  1995, the total  debt-to-total book capitalization ratio  of DUAL and ENSCO
combined (after estimated one-time costs and other adjustments) was 30%.  DUAL's
stand-alone  total debt-to-total book  capitalization ratio was  51% at December
31, 1995.
 
    In evaluating the fairness of the conversion rate to be used in the  Merger,
Simmons also considered certain other analyses which are set forth below.
 
    ANALYSIS OF SELECTED PUBLICLY-TRADED COMPARABLE COMPANIES.  Simmons reviewed
certain  financial information  for the  12 months  ended December  31, 1995 and
stock market information as of January 24,  1996 and February 20, 1996 for  DUAL
and  certain publicly  available financial information  as of  the most recently
reported period and stock market information as of February 20, 1996 for certain
selected publicly traded companies. For the purposes of this analysis the  group
of  similar  companies was  comprised  of Arethusa  (Off-shore)  Limited, Atwood
Oceanics, Inc., Cliffs Drilling Company, Diamond Offshore Drilling, Inc., Falcon
Drilling Company, Inc.,  Global Marine, Inc.,  Marine Drilling Companies,  Inc.,
Noble  Drilling Corporation, Reading & Bates Corporation, Rowan Companies, Inc.,
Sonat Offshore  Drilling Inc.,  and ENSCO  (collectively, the  "DUAL  Comparable
Companies").
 
    For  DUAL  and each  of the  DUAL  Comparable Companies,  Simmons calculated
multiples of market stock  price to LTM  earnings and cash  flow per share,  and
projected  earnings and  cash flow  per share  (derived from  publicly available
information concerning  the  estimates of  the  future operating  and  financial
performance  of  DUAL and  the DUAL  Comparable  Companies prepared  by industry
analysts unaffiliated  with  either  DUAL  or ENSCO)  for  1996  and  1997,  and
multiples  of adjusted  market capitalization  to LTM  EBDIT and  projected 1996
EBDIT (derived from publicly available  information concerning the estimates  of
the  future operating and financial performance  of DUAL and the DUAL Comparable
Companies prepared by industry analysts unaffiliated with either DUAL or ENSCO).
An analysis of market stock price to  LTM earnings per share, to projected  1996
earnings  per share and to projected 1997  earnings per share yielded for DUAL a
not meaningful result, 25.6x and 14.7x, respectively, (a not meaningful  result,
32.3x  and 18.5x, respectively, at the  Implied Consideration) with a mean value
(excluding  the  highest  and   lowest  value)  of   39.7x,  22.6x  and   14.1x,
respectively,  for the  DUAL Comparable Companies.  An analysis  of market stock
price to LTM cash flow per share, to  projected 1996 cash flow per share and  to
projected  1997  cash flow  per share  yielded  for DUAL  48.5x, 7.7x  and 6.3x,
respectively, (61.2x, 9.7x and 7.9x, respectively, at the Implied Consideration)
with a mean value (excluding the highest  and lowest value) of 24.3x, 12.5x  and
9.8x,  respectively, for the DUAL Comparable  Companies. An analysis of adjusted
market capitalization to LTM EBDIT and to projected 1996 EBDIT yielded for  DUAL
18.9x  and 7.2x,  respectively, (22.1x  and 8.4x,  respectively, at  the Implied
Consideration) with a  mean value (excluding  the highest and  lowest value)  of
18.7x and 10.3x, respectively, for the DUAL Comparable Companies.
 
    Simmons  performed  a similar  analysis with  respect  to ENSCO  and certain
selected  publicly  traded  companies   (collectively,  the  "ENSCO   Comparable
Companies").  For  ENSCO and  each of  the  ENSCO Comparable  Companies, Simmons
calculated multiples of  market stock price  to LTM earnings  and cash flow  per
share,  and projected  earnings and cash  flow per share  (derived from publicly
available information  concerning  the estimates  of  the future  operating  and
financial  performance of ENSCO  and the ENSCO  Comparable Companies prepared by
industry analysts unaffiliated with either DUAL or ENSCO) for 1996 and 1997, and
multiples of  adjusted market  capitalization to  LTM EBDIT  and projected  1996
EBDIT  (derived from publicly available  information concerning the estimates of
the future operating and financial performance of ENSCO and the ENSCO Comparable
Companies prepared by industry analysts unaffiliated with either DUAL or ENSCO).
An analysis of market stock price to  LTM earnings per share, to projected  1996
earnings  per share and to  projected 1997 earnings per  share yielded for ENSCO
38.3x, 22.7x and  17.7x, respectively,  with a mean  value of  42.6x, 23.1x  and
13.9x,  respectively, for the ENSCO Comparable  Companies. An analysis of market
stock price to LTM cash  flow per share, to projected  1996 cash flow per  share
and  to projected 1997  cash flow per  share yielded for  ENSCO 16.0x, 12.4x and
10.7x, respectively, with a mean value of
 
                                       30
<PAGE>
29.1x, 13.5x  and 9.7x,  respectively, for  the ENSCO  Comparable Companies.  An
analysis  of adjusted market  capitalization to LTM EBDIT  and to projected 1996
EBDIT yielded for ENSCO 15.4x and 9.9x, respectively, with a mean value of 23.0x
and 11.2x, respectively, for the ENSCO Comparable Companies.
 
    Because of the inherent differences between  the operations of DUAL and  the
DUAL  Comparable Companies and ENSCO and the ENSCO Comparable Companies, Simmons
believed that a  purely quantitative  comparable company analysis  would not  be
exclusively  dispositive in the context of  the Merger. Simmons further believed
than an appropriate use of a comparable company analysis in this instance  would
involve  qualitative judgments concerning differences  between the financial and
operating characteristics of DUAL  and the DUAL  Comparable Companies and  ENSCO
and  the ENSCO Comparable  Companies, which judgments  are reflected in Simmons'
opinion.
 
    ANALYSIS OF SELECTED COMPARABLE ACQUISITION TRANSACTIONS.  Simmons  reviewed
transactions  involving  the  acquisition of  all  or part  of  certain offshore
drilling companies (seven transactions) and certain oil field service  companies
(eleven  transactions). Simmons calculated the multiples of acquisition price to
LTM revenues, LTM EBDIT and Adjusted Book Value of such companies.
 
    For offshore  drilling  companies, these  calculations  yielded a  range  of
acquisition  price to LTM revenues  of 1.6x to 5.1x  with a mean (excluding high
and low) of 2.8x,  a range of acquisition  price to LTM EBDIT  of 7.7x to  26.6x
with  a mean (excluding high and low) of 11.8x, and a range of acquisition price
to Adjusted Book Value of 1.2x to 2.1x  with a mean (excluding high and low)  of
1.9x.  Simmons  then compared  the results  of  these calculations  to multiples
calculated using the closing price  of DUAL Common Stock  on January 24, 1996  ,
which  was $13  1/16, and  the Implied  Consideration. The  calculations yielded
multiples of acquisition price to LTM  revenues of 3.8x and 4.5x,  respectively,
of  acquisition price  to LTM  EBDIT of  18.9x and  22.1x, respectively,  and of
acquisition price to Adjusted Book Value of 1.3x and 1.5x, respectively.
 
    For oil  field service  companies,  these calculations  yielded a  range  of
acquisition  price to LTM revenues  of 0.5x to 4.8x  with a mean (excluding high
and low) of 1.5x,  a range of acquisition  price to LTM EBDIT  of 6.7x to  13.8x
with  a mean (excluding high and low) of  8.8x, and a range of acquisition price
to Adjusted Book Value of 1.3x to 5.0x  with a mean (excluding high and low)  of
2.4x.  Simmons  then compared  the results  of  these calculations  to multiples
calculated using the closing price of DUAL Common Stock on January 24, 1996  and
the Implied Consideration listed above.
 
    Because  the  reasons  for,  and  circumstances  surrounding,  each  of  the
transactions analyzed  were  so diverse  and  due to  the  inherent  differences
between the operations of DUAL and the selected companies, Simmons believed that
a  purely quantitative comparable transaction  analysis would not be exclusively
dispositive in  the context  of the  Merger. Simmons  further believed  that  an
appropriate  use of  a comparable  transaction analysis  in this  instance would
involve qualitative judgments concerning differences between the characteristics
of these transactions and the Merger that would affect the acquisition value  of
the acquired companies and businesses and DUAL, which judgments are reflected in
Simmons' opinion.
 
    The  foregoing summary does not purport to  be a complete description of the
analyses performed by Simmons or of its presentations to the Board of  Directors
of  DUAL.  The preparation  of  financial analyses  and  fairness opinions  is a
complex process  and  is not  necessarily  susceptible to  partial  analysis  or
summary  description. Simmons  believes that its  analyses (and  the summary set
forth above) must be considered as a whole, and that selecting portions of  such
analyses  and of the  factors considered by Simmons,  without considering all of
such analyses and  factors, could  create an  incomplete view  of the  processes
underlying  the analyses conducted  by Simmons and its  opinion. Simmons made no
attempt to  assign  specific  weights  to  particular  analyses.  Any  estimates
contained  in Simmons' analyses are not necessarily indicative of actual values,
which may be  significantly more or  less favorable than  as set forth  therein.
Estimates  of values of companies do not purport to be appraisals or necessarily
reflect the  prices  at which  companies  may  actually be  sold.  Because  such
estimates  are  inherently  subject  to  uncertainty,  Simmons  does  not assume
responsibility for their accuracy.
 
                                       31
<PAGE>
    Simmons is a specialized energy-related  investment banking firm engaged  in
the  valuation of businesses and their securities in connection with mergers and
acquisitions, private  placements of  debt  and equity  and the  management  and
underwriting  of sales of equity and debt  to the public. Simmons has previously
rendered investment banking  services to  DUAL and  ENSCO in  connection with  a
number  of transactions  for which  Simmons received  customary compensation. In
addition, in the  ordinary course of  business, Simmons may  actively trade  the
securities  of  DUAL and  ENSCO  for its  own account  and  for the  accounts of
customers, and, accordingly, may at  any time hold a  long or short position  in
such  securities. Simmons acted as financial  advisor to DUAL in connection with
the transactions contemplated by the Merger Agreement and will receive a fee for
its services that is contingent upon the consummation of the Merger.
 
    DUAL has agreed  to pay Simmons  a contingent fee  totaling $3 million  upon
consummation  of the Merger, which fee is inclusive of certain expenses incurred
in connection  with  Simmons'  engagement  by DUAL.  DUAL  has  also  agreed  to
indemnify  Simmons and certain  related persons against  certain liabilities and
expenses relating  to  or  arising  out of  its  engagement,  including  certain
liabilities under Federal securities laws.
 
    During  the past  five years,  DUAL and  ENSCO have  each paid  Simmons fees
totaling $50,000  in  connection  with previously  rendered  investment  banking
services.
 
CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES
 
    THE  FOLLOWING DISCUSSION IS A SUMMARY OF THE MATERIAL UNITED STATES FEDERAL
INCOME TAX CONSEQUENCES  OF THE  MERGER AND  IS NOT  INTENDED TO  BE A  COMPLETE
DISCUSSION  OF ALL POTENTIAL TAX  EFFECTS THAT MIGHT BE  RELEVANT TO THE MERGER.
SUCH DISCUSSION DEALS ONLY WITH A CITIZEN OR RESIDENT OF THE UNITED STATES OR  A
DOMESTIC  CORPORATION  (A "U.S.  HOLDER"). THIS  SUMMARY  ASSUMES THAT  THE DUAL
COMMON STOCK HAS  BEEN HELD  AS A  CAPITAL ASSET. IT  MAY NOT  BE APPLICABLE  TO
CERTAIN   CLASSES  OF   TAXPAYERS,  INCLUDING,   WITHOUT  LIMITATION,  INSURANCE
COMPANIES, TAX-EXEMPT ORGANIZATIONS, FINANCIAL INSTITUTIONS, SECURITIES DEALERS,
BROKER-DEALERS, FOREIGN PERSONS, PERSONS WHO HOLD DUAL COMMON STOCK AS PART OF A
CONVERSION TRANSACTION, AND PERSONS WHO  ACQUIRED DUAL COMMON STOCK PURSUANT  TO
THE  EXERCISE OF EMPLOYEE STOCK OPTIONS  OR RIGHTS OR OTHERWISE AS COMPENSATION.
MOREOVER, THE  STATE,  LOCAL,  FOREIGN,  AND ESTATE  TAX  CONSEQUENCES  TO  DUAL
STOCKHOLDERS OF THE MERGER ARE NOT DISCUSSED.
 
    THIS  SUMMARY IS BASED ON LAWS, REGULATIONS, RULINGS, PRACTICE, AND JUDICIAL
DECISIONS IN EFFECT AT  THE DATE OF THIS  PROSPECTUS/PROXY STATEMENT AND ON  THE
OPINION  OF AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P., COUNSEL TO DUAL. HOWEVER,
LEGISLATIVE, JUDICIAL,  OR  ADMINISTRATIVE  CHANGES OR  INTERPRETATIONS  MAY  BE
FORTHCOMING  THAT COULD ALTER OR MODIFY THE STATEMENTS AND CONCLUSIONS SET FORTH
HEREIN. ANY SUCH CHANGES  OR INTERPRETATIONS MAY OR  MAY NOT BE RETROACTIVE  AND
COULD  AFFECT  THE  TAX  CONSEQUENCES  DESCRIBED  HEREIN  TO  STOCKHOLDERS. EACH
STOCKHOLDER IS URGED TO  CONSULT WITH SUCH STOCKHOLDER'S  OWN TAX ADVISER AS  TO
THE  PARTICULAR TAX  CONSEQUENCES TO SUCH  HOLDER OF  THE TRANSACTIONS DESCRIBED
HEREIN, INCLUDING THE APPLICABILITY AND EFFECT  OF ANY STATE, LOCAL, OR  FOREIGN
TAX LAWS, AND OF CHANGES IN APPLICABLE TAX LAWS.
 
    It  is  intended that  the  Merger qualify  as  a reorganization  within the
meaning of Section  368(a)(1)(A) of  the Code by  reason of  the application  of
Section  368(a)(2)(E) of the Code, and that, for federal income tax purposes, no
gain or loss will be recognized by ENSCO or DUAL as a result of the Merger.
 
    DUAL  has  received  the  Tax  Opinion   dated  as  of  the  date  of   this
Prospectus/Proxy   Statement,   that  the   Merger  should   be  treated   as  a
reorganization within the meaning  of Section 368(a) of  the Code and that  DUAL
and  ENSCO will each be a party to the reorganization pursuant to Section 368(b)
of the Code. Such an opinion is  not binding on the Internal Revenue Service  or
the  courts, and, therefore, the delivery of  the Tax Opinion cannot assure that
the Internal  Revenue  Service  or  the  courts  will  treat  the  Merger  as  a
reorganization within the meaning of Section 368(a) of the Code. The Tax Opinion
is  based,  among other  things, on  assumptions relating  to certain  facts and
circumstances of,  and the  intentions  of the  parties  to, the  Merger,  which
assumptions have been made with the consent of DUAL
 
                                       32
<PAGE>
and  ENSCO. The Tax Opinion is based on  the assumption that the amount of ENSCO
Common Stock  (determined by  value as  of  the Effective  Time) which  will  be
received  by  those  stockholders of  DUAL  (including the  stockholders  of the
Principal  Stockholder  who  will  receive   ENSCO  Common  Stock  pursuant   to
distributions  by, and the  liquidation of, the  Principal Stockholder after the
Merger, and who owned less than 5% of the value of the Principal Stockholder  as
of  the Effective Time) who  each owned, at the Effective  Time, less than 5% of
the value of DUAL  (taking into account such  stockholder's direct and  indirect
stockholding  in DUAL)  plus the  amount of  ENSCO Common  Stock received  by B.
Skaugen Shipping ASA, will be greater than or equal to 40% of the total value of
all of the formerly outstanding shares of DUAL Common Stock as of the  Effective
Time. In addition, the Tax Opinion is based on a representation from DUAL (and a
similar  representation from B. Skaugen Shipping ASA)  to the effect that to its
knowledge there  is  no plan  or  intention on  the  part of  DUAL  stockholders
described  in the immediately preceding  sentence (or on the  part of B. Skaugen
Shipping ASA) to  sell, exchange,  or otherwise dispose  of a  number of  shares
received  in the Merger that would  reduce such stockholders' ownership of ENSCO
Common Stock received in the Merger to a number of shares of ENSCO Common  Stock
having  a value, as of the Effective Time, of  less than 40% of the value of all
the formerly outstanding shares of  DUAL Common Stock as  of the same time.  The
Principal Stockholder has indicated that it intends to sell, for cash, a portion
of  the ENSCO Common Stock received in  the Merger. Because the number of shares
sold will vary with changes in value of the ENSCO Common Stock after the closing
of the  Merger and  fluctuations in  the exchange  rate for  U.S. and  Norwegian
currency,  there is no assurance that the  assumption upon which the Tax Opinion
is in part based will  be correct, in which event  (i) the Tax Opinion would  be
rendered  inapplicable  and (ii)  the  Merger may  or may  not  be treated  as a
tax-free "reorganization" depending on all the facts and circumstances.
 
    The Tax Opinion provides, assuming the  Merger is a reorganization with  the
meaning  of Section 368(a)  of the Code, that  (i) a U.S.  Holder of DUAL Common
Stock who, pursuant  to the  Merger, exchanges  such U.S.  Holder's DUAL  Common
Stock  solely for  ENSCO Common  Stock will  not recognize  gain or  loss on the
exchange, (ii) the holding  period of ENSCO Common  Stock received by each  U.S.
Holder of DUAL Common Stock in the Merger will include the holding period of the
DUAL  Common Stock surrendered therefor, (iii)  the aggregate adjusted tax basis
of such ENSCO Common Stock  will equal the U.S.  Holder's adjusted tax basis  in
the  DUAL  Common  Stock  surrendered  plus  the  amount  of  any  gain  or loss
recognized, less the amount  of cash received,  and (iv) that  a U.S. Holder  of
DUAL  Common  Stock who,  pursuant to  the Merger,  receives cash  in lieu  of a
fractional share of ENSCO Common Stock  will be treated as having received  such
fractional share of ENSCO Common Stock pursuant to the Merger and then as having
received  such cash  in a  redemption of such  fractional share  of ENSCO Common
Stock. Such a U.S. Holder of DUAL Common Stock generally will recognize  capital
gain  or loss  on such  deemed redemption  equal to  the difference  between the
amount of  cash  received  and the  U.S.  Holder's  adjusted tax  basis  in  the
fractional share of ENSCO Common Stock.
 
    The  Merger is not contingent  on the updating of the  Tax Opinion as of the
Effective Time. Further, whether the assumptions counsel was directed to make in
rendering the  Tax Opinion  are accurate,  will  not be  known until  after  the
Effective  Time. In the  event that the  40% continuity representation described
above cannot be  satisfied as of  the Effective  Time, it is  possible that  the
Merger  might not be treated  as a reorganization within  the meaning of Section
368(a) of the Code. The Tax Opinion does not address the tax consequences to the
DUAL stockholders in  the event the  Merger is not  treated as a  reorganization
within  the meaning of Section 368(a) of the Code. Generally in such event, each
stockholder of DUAL would recognize gain in the amount of the difference between
(i) the fair  market value  of the  ENSCO Common Stock  and the  amount of  cash
received  in lieu  of a  fractional share  of ENSCO  Common Stock  and (ii) such
holder's adjusted tax basis in the  DUAL Common Stock surrendered therefor.  Any
such  recognized gain generally would be  characterized as capital gain or loss.
If the Merger is  not considered a reorganization,  the holding period of  ENSCO
Common  Stock received by  each U.S. Holder  of DUAL Common  Stock in the Merger
would not  include the  holding  period of  the  DUAL Common  Stock  surrendered
therefor  and the aggregate adjusted tax basis  of such ENSCO Common Stock would
equal the fair market value of ENSCO Common Stock received.
 
                                       33
<PAGE>
REGULATORY APPROVALS
 
    ENSCO and  DUAL  are  aware  of no  government  regulatory  approvals  still
required for consummation of the Merger, other than compliance with notification
requirements of environmental agencies, with federal securities laws, with state
securities  or "Blue Sky" laws,  and with the requirements  of the U.S. Maritime
Administration. The Merger was subject to the pre-merger notification provisions
of the HSR Act. Filings were made under  the HSR Act by ENSCO and DUAL with  the
Federal  Trade Commission and  the Department of  Justice Antitrust Division. On
February 28, 1996, early termination of the waiting period under the HSR Act was
granted, thus permitting the Merger to proceed pursuant to the HSR Act,  subject
to stockholder approval and the other conditions described herein.
 
    Under the Merger Agreement, DUAL, ENSCO and Merger Subsidiary have agreed to
use  their  best efforts  to (i)  take, or  cause to  be taken,  all appropriate
action, and do, or cause to be  done, all things necessary, proper or  advisable
under  applicable law or required  to be taken by  any governmental authority or
otherwise to consummate and make effective the transactions contemplated by  the
Merger  Agreement as promptly as practicable,  (ii) obtain from any governmental
authorities any consents, licenses, permits, waivers, approvals,  authorizations
or  orders required  to be obtained  or made  by ENSCO or  DUAL or  any of their
subsidiaries in connection with the authorization, execution and delivery of the
Merger  Agreement  and  the   consummation  of  the  transactions   contemplated
thereunder,  including, without limitation, the Merger, and (iii) as promptly as
practicable, make all necessary filings, and thereafter make any other  required
submissions,  with respect to the Merger Agreement and the Merger required under
(a) the Securities Act and the Exchange Act, and any other applicable federal or
state securities laws, (b) the rules  and regulations of the NYSE, (c)  Delaware
law,  (d) the HSR Act  and any related governmental  request thereunder, and (e)
any other applicable law; provided that ENSCO and DUAL shall cooperate with each
other in connection  with the making  of all such  filings, including  providing
copies  of all such documents to the  non-filing party and its advisors prior to
filing and,  if  requested, accepting  all  reasonable additions,  deletions  or
changes  suggested in  connection therewith. DUAL  and ENSCO have  agreed to use
reasonable best efforts to  furnish to each other  all information required  for
any application or other filing to be made pursuant to the rules and regulations
of any applicable law (including all information required to be included in this
Prospectus/  Proxy Statement and the  Registration Statement) in connection with
the transactions contemplated by the Merger Agreement.
 
ACCOUNTING TREATMENT
 
    It is intended that ENSCO  will account for the  Merger as a purchase  under
GAAP.   The  effect  of  purchase  accounting  treatment  is  that  the  assets,
liabilities, and stockholders' equity  accounts will be  recorded at their  fair
values at the Effective Time. Any excess of consideration paid by ENSCO over the
fair value of DUAL's assets will be recorded as goodwill.
 
NYSE LISTING
 
    The  shares of ENSCO Common Stock to be issued in connection with the Merger
will be listed  on the NYSE.  Evidence from the  NYSE that the  shares of  ENSCO
Common  Stock to be issued  in connection with the Merger  will be listed on the
NYSE immediately following the Effective Time is a condition to the consummation
of the Merger. See "Certain Provisions of the Merger Agreement -- Conditions  to
the Consummation of the Merger."
 
INTERESTS OF CERTAIN PERSONS; POSSIBLE CONFLICTS OF INTEREST
 
    INTERESTS   OF  CERTAIN  PERSONS   IN  THE  MERGER.     In  considering  the
recommendation of the  Board of  Directors of DUAL  with respect  to the  Merger
Agreement  and  the transactions  contemplated  thereby, stockholders  should be
aware that  certain members  of  the management  of DUAL  and  of its  Board  of
Directors  have certain  interests in  the Merger  that are  in addition  to the
interests of the stockholders of DUAL generally (including, without  limitation,
the  potential  receipt  of  severance  and/or  other  employment  benefits  and
compensation for stock options).
 
                                       34
<PAGE>
    EMPLOYMENT AGREEMENTS.   Each of  DUAL's executive officers  named below  is
currently  employed by DUAL  pursuant to an employment  agreement. Each of these
agreements provides for a payment  to the employee in the  event of a change  of
control  of DUAL. Pursuant to  the Merger and the  DUAL Executive Agreements (as
discussed and defined below), the  following persons will receive the  following
payments  under their respective employment agreements at the Effective Time, in
addition to the other amounts described below: L.H. Dick Robertson ($1,080,000);
Robert F. Chrone ($200,000);  William R. Dudark  ($290,000); Dudley M.  Haralson
($600,000);  Lewis W. Kreps ($300,000); Robert C. McCoy ($320,000); and W. Allen
Parks ($738,750).
 
    STOCK OPTIONS.   Certain  officers and  directors of  DUAL hold  options  to
purchase  shares of DUAL Common Stock pursuant to the Non-Employee Director Plan
and  the  1993  Plan.  Under  the  Merger  Agreement,  these  options  will   be
extinguished in return for shares of ENSCO Common Stock. See "Certain Provisions
of  the Merger Agreement  -- Effect of  the Merger on  Stock Options." The stock
options granted under the  1993 Plan were  granted during 1995  and vest over  a
five-year  period. Under  the terms of  the option agreements  granted under the
1993 Plan and the  DUAL Executive Agreements (as  discussed and defined  below),
all  of  the stock  options  will vest  as of  May  15, 1996.  Each non-employee
director of DUAL is  granted an option  to acquire 2,000  shares of DUAL  Common
Stock  on  the day  of  the annual  meeting of  stockholders  of DUAL  under the
Non-Employee Director Plan. These options vest over a one-year period. Under the
terms of the option  agreements granted under  the 1993 Plan,  all of the  stock
options  will vest  as a result  of the  Merger. See "Certain  Provisions of the
Merger Agreement -- Effect of the Merger on Stock Options."
 
    RESTRICTED STOCK.  Certain  executives of DUAL have  been granted awards  of
restricted  stock  which  vest  on  August 12,  1996.  Under  the  terms  of the
restricted stock awards, all such  awards will vest as  a result of the  Merger.
The following persons have the aggregate number of restricted stock awards which
will  vest on the Effective Time: L.H. Dick Robertson (9,000); William R. Dudark
(3,900); Dudley M.  Haralson (3,900); Lewis  W. Kreps (3,900);  Robert C.  McCoy
(4,500); and W. Allen Parks (4,800).
 
    CERTAIN  BENEFIT PLANS.  DUAL has agreed to amend the provisions of the DUAL
DRILLING COMPANY Supplemental Executive Retirement  Plan (the "SERP") to  freeze
all  benefits under  the SERP  as of the  day before  the Effective  Time and to
permit ENSCO or the Surviving Corporation,  as successor to DUAL's rights  under
the  SERP, to distribute the  determined benefit under the  SERP payable to each
participant covered  by  the  SERP in  one  lump  sum payment  in  ENSCO's  sole
discretion  as  soon as  practicable  after January  1,  1997, to  determine the
benefits payable  thereunder to  each  participant, and  to terminate  the  SERP
effective  as of the day before the Effective Time. The benefits so payable have
been established  by DUAL  and  ENSCO under  the  DUAL Executive  Agreements  as
follows:  L.H. Dick Robertson ($1,527,227);  William R. Dudark ($94,683); Dudley
M. Haralson ($153,671); Lewis  W. Kreps ($88,270);  Robert C. McCoy  ($202,309);
and W. Allen Parks ($197,623).
 
    DUAL has agreed to amend the provisions of the DUAL DRILLING COMPANY Benefit
Restoration  Plan (the "Benefit Restoration Plan")  to freeze all benefits under
the Benefit Restoration Plan,  to determine the  benefits payable thereunder  to
each  participant, and to terminate the Benefit Restoration Plan effective as of
the day  before  the Effective  Time.  The Benefit  Restoration  Plan  currently
permits   certain  officers  and  other  employees  of  DUAL  to  defer  current
compensation which cannot be redirected into the DUAL DRILLING COMPANY Employees
Tax Deferred/Thrift  Savings Plan  and  Trust due  to contribution  and  funding
limitations  imposed by the Code. The payments shall be made on or before ninety
(90) days from and after the Effective Time. The benefits so payable, calculated
through December 31,  1995, have been  established by DUAL  and ENSCO under  the
DUAL Executive Agreements (as discussed and defined below) as follows: L.H. Dick
Robertson  ($152,067);  William R.  Dudark ($13,335);  Lewis W.  Kreps ($4,976);
Robert C.  McCoy ($2,475);  and  W. Allen  Parks  ($61,091). The  above  benefit
amounts  do not include the earnings retained  on the balance of each employee's
account which will accrue between January 1, 1996 and the Effective Time, DUAL's
matching contributions,  if  any,  for 1996,  or  the  employee's  contributions
subsequent to December 31, 1995.
 
                                       35
<PAGE>
    DUAL  has agreed to freeze all benefits under the Discontinued Executive and
Manager Team Incentive  Program ("Team Incentive  Program") maintained by  DUAL,
and ENSCO has agreed to assume the benefit payment obligations of DUAL under the
Team Incentive Program. After all such benefit payments have been made, the Team
Incentive  Program will be terminated. An obligation to make payments to certain
officers of DUAL aggregating $359,792 will be triggered by the Merger under  the
Team  Incentive Program pursuant to deferred bonuses earned thereunder. Pursuant
to the  Merger and  the  DUAL Executive  Agreements  (as discussed  and  defined
below), the following persons will receive the following payments under the Team
Incentive Program at the Effective Time: L.H. Dick Robertson ($181,428); William
R. Dudark ($19,926); Dudley M. Haralson ($14,064); Lewis W. Kreps ($31,854); and
W. Allen Parks ($112,520).
 
    DUAL has agreed to terminate its Annual Incentive Plan as of the date of the
commencement  of such plan and accordingly  no benefits shall accrue thereunder.
The executive officers  of DUAL which  participated in this  plan agreed to  the
termination  thereof in their respective DUAL Executive Agreements (as discussed
and defined below).
 
    DUAL SPECIAL PERFORMANCE UNIT  PLAN.  Pursuant to  the Unit Plan,  effective
August 21, 1995, maintained by DUAL, L.H. Dick Robertson, W. Allen Parks, Dudley
M.  Haralson, Robert C. McCoy, Lewis W.  Kreps, William R. Dudark, and Robert F.
Chrone will receive  payments upon occurrence  of the Merger.  The Unit Plan  is
subject  to  approval  by  the  stockholders of  DUAL.  Pursuant  to  the Merger
Agreement, DUAL has  agreed that the  aggregate of these  payments and the  cash
bonus  amount payable  to David W.  Skarke pursuant to  his employment agreement
will not exceed a total of $2 million. See "Skarke Agreement" below. The amounts
payable under  the Unit  Plan, as  set forth  in the  DUAL Executive  Agreements
(defined  below)  will be  paid  at the  Effective  Time as  follows:  L.H. Dick
Robertson ($469,972); Robert F. Chrone ($78,329); William R. Dudark  ($208,877);
Dudley  M.  Haralson  ($271,540); Lewis  W.  Kreps ($208,877);  Robert  C. McCoy
($229,765); and  W. Allen  Parks  ($271,540). ENSCO  has  agreed to  assume  the
benefit  payment  obligations of  DUAL under  the  Unit Plan  and to  cause such
payments to be made. After all such payments have been made, the Unit Plan  will
be  deemed terminated. See "Proposal to Approve the Adoption of the DUAL Special
Performance Unit Plan."
 
    DUAL EXECUTIVE AGREEMENTS.   Each of DUAL's  executive officers named  below
and  DUAL entered  into an Agreement  and Consent and  Compromise and Settlement
each dated  March  21,  1996,  and  amended on  May  9,  1996  ("DUAL  Executive
Agreements").  Under  the terms  of the  DUAL Executive  Agreements each  of the
executive officers of  DUAL have  agreed to accept  the payments  and shares  of
ENSCO  Common Stock discussed above in  consideration of the executive officers'
agreement to terminate their respective  rights under the employment  agreements
and  make amendments to certain benefit plans of DUAL. The executive officers of
DUAL which entered into the DUAL Executive Agreements are as follows: L.H.  Dick
Robertson,  Robert F.  Chrone, William R.  Dudark, Dudley M.  Haralson, Lewis W.
Kreps, Robert C. McCoy and W. Allen Parks.
 
    INDEMNIFICATION.   Under the  Merger Agreement,  ENSCO has  agreed that  the
Surviving  Corporation's Certificate of Incorporation  and By-laws following the
Merger will be no  less favorable with respect  to indemnification of  officers,
directors,  employees and agents than DUAL's  current Bylaws. In addition, ENSCO
has agreed not to amend or repeal for a period of six years after the Merger the
provisions of  the  Surviving  Corporation's Certificate  of  Incorporation  and
Bylaws providing for indemnification of individuals who were directors, officers
or  employees  of DUAL  or any  of  its subsidiaries  (each a  "Subsidiary" and,
collectively,  the  "Subsidiaries")  in  any  manner  that  would  affect   such
individuals'  rights  thereunder,  except as  required  by Delaware  law  and to
indemnify such directors and  officers for six years  after the Effective  Time.
Pursuant  to the  Merger Agreement, the  Surviving Corporation  will maintain in
effect for six years the current policies of directors' and officers'  insurance
maintained  by DUAL  and the  Subsidiaries with  respect to  claims arising from
facts and events which occurred before the Effective Time.
 
                                       36
<PAGE>
    SKARKE AGREEMENT.   Pursuant  to an  employment agreement  between DUAL  and
David  W. Skarke,  Chairman of the  Board of  DUAL, dated October  2, 1995, DUAL
agreed to pay Mr. Skarke a cash  bonus equal to 15.02% of the Performance  Bonus
Pool upon consummation of the Merger in exchange for his services as Chairman of
the  Board  of  Directors of  DUAL.  Mr.  Skarke's cash  bonus,  along  with the
Performance Bonus  Pool  of  the Unit  Plan,  has  been limited  by  the  Merger
Agreement. As so limited, Mr. Skarke will receive a cash bonus equal to $261,100
upon  consummation of the Merger.  See "Proposal to Approve  the Adoption of the
DUAL Special Performance Unit Plan."  Pursuant to the employment agreement,  Mr.
Skarke was also granted 100,000 options under the 1993 Plan at an exercise price
of  $10 3/8 per share, vesting at a rate of 20% per year, beginning on September
14, 1996. Upon  consummation of the  Merger, Mr. Skarke  will receive shares  of
ENSCO  Common Stock, as described in "Certain Provisions of the Merger Agreement
- -- Effect of the Merger on Stock Options."
 
    EXECUTIVE OFFICERS;  BOARD OF  DIRECTORS.   The  officers and  directors  of
Merger  Subsidiary immediately prior  to the Effective Time  will be the initial
officers of the Surviving Corporation. The respective terms and responsibilities
of each officer and director of the  Surviving Corporation will be as set  forth
in  the Certificate  of Incorporation and  Bylaws of  the Surviving Corporation.
DUAL has agreed  to terminate the  employment of any  of its executive  officers
identified  by  ENSCO  at  least  one day  prior  to  the  Effective  Time. This
termination will  not adversely  affect  any Long-Term  Options (as  defined  in
"Certain  Provisions of the  Merger Agreement --  Effect of the  Merger on Stock
Options"). ENSCO intends to terminate all of DUAL's current executive officers.
 
RESTRICTIONS ON RESALES BY AFFILIATES
 
    ENSCO has registered the  shares of ENSCO Common  Stock that the holders  of
DUAL  Common Stock will be  entitled to receive upon  consummation of the Merger
under  the  Securities  Act.  DUAL  stockholders  who  are  not  deemed  to   be
"affiliates"  of DUAL  may freely  sell the  shares of  ENSCO Common  Stock they
receive in the Merger. Stockholders of DUAL who are deemed to be "affiliates" of
DUAL may resell the shares of ENSCO  Common Stock received in the Merger (i)  in
transactions in compliance with Rule 145 under the Securities Act, (ii) pursuant
to  an  effective  registration statement  under  the Securities  Act,  or (iii)
pursuant to any other exemption from registration which may be available to such
stockholder. Such stockholders  may not use  this Prospectus/Proxy Statement  to
effect resales of the shares of ENSCO Common Stock they receive.
 
    Rule  145, as  currently in  effect, imposes  restrictions on  the manner in
which such affiliates may make  resales and also on  the volume of resales  that
such  affiliates, and others with whom they may  act in concert, may make in any
three-month period. The term  "affiliates" is defined in  the Securities Act  to
include  any person who, directly or  indirectly, controls, is controlled by, or
is under common control with, DUAL at the time the Merger is submitted to a vote
of the  stockholders of  DUAL. ENSCO  has been  advised by  DUAL that  David  W.
Skarke,  L.H.  Dick Robertson,  W. Allen  Parks, Dudley  M. Haralson,  Robert C.
McCoy, Lewis W.  Kreps and William  R. Dudark (each  a DUAL executive  officer),
Aage  Figenschou, Magne Kristiansen, Frank Jungers  and Edward O. Vetter (each a
director of  DUAL),  the Principal  Stockholder,  and B.  Skaugen  Shipping  ASA
("Skaugen")  may be deemed  "affiliates" of DUAL for  purposes of the Securities
Act. Pursuant to the Merger Agreement,  DUAL has agreed to use all  commercially
reasonable   efforts  to  cause  each  such  person  other  than  the  Principal
Stockholder and Skaugen to deliver an affiliates' agreement to ENSCO on or prior
to the Effective Time in  substantially the form attached  as an exhibit to  the
Merger Agreement.
 
    Pursuant to the Principal Stockholder Agreement, ENSCO has agreed to use all
commercially  reasonable efforts to effect the registration under the Securities
Act of the transfer to the stockholders of the Principal Stockholder of  certain
of  the shares of ENSCO Common Stock to be received by the Principal Stockholder
in exchange for  the Principal Stockholder  Shares and the  resale of shares  of
ENSCO  Common Stock by the Principal  Stockholder or Skaugen, subject to certain
restrictions. See  "Principal Stockholder  Agreement --  Registration of  Merger
Shares."
 
                                       37
<PAGE>
MANAGEMENT
 
    Pursuant  to the Merger Agreement, upon  the consummation of the Merger, the
officers and directors of Merger  Subsidiary immediately prior to the  Effective
Time will be the initial officers and directors of the Surviving Corporation, in
each  case until their  respective successors are duly  elected or appointed and
qualified, or  as otherwise  provided in  the Certificate  of Incorporation  and
Bylaws of the Surviving Corporation.
 
EFFECT OF MERGER ON CERTAIN DEBT OF DUAL
 
    DUAL  currently has outstanding  $100,000,000 in principal  amount of 9 7/8%
Senior Subordinated Notes due 2004 (the "Senior Notes"). Upon the occurrence  of
a  Change of Control as defined in the indenture (the "Indenture") providing for
the Senior Debt, DUAL will be obligated to make an offer to purchase (a  "Change
of  Control  Offer")  and  shall, subject  to  the  provisions  described below,
purchase, on a  business day (the  "Change of Control  Purchase Date") not  more
than  90 or less than 30 days following the occurrence of the Change of Control,
all of the then  outstanding Senior Notes  at a purchase  price (the "Change  of
Control  Purchase Price") equal to 101% of the aggregate principal amount of the
Senior Notes, plus accrued and unpaid interest, if any, to the Change of Control
Purchase Date.  DUAL  shall,  subject  to the  provisions  described  below,  be
required  to  purchase all  Senior Notes  properly tendered  into the  Change of
Control Offer and  not withdrawn.  The Change of  Control Offer  is required  to
remain open for at least 20 business days and until the close of business on the
fifth business day prior to the Change of Control Purchase Date.
 
    In  order to effect such Change of  Control Offer, DUAL will, not later than
the 30th  day  after the  Change  of Control,  mail  to the  trustee  under  the
Indenture  and to each  holder of Senior  Notes notice of  the Change of Control
Offer, which notice shall govern  the terms of the  Change of Control Offer  and
shall  state, among  other things, the  procedures that holders  of Senior Notes
must follow to accept the Change of Control Offer.
 
    The Merger will constitute  a Change of Control  and therefore DUAL will  be
required  to make a Change of Control Offer after the Effective Time. The Merger
is not  conditioned on  ENSCO receiving  any  waiver of  the Change  in  Control
provisions of the Indenture. ENSCO believes it is not likely that holders of the
Senior  Notes will  tender for  purchase any  significant portion  of the Senior
Notes since such Senior Notes  currently have a value in  excess of 101% of  the
aggregate  principal amount of the Senior Notes. In the event all or any portion
of the Senior Notes are tendered by  the holders thereof, ENSCO believes it  has
adequate  liquidity from existing financial resources  to pay the purchase price
of such Senior Notes so tendered.
 
    DUAL has entered into a $32.0 million term loan with a group of banks led by
Citibank, N.A.  (the  "Citibank Facility").  The  Citibank Facility  includes  a
revolving  credit facility, a term loan and a $15 million guarantee facility for
the issuance of bonds and letters  of credit. The revolving credit facility  and
the  term loan under the Citibank Facility bears interest at LIBOR plus a margin
of 2%. As of April 30,  1996, the aggregate outstanding principal balance  under
the  revolving credit facility and the term loan under the Citibank Facility was
approximately  $28.8  million.  In  addition,  at  April  30,  1996  there  were
outstanding letters of credit for approximately $7.9 million under the guarantee
facility  of the Citibank Facility.  DUAL has also entered  into a $17.0 million
term loan with Christiania  Bank og Kreditkasse (the  "CBK Loan"). The CBK  Loan
bears interest at LIBOR plus 2%. As of April 30, 1996, the outstanding principal
balance under the CBK Loan was approximately $12.9 million.
 
    Both  the Citibank  Facility and the  CBK Loan  contain customary provisions
restricting changes in control of DUAL.  Pursuant to the Merger Agreement,  DUAL
has  covenanted to  use commercially reasonable  efforts to  obtain waivers from
certain third parties, including  the parties to the  Citibank Facility and  the
CBK  Loan, regarding  any change  in control resulting  from the  Merger. In the
event that  DUAL  is unable  to  obtain these  waivers,  ENSCO believes  it  has
adequate  liquidity from  existing financial  resources to  pay any  amounts due
under the Citibank Facility and the CBK Loan upon consummation of the Merger.
 
                                       38
<PAGE>
                   CERTAIN PROVISIONS OF THE MERGER AGREEMENT
 
    THE FOLLOWING  IS  A  SUMMARY  OF THE  MATERIAL  PROVISIONS  OF  THE  MERGER
AGREEMENT  NOT  SUMMARIZED  ELSEWHERE IN  THIS  PROSPECTUS/PROXY  STATEMENT. THE
MERGER AGREEMENT IS ATTACHED  AS APPENDIX A  TO THIS PROSPECTUS/PROXY  STATEMENT
AND IS INCORPORATED HEREIN BY REFERENCE.
 
EXCHANGE PROCEDURES
 
    The  Merger Agreement  provides that,  as of  or before  the Effective Time,
ENSCO will deposit with a bank or trust company organized under the laws of, and
having an office in, the  United States or any  state thereof and designated  by
ENSCO  (the "Exchange  Agent"), for  the benefit of  the holders  of DUAL Common
Stock, for  exchange  in  accordance  with the  Merger  Agreement,  through  the
Exchange  Agent, (i) certificates evidencing the number of whole shares of ENSCO
Common Stock issuable  in exchange for  DUAL Common  Stock and (ii)  cash in  an
amount sufficient to permit payment of cash payable in lieu of fractional shares
(such  certificates and  cash, the  "Exchange Fund").  The Exchange  Agent will,
pursuant to irrevocable  instructions from  ENSCO, deliver the  shares of  ENSCO
Common Stock and cash contemplated to be issued out of the Exchange Fund.
 
    As  soon  as reasonably  practicable after  the  Effective Time,  ENSCO will
instruct the Exchange Agent to mail to each holder of record of a certificate or
certificates which immediately prior to the Effective Time evidenced outstanding
shares of DUAL Common Stock (other than Cancelable Shares) (the  "Certificates")
(i)  a letter  of transmittal  and (ii)  instructions for  use in  effecting the
surrender of the Certificates in exchange for certificates evidencing shares  of
ENSCO  Common Stock.  Upon surrender  of a  Certificate for  cancellation to the
Exchange Agent together with such letter of transmittal, duly executed, and such
other customary documents as may be required pursuant to such instructions,  the
holder  of such Certificate will  be entitled to receive  in exchange therefor a
certificate representing the number of whole shares of ENSCO Common Stock  which
such  holder has the  right to receive in  respect of the  shares of DUAL Common
Stock formerly represented by  such Certificate (after  taking into account  all
shares  of DUAL Common  Stock then held  by such holder),  together with cash in
lieu of fractional shares of ENSCO Common Stock to which such holder is entitled
and any dividends  or distributions to  which such holder  is entitled, and  the
Certificate  so surrendered shall forthwith  be canceled. Under no circumstances
will any holder of a Certificate (except for certain lost, stolen, or  destroyed
Certificates)  be entitled  to receive  any part of  the shares  of ENSCO Common
Stock into which the shares  of DUAL Common Stock  were converted in the  Merger
until  such holder shall  have surrendered such  Certificate. In the  event of a
transfer of ownership of shares of DUAL Common Stock which is not registered  in
the  transfer records of DUAL,  the shares of ENSCO  Common Stock into which the
DUAL Common Stock were converted in the  Merger may be issued to the  transferee
if  the Certificate evidencing such shares of  DUAL Common Stock is presented to
the Exchange Agent, accompanied by all documents required to evidence and effect
such transfer and by evidence that any applicable stock transfer taxes have been
paid. Until so surrendered,  each Certificate will be  deemed at any time  after
the Effective Time to evidence only the right to receive upon such surrender the
certificate  representing the number of whole shares of ENSCO Common Stock which
such holder has the  right to receive  in respect of the  shares of DUAL  Common
Stock  formerly represented by  such Certificate (after  taking into account all
shares of DUAL Common  Stock then held  by such holder),  together with cash  in
lieu of fractional shares of ENSCO Common Stock to which such holder is entitled
and  any dividends or distributions to which  such holder is entitled. ENSCO has
agreed, from and after the Effective Time, to treat the holders of  certificates
formerly representing shares of DUAL Common Stock as holding of record the whole
number of shares of ENSCO Common Stock for purposes of voting and determinations
of quorums for voting.
 
    No  dividends or  other distributions declared  or made  after the Effective
Time with respect to shares of ENSCO  Common Stock with a record date after  the
Effective  Time will be paid to the holder of any unsurrendered Certificate with
respect to the  shares of  ENSCO Common  Stock into  which such  shares of  DUAL
Common  Stock were converted in the Merger, until the holder of such Certificate
shall surrender  such  Certificate  for  exchange  as  provided  in  the  Merger
Agreement. Subject to the effect of
 
                                       39
<PAGE>
applicable laws, following surrender of any such Certificate, there will be paid
to  the holder  of such Certificate,  in addition  to the whole  shares of ENSCO
Common Stock to which such holder  is entitled, without interest, the amount  of
dividends  or other  distributions with a  record date after  the Effective Time
theretofore paid  with  respect  to  the whole  shares  of  ENSCO  Common  Stock
evidenced by such Certificate.
 
    All  shares of ENSCO  Common Stock issued  upon conversion of  the shares of
DUAL Common  Stock  in  accordance  with  the  terms  of  the  Merger  Agreement
(including  any cash paid  or other distributions)  will be deemed  to have been
issued in full  satisfaction of  all rights pertaining  to such  shares of  DUAL
Common Stock.
 
    Neither  ENSCO nor the Surviving Corporation will be liable to any holder of
shares of DUAL Common  Stock for any  shares of ENSCO Common  Stock or cash  (or
dividends  or distributions with respect thereto) delivered to a public official
pursuant to any applicable abandoned property, escheat or similar law.
 
    If any Certificate has been lost, stolen or destroyed, upon the making of an
affidavit of  that fact  by the  person claiming  such Certificate  to be  lost,
stolen  or destroyed and, if required  by the Surviving Corporation, the posting
by such person of a bond in such reasonable amount as the Surviving  Corporation
may  direct as  indemnity against  any claim  that may  be made  against it with
respect to such Certificate, the Exchange Agent will issue in exchange for  such
lost,  stolen or destroyed Certificate the shares of ENSCO Common Stock, cash in
lieu of  fractional  shares of  ENSCO  Common  Stock and  unpaid  dividends  and
distributions  on shares  of ENSCO Common  Stock deliverable  in respect thereof
pursuant to the Merger Agreement.
 
    At the Effective Time, the stock transfer  books of DUAL will be closed  and
there  will be  no further  registration of transfers  of shares  of DUAL Common
Stock thereafter on the  records of DUAL.  At or after  the Effective Time,  any
Certificates  presented to the  Exchange Agent or  ENSCO for any  reason will be
converted into the  shares of  ENSCO Common Stock,  cash in  lieu of  fractional
shares  of ENSCO Common Stock and any  dividends or other distributions to which
they are entitled.
 
    No certificates or scrip evidencing fractional shares of ENSCO Common  Stock
will  be issued, but in lieu thereof each  holder of shares of DUAL Common Stock
who would otherwise be entitled to receive a fraction of a share of ENSCO Common
Stock, after aggregating  all shares  of ENSCO  Common Stock  which such  holder
would  be otherwise  entitled to  receive, will receive  an amount  equal to the
Average Trading Price (as defined below)  multiplied by the fraction of a  share
of  ENSCO Common Stock to which such holder would otherwise be entitled, without
interest. The "Average Trading  Price" will be the  average of the closing  sale
prices  of the shares of ENSCO Common Stock on the NYSE or, if not listed on the
NYSE, any  exchange on  which  the shares  of ENSCO  Common  Stock may  then  be
principally  listed (as reported by THE WALL  STREET JOURNAL or, if not reported
thereby, by another authoritative source) over the ten business days immediately
preceding the Effective Date.
 
CERTAIN REPRESENTATIONS AND WARRANTIES
 
    The Merger  Agreement contains  various  representations and  warranties  of
ENSCO,  the  Merger Subsidiary  and DUAL  relating to,  among other  things, the
following matters (which representations and warranties are subject, in  certain
cases,   to  specified  exceptions  and  generally   apply  only  to  facts  and
circumstances existing as  of the  date of the  Merger Agreement):  (i) the  due
organization,  valid  existence, good  standing,  power, and  authority  of, and
necessary governmental approvals secured by, and similar corporate matters  with
respect  to, each of ENSCO, DUAL,  Merger Subsidiary, and the Subsidiaries; (ii)
the delivery of, and absence of  violation of, the Certificate of  Incorporation
and Bylaws or equivalent organizational documents (collectively, "Organizational
Documents")  of each  of ENSCO, DUAL,  Merger Subsidiary,  and the Subsidiaries;
(iii) the capital structure of each  of ENSCO, DUAL, Merger Subsidiary, and  the
Subsidiaries;  (iv) the power and authority of each of the parties to the Merger
Agreement to  execute  the  Merger  Agreement and  to  perform  its  obligations
thereunder,  and its due  authorization, execution, delivery,  and validity; (v)
the absence  of conflict  with the  Organizational Documents,  with domestic  or
foreign laws, rules, regulations, orders, judgments, or
 
                                       40
<PAGE>
decrees,  and with  notes, bonds, indentures,  mortgages, contracts, agreements,
permits, leases,  licenses, franchises,  and other  instruments or  obligations;
(vi)  the  absence  of certain  governmental  consent,  approval, authorization,
permit, notification,  and  filing requirements;  (vii)  the possession  of  all
franchises,  grants,  authorizations, licenses,  permits,  easements, variances,
exceptions, consents, certificates,  approvals, and orders  of any  governmental
authority  necessary  to carry  on the  business of  DUAL and  the Subsidiaries;
(viii) reports and other documents filed with the Commission and the accuracy of
the information contained therein; (ix) the absence of certain changes or events
prior to the date of  the Merger Agreement having  a material adverse effect  on
DUAL,  the Subsidiaries, ENSCO, or Merger Subsidiary; (x) the absence of certain
pending or  threatened litigation  against DUAL  or the  Subsidiaries; (xi)  the
existence  and  content of  employee  benefit plans,  employment  contracts, and
severance agreements of  DUAL and  the Subsidiaries;  (xii) the  absence of  any
collective bargaining agreement or other labor union contract applicable to DUAL
or  the Subsidiaries;  (xiii) the  intellectual property  rights used, employed,
exploited,  or  licensed  by  DUAL   or  any  Subsidiary;  (xiv)  currency   and
completeness   of  tax  filings,  payments,   and  accruals  by  ENSCO,  ENSCO's
subsidiaries, DUAL, and the Subsidiaries; (xv) the absence of actions by DUAL or
any Subsidiary preventing the Merger  from qualifying as a reorganization  under
Section  368(a) of  the Code;  (xvi) certain  environmental matters;  (xvii) the
engagement and opinion of Simmons; (xviii) the vote required by stockholders  of
DUAL;  (xix)  the absence  of brokers',  finders',  or investment  bankers' fees
incurred by DUAL, the Subsidiaries, ENSCO,  and the Merger Subsidiary; (xx)  the
validity  of DUAL's and  the Subsidiaries' title and  interest in properties and
assets; (xxi) the material contracts to which DUAL or any Subsidiary is a party;
(xxii) the absence of  "parachute payments" by DUAL  or any Subsidiary;  (xxiii)
the  absence of certain  unlawful business practices  by DUAL, the Subsidiaries,
and their  officers,  directors, agents,  and  employees; (xxiv)  real  property
leased  by DUAL and the Subsidiaries; (xxv)  insurance policies to which DUAL or
any Subsidiary has  been a party,  named insured, or  other beneficiary;  (xxvi)
certain  accounting and tax matters; (xxvii) the recommendation of the Merger by
the Board  of  Directors  of  DUAL; (xxviii)  contractual  or  other  provisions
relating  to "changes in control" or similar occurrences; (xxix) the description
and documentation of all  drilling rigs owned, leased,  or chartered by DUAL  or
any   Subsidiary,  and  (xxx)  certain  matters  concerning  DUAL's  contractual
relationships with  various parties,  including Sime-Dual  Drilling Sdn  Bhd,  a
Malaysian company.
 
CONDUCT OF BUSINESS PENDING THE MERGER
 
    DUAL  has agreed  that, between  the date  of the  Merger Agreement  and the
Effective Time, except for certain transactions previously disclosed to ENSCO or
as contemplated  by  the Merger  Agreement,  unless ENSCO  otherwise  agrees  in
writing  (which agreement may not be  unreasonably withheld), (i) the businesses
of DUAL  and the  Subsidiaries  will be  conducted only  in,  and DUAL  and  the
Subsidiaries will not take any action except in, the ordinary course of business
consistent  with past practice and in  accordance with all applicable laws, (ii)
DUAL will  use  all reasonable  efforts  to preserve  substantially  intact  its
business  organization, to keep available the  services of the current officers,
employees and  consultants of  DUAL and  the Subsidiaries  and to  preserve  the
current relationships of DUAL and the Subsidiaries with customers, suppliers and
other  persons  with  which  DUAL or  any  Subsidiary  has  significant business
relations, (iii) DUAL will not, and will not permit any Subsidiary to, engage in
any practice, take  any action,  or enter  into any  of a  number of  enumerated
transactions  relating to changes or events  having a material adverse effect on
DUAL or the Subsidiaries,  and (iv) DUAL  and each Subsidiary  will cause to  be
maintained  in full force and effect,  and without modification or amendment, or
any lapse  of  coverage  under,  all insurance  policies  contained  in  a  list
previously provided to ENSCO.
 
    ENSCO  has agreed  that, between  the date of  the Merger  Agreement and the
Effective Time, except as previously disclosed to DUAL or as contemplated by the
Merger Agreement, unless DUAL otherwise  agrees in writing (which agreement  may
not be unreasonably withheld), (i) the businesses of ENSCO and Merger Subsidiary
will  be conducted  only in,  and ENSCO  shall not,  and shall  cause the Merger
Subsidiary not to,  take any action  except in the  ordinary course of  business
consistent  with past practice  and in accordance with  all applicable laws, and
(ii) ENSCO will not (a) amend other
 
                                       41
<PAGE>
otherwise change its Certificate  of Incorporation or  Bylaws, (b) declare,  set
aside,  make or pay any dividend or  other distribution, payable in cash, stock,
property or  otherwise,  with  respect to  any  of  its capital  stock,  or  (c)
reclassify,  combine, split or  divide its capital stock  or redeem, purchase or
otherwise  acquire,  directly  or  indirectly,  any  of  its  capital  stock  or
securities  or  obligations convertible  into or  exchangeable for  such capital
stock.
 
NO SOLICITATION OF TRANSACTIONS
 
    The Merger Agreement provides  that DUAL will  not, directly or  indirectly,
negotiate  with any person other  than ENSCO with respect  to the acquisition of
DUAL or the shares of DUAL Common  Stock owned by the Principal Stockholder  and
it  will not,  and will  not permit any  of its  officers, directors, employees,
agents or representatives  (including, without  limitation, investment  bankers,
attorneys  and accountants) to (i) initiate  contact with, (ii) make, solicit or
encourage any inquiries or proposals, (iii)  enter into, or participate in,  any
discussions  or negotiations  with, (iv)  disclose, directly  or indirectly, any
information not customarily disclosed concerning the business and properties  of
DUAL  or any  Subsidiary to or  (v) afford  any access to  any of  DUAL's or any
Subsidiary's properties, books and records to any person in connection with  any
possible proposal relating to (a) the disposition of their respective businesses
or  substantially all  or their  assets, (b) the  acquisition of  equity or debt
securities of DUAL including  equity or debt securities  owned by the  Principal
Stockholder,  or  (c) the  merger, share  exchange  or business  combination, or
similar acquisition transaction of or involving DUAL or any Subsidiary with  any
person  other  than ENSCO.  However, the  Merger Agreement  allows the  Board of
Directors of DUAL to take and disclose to stockholders of DUAL certain positions
regarding a  tender offer  or an  exchange offer.  DUAL agrees  to notify  ENSCO
promptly  if any proposal  or offer, or  any inquiry or  contact with any person
with respect thereto, is made and must, in any such notice to ENSCO, indicate in
reasonable detail  the  identity of  the  person making  such  proposal,  offer,
inquiry or contact and the terms and conditions of such proposal, offer, inquiry
or  contact.  DUAL agrees  not to  release any  third party  from, or  waive any
provision of, any  confidentiality or standstill  agreement to which  DUAL is  a
party.  DUAL  also agrees  to  cease and  cause  to be  terminated  all existing
discussions or negotiations with any parties  conducted up to and including  the
date of the Merger Agreement with respect to any of the foregoing.
 
CONDITIONS TO THE CONSUMMATION OF THE MERGER
 
    The Merger Agreement provides that the obligations of DUAL, ENSCO and Merger
Subsidiary  to  consummate the  Merger are  subject to  the satisfaction  of the
following conditions: (i) the Merger Agreement and the transactions contemplated
thereunder must have been  approved and adopted by  the affirmative vote of  the
stockholders  of DUAL in accordance with  Delaware Law and DUAL's Certificate of
Incorporation and Bylaws and the rules of the National Association of Securities
Dealers; (ii) no governmental authority, regulatory authority, or court may have
enacted, issued, promulgated,  enforced or entered  any order, executive  order,
stay,  decree, judgment,  injunction, statute,  rule or  regulation which  is in
effect and  which has  the effect  of  making the  Merger illegal  or  otherwise
prohibiting  consummation of the  Merger; (iii) the  Registration Statement must
have been declared effective, and no stop order suspending the effectiveness  of
the  Registration Statement shall  be in effect;  (iv) ENSCO and  DUAL must have
received from the  NYSE evidence that  the shares  of ENSCO Common  Stock to  be
issued  to the stockholders  of DUAL in the  Merger shall be  listed on the NYSE
immediately following the Effective Time; and (v) any applicable waiting  period
under the HSR Act relating to the Merger must have expired or been terminated.
 
    Under  the Merger Agreement, the obligations  of ENSCO and Merger Subsidiary
to consummate  the Merger  are  subject to  the  satisfaction of  the  following
further  conditions: (i)  DUAL must have  performed or complied  in all material
respects with all agreements and covenants required by the Merger Agreement  and
each  of  the representations  and warranties  of DUAL  contained in  the Merger
Agreement must be  true and correct  in all material  respects; (ii) ENSCO  must
have received from each affiliate of DUAL and any other person who may be deemed
to  have become an affiliate  of DUAL (under Rule  145 under the Securities Act)
after the date of the Agreement and at  or prior to the Effective Time a  signed
Affiliate  Agreement (a  form of  which is attached  to the  Merger Agreement as
Exhibit B); (iii) since  the date of the  Merger Agreement, no material  adverse
change in the financial
 
                                       42
<PAGE>
condition, results of operations or business of DUAL and the Subsidiaries, taken
as  a whole,  may have occurred,  and neither  DUAL nor any  Subsidiary may have
suffered any damage, destruction  or loss materially  affecting the business  or
properties  of DUAL and the  Subsidiaries, taken as a  whole; (iv) DUAL and each
Subsidiary must have delivered  to ENSCO, each  dated as of  a date not  earlier
than  30 days prior to the Effective Time, certain documents; and (v) ENSCO must
have received from Akin, Gump, Strauss,  Hauer & Feld, L.L.P. a written  opinion
dated  as of the date of the closing  to be held immediately prior to the filing
of the Certificate of Merger covering the matters set forth on Exhibit C to  the
Merger Agreement.
 
    Under the Merger Agreement, the obligations of DUAL to consummate the Merger
are  subject to the satisfaction of  the following further conditions: (i) ENSCO
and Merger Subsidiary must have performed  or complied in all material  respects
with  all agreements and covenants required by  the Merger Agreement and each of
the representations and warranties of  ENSCO and Merger Subsidiary contained  in
the  Merger Agreement must  be true and  correct in all  material respects; (ii)
since the  date of  the Merger  Agreement,  no material  adverse change  in  the
financial  condition,  results  of  operations  or  business  of  ENSCO  and its
subsidiaries,  taken  as  a  whole,  may  have  occurred,  and  ENSCO  and   its
subsidiaries  may not have  suffered any damage,  destruction or loss materially
affecting the business or properties of  ENSCO and its subsidiaries, taken as  a
whole; and (iii) DUAL must have received from Baker & McKenzie a written opinion
dated  as of the date of the closing  to be held immediately prior to the filing
of the Certificate of Merger covering the matters set forth on Exhibit D to  the
Merger Agreement.
 
TERMINATION
 
    The  Merger Agreement provides that it may  be terminated and the Merger and
the other transactions contemplated by the Merger Agreement may be abandoned  at
any time prior to the Effective Time, notwithstanding any requisite approval and
adoption  of the Merger Agreement and  the transactions contemplated thereby, as
follows: (i) by mutual written consent duly authorized by the Board of Directors
of DUAL and  the Boards of  Directors of  ENSCO and Merger  Subsidiary; (ii)  by
either ENSCO or DUAL, if either the Effective Time has not occurred on or before
July  31,  1996  (provided, however,  that  the  right to  terminate  the Merger
Agreement described in this clause (ii) will not be available to any party whose
failure to fulfill any obligation under the Merger Agreement has been the  cause
of, or resulted in, the failure of the Effective Time to occur on or before such
date)  or there  is any  Order which is  final and  nonappealable preventing the
consummation of the Merger, except  if the party relying  on such Order has  not
complied  with  its obligations  relating to  obtaining any  consents, licenses,
permits, waivers, approvals, authorizations, or  orders required to be  obtained
from  governmental authorities in connection with the Merger Agreement (see "The
Merger -- Regulatory Approvals"); (iii) by ENSCO, if a tender offer or  exchange
offer  for 50%  or more of  the outstanding shares  of capital stock  of DUAL is
commenced, and the Board  of Directors of DUAL  has failed to recommend  against
the  stockholders  of DUAL  tendering  their shares  into  such tender  offer or
exchange offer; (iv) by ENSCO, if the  stockholders of DUAL fail to approve  and
adopt  the Merger Agreement, the Merger and the transactions contemplated by the
Merger Agreement  at a  meeting duly  convened therefor;  (v) by  ENSCO, upon  a
breach  of any  representation, warranty, covenant  or agreement on  the part of
DUAL set forth in the Merger Agreement, or if any representation or warranty  of
DUAL  has  become  untrue,  in  either case  such  that  the  conditions  to the
consummation of the Merger would not be satisfied (a "Terminating DUAL  Breach")
(provided,  however, that,  if such Terminating  DUAL Breach is  curable by DUAL
through the exercise of its  best efforts and for so  long as DUAL continues  to
exercise  such best  efforts, ENSCO  may not  terminate the  Merger Agreement as
provided  in  this  clause   (v));  or  (vi)  by   DUAL,  upon  breach  of   any
representation,  warranty, covenant or agreement on  the part of ENSCO set forth
in the  Merger Agreement,  or if  any representation  or warranty  of ENSCO  has
become  untrue, in either case  such that the conditions  to the consummation of
the Merger  would not  be satisfied  (a "Terminating  ENSCO Breach")  (provided,
however, that, if such Terminating ENSCO Breach is curable by ENSCO through best
efforts  and for so long as ENSCO  continues to exercise such best efforts, DUAL
may not terminate the Merger Agreement as provided in this clause (vi)).
 
                                       43
<PAGE>
ALTERNATIVE PROPOSAL FEE; EXPENSES; DAMAGES
 
    The  Merger  Agreement  provides  that  DUAL  will  pay  ENSCO  a  fee   (an
"Alternative  Proposal Fee") of $5 million, which  amount is inclusive of all of
ENSCO's expenses related  to the Merger  Agreement, if the  Merger Agreement  is
terminated  as described in clause (iii) in the immediately preceding paragraph,
or if the  Merger Agreement is  terminated as  described in clause  (iv) in  the
immediately  preceding paragraph as a result  of the failure of the stockholders
of DUAL  to  approve the  Merger  and  a proposal  concerning  certain  business
combination  transactions has been made prior  to such termination, and any such
business  combination  is  thereafter  consummated  within  12  months  of  such
termination.
 
    Under  the Merger Agreement, ENSCO will  be entitled to receive its expenses
(but not the Alternative Proposal Fee) in the event that the Merger Agreement is
terminated by  ENSCO  as  described  in clause  (ii)  in  the  second  preceding
paragraph (subject to the proviso thereof) or terminated by DUAL as described in
clause  (v) in the second preceding  paragraph. Similarly, DUAL will be entitled
to receive  its  expenses if  the  Merger Agreement  is  terminated by  DUAL  as
described in clause (vi) in the second preceding paragraph.
 
    The Merger Agreement provides that no termination of the Merger Agreement as
a  result  of the  matters as  described in  clauses  (v) or  (vi) in  the third
preceding paragraph, shall prejudice the  ability of a non-breaching party  from
seeking  damages  from  any  other  party  for  any  breach  of  this Agreement,
including, without  limitation, attorneys'  fees  and the  right to  pursue  any
remedy at law or in equity.
 
    The  Merger Agreement provides  that, except as  set forth above  and in the
sections of the Merger Agreement relating to the preparation of the Registration
Statement, all  costs  and  expenses  incurred in  connection  with  the  Merger
Agreement  and the transactions  contemplated thereby will be  paid by the party
incurring such expenses,  whether or  not any such  transaction is  consummated,
except  that from and after the Effective Time DUAL will reimburse ENSCO for all
out-of-pocket expenses and fees incurred or accrued by ENSCO in connection  with
the transactions contemplated by the Merger Agreement.
 
AMENDMENT AND WAIVER
 
    The  Merger Agreement may be amended by  the parties thereto by action taken
by or on behalf of their respective Boards of Directors at any time prior to the
Effective Time. However, after the approval and adoption of the Merger Agreement
and the  transactions  contemplated thereby  by  the stockholders  of  DUAL,  no
amendment  may  be made  which would  reduce the  amount or  change the  type of
consideration into which each share of DUAL Common Stock is to be converted upon
consummation of the Merger.
 
    The Merger Agreement also provides that, at any time prior to the  Effective
Time,  any party  thereto may  (i) extend  the time  for the  performance of any
obligation or other act of any other party thereto, (ii) waive any inaccuracy in
the  representations  and  warranties  contained  therein  or  in  any  document
delivered  pursuant thereto  and (iii)  waive compliance  with any  agreement or
condition contained therein.
 
EFFECT OF THE MERGER ON STOCK OPTIONS
 
    Under  the  Merger  Agreement,  all  outstanding  options  (the   "Long-Term
Options")  issued  pursuant to  the  1993 Plan  must  be surrendered  as  of the
Effective Time and the  Surviving Corporation must,  within three business  days
following  the Effective Time,  deliver to each  holder of a  Long-Term Option a
number of whole shares  of ENSCO Common  Stock equal to the  excess, if any,  of
0.625  over a fraction, the numerator of  which is the exercise price under such
Long-Term Option, and  the denominator of  which is the  average of the  closing
prices  of ENSCO Common Stock  on the NYSE for  the five business days preceding
the Effective Time, multiplied by the number of shares covered by such Long-Term
Option.
 
                                       44
<PAGE>
    Under  the  Merger Agreement,  on or  before the  Effective Time,  DUAL will
endeavor to  enter  into  one  or  more  agreements  with  the  holders  of  all
outstanding  options  under the  Non-Employee  Director Plan  (the "Non-Employee
Options"), pursuant to which such  holders must surrender all such  Non-Employee
Options  to DUAL no later than three  business days prior to the Effective Time.
Pursuant to each  such agreement and  as consideration for  such surrender,  the
Surviving  Corporation will, within two business  days after the Effective Time,
deliver to the holder  of any Non-Employee  Option a number  of whole shares  of
ENSCO  Common Stock equal to  the excess, if any, of  0.625 over a fraction, the
numerator of which is the exercise price under such Non-Employee Option, and the
denominator of which is the average of the closing prices of ENSCO Common  Stock
on  the NYSE for the five business days preceding the Effective Time, multiplied
by the number of shares covered by such Non-Employee Option.
 
    In performing  its  obligations in  respect  of the  Long-Term  Options  and
Non-Employee  Options pursuant to  the Merger Agreement,  DUAL must fully comply
with the terms  and conditions of  the 1993 Plan  and the Non-Employee  Director
Plan.
 
    The  following table sets forth,  as of March 31,  1996, with respect to the
executive officers  and directors  of DUAL,  (i) the  number of  shares of  DUAL
Common Stock subject to Long-Term Options and Non-Employee Options that are held
by  such officer or  director and (ii)  the estimated number  of shares of ENSCO
Common Stock deliverable to such officer or director in respect of such  options
pursuant  to  the Merger  Agreement, assuming  that the  average of  the closing
prices of ENSCO Common Stock for the five business days preceding the  Effective
Time is $26.00, which was the closing sale price of ENSCO Common Stock on May 8,
1996 as reported by the NYSE:
 
<TABLE>
<CAPTION>
                                                                         NUMBER OF         ESTIMATED
                                                                       SHARES SUBJECT  NUMBER OF SHARES
NAME                                                                     TO OPTIONS     DELIVERABLE (1)
- ---------------------------------------------------------------------  --------------  -----------------
<S>                                                                    <C>             <C>
David W. Skarke......................................................       100,000           22,596
L.H. Dick Robertson..................................................       180,000           43,269
William R. Dudark....................................................        80,000           19,230
Dudley M. Haralson...................................................       104,000           25,000
Lewis W. Kreps.......................................................        80,000           19,230
Robert C. McCoy......................................................        88,000           21,153
W. Allen Parks.......................................................       104,000           25,000
Aage Figenschou......................................................         2,000              500
Frank Jungers........................................................         2,000              538
Magne Kristiansen....................................................         2,000              500
Edward O. Vetter.....................................................         2,000              538
</TABLE>
 
- ------------------------
(1) Based  on the  closing sale price  of ENSCO Common  Stock on May  8, 1996 of
    $26.00 as reported by the NYSE.
 
EFFECT OF THE MERGER ON CERTAIN BENEFIT PLANS
 
    Pursuant to the Merger Agreement, ENSCO has agreed to assume or to cause the
Surviving Corporation  to  assume  the DUAL  DRILLING  COMPANY  Employee  Health
Benefit  Plan (the  "DUAL Medical Plan"),  the DUAL DRILLING  COMPANY Group Life
Insurance Plan, the DUAL DRILLING COMPANY Long-Term Disability Plan and the DUAL
DRILLING COMPANY  Group  Travel  Accident  Insurance  Plan,  Voluntary  Personal
Accident  Insurance Plan, Long-Term Disability Plan for Third Country Nationals,
and Premium Conversion Cafeteria Plan unless any such plan is terminated by DUAL
as directed  by ENSCO  prior to  the Effective  Time (collectively,  the  "Group
Insurance  Plans"). ENSCO has also agreed to provide to former employees of DUAL
and its subsidiaries certain  group health coverage under  Section 4980B of  the
Code  and Sections 601 to 608 of  the Employee Retirement Income Security Act of
1974 ("COBRA Coverage") for any  individuals receiving COBRA Coverage under  the
DUAL Medical Plan as of the Effective Time and for any employees of DUAL and its
subsidiaries  and  their  dependents who  become  eligible for  and  elect COBRA
Coverage as a result of
 
                                       45
<PAGE>
the  transactions  contemplated  by  the  Merger  Agreement;  however,   nothing
contained  in  the Merger  Agreement obligates  ENSCO  to extend  COBRA Coverage
beyond its normal expiration period. ENSCO  and DUAL have agreed that ENSCO  may
direct  DUAL to  terminate any  Group Insurance Plan  as of  the Effective Time,
liquidate the assets of any trust or  funding arrangement for any such plan  and
settle all claims for benefits under any such plan.
 
    DUAL   has  agreed  to  amend  the   DUAL  DRILLING  COMPANY  Employees  Tax
Deferred/Thrift Savings  Plan  and Trust  (the  "401(k) Plan")  to  comply  with
certain  provisions of the Code. Subject to certain conditions, ENSCO has agreed
to sponsor the 401(k) Plan as of the Effective Time and to take certain steps to
merge the 401(k)  Plan into the  ENSCO Savings Plan.  Amounts payable under  the
401(k)  Plan to employees whose employment is terminated for any reason prior to
December 31, 1996  shall be  paid promptly after  any such  termination and  not
after  the end  of the  1996 plan year,  if requested  by the  employee and such
payment is permitted by the terms of the 401(k) Plan.
 
    DUAL has agreed to amend the  terms of the post-retirement medical  coverage
provided  by DUAL under  the DUAL Medical  Plan (the "Retiree  Medical Plan") to
provide that no employee of DUAL or any of its subsidiaries, or any  beneficiary
or dependent of such an employee, may become entitled to post-retirement medical
coverage under the Retiree Medical Plan due to a retirement or other termination
of  employment after the Effective Time. Similarly, DUAL has agreed to terminate
the Retiree Medical Plan. Effective as  of the Effective Time, ENSCO has  agreed
to  provide post-retirement medical coverage under the ENSCO Medical Plan to any
former employee  of DUAL  or any  Subsidiary who  as of  the Effective  Time  is
receiving  post-retirement  medical  coverage under  the  Retiree  Medical Plan,
provided that such coverage under  the ENSCO Medical Plan  shall be on the  same
terms  and conditions and shall provide the same benefits as currently available
to retired former employees of ENSCO  under the ENSCO Medical Plan, but  without
regard  to  the  age and  service  provisions  of the  ENSCO  Medical  Plan that
determine initial eligibility for  post-retirement medical coverage and  without
regard  to any pre-existing condition limitations contained in the ENSCO Medical
Plan.  Any  employee  of  DUAL  or  any  Subsidiary  who  is  not  entitled   to
post-retirement  medical  coverage  under the  Retiree  Medical Plan  as  of the
Effective Time will be eligible  for post-retirement medical coverage under  the
ENSCO  Medical Plan only if the terms and conditions for such coverage under the
ENSCO Medical Plan are satisfied.
 
    Effective as of  the Effective  Time, ENSCO has  agreed to  assume the  DUAL
DRILLING  COMPANY Severance  Pay Plan  for Office  Employees, the  DUAL DRILLING
COMPANY Severance Pay Plan for Key Operating and Support Staff Employees and the
DUAL DRILLING  COMPANY Severance  Pay  Plan for  Key Operating  and  Engineering
Managers, provided that ENSCO shall not be obligated to continue these plans for
any specified period of time.
 
    Effective  as of the day before the  Effective Time, DUAL has agreed to take
all such action necessary  to freeze all benefits  under the Employee  Incentive
Plan,  the Safety Bonus Plan, and the Pro-Performance Bonus Plan for Toolpushers
maintained by  DUAL,  and  ENSCO  has  agreed  to  assume  the  benefit  payment
obligations  of DUAL under such  plans and cause such  payments to be made after
the Effective Time, and after all such payments have been made, the plans  shall
be  deemed  terminated  and ENSCO  shall  take  such other  action  as  it deems
appropriate to accomplish such termination.
 
    Prior to  the Effective  Time, DUAL  has agreed  to file  with the  Internal
Revenue  Service  certain  documents  with  respect  to  the  Premium Conversion
Cafeteria Plan for each plan year since the inception of the Premium  Conversion
Cafeteria  Plan. DUAL has also agreed to  file certain documents pursuant to the
requirements established by  the U.S. Department  of Labor ("DOL")  and to  make
certain  payments in respect of certain  employment contracts maintained by DUAL
with its officers.
 
    Pursuant to the  Merger Agreement, certain  employment agreements listed  by
DUAL  will be terminated by  DUAL as of the  Effective Time. Notwithstanding the
foregoing, ENSCO may offer
 
                                       46
<PAGE>
employment prior  to the  Effective Time  to persons  who have  such  employment
agreements  and the terms  of such employment,  if accepted by  such employee of
DUAL, may affect DUAL's obligations to the affected employee.
 
                        PRINCIPAL STOCKHOLDER AGREEMENT
 
VOTING AND PROXY
 
    ENSCO  has  entered  into  the  Principal  Stockholder  Agreement  with  the
Principal  Stockholder, which owns approximately 59.6% of the outstanding shares
of DUAL  Common Stock,  a copy  of which  is attached  to this  Prospectus/Proxy
Statement  as Appendix  D. Pursuant  to the  terms of  the Principal Stockholder
Agreement, the Principal Stockholder has agreed to vote, and has granted a proxy
to allow ENSCO to  vote, the Principal  Stockholder Shares (i)  in favor of  the
Merger  and the Merger Agreement, (ii) in  favor of adoption and approval of the
Unit Plan,  and (iii)  against  any proposal  for any  recapitalization,  merger
(other  than the Merger),  sale of assets or  other business combination between
DUAL and any person  or entity (other  than ENSCO or  Merger Subsidiary) or  any
other  action  or agreement  that ENSCO  notifies  the Principal  Stockholder in
writing before any vote would result in a breach of any covenant, representation
or warranty  or any  other obligation  or  agreement of  DUAL under  the  Merger
Agreement or which could result in any of the conditions to the Merger Agreement
not  being  fulfilled. ENSCO  and Merger  Subsidiary  intend that  the Principal
Stockholder Shares be  so voted. The  Principal Stockholder has  agreed that  it
shall  not, and  shall not  offer or agree  to, sell,  transfer, tender, assign,
hypothecate or otherwise dispose  of, or create or  permit to exist any  pledge,
lien,   security  interest,  mortgage,  charge,  claim,  option,  proxy,  voting
restriction, right of first refusal,  limitation on disposition, or  encumbrance
of  any kind  on or with  respect to  the Principal Stockholder  Shares or other
voting securities of  DUAL, whether  issued heretofore or  hereafter, which  are
held of record or beneficially by the Principal Stockholder.
 
ABSTENTION FROM VOTING
 
    Pursuant  to the Principal Stockholder  Agreement, the Principal Stockholder
has agreed  that until  July 31,  1996 it  will not  vote any  of the  Principal
Stockholder   Shares  at  any  annual,  special  or  adjourned  meeting  of  the
stockholders of DUAL, including the right  to sign its name (as stockholder)  to
any  consent, certificate or other document relating to DUAL that the law of the
State of Delaware  may permit or  require, (i)  to approve of  the adoption  and
approval  of the Unit  Plan, effective August  21, 1995 in  any manner except as
contemplated by the Merger Agreement, or (ii) in any manner that is intended, or
could reasonably be  expected, to  impede, interfere with,  delay, postpone,  or
materially   adversely  affect  the  transactions  contemplated  by  the  Merger
Agreement.
 
COVENANTS AND REPRESENTATIONS
 
    Pursuant to the Principal  Stockholder Agreement, the Principal  Stockholder
has agreed that it has been advised that (i) the offer, sale and delivery of the
ENSCO  Common Stock to the Principal Stockholder  pursuant to the Merger may not
be registered  under the  Securities  Act, despite  ENSCO's obligations  to  use
commercially  reasonable efforts to effect such registration; (ii) if the offer,
sale and  delivery  of the  ENSCO  Common  Stock to  the  Principal  Stockholder
pursuant  to the Merger has  not been registered under  the Securities Act, then
such shares  may  not  be  offered, sold,  pledged,  hypothecated  or  otherwise
transferred  unless  subsequently  registered  under the  Securities  Act  or an
exemption from such registration is available;  and (iii) even if such sale  and
delivery  to  the  Principal Stockholder  of  shares  of ENSCO  Common  Stock is
registered under the Securities Act, to the extent the Principal Stockholder  is
considered  an "affiliate" of DUAL at the time the Merger Agreement is submitted
for a vote  of the  stockholders of  DUAL, any public  offering or  sale by  the
Principal  Stockholder of its  shares of ENSCO Common  Stock will, under current
law, require  (a) the  further registration  under the  Securities Act  of  such
shares,  which  ENSCO is  obligated to  use  commercially reasonable  efforts to
effect, (b) compliance  with Rule 145  promulgated by the  Commission under  the
Securities  Act,  or  (c)  the  availability  of  another  exemption  from  such
registration under the  Securities Act. Furthermore,  the Principal  Stockholder
also  has agreed  that it  understands that  stop transfer  instructions will be
given to  ENSCO's transfer  agent with  respect to  the shares  of ENSCO  Common
 
                                       47
<PAGE>
Stock  receivable by the Principal Stockholder and  that a legend will be placed
on the certificates for such shares issued to the Principal Stockholder, to  the
extent  the Principal  Stockholder is considered  an "affiliate" of  DUAL at the
time the Merger Agreement is submitted for a vote of the stockholders of DUAL.
 
REGISTRATION OF MERGER SHARES
 
    Pursuant to the Principal Stockholder Agreement, ENSCO has agreed to use all
commercially reasonable efforts to effect the registration under the  Securities
Act  of the transfer to the stockholders of the Principal Stockholder of certain
of the shares of ENSCO Common Stock to be received by the Principal  Stockholder
in  exchange for the  Principal Stockholder Shares  and the resale  of shares of
ENSCO Common  Stock by  the  Principal Stockholder  or  one of  its  significant
stockholders,  subject  to  certain  restrictions.  ENSCO  has  agreed  to  keep
continuously effective the  registration statement in  respect of the  Principal
Stockholder   Shares  for  the  period  of  time  necessary  for  the  Principal
Stockholder to complete the intended method  or methods of distribution for  the
Principal Stockholder Shares.
 
NO SOLICITATION
 
    Pursuant  to the Principal Stockholder  Agreement, the Principal Stockholder
has agreed that until July 31, 1996, it will not negotiate with any person other
than ENSCO with  respect to the  acquisition of  DUAL or the  DUAL Common  Stock
owned  by the Principal Stockholder and it will  not, and will not permit any of
its officers, directors, employees, agents or representatives (including without
limitation, investment  bankers,  attorneys  and accountants)  to  (i)  initiate
contact  with, (ii) make, solicit or encourage any inquiries or proposals, (iii)
enter into  or  participate  in  any  discussions  or  negotiations  with,  (iv)
disclose,  directly  or indirectly,  any  information not  customarily disclosed
concerning the business and  properties of DUAL  or the Principal  Stockholder's
interest  in DUAL under the control of  the Principal Stockholder, or (v) afford
any access to DUAL's  properties, books and records  in its possession or  under
its  control to any person in connection  with any possible proposal relating to
(a) the  disposition of  DUAL's  or the  Principal Stockholder's  businesses  or
substantially  all of their respective assets,  (b) the acquisition of equity or
debt securities of DUAL or the  Principal Stockholder, including equity or  debt
securities  in DUAL owned by the Principal Stockholder, or (c) the merger, share
exchange or  business  combination, or  similar  acquisition transaction  of  or
involving DUAL or the Principal Stockholder with any person other than ENSCO. In
addition,  until  July  31,  1996,  the  Principal  Stockholder  has  agreed  to
immediately notify ENSCO orally,  and subsequently confirm  in writing, all  the
relevant  details relating to  all inquiries and proposals  which it may receive
relating to any such  matters. Furthermore, until July  31, 1996, the  Principal
Stockholder  has  agreed  that it  will  not, and  will  not permit  any  of its
representatives, at any time, to enter into or participate in any discussions or
negotiations regarding, or accept, any proposal for such a transaction  received
by  them  from  a third  party  or that  a  third  party expresses  a  desire to
communicate to it.
 
                                 THE COMPANIES
 
ENSCO INTERNATIONAL INCORPORATED
 
    ENSCO is  an  international offshore  contract  drilling company  that  also
provides  marine transportation  services in  the U.S.  Gulf of  Mexico. ENSCO's
complement of  offshore drilling  rigs  includes 24  jackup  rigs and  10  barge
drilling  rigs, and ENSCO's marine transportation  fleet consists of 37 vessels.
ENSCO's operations are integral to  the exploration, development and  production
of oil and gas.
 
    Since  1987, ENSCO has pursued a strategy  of building its fleet of offshore
drilling rigs.  This strategy  was  exemplified by  ENSCO's acquisition  of  the
remainder  of  Penrod  Holding Corporation  ("Penrod")  in August  1993  and the
expansion of ENSCO's Venezuelan rig fleet during 1993 and 1994 with the delivery
of four new  barge drilling rigs  in each  year. ENSCO also  added three  harsh-
environment jackup rigs to its North Sea fleet, two in 1994 and one in 1995.
 
                                       48
<PAGE>
    With  ENSCO's increasing  emphasis on offshore  markets, it  has disposed of
businesses that are not offshore-oriented or that management believed would  not
meet  its standards for  financial performance. Accordingly,  in 1995 ENSCO sold
its technical services  business, in 1994  ENSCO sold substantially  all of  its
land rigs and in 1993 ENSCO's supply business was sold.
 
    ENSCO  was formed as a  Texas corporation in 1975  and was reincorporated in
Delaware in 1987.  At ENSCO's  Annual Meeting of  Stockholders held  on May  23,
1995,  the stockholders  approved the  change in the  name of  ENSCO from Energy
Service Company,  Inc. to  ENSCO International  Incorporated. ENSCO's  principal
executive  office is located  at 2700 Fountain Place,  1445 Ross Avenue, Dallas,
Texas 75202-2792, and its telephone number is (214) 922-1500.
 
DUAL DRILLING COMPANY
 
    DUAL is a domestic and international offshore drilling contractor. DUAL  was
formed  in 1951 as a domestic onshore drilling contractor. In 1975, DUAL entered
the offshore contract  drilling business  and built  a fleet  of deep  drilling,
self-contained platform rigs. During 1981, DUAL entered the international sector
of  the offshore drilling business  when it began to  acquire a fleet of premium
independent-leg cantilever jackup rigs. Since  that time, DUAL has expanded  its
offshore  fleet  by  purchasing  additional platform  and  jackup  rigs,  and by
bareboat chartering  jackup  rigs. DUAL  now  operates  a fleet  of  20  premium
offshore   rigs,  consisting   of  10   self-contained  platform   rigs  and  10
independent-leg cantilever jackup rigs.
 
    The Principal  Stockholder  acquired DUAL  in  1990 through  a  wholly-owned
subsidiary,  and operated DUAL as an independent business focused exclusively on
offshore operations. Historically, the  Principal Stockholder had conducted  its
offshore  drilling operations  through three  independent, indirect wholly-owned
subsidiaries, and  through direct  ownership  of a  limited number  of  offshore
drilling rigs. In August 1993, DUAL completed an initial public offering of 6.25
million  shares of DUAL Common Stock,  which reduced the Principal Stockholder's
ownership interest in DUAL to approximately 59.6% of the outstanding DUAL Common
Stock. With  the consummation  of  the initial  public offering,  the  Principal
Stockholder  restructured  its  offshore  drilling operations  so  that  (i) the
subsidiaries became a consolidated group which combined the drilling  operations
of the Principal Stockholder, which were distinct from DUAL, with those of DUAL,
(ii)  ownership of  certain rigs,  along with  related debt,  was transferred to
DUAL, and (iii) substantially  all of the  intercompany indebtedness payable  to
the  Principal Stockholder  was satisfied  as a  contribution to  the capital of
DUAL. DUAL's principal executive  office is located at  5956 Sherry Lane,  Suite
1500, Dallas, Texas 75225, and its telephone number is (214) 373-6200.
 
MERGER SUBSIDIARY
 
    Merger  Subsidiary was  formed by  ENSCO on  March 5,  1996, solely  for the
purpose of effecting the Merger. It has  no material assets and has not  engaged
in  any activities except in connection with  the Merger. All of its outstanding
capital stock is owned by ENSCO. Merger Subsidiary's principal executive  office
is  located at 2700 Fountain Place,  1445 Ross Avenue, Dallas, Texas 75202-2792,
and its telephone number is (214) 922-1500.
 
                                       49
<PAGE>
                             PRINCIPAL STOCKHOLDERS
 
    The following table sets  forth certain information, as  of April 30,  1996,
regarding  beneficial ownership and percentage of total voting power of the DUAL
Common Stock by each stockholder who is known by DUAL to own more than 5% of the
outstanding DUAL Common  Stock, each  director, each executive  officer and  all
directors  and executive officers  as a group. Each  stockholder has sole voting
and investment power with  respect to such stockholder's  shares of DUAL  Common
Stock, except to the extent that authority is shared by spouses under applicable
law and as set forth in footnotes below.
 
<TABLE>
<CAPTION>
                                                                                  NUMBER OF
                                                                                   SHARES
                                                                                 BENEFICIALLY  PERCENT OF
NAME OF BENEFICIAL OWNER                                                          OWNED (4)       CLASS
- -------------------------------------------------------------------------------  -----------  -------------
<S>                                                                              <C>          <C>
Dual Invest ASA (1)............................................................    9,382,354        59.6%
David W. Skarke................................................................      100,000        *
Aage Figenschou................................................................      --             *
Magne Kristiansen..............................................................        2,000        *
Frank Jungers..................................................................        3,000        *
Edward O. Vetter...............................................................        3,000        *
L.H. Dick Robertson (2)........................................................      223,920         1.4%
W. Allen Parks (2)(3)..........................................................      125,972        *
Dudley M. Haralson (2).........................................................      113,502        *
Robert C. McCoy (2)............................................................      105,772        *
Lewis W. Kreps (2).............................................................       93,424        *
William R. Dudark (2)..........................................................       93,424        *
All directors and executive officers as a group (10 persons)...................      864,014         5.2%
</TABLE>
 
- ------------------------
*   Less than 1%
 
(1)  The address of Dual  Invest ASA is P.O. Box  1611, Vika 0119, Oslo, Norway.
    Dual Invest ASA is publicly traded on the Oslo Stock Exchange.
 
<TABLE>
<CAPTION>
                                                                                          % OF CAPITAL
                                                                                         STOCK OWNED OF
                                                                            NUMBER OF    RECORD OF DUAL
NAME OF STOCKHOLDER                               ADDRESS                 SHARES OWNED     INVEST ASA
- ---------------------------------  -------------------------------------  -------------  ---------------
<S>                                <C>                                    <C>            <C>
B. Skaugen Shipping ASA            Dronning Maudsgt. 1                       3,295,597          22.22
                                   P.O. Box 1611, Vika
                                   0119 Oslo, Norway
 
Morgan Guaranty Trust              Securities Reconsiliation Dept.           1,972,000          13.30
Co. of N.Y.                        Boulevard Emile Jacqmain 151
                                   1210 Brussels, Belgium
</TABLE>
 
(2) Includes remaining unvested shares  of restricted stock granted directly  to
    the officers of DUAL in 1993, or an aggregate of 30,000 shares of a total of
    172,857  shares granted to  DUAL's executive officers  under the Dual Invest
    ASA Stock Option Compensation Plan 1991 and the 1993 Plan. The holders  have
    the  right to vote and receive dividends of such shares, but do not have the
    power to dispose  of, or direct  the disposition of,  such shares until  the
    shares  are vested pursuant to the terms  of such plan. The remaining 30,000
    shares vest in August 1996.
 
(3) Includes 100 shares  owned by his  parents as to  which Mr. Parks  disclaims
    beneficial ownership, and 300 shares owned by his children.
 
                                       50
<PAGE>
(4) Includes outstanding options issued under the 1993 Plan (the "Options"), and
    that  vest within 60  days giving each  Option holder the  right to purchase
    shares of DUAL  Common Stock equal  to the  number of shares  listed in  the
    table below:
 
<TABLE>
<S>                                                         <C>
David W. Skarke                                               100,000
Magne Kristiansen                                               2,000
Frank Jungers                                                   2,000
Edward O. Vetter                                                2,000
L. H. Dick Robertson                                          180,000
W. Allen Parks                                                104,000
Dudley M. Haralson                                            104,000
Robert C. McCoy                                                88,000
Lewis W. Kreps                                                 80,000
William R. Dudark                                              80,000
</TABLE>
 
    Mr.  Aage Figenschou has an  Option to purchase 2,000  shares of DUAL Common
    Stock that vests fully on July 23, 1996. In the event of a Change of Control
    (as defined in  the Plan) the  Options vest fully  on the  Change-of-Control
    date.
 
                                       51
<PAGE>
                     UNAUDITED COMBINED CONDENSED PRO FORMA
                             FINANCIAL INFORMATION
 
    The   following  unaudited  combined  condensed   pro  forma  statements  of
operations for the  three months ended  March 31,  1996 and for  the year  ended
December  31, 1995,  and the  following unaudited  combined condensed  pro forma
balance sheet  at March  31,  1996, present,  for informational  purposes  only,
certain  unaudited combined condensed pro  forma financial information for ENSCO
giving effect to  the Merger  and related purchase  accounting adjustments.  The
unaudited  combined condensed pro forma statements  of operations give pro forma
effect to  the Merger  as if  it  had been  completed on  January 1,  1995.  The
unaudited  combined condensed pro forma balance  sheet gives pro forma effect to
the Merger as if it had been completed on March 31, 1996.
 
    The unaudited combined  condensed pro forma  financial information has  been
prepared  in accordance with GAAP and gives effect to the Merger, as a result of
which DUAL would become a wholly-owned  subsidiary of ENSCO, using the  purchase
method of accounting.
 
    The  unaudited combined condensed pro  forma financial information should be
read in conjunction with, and is qualified in its entirety by reference to,  the
unaudited  Consolidated Financial Statements of ENSCO and Notes thereto included
in the ENSCO Form 10-Q for the three months ended March 31, 1996 and the audited
Consolidated Financial Statements  of ENSCO  and Notes thereto  included in  the
ENSCO   1995  Form   10-K,  which   are  incorporated   by  reference   in  this
Prospectus/Proxy  Statement,  and  to   the  unaudited  Consolidated   Financial
Statements  of DUAL  and Notes thereto  included in  the DUAL Form  10-Q for the
three months  ended  March  31,  1996 and  the  audited  Consolidated  Financial
Statements  of DUAL and Notes thereto included in the DUAL 1995 Form 10-K, which
are incorporated by reference in this Prospectus/Proxy Statement.
 
    The unaudited combined  condensed pro forma  financial information does  not
purport  to present  what ENSCO's  financial position  or results  of operations
would actually  have been  had the  Merger occurred  as of  the date  or at  the
beginning  of the periods indicated, nor does such financial information purport
to project ENSCO's financial  position or results of  operations for any  future
date or period.
 
                                       52
<PAGE>
                        ENSCO INTERNATIONAL INCORPORATED
 
                   COMBINED CONDENSED PRO FORMA BALANCE SHEET
 
                              AS OF MARCH 31, 1996
                                 (IN THOUSANDS)
                                  (UNAUDITED)
 
                                     ASSETS
 
<TABLE>
<CAPTION>
                                                               HISTORICAL
                                                        -------------------------
                                                           ENSCO         DUAL
                                                        INTERNATIONAL  DRILLING       PRO FORMA
                                                        INCORPORATED    COMPANY      ADJUSTMENTS      PRO FORMA
                                                        ------------  -----------  ---------------  -------------
<S>                                                     <C>           <C>          <C>              <C>
Current assets
  Cash and short-term investments.....................   $   75,154   $    45,217  $    (13,023)(b) $     107,348
  Accounts and notes receivable, net..................       65,071        20,884                          85,955
  Prepaid expenses and other..........................       21,587        12,722        (3,708)(a)        30,601
                                                        ------------  -----------  ---------------  -------------
    Total current assets..............................      161,812        78,823       (16,731)          223,904
                                                        ------------  -----------  ---------------  -------------
Property and equipment, net...........................      642,493       193,480        85,820(a)        921,793
Goodwill..............................................        7,140        24,600        28,178(a)         81,754
                                                                                         13,843(b)
                                                                                          7,993(c)
Other assets..........................................       13,175         6,857        (3,565)(a)        16,467
                                                        ------------  -----------  ---------------  -------------
                                                         $  824,620   $   303,760  $    115,538     $   1,243,918
                                                        ------------  -----------  ---------------  -------------
                                                        ------------  -----------  ---------------  -------------
 
                                      LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities
  Accounts payable and accrued liabilities............   $   45,071   $    16,222  $      7,500(a)  $      69,613
                                                                                            820(b)
  Current maturities of long-term debt................       32,851         7,178                          40,029
                                                        ------------  -----------  ---------------  -------------
    Total current liabilities.........................       77,922        23,400         8,320           109,642
                                                        ------------  -----------  ---------------  -------------
Long-term debt........................................      150,518       135,249         5,106(a)        290,873
Deferred income taxes.................................       29,251         1,562        18,477(a)         49,290
Other liabilities.....................................       20,092         2,428         3,000(a)         25,520
Stockholders' equity
  Common stock........................................        6,695           158           827(a)          7,709
                                                                                             29(c)
  Additional paid-in capital..........................      616,300       173,793        38,985(a)        837,042
                                                                                          7,964(c)
  Accumulated deficit.................................       (8,908)      (32,411)       32,411(a)         (8,908)
  Other...............................................       (6,113)      --                               (6,113)
  Treasury stock......................................      (61,137)         (419)          419(a)        (61,137)
                                                        ------------  -----------  ---------------  -------------
    Total stockholders' equity........................      546,837       141,121        80,635           768,593
                                                        ------------  -----------  ---------------  -------------
                                                         $  824,620   $   303,760  $    115,538     $   1,243,918
                                                        ------------  -----------  ---------------  -------------
                                                        ------------  -----------  ---------------  -------------
</TABLE>
 
                             See accompanying notes
 
                                       53
<PAGE>
                        ENSCO INTERNATIONAL INCORPORATED
              COMBINED CONDENSED PRO FORMA STATEMENT OF OPERATIONS
 
                       THREE MONTHS ENDED MARCH 31, 1996
                                 (IN THOUSANDS)
                                  (UNAUDITED)
 
<TABLE>
<CAPTION>
                                                                    HISTORICAL
                                                             -------------------------
                                                                ENSCO         DUAL
                                                             INTERNATIONAL  DRILLING     PRO FORMA
                                                             INCORPORATED    COMPANY    ADJUSTMENTS    PRO FORMA
                                                             ------------  -----------  ------------  -----------
<S>                                                          <C>           <C>          <C>           <C>
Revenues
  Contract drilling........................................   $   72,883    $  29,461   $             $   102,344
  Marine transportation....................................       11,663       --                          11,663
                                                             ------------  -----------                -----------
                                                                  84,546       29,461                     114,007
                                                             ------------  -----------                -----------
Operating expenses
  Contract drilling........................................       37,337       18,589                      55,926
  Marine transportation....................................        6,187       --                           6,187
  Depreciation and amortization............................       16,374        4,813       1,234(b)       22,455
                                                                                               34(c)
  General and administrative...............................        2,215        2,040                       4,255
                                                             ------------  -----------  ------------  -----------
                                                                  62,113       25,442       1,268          88,823
                                                             ------------  -----------  ------------  -----------
Operating income...........................................       22,433        4,019      (1,268)         25,184
                                                             ------------  -----------  ------------  -----------
Other income (expense)
  Interest income..........................................        1,236          527         146(d)        1,909
  Interest expense.........................................       (4,049)      (3,634)         99(e)       (7,584)
  Other, net...............................................          264           92                         356
                                                             ------------  -----------  ------------  -----------
                                                                  (2,549)      (3,015)        245          (5,319)
                                                             ------------  -----------  ------------  -----------
Income from continuing operations before income taxes and
 minority interest.........................................       19,884        1,004      (1,023)         19,865
Provision for income taxes.................................        4,767           30         (22)(f)       4,775
Minority interest..........................................          427       --                             427
                                                             ------------  -----------  ------------  -----------
Net income.................................................   $   14,690    $     974   $  (1,001)    $    14,663
                                                             ------------  -----------  ------------  -----------
                                                             ------------  -----------  ------------  -----------
Earnings per share from continuing operations..............   $     0.24                              $      0.21
                                                             ------------                             -----------
                                                             ------------                             -----------
Average common shares outstanding..........................       60,651                   10,138(g)       70,789
                                                             ------------               ------------  -----------
                                                             ------------               ------------  -----------
</TABLE>
 
                             See accompanying notes
 
                                       54
<PAGE>
                        ENSCO INTERNATIONAL INCORPORATED
 
              COMBINED CONDENSED PRO FORMA STATEMENT OF OPERATIONS
 
                          YEAR ENDED DECEMBER 31, 1995
                                 (IN THOUSANDS)
                                  (UNAUDITED)
 
<TABLE>
<CAPTION>
                                                                    HISTORICAL
                                                             -------------------------
                                                                ENSCO         DUAL
                                                             INTERNATIONAL  DRILLING      PRO FORMA
                                                             INCORPORATED    COMPANY     ADJUSTMENTS    PRO FORMA
                                                             ------------  -----------  -------------  -----------
<S>                                                          <C>           <C>          <C>            <C>
Revenues
  Contract drilling........................................   $  240,775    $  85,889   $              $   326,664
  Marine transportation....................................       38,339       --                           38,339
  Gain on disposition of assets............................       --           --            5,127(a)        5,127
                                                             ------------  -----------  -------------  -----------
                                                                 279,114       85,889        5,127         370,130
                                                             ------------  -----------  -------------  -----------
Operating expenses
  Contract drilling........................................      132,558       60,229                      192,787
  Marine transportation....................................       23,402       --                           23,402
  Depreciation and amortization............................       58,390       19,608        6,302(b)       84,537
                                                                                               237(c)
  General and administrative...............................        9,569        7,563                       17,132
                                                             ------------  -----------  -------------  -----------
                                                                 223,919       87,400        6,539         317,858
                                                             ------------  -----------  -------------  -----------
Operating income (loss)....................................       55,195       (1,511)      (1,412)         52,272
                                                             ------------  -----------  -------------  -----------
Other income (expense)
  Interest income..........................................        6,310        2,400          567(d)        9,277
  Interest expense.........................................      (16,564)     (14,705)         615(e)      (30,654)
  Gain on disposition of assets............................       --            5,127       (5,127)(a)
  Other, net...............................................        2,398          336                        2,734
                                                             ------------  -----------  -------------  -----------
                                                                  (7,856)      (6,842)      (3,945)        (18,643)
                                                             ------------  -----------  -------------  -----------
Income (loss) from continuing operations before income
 taxes and minority interest...............................       47,339       (8,353)      (5,357)         33,629
Provision for income taxes.................................        3,397          885       (1,792)(f)       2,490
Minority interest..........................................        2,179       --                            2,179
                                                             ------------  -----------  -------------  -----------
Income (loss) from continuing operations...................   $   41,763    $  (9,238)  $   (3,565)    $    28,960
                                                             ------------  -----------  -------------  -----------
                                                             ------------  -----------  -------------  -----------
Earnings per share from continuing operations..............   $     0.69                               $      0.41
                                                             ------------                              -----------
                                                             ------------                              -----------
Average common shares outstanding..........................       60,527                    10,138(g)       70,665
                                                             ------------               -------------  -----------
                                                             ------------               -------------  -----------
</TABLE>
 
                             See accompanying notes
 
                                       55
<PAGE>
NOTES TO UNAUDITED COMBINED CONDENSED PRO FORMA FINANCIAL STATEMENTS
 
    The accompanying unaudited combined condensed pro forma financial statements
are  based upon adjustments to  the historical consolidated financial statements
of ENSCO and DUAL to give effect to the Merger.
 
    The unaudited combined condensed pro forma balance sheet assumes the  Merger
was  consummated on  March 31,  1996, and  the unaudited  combined condensed pro
forma statements of operations assume the  Merger was consummated as of  January
1,  1995. The unaudited combined condensed proforma statements of operations are
not necessarily indicative of the results that would have been obtained had  the
Merger  been  consummated on  January 1,  1995 or  that may  be obtained  in the
future.
 
    Certain information  and notes  normally  included in  financial  statements
prepared  in accordance with  GAAP have been  omitted pursuant to  the rules and
regulations of  the  Commission.  The unaudited  combined  condensed  pro  forma
financial  statements  should  be  read  in  conjunction  with  the consolidated
financial statements of ENSCO and DUAL for the three months ended March 31, 1996
previously filed separately with the Commission under Form 10-Q and incorporated
by reference in this Prospectus/Proxy  Statement and the consolidated  financial
statements  of ENSCO and  DUAL for the  year ended December  31, 1995 previously
filed separately  with  the  Commission  under Form  10-K  and  incorporated  by
reference in this Prospectus/Proxy Statement.
 
PRO FORMA ADJUSTMENTS TO HISTORICAL COMBINED CONDENSED BALANCE SHEET
 
    The  following pro forma adjustments have been made to reflect the Merger as
if it had been consummated as of March 31, 1996:
 
    (a) To reflect the issuance of 9,853,571 shares of ENSCO Common Stock to  be
       issued  in the Merger based upon an estimated average price for shares of
       ENSCO Common Stock  of $21.694  per share. The  adjustments allocate  the
       purchase  price  to the  fair value  of  assets acquired  and liabilities
       assumed and eliminate the historical equity of DUAL.
 
    (b) To reflect  other direct costs  of the Merger,  including severance  and
       other   employee-related  costs,  professional  services,  and  fees  and
       commissions. The majority of the costs are assumed to be paid prior to or
       at closing.
 
    (c) To reflect the issuance  of 284,186 shares of  ENSCO Common Stock to  be
       issued in the Merger related to outstanding options to purchase shares of
       DUAL Common Stock. The above number of shares of ENSCO Common Stock to be
       issued  is based upon the excess of  0.625 over a fraction, the numerator
       of which is the exercise price under the DUAL Options and the denominator
       of which  is  the  approximate  current  price  of  ENSCO  Common  Stock,
       multiplied by the number of shares covered by the DUAL Options.
 
PRO FORMA ADJUSTMENTS TO HISTORICAL COMBINED CONDENSED STATEMENTS OF OPERATIONS
 
    The  following pro forma adjustments have been made to reflect the Merger as
if it had been consummated as of January 1, 1995:
 
    (a) To conform  DUAL's accounting policy  with ENSCO's with  respect to  the
       reporting of gains on disposition of assets.
 
    (b)  To adjust depreciation and amortization for the effects of the increase
       in accounting basis of the property and equipment acquired and adjustment
       of remaining useful lives.
 
                                       56
<PAGE>
    (c) To reflect the amortization of the amount of purchase price in excess of
       the fair value of the net assets acquired over 40 years, as follows:
 
<TABLE>
<S>                                                                 <C>
Total pro forma goodwill at December 31, 1995.....................  $  85,730
Less: Original ENSCO goodwill at December 31, 1995................     (7,252)
                                                                    ---------
    Goodwill attributable to Dual acquisition.....................     78,478
Amortization period (in years)....................................         40
                                                                    ---------
    Annual amortization of new goodwill...........................      1,962
Less: Amortization of goodwill per Dual historical books..........     (1,725)
                                                                    ---------
    Increase in goodwill amortization.............................  $     237
                                                                    ---------
                                                                    ---------
</TABLE>
 
    (d)  To  reflect  the  amortization  of  the  premium  associated  with  the
       subordinated debt.
 
    (e)  To  eliminate the  amortization of  loan  origination costs  which were
       assigned no value in the purchase price allocation.
 
    (f) To adjust deferred tax expense as  a result of the change in book  basis
       of various assets and liabilities.
 
    (g) To reflect the issuance of 10,137,757 shares of ENSCO Common Stock.
 
                      COMPARISONS OF STOCKHOLDERS' RIGHTS
 
    Upon   consummation  of  the  Merger,   stockholders  of  DUAL  will  become
stockholders of ENSCO. The rights of  former DUAL stockholders will continue  to
be governed by applicable Delaware law ("Delaware Law"), and will be governed by
the  Certificate of Incorporation,  as amended, and Bylaws  of ENSCO (the "ENSCO
Certificate" and the "ENSCO Bylaws,"  respectively). The following is a  summary
of  the material differences between the  ENSCO Certificate and ENSCO Bylaws, on
the one hand, and the Certificate of Incorporation and Bylaws of DUAL (the "DUAL
Certificate" and the "DUAL Bylaws," respectively),  on the other hand, that  may
affect  the rights  of DUAL's  stockholders who  become holders  of ENSCO Common
Stock.
 
CAPITAL STOCK
 
    The authorized  capital stock  of ENSCO  consists of  125,000,000 shares  of
Common  Stock, $.10  par value,  5,000,000 shares  of First  Preferred Stock (as
defined in the  ENSCO Certificate),  par value  $1.00 and  15,000,000 shares  of
Serial  Preferred Stock (as defined in  the ENSCO Certificate), par value $1.00.
No shares  of First  Preferred Stock  or Serial  Preferred Stock  are  currently
outstanding.  All shares of ENSCO  Common Stock are identical  and have one vote
per share.
 
    The authorized capital stock of DUAL consists of 50,000,000 shares of Common
Stock, $0.01 par value, and 10,000,000 shares of Preferred Stock (as defined  in
the  DUAL Certificate), $0.01 par value. No shares of DUAL's Preferred Stock are
currently outstanding. All shares  of DUAL Common Stock  are identical and  have
one vote per share.
 
SPECIAL MEETINGS OF STOCKHOLDERS
 
    Under  Delaware Law, a special meeting of  stockholders may be called by the
board of directors or by any other person authorized to do so in the certificate
of incorporation or the bylaws.
 
    The  DUAL  Certificate  contains  provisions  concerning  special   meetings
consistent  with  the  provisions  of Delaware  Law  described  above,  and also
empowers holders of at least 33 1/3% of the stock entitled to vote on the matter
or matters to be  considered at a  special meeting to call  such a meeting.  The
ENSCO  Certificate  contains provisions  concerning special  meetings consistent
with the  provisions of  Delaware Law  described above,  and also  empowers  the
chairman of the board or the president to call a special meeting.
 
                                       57
<PAGE>
NUMBER OF DIRECTORS
 
    Under  Delaware law,  the minimum number  of directors is  one. Delaware law
permits the board  of directors alone  to change the  authorized number, or  the
range,  of directors by  amendment to the  bylaws, unless the  directors are not
authorized in the certificate of incorporation to amend the bylaws or the number
of directors is  fixed in  the certificate of  incorporation, in  which cases  a
change  in the number of directors may be made only upon approval of such change
by the stockholders.
 
    The DUAL Bylaws provide for a variable number of directors between five  and
seven,  with the exact number currently fixed  at six. The ENSCO Certificate and
the ENSCO Bylaws also provide for  a variable number of directors between  three
and 15, with the exact number currently fixed at eight.
 
CLASSIFICATION OF BOARD
 
    A  classified board is  one in which  a certain number  of the directors are
elected on a rotating basis each  year. This method of electing directors  makes
changes  in the  composition of  the board  of directors,  and thus  a potential
change in control  of a  corporation, a  lengthier and  more difficult  process.
Delaware  law permits,  but does not  require, a classified  board of directors,
with staggered terms  under which  one-half or  one-third of  the directors  are
elected for terms of two or three years, respectively.
 
    DUAL  does not have  a classified board of  directors. The ENSCO Certificate
and Bylaws divide  the ENSCO board  of directors into  three classes, with  each
class serving a staggered three-year term.
 
REMOVAL OF DIRECTORS
 
    Under  Delaware  law, a  director  of a  corporation  that does  not  have a
classified board of  directors may  be removed with  or without  cause with  the
approval of a majority of the outstanding shares entitled to vote at an election
of  directors, and a  director of a  corporation that has  a classified board of
directors may be removed only for cause unless the corporation's certificate  of
incorporation provides otherwise.
 
    The  ENSCO Certificate  provides for  a classified  board of  directors. The
ENSCO Bylaws provide that  a director may  be removed at any  time and only  for
cause  by the  holders of  stock having  more than  50% of  the voting  power of
outstanding stock. The DUAL Certificate provides that a director may be  removed
at  an  annual or  special meeting  of stockholders  and only  for cause  by the
holders of a majority of DUAL's voting capital stock.
 
ANTI-TAKEOVER PROVISIONS
 
    ENSCO.   On February  21, 1995,  the  ENSCO Board  of Directors  declared  a
dividend  of one preferred share purchase right (a "Right") for each outstanding
share of ENSCO  Common Stock. The  dividend was  payable on March  6, 1995  (the
"Rights  Record Date") to  the stockholders of  record on that  date. Each Right
entitles the registered holder to purchase  from ENSCO one-hundredth of a  share
of Series A Junior Participating Preferred Stock, par value $1.00 per share (the
"Series  A Preferred Stock") of ENSCO at a price of $50.00 per one one-hundredth
of a  share of  Series A  Preferred  Stock (the  "Purchase Price"),  subject  to
adjustment.  The Rights are described on ENSCO's Form 8-A Registration Statement
filed with the Commission on February 23, 1995, which is incorporated herein  by
reference.
 
    DUAL.    DUAL  does  not  have a  stockholders'  rights  plan  and  the DUAL
Certificate  and  the  DUAL  Bylaws   do  not  contain  specific   anti-takeover
provisions.
 
INDEMNIFICATION AND LIMITATION OF MONETARY LIABILITIES
 
    Section 145 of the General Corporation Law of the State of Delaware provides
generally  and in pertinent  part that a Delaware  corporation may indemnify its
directors and  officers  against  expenses,  judgments,  fines  and  settlements
actually and reasonably incurred by them in connection with any civil, criminal,
administrative,  or investigative  suit or  action except  actions by  or in the
right of the  corporation if,  in connection with  the matters  in issues,  they
acted  in good faith  and in a manner  they reasonably believed to  be in or not
opposed  to  the  best   interests  of  the   corporation,  and  in   connection
 
                                       58
<PAGE>
with  any criminal  suit or  proceeding, if  in connection  with the  matters in
issue, they  had no  reasonable cause  to believe  their conduct  was  unlawful.
Section  145 further provides that in  connection with the defense or settlement
of any action by or in the right of the corporation, a Delaware corporation  may
indemnify  its directors and  officers against expenses  actually and reasonably
incurred by them if, in connection with the matters in issue, they acted in good
faith, in a manner they reasonably believed to be in or not opposed to the  best
interests  of the corporation,  except that no indemnification  may be made with
respect to any claim, issue or matter as to which such person has been  adjudged
liable for negligence or misconduct unless the Court of Chancery or the court in
which  such action or suit to brought approves such indemnification. Section 145
further permits  a Delaware  corporation  to grant  its directors  and  officers
additional rights of indemnification through bylaw provisions and otherwise, and
to purchase indemnity insurance on behalf of its directors and officers.
 
    Article  Fifteen of the  ENSCO Certificate provides,  in general, that ENSCO
must indemnify its  directors and  officers under certain  of the  circumstances
defined  in Section 145, and that no director of ENSCO will be personally liable
to ENSCO  or  its stockholders  for  monetary damages  for  any breach  of  such
director's  fiduciary duty, with certain exceptions. This Article further allows
ENSCO to purchase and maintain insurance  on behalf of its directors,  officers,
employees, or agents and to provide for such indemnification by means of a trust
fund, security interest, letter of credit, surety bond, contract, and/or similar
arrangement.  The  directors  and officers  of  ENSCO and  its  subsidiaries are
insured (subject to certain exceptions and deductions) against liabilities which
they may  incur in  their  capacity as  such,  including liabilities  under  the
Securities  Act, under a liability insurance  policy carried by ENSCO. The ENSCO
Bylaws require ENSCO to indemnify its officers, directors, employees and  agents
to  the full  extent permitted by  the General  Corporation Law of  the State of
Delaware.
 
    Article Eight  of the  DUAL  Bylaws provides,  in  general, that  DUAL  must
indemnify  its directors and officers under certain of the circumstances defined
in Section  145, and  Article Sixth  of the  DUAL Certificate  provides that  no
director  of DUAL  will be  personally liable  to DUAL  or its  stockholders for
monetary damages for any breach of such director's fiduciary duty, with  certain
exceptions  identical to those set forth in the ENSCO Certificate. Article Eight
of the DUAL  Bylaws further allows  DUAL to purchase  and maintain insurance  on
behalf  of  its directors,  officers, employees,  or  agents. The  directors and
officers of DUAL and its subsidiaries are insured (subject to certain exceptions
and deductions) against liabilities  which they may incur  in their capacity  as
such,  including  liabilities  under  the  Securities  Act,  under  a  liability
insurance policy  carried by  DUAL.  The DUAL  Bylaws  further require  DUAL  to
indemnify  its  officers, directors,  employees and  agents  to the  full extent
permitted by the General Corporation Law of the State of Delaware.
 
DISSOLUTION
 
    The DUAL  Certificate  provides  that,  in the  event  of  the  dissolution,
liquidation  or winding  up of  DUAL, the  DUAL Preferred  Stock shall  have the
rights fixed for it  by the Board  of Directors of  DUAL. The ENSCO  Certificate
provides  that, in the event of dissolution, liquidation or winding up of ENSCO,
holders of the First Preferred Stock shall have the rights fixed for them by the
ENSCO Board of Directors; provided that rights of any series of Serial Preferred
Stock to receive  any such distribution  will be junior  and subordinate to  the
liquidation  preference of all  outstanding shares of  the $1.50 Preferred Stock
(as  defined  in  the  ENSCO  Certificate  and  none  of  which  are   currently
outstanding),  the Series A Preferred Stock (as defined in the ENSCO Certificate
and none of  which are  currently outstanding), and  any other  series of  First
Preferred  Stock which by its terms is  senior to the shares of Serial Preferred
Stock.
 
PREEMPTIVE RIGHTS
 
    Unless the certificate of incorporation provides otherwise, the stockholders
of a Delaware corporation do not  have preemptive rights. Neither the ENSCO  nor
the DUAL Certificate provides for any preemptive rights.
 
                                       59
<PAGE>
PAYMENT OF DIVIDENDS
 
    ENSCO  has never paid dividends to the holders of its Common Stock. DUAL has
not paid dividends  to holders  of DUAL Common  Stock since  its initial  public
offering  in August 1993. ENSCO intends to retain all of its earnings for use in
its business and, therefore, does not anticipate paying cash dividends on  ENSCO
Common Stock in the foreseeable future.
 
RESTRICTIONS ON ALIEN OWNERSHIP
 
    The  ENSCO Certificate  contains certain  provisions to  limit ownership and
control of shares of  any class of  capital stock of  ENSCO by certain  non-U.S.
citizens  in order  to permit ENSCO  to hold,  obtain or reinstate  a license or
franchise from a  governmental agency necessary  to conduct its  business as  an
owner  and operator  of U.S.-flag vessels.  The ENSCO  Certificate restricts the
transfer of shares of ENSCO Common Stock when such transfer would result in  the
ownership or control by one or more non-U.S. citizens of an aggregate percentage
of the shares of ENSCO Common Stock in excess of a specified percentage.
 
DISSENTING STOCKHOLDERS' RIGHTS
 
    No  stockholder  of  DUAL  or  ENSCO will  have  any  dissenters'  rights in
connection with, or as a result of, the matters to be acted upon at the  Special
Meeting.
 
STOCKHOLDERS' LISTS AND INSPECTION OF BOOKS AND RECORDS
 
    Both  the ENSCO Bylaws and the DUAL  Bylaws provide that any stockholder may
examine the corporation's list of shareholders for a period of at least 10  days
prior  to any meeting  of stockholders, if  such examination is  for any purpose
germane to the meeting. The list may also be inspected at the time and place  of
the  meeting by  an stockholder who  is present.  Except as stated  above and as
provided by applicable law, no stockholder  of ENSCO or DUAL has any  additional
rights to inspect the corporation's books and records.
 
                    PROPOSAL TO APPROVE THE ADOPTION OF THE
                       DUAL SPECIAL PERFORMANCE UNIT PLAN
 
    Effective  August 21, 1995, DUAL's Board of Directors, upon a recommendation
of its Compensation Committee, adopted the  Unit Plan and the performance  goals
set  forth therein, subject  to approval by  the stockholders of  DUAL. The Unit
Plan is designed to retain  and reward key executives  of DUAL by granting  them
special  cash bonuses  in the  event of  a Sale  Transaction (as  defined below)
involving DUAL during the term  of the Unit Plan. The  Unit Plan is intended  to
provide  the participants  in the  Unit Plan with  an incentive  to increase the
value of DUAL's business by  allowing them to participate  in a cash bonus  pool
that is commensurate with the sale price of DUAL.
 
    The  Unit Plan appears as Appendix  C to this Prospectus/Proxy Statement and
is incorporated herein  by reference.  The Unit Plan  is intended  to allow  the
award  of  benefits that  qualify as  performance-based compensation  within the
meaning of Section 162(m) of the Code. Stockholder approval of the Unit Plan  is
sought in order to allow the Unit Plan to comply with the requirements set forth
in Section 162(m) of the Code.
 
    The  participants in the Unit Plan are  L.H. Dick Robertson, W. Allen Parks,
Dudley M. Haralson,  Robert C.  McCoy, Lewis W.  Kreps, William  R. Dudark,  and
Robert F. Chrone.
 
    The  Unit Plan will be administered by a committee appointed by the Board of
Directors of DUAL (the "Plan Committee"). The Plan Committee must be constituted
so as to permit the Unit Plan to comply with Rule 16b-3 under the Exchange  Act.
Additionally,  all members of the Plan  Committee must be "outside directors" as
defined in Section 162(m) of the  Code. The Compensation Committee of the  Board
of Directors of DUAL currently serves as the Plan Committee.
 
    Receipt of benefits pursuant to the Unit Plan is conditioned upon occurrence
of  a  merger,  consolidation, or  other  reorganization  of DUAL  in  which the
outstanding DUAL Common Stock is converted  into or exchanged for securities  of
another issuer, cash, or other property, or upon occurrence of a sale, lease, or
exchange  of all or substantially all of the assets of DUAL (collectively, "Sale
Transactions").
 
                                       60
<PAGE>
    The bonus pool to be distributed pursuant to the Unit Plan (the "Performance
Bonus Pool")  is a  cash amount  based on  the Equivalent  Share Price  (defined
below), which amount is determined as follows:
 
<TABLE>
<CAPTION>
    IF THE EQUIVALENT SHARE PRICE IS:               THEN THE PERFORMANCE BONUS POOL AMOUNT IS:
- ------------------------------------------  ----------------------------------------------------------
<S>                                         <C>
At least $12.00 but less than $13.00        The product of (i) the excess of the Equivalent Share
                                             Price over $10.00 multiplied by (ii) 100,000.
At least $13.00 but less than $14.00        The product of (i) the excess of the Equivalent Share
                                             Price over $10.00 multiplied by (ii) 200,000.
At least $14.00 but less than $15.00        The product of (i) the excess of the Equivalent Share
                                             Price over $10.00 multiplied by (ii) 300,000.
$15.00 and above                            The product of (i) the excess of the Equivalent Share
                                             Price over $10.00 multiplied by (ii) 400,000.
</TABLE>
 
    Upon  occurrence of  a Sale Transaction,  each participant will  be paid, in
cash, the following  proportions of  the Performance  Bonus Pool:  27.0% to  Mr.
Robertson;  15.6% to  each of Mr.  Parks and  Mr. Haralson; 13.2%  to Mr. McCoy;
12.0% to each of Mr. Kreps and Dudark; and 4.5% to Mr. Chrone.
 
    Pursuant to the  Merger Agreement, DUAL  has agreed to  set the  Performance
Bonus Pool associated with the Merger, together with certain payments due to Mr.
Skarke  under his employment agreement with DUAL, at $2,000,000. See "The Merger
- -- Interests  of  Certain Persons;  Possible  Conflicts of  Interest  --  Skarke
Agreement."  As so limited,  and as limited  by the terms  of the DUAL Executive
Agreements,  the  amounts  to  be  received  by  Unit  Plan  participants   upon
consummation  of the Merger,  if the Merger  is approved by  the stockholders of
DUAL, would be as follows:
 
                               NEW PLAN BENEFITS
 
<TABLE>
<CAPTION>
                                                                                            UNIT PLAN
                                                                               -----------------------------------
NAME AND POSITION                                                              DOLLAR VALUE($)   NUMBER OF UNITS
- -----------------------------------------------------------------------------  ---------------  ------------------
<S>                                                                            <C>              <C>
L. H. Dick Robertson.........................................................   $     469,972    Not applicable.
  CHIEF EXECUTIVE OFFICER, PRESIDENT, AND DIRECTOR
W. Allen Parks...............................................................         271,540    Not applicable.
  EXECUTIVE VICE PRESIDENT AND CHIEF FINANCIAL OFFICER
Robert C. McCoy..............................................................         229,765    Not applicable.
  SENIOR VICE PRESIDENT, MARKETING
Dudley M. Haralson...........................................................         271,540    Not applicable.
  SENIOR VICE PRESIDENT, OPERATIONS
Lewis W. Kreps...............................................................         208,877    Not applicable.
  VICE PRESIDENT, BUSINESS DEVELOPMENT
                                                                               ---------------
All current executive officers as a group....................................       1,451,694    Not applicable.
                                                                               ---------------
                                                                               ---------------
All current directors who are not executive officers
 as a group (1)..............................................................         261,100    Not applicable.
                                                                               ---------------
                                                                               ---------------
All employees, including all current officers who are not executive officers,
 as a group..................................................................         287,206    Not applicable.
                                                                               ---------------
                                                                               ---------------
</TABLE>
 
- ------------------------
(1) Consists of the cash bonus payable to Mr. Skarke pursuant to his  employment
    agreement.  Although Mr. Skarke is  not a participant in  the Unit Plan, his
    cash bonus is measured  as a percentage of  the Performance Bonus Pool.  See
    "The  Merger -- Interests of Certain Persons; Possible Conflicts of Interest
    -- Skarke Agreement."
 
                                       61
<PAGE>
    Approval of the Unit Plan is not a condition to consummation of the  Merger,
and approval of the Merger is not a condition to effectiveness of the Unit Plan.
 
    The "Equivalent Share Price" is a dollar amount which is equal to (A)(i) the
value of the consideration (as reasonably determined by the Plan Committee) paid
or  given for the  stock or assets of  DUAL sold or  transferred pursuant to the
Sale Transaction divided by (ii) the decimal representing the percentage of  the
DUAL  Common Stock or assets of DUAL that is sold or transferred pursuant to the
Sale Transaction, divided by (B) the number of outstanding shares of DUAL Common
Stock on August  21, 1995, as  adjusted by  the Plan Committee  pursuant to  the
terms of the Unit Plan.
 
    Unless  the Unit Plan is required by  applicable law to be extended, it will
terminate on August  20, 1997,  and payments  under it  will only  be made  with
respect  to a Sale Transaction  which is effective, or  as to which a definitive
binding agreement is in effect, on or before that date.
 
    The Unit Plan may be amended by  the Board of Directors of DUAL, subject  to
any  stockholder  approval  required by  Section  162(m)  of the  Code  or other
applicable law.
 
    The proposal to approve the Unit Plan  must receive the favorable vote of  a
majority  of the  total number  of shares of  DUAL Common  Stock represented and
entitled to vote at the Special Meeting or any adjournment thereof for approval.
An affirmative vote by a stockholder shall also be deemed to be approval of  the
performance  goals under  the Unit  Plan for purposes  of Section  162(m) of the
Code. Pursuant to the Principal Stockholder Agreement, the Principal Stockholder
Shares (representing  approximately  59.6% of  the  outstanding shares  of  DUAL
Common  Stock) will be voted  at the Special Meeting in  favor of the Unit Plan.
Therefore, no additional  stockholder votes  are necessary to  approve the  Unit
Plan.
 
    THE BOARD OF DIRECTORS OF DUAL HAS ADOPTED THE UNIT PLAN AND RECOMMENDS THAT
DUAL  STOCKHOLDERS VOTE  FOR THE  PROPOSAL TO APPROVE  THE ADOPTION  OF THE UNIT
PLAN.
 
                                       62
<PAGE>
                   COMPENSATION OF DUAL'S EXECUTIVE OFFICERS
 
    The following table reflects, for the fiscal years ended December 31,  1995,
1994  and 1993, cash compensation paid by DUAL or the Principal Stockholder, and
a summary of certain other compensation paid or accrued for such year, to DUAL's
Chief Executive  Officer  and the  five  other executive  officers  (the  "Named
Executive  Officers") of DUAL  for service in all  capacities with the Principal
Stockholder or DUAL and their subsidiaries.
 
                           SUMMARY COMPENSATION TABLE
 
<TABLE>
<CAPTION>
                                                                                    LONG TERM COMPENSATION
                                                                                            AWARDS
                                                                                    ----------------------
                                                            ANNUAL COMPENSATION     RESTRICTED    OPTIONS
NAME AND                                                  ------------------------     STOCK      (NO. OF      ALL OTHER
PRINCIPAL POSITION                               YEAR     SALARY (1)    BONUS (2)     AWARDS      SHARES)   COMPENSATION (4)
- ---------------------------------------------  ---------  -----------  -----------  -----------  ---------  ----------------
<S>                                            <C>        <C>          <C>          <C>          <C>        <C>
L. H. Robertson..............................       1995  $   360,000  $   115,000                 180,000     $   24,192
  President and Chief                               1994      430,259       52,254                 120,000         28,887
  Executive Officer                                 1993      345,000      172,500  $   766,499     60,000         20,603
W. Allen Parks...............................       1995      246,250       10,000                 104,000         16,903
  Executive Vice President                          1994      295,360       36,194                  65,000         15,416
  and Chief Financial Officer                       1993      228,000      112,500      358,335     39,000         13,491
Dudley M. Haralson...........................       1995      171,051       10,000                 104,000         10,478
  Senior Vice President                             1994      187,028       22,673                  65,000          9,704
  Operations                                        1993      125,000       62,500      229,285     27,000          7,419
Robert C. McCoy..............................       1995      156,000       10,000                  88,000         10,347
  Senior Vice President                             1994      187,362       22,980                  58,000          8,982
  Marketing                                         1993      140,000       70,000      354,135     30,000          8,382
Lewis W. Kreps...............................       1995      145,688       10,000                  80,000         10,000
  Senior Vice President                             1994      169,634       20,682                  53,000          9,528
  Business Development                              1993      125,000       62,500      229,285     27,000          7,419
William R. Dudark............................       1995      142,500       10,000                  80,000          9,781
  Vice President                                    1994      169,634       20,682                  53,000          9,685
  Operations and Engineering                        1993      125,000       62,500      229,285     27,000          7,419
</TABLE>
 
- ------------------------
(1) Salary figures  for  1994  include amounts  paid  for  accumulated  vacation
    benefits  upon termination of  DUAL's previous vacation  policy. The amounts
    paid to Messrs.  Robertson, Parks,  Haralson, McCoy, Kreps  and Dudark  were
    $82,759; $56,610; $37,472; $35,862; $32,759; and $32,759, respectively.
 
(2)  Bonuses earned in  1993 were payable  in installments equal  to 50% of such
    bonus in January 1994, 25% in January 1995 and 25% in January 1996. The 1994
    bonuses were earned in 1994 and were paid in full in February 1995. The 1995
    bonuses were earned and paid in 1995.
 
(3) At December 31, 1995,  the number of shares  of restricted stock granted  by
    DUAL  as  restricted stock  awards under  the  1993 Plan  ("Restricted Stock
    Awards") which had not vested and the  total value of such shares, based  on
    the  last reported sales price of the DUAL Common Stock on December 29, 1995
    (the final trading day  of 1995), held by  each Named Executive Officer  are
 
                                       63
<PAGE>
    included  in the table  below. All shares  of restricted stock  that had not
    vested as of December 31, 1995 are attributable to a Restricted Stock  Award
    granted  by DUAL on August 12, 1993.  All such unvested shares vest fully on
    August 12, 1996.
 
<TABLE>
<CAPTION>
                                                                               SHARES       VALUE
                                                                              ---------  -----------
<S>                                                                           <C>        <C>
Mr. Robertson...............................................................      9,000  $   102,375
Mr. Parks...................................................................      4,800       54,600
Mr. Haralson................................................................      3,900       44,363
Mr. McCoy...................................................................      4,500       51,188
Mr. Kreps...................................................................      3,900       44,363
Mr. Dudark..................................................................      3,900       44,363
</TABLE>
 
    On December 9, 1993, DUAL granted a Restricted Stock Award of 8,559;  3,891;
    2,335;  3,891;  2,335 and  2,335 shares  of DUAL's  Common Stock  to Messrs.
    Robertson, Parks,  Haralson,  McCoy,  Kreps and  Dudark,  respectively.  The
    shares  of Common Stock  vested 50% on June  9, 1994 and  50% on December 9,
    1994.
 
    On August 12, 1993, DUAL granted a Restricted Stock Award of 39,942; 18,155;
    10,893; 18,155;  10,893  and  10,893  shares  of  Common  Stock  to  Messrs.
    Robertson,  Parks,  Haralson,  McCoy, Kreps  and  Dudark,  respectively. The
    shares of Common Stock vested 50% on February 12, 1994 and 50% on August 12,
    1994.
 
    All Restricted Stock Awards granted by DUAL entitle the beneficiaries to all
    rights as  a stockholder  from the  date of  grant (including  the right  to
    receive  dividends when, as,  and if declared)  other than (i)  the right to
    transfer or sell the shares prior to the vesting date, and (ii) the right to
    possession prior to the vesting date.
 
(4) For 1995, All Other Compensation includes (i) Company matching contributions
    under DUAL's 401(K) Plan in the amount of $9,000 each for Messrs. Robertson,
    Parks, Haralson and McCoy, $8,741 for  Mr. Kreps and $8,550 for Mr.  Dudark,
    (ii) Company Matching Contributions under DUAL's Benefit Restoration Plan in
    the  amount of $12,600 for Mr. Robertson and $5,775 for Mr. Parks, and (iii)
    the dollar value  of insurance premiums  paid by DUAL  with respect to  term
    life insurance in the amounts of $2,592; $2,128; $1,478; $1,347; $1,259; and
    $1,231  for  Messrs. Robertson,  Parks, Haralson,  McCoy, Kreps  and Dudark,
    respectively.
 
OPTION CANCELLATIONS
 
    The table below sets forth  information regarding stock options,  previously
issued to the Named Executive Officers under the 1993 Plan, that were authorized
for cancellation during 1995 by DUAL's Board of Directors.
 
                                       64
<PAGE>
                         TEN YEAR OPTION/SAR REPRICINGS
 
<TABLE>
<CAPTION>
                                                          NUMBER OF     MARKET
                                                         SECURITIES    PRICE OF     EXERCISE                  LENGTH OF
                                                         UNDERLYING    STOCK AT     PRICE AT               ORIGINAL OPTION
                                                          OPTIONS/      TIME OF      TIME OF               TERM REMAINING
                                                            SARS       REPRICING    REPRICING      NEW       AT DATE OF
                                                         REPRICED OR      OR           OR       EXERCISE    REPRICING OR
NAME AND PRINCIPAL POSITION                     DATE       AMENDED     AMENDMENT    AMENDMENT     PRICE       AMENDMENT
- --------------------------------------------  ---------  -----------  -----------  -----------  ---------  ---------------
<S>                                           <C>        <C>          <C>          <C>          <C>        <C>
L. H. Robertson.............................    8/21/95      60,000    $   10.00    $  14.000   $   10.00        8 years
  Director, President                           8/21/95     120,000        10.00       12.125       10.00        9 years
  and CEO
W. Allen Parks..............................    8/21/95      39,000        10.00       14.000       10.00        8 years
  Executive Vice                                8/21/95      65,000        10.00       12.125       10.00        9 years
  President and CFO
Dudley M. Haralson..........................    8/21/95      27,000        10.00       14.000       10.00        8 years
  Senior Vice President                         8/21/95      65,000        10.00       12.125       10.00        9 years
  USA/International
  Operations
Robert C. McCoy.............................    8/21/95      30,000        10.00       14.000       10.00        8 years
  Senior Vice President                         8/21/95      58,000        10.00       12.125       10.00        9 years
  International Marketing
Lewis W. Kreps..............................    8/21/95      27,000        10.00       14.000       10.00        8 years
  Senior Vice President                         8/21/95      53,000        10.00       12.125       10.00        9 years
  Business Development
William R. Dudark...........................    8/21/95      27,000        10.00       14.000       10.00        8 years
  Vice President                                8/21/95      53,000        10.00       12.125       10.00        9 years
  International Operations
  and Engineering
</TABLE>
 
    In  August 1995,  the Compensation  Committee of  DUAL's Board  of Directors
approved the  granting of  options to  the executive  officers and  certain  key
employees  of DUAL in exchange for the  cancellation of then existing options to
purchase Company common stock.  The Compensation Committee carefully  considered
its  decision to grant new options and  effectively reduce the exercise price of
then existing  options.  In  making its  decision,  the  Compensation  Committee
consulted  with an independent  compensation consultant and  also considered the
then current market price of the DUAL Common Stock, the exercise prices at which
the options had  previously been granted,  the overall performance  of the  DUAL
Common  Stock since the previous  options had been issued  and the importance of
providing renewed incentive for the executive officers and employees to continue
in the service of DUAL and renew interest in the success of DUAL.
 
    Compensation Committee:                            Frank Jungers
                                                    Aage Figenschou
                                                    Edward O. Vetter
                                                    Magne Kristiansen
 
                                       65
<PAGE>
OPTION GRANTS
 
    The table below sets forth information regarding Stock Options granted under
the 1993 Plan to the Named Executive Officers during 1995:
 
                               INDIVIDUAL GRANTS
 
<TABLE>
<CAPTION>
                                                                                                  POTENTIAL REALIZABLE VALUE
                                                         PERCENTAGE OF                            AT ASSUMED ANNUAL RATES OF
                                                         TOTAL OPTIONS                           STOCK PRICE APPRECIATION FOR
                                             NUMBER OF    GRANTED TO    EXERCISE OR                  FULL OPTION TERM (2)
                                              OPTIONS    EMPLOYEES IN   BASE PRICE   EXPIRATION  ----------------------------
NAME                                        GRANTED (1)   FISCAL 1995    PER SHARE      DATE          5%             10%
- ------------------------------------------  -----------  -------------  -----------  ----------  -------------  -------------
<S>                                         <C>          <C>            <C>          <C>         <C>            <C>
L. H. Robertson...........................     180,000         17.1%     $  10.000     08/21/05  $   1,132,200  $   2,869,200
W. Allen Parks............................     104,000         13.3%        10.000     08/21/05        654,160      1,657,760
Dudley M. Haralson........................     104,000         13.3%        10.000     08/21/05        654,160      1,657,760
Robert C. McCoy...........................      88,000          8.4%        10.000     08/21/05        553,520      1,402,720
William R. Dudark.........................      80,000          7.6%        10.000     08/21/05        503,200      1,275,200
Lewis W. Kreps............................      80,000          7.6%        10.000     08/21/05        503,200      1,275,200
</TABLE>
 
- ------------------------
(1) The options are  exercisable in increments  of 20% on  August 21, 1996,  the
    first  anniversary of  the date  of grant,  and 20%  on each  of the second,
    third, fourth and fifth anniversary dates of the grant.
 
(2) Potential Realizable Value is  based on the assumed  annual growth rates  of
    Common Stock for the 10-year option term. A 5% annual growth rate results in
    a  stock price of $16.29 per share and a 10% annual growth rate results in a
    stock price of  $25.94 per  share. Actual gains,  if any,  on stock  options
    exercised  are dependent on  the future performance  of DUAL's common stock.
    There can be no assurance that the  amounts reflected in this table will  be
    achieved.
 
AGGREGATED EXERCISES OF OPTIONS/SARS AND FISCAL YEAR-END OPTION/SAR VALUE TABLE
 
    For  each of the  Named Executive Officers, the  information set forth below
reflects for the  fiscal year ended  December 31, 1995,  options under the  1993
Plan  and the  value realized thereon  as well as  exercisable and unexercisable
options which were unexercised at year-end 1995 and the realizable value thereon
at such date:
 
<TABLE>
<CAPTION>
                                                                            NUMBER OF UNEXERCISED           VALUE OF UNEXERCISED
                                              NUMBER OF                        OPTIONS HELD AT              IN-THE-MONEY OPTIONS
                                               SHARES                         DECEMBER 31, 1995             AT DECEMBER 31, 1995
                                             ACQUIRED ON     VALUE     --------------------------------  --------------------------
NAME                                          EXERCISE     REALIZED       EXERCISABLE     UNEXERCISABLE  EXERCISABLE  UNEXERCISABLE
- -------------------------------------------  -----------  -----------  -----------------  -------------  -----------  -------------
<S>                                          <C>          <C>          <C>                <C>            <C>          <C>
L. H. Robertson............................         N/A          N/A             -0-           180,000          N/A    $   247,500
W. Allen Parks.............................         N/A          N/A             -0-           104,000          N/A        143,000
Dudley M. Haralson.........................         N/A          N/A             -0-           104,000          N/A        143,000
Robert C. McCoy............................         N/A          N/A             -0-            88,000          N/A        121,000
William R. Dudark..........................         N/A          N/A             -0-            80,000          N/A        110,000
Lewis W. Kreps.............................         N/A          N/A             -0-            80,000          N/A        110,000
</TABLE>
 
- ------------------------
 *  Represents the number  of unexercised options  multiplied by the  difference
    between  $11.375, which was the last reported sales price of the DUAL Common
    Stock on the Nasdaq Stock Market as of December 31, 1995, and the  per-share
    exercise price of $10.00 for the options granted in 1995.
 
SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN
 
    In  June 1993,  DUAL implemented  the SERP,  a defined  benefit pension plan
covering certain of  its executive  officers. The  SERP is  administered by  the
Board  of Directors or by  the Compensation Committee and  is intended to assist
DUAL  in   attracting   and   retaining   employees   of   exceptional   ability
 
                                       66
<PAGE>
by  providing certain  benefits. The  SERP provides  that upon  reaching age 65,
after 10 years  of service  with DUAL and  upon termination  of employment  from
DUAL,  a participant  will receive annual  benefits, which when  added to Social
Security Benefits,  equal  30%  of  the average  of  the  participant's  highest
consecutive  36-month base  compensation during  the participant's  service with
DUAL ("Average  Compensation"). During  June  1994, the  Compensation  Committee
approved  an amendment  to the  SERP that raised  the annual  benefit for DUAL's
president to 50% of Average Compensation when added to Social Security benefits.
DUAL is the sole contributor to the SERP and the cost of benefits under the SERP
are recognized as they accrue based on certain future salary and social security
benefit assumptions.
 
    The table  below provides  certain  information with  respect to  the  Named
Executive  Officers  and  the  estimated  benefits  payable  to  the  respective
executives under the SERP.
 
                                 SERP BENEFITS
 
<TABLE>
<CAPTION>
                                                                           ESTIMATED
                                                                       ANNUAL RETIREMENT
                                                    CURRENT YEARS OF    BENEFIT AT AGE
NAME                                                     SERVICE              65*
- --------------------------------------------------  -----------------  -----------------
<S>                                                 <C>                <C>
L. H. Robertson...................................             13         $   180,000
W. Allen Parks....................................             18              73,875
Dudley M. Haralson................................              8              60,000
Robert W McCoy....................................              7              48,000
Lewis W. Kreps....................................             14              45,000
William R. Dudark.................................              9              43,500
</TABLE>
 
- ------------------------
 * Amounts to be reduced by Social Security benefits.
 
                                 LEGAL MATTERS
 
    The legality of the shares of  ENSCO Common Stock being offered hereby  will
be  passed upon for ENSCO by Baker & McKenzie, Dallas, Texas. The federal income
tax consequences in  connection with  the Merger will  be passed  upon by  Akin,
Gump, Strauss, Hauer & Feld, L.L.P., Dallas, Texas.
 
                                    EXPERTS
 
    The  financial statements incorporated in this Prospectus/Proxy Statement by
reference from the  ENSCO 1995 Form  10-K have been  audited by two  independent
accountants.  The companies and periods covered by these audits are indicated in
the individual  accountants' reports.  Such financial  statements have  been  so
included  in reliance on the reports of the two independent accountants given on
the authority of such firms as experts in auditing and accounting.
 
    The consolidated  financial statements  and 1995  schedules incorporated  by
reference  in this Prospectus/Proxy Statement from  the DUAL 1995 Form 10-K have
been so  incorporated  in  reliance  on the  report  of  Price  Waterhouse  LLP,
independent  accountants,  given on  the authority  of said  firm as  experts in
auditing and accounting.
 
    The  fairness  opinion  of  Simmons   incorporated  by  reference  in   this
Prospectus/Proxy  Statement  has  been  so  incorporated  in  reliance  upon the
authority of Simmons as experts in investment banking.
 
                             STOCKHOLDER PROPOSALS
 
    If the Merger is not consummated, DUAL will hold its 1996 Annual Meeting  of
Stockholders  in August 1996.  DUAL stockholders who  intend to submit proposals
for inclusion in DUAL's 1996 Proxy Statement and Proxy for stockholder action at
the DUAL 1996 Annual Meeting of Stockholders must do so by sending the  proposal
and  supporting statements, if  any, to DUAL  at its corporate  offices no later
than May 31, 1996.
 
                                       67
<PAGE>
                                     INDEX
                                 DEFINED TERMS
<TABLE>
<CAPTION>
                                                    PAGE
                                                    -----
<S>                                              <C>
1993 Plan......................................           9
401(k) Plan....................................          46
affiliates.....................................          10
Alternative Proposal Fee.......................           9
Average Compensation...........................          66
Average Trading Price..........................          40
Benefit Restoration Plan.......................          35
Cancelable Shares..............................           7
CBK Loan.......................................          38
Certificates...................................          39
Change of Control Offer........................          38
Change of Control Purchase Date................          38
Change of Control Purchase Price...............          38
Citibank Facility..............................          38
COBRA Coverage.................................          45
Code...........................................           7
Commission.....................................           1
Delaware Law...................................          56
DOL............................................          46
DUAL...........................................           1
DUAL 1995 Form 10-K............................           2
DUAL Bylaws....................................          56
DUAL Certificate...............................          56
DUAL Common Stock..............................           1
DUAL Comparable Companies......................          29
DUAL Executive Agreements......................          36
DUAL Medical Plan..............................          45
DUAL Options...................................           9
EBDIT..........................................          29
Effective Time.................................           1
ENSCO..........................................           1
ENSCO 1995 Form 10-K...........................           2
ENSCO Bylaws...................................          56
ENSCO Certificate..............................          56
ENSCO Common Stock.............................           1
ENSCO Comparable Companies.....................          30
Equivalent Share Price.........................          16
Exchange Act...................................           2
Exchange Agent.................................          39
Exchange Fund..................................          38
Final Candidates...............................          23
Final Three Candidates.........................          23
GAAP...........................................           8
Group Insurance Plans..........................          45
HSR Act........................................           8
Implied Consideration..........................          28
Indenture......................................          38
Long-Term Options..............................          44
 
<CAPTION>
                                                    PAGE
                                                    -----
<S>                                              <C>
LTM............................................          29
MARAD..........................................          18
Merger.........................................           1
Merger Agreement...............................           1
Merger Subsidiary..............................           1
Named Executive Officers.......................          62
Non-Employee Director Plan.....................           9
Non-Employee Options...........................          45
NYSE...........................................           2
OPA'90.........................................          17
Order..........................................           9
Organizational Documents.......................          40
Penrod.........................................          48
Performance Bonus Pool.........................          60
Plan Committee.................................          59
Potential Candidates...........................          22
Potential Sale or Merger Transaction...........          21
Principal Stockholder..........................          10
Principal Stockholder Agreement................          10
Principal Stockholder Shares...................          10
Prospectus/Proxy Statement.....................           1
Purchase Price.................................          57
Record Date....................................           6
Registration Statement.........................           1
Restricted Stock Awards........................          62
Retiree Medical Plan...........................          46
Right..........................................          57
Rights Record Date.............................          57
Sale Transactions..............................          16
Securities Act.................................           2
Selected Candidates............................          23
Senior Notes...................................          38
Series A Preferred Stock.......................          57
SERP...........................................          35
Simmons........................................           7
Simmons Engagement Letter......................          22
Simmons Transaction Fee........................          22
Skaugen........................................          37
Special Meeting................................           1
Subsidiaries...................................          36
Subsidiary.....................................          36
Surviving Corporation..........................           6
Tax Opinion....................................           7
Team Incentive Program.........................          36
Terminating DUAL Breach........................          43
Terminating ENSCO Breach.......................          43
U.S. Holder....................................          32
Unit Plan......................................           5
</TABLE>
 
                                       68
<PAGE>
                                   APPENDIX A
                                MERGER AGREEMENT
 
                                      A-1
<PAGE>
- ----------------------------------------------------------------------
- ----------------------------------------------------------------------
 
                         AGREEMENT AND PLAN OF MERGER*
                                     AMONG
                       ENSCO INTERNATIONAL INCORPORATED,
                            DDC ACQUISITION COMPANY
                                      AND
                             DUAL DRILLING COMPANY
                              DATED MARCH 21, 1996
 
*As amended and restated by Amendment No. 1 to Agreement and Plan of Merger
 dated as of May 7, 1996.
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                                      A-2
<PAGE>
                                      a-i
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                             ARTICLE I
                                             THE MERGER
 
                                                                               SECTION       PAGE
- -----------------------------------------------------------------------------------------  ---------
    1.01.  The Merger....................................................................        A-9
<C>        <S>                                                                             <C>
    1.02.  Effective Time; Closing.......................................................        A-9
    1.03.  Effect of the Merger..........................................................        A-9
    1.04.  Certificate of Incorporation; Bylaws..........................................        A-9
    1.05.  Directors and Officers........................................................       A-10
 
                                             ARTICLE II
                         CONVERSION OF SECURITIES; EXCHANGE OF CERTIFICATES
    2.01.  Conversion of Securities......................................................       A-10
    2.02.  Exchange of Certificates......................................................       A-11
    2.03.  Stock Transfer Books..........................................................       A-12
    2.04.  Stock Options.................................................................       A-13
 
                                            ARTICLE III
                            REPRESENTATIONS AND WARRANTIES OF THE TARGET
    3.01.  Organization and Qualification; Subsidiaries..................................       A-13
    3.02.  Certificate of Incorporation and Bylaws.......................................       A-14
    3.03.  Capitalization................................................................       A-14
    3.04.  Authority Relative to This Agreement..........................................       A-14
    3.05.  No Conflict; Required Filings and Consents....................................       A-15
    3.06.  Permits; Compliance...........................................................       A-15
    3.07.  SEC Filings; Financial Statements.............................................       A-16
    3.08.  Absence of Certain Changes or Events..........................................       A-16
    3.09.  Absence of Litigation.........................................................       A-17
    3.10.  Employee Benefit Matters......................................................       A-17
    3.11.  Labor Matters.................................................................       A-21
    3.12.  Intellectual Property.........................................................       A-21
    3.13.  Taxes.........................................................................       A-21
    3.14.  Environmental Matters.........................................................       A-22
    3.15.  Opinion of Financial Advisor..................................................       A-23
    3.16.  Vote Required.................................................................       A-23
    3.17.  Brokers.......................................................................       A-23
    3.18.  Tangible Property.............................................................       A-23
    3.19.  Bareboat Charter..............................................................       A-23
    3.20.  Material Contracts............................................................       A-23
    3.21.  Parachute Payments............................................................       A-24
    3.22.  Certain Business Practices....................................................       A-24
    3.23.  Real Property and Leases......................................................       A-24
    3.24.  Insurance.....................................................................       A-24
    3.25.  Accounting and Tax Matters....................................................       A-25
    3.26.  Board Recommendation..........................................................       A-25
    3.27.  Change in Control.............................................................       A-26
    3.28.  Target Drilling Rigs..........................................................       A-26
    3.29.  Sime-Dual Sdn Bhd.............................................................       A-27
</TABLE>
 
                                      A-3
<PAGE>
                                      a-ii
 
<TABLE>
<CAPTION>
                                                                                                 SECTION        PAGE
- ------------------------------------------------------------------------------------------------------------  ---------
<C>        <S>                                                                                                <C>
    3.30.  Accounts Receivable..............................................................................       A-27
 
                                                      ARTICLE IV
                              REPRESENTATIONS AND WARRANTIES OF ACQUIROR AND ACQUIROR SUB
    4.01.  Corporate Organization and Qualification.........................................................       A-27
    4.02.  Certificate of Incorporation and Bylaws..........................................................       A-28
    4.03.  Capitalization...................................................................................       A-28
    4.04.  Authority Relative to This Agreement.............................................................       A-28
    4.05.  No Conflict; Required Filings and Consents.......................................................       A-29
    4.06.  SEC Filings; Financial Statements................................................................       A-29
    4.07.  Absence of Certain Changes or Events.............................................................       A-30
    4.08.  Brokers..........................................................................................       A-30
    4.09.  Taxes............................................................................................       A-30
 
                                                       ARTICLE V
                                        CONDUCT OF BUSINESS PENDING THE MERGER
    5.01.  Conduct of Business by the Target Pending the Merger.............................................       A-31
    5.02.  Conduct of Business by Acquiror Pending the Merger...............................................       A-31
 
                                                      ARTICLE VI
                                                 ADDITIONAL AGREEMENTS
    6.01.  Registration Statement; Proxy Statement..........................................................       A-32
    6.02.  Target Stockholder Meeting.......................................................................       A-33
    6.03.  Appropriate Action; Consents; Filings............................................................       A-33
    6.04.  Access to Information; Confidentiality...........................................................       A-34
    6.05.  No Solicitation of Transactions..................................................................       A-34
    6.06.  Directors' and Officers' Indemnification.........................................................       A-35
    6.07.  Obligations of Acquiror Sub......................................................................       A-36
    6.08.  Public Announcements.............................................................................       A-36
    6.09.  Delivery of SEC Documents........................................................................       A-36
    6.10.  Environmental Assessment.........................................................................       A-36
    6.11.  Notification of Certain Matters..................................................................       A-36
    6.12.  Further Action...................................................................................       A-36
    6.13.  Employee Benefits................................................................................       A-36
    6.14.  Affiliates; Accounting and Tax Treatment.........................................................       A-39
    6.15.  Certain Employees................................................................................       A-40
 
                                                      ARTICLE VII
                                               CONDITIONS TO THE MERGER
    7.01.  Conditions to the Obligations of Each Party......................................................       A-40
    7.02.  Conditions to the Obligations of Acquiror and Acquiror Sub.......................................       A-40
    7.03.  Conditions to the Obligations of the Target......................................................       A-41
 
                                                     ARTICLE VIII
                                           TERMINATION, AMENDMENT AND WAIVER
    8.01.  Termination......................................................................................       A-42
    8.02.  Fees and Expenses................................................................................       A-42
    8.03.  Amendment........................................................................................       A-43
</TABLE>
 
                                      A-4
<PAGE>
                                     a-iii
 
<TABLE>
<CAPTION>
                                                                                                 SECTION        PAGE
- ------------------------------------------------------------------------------------------------------------  ---------
<C>        <S>                                                                                                <C>
    8.04.  Waiver...........................................................................................       A-43
 
                                                      ARTICLE IX
                                                  GENERAL PROVISIONS
    9.01.  Non-Survival of Representations, Warranties and Agreements.......................................       A-44
    9.02.  Notices..........................................................................................       A-44
    9.03.  Certain Definitions..............................................................................       A-44
    9.04.  Severability.....................................................................................       A-45
    9.05.  Assignment; Binding Effect; Benefit..............................................................       A-45
    9.06.  Incorporation of Schedules.......................................................................       A-45
    9.07.  Specific Performance.............................................................................       A-45
    9.08.  Governing Law....................................................................................       A-45
    9.09.  Headings.........................................................................................       A-46
    9.10.  Counterparts.....................................................................................       A-46
    9.11.  Waiver of Jury Trial.............................................................................       A-46
    9.12.  Entire Agreement.................................................................................       A-46
</TABLE>
 
EXHIBIT A -- Certificate of Incorporation
EXHIBIT B -- Affiliate Agreement
EXHIBIT C -- Legal Opinion for Counsel for the Target
EXHIBIT D -- Legal Opinion for Counsel for Acquiror
 
                                      A-5
<PAGE>
                                 DEFINED TERMS
 
<TABLE>
<CAPTION>
TERM                                                                                                 LOCATION
- ------------------------------------------------------------------------------------------------  ---------------
<S>                                                                                               <C>
Acquiror........................................................................................  Recitals
Acquiror Common Stock...........................................................................  2.01(a)(i)
Acquiror Disclosure Schedules...................................................................  Article IV
Acquiror Preferred Stock........................................................................  4.03
Acquiror SEC Reports............................................................................  4.06(a)
Acquiror Sub....................................................................................  Recitals
Acquiror Sub Common Stock.......................................................................  2.01(a)(iii)
Acquiror Subsidiary.............................................................................  4.03
affiliate.......................................................................................  9.03(a)
Affiliate Agreements............................................................................  6.14
Agreement.......................................................................................  Recitals
AICPA Statement.................................................................................  7.02(b)
Alternative Proposal Fee........................................................................  8.02(a)
Average Trading Price...........................................................................  2.02(e)
beneficial owner................................................................................  9.03(b)
Benefit Restoration Plan........................................................................  6.13(e)
Board...........................................................................................  6.05(iii)
Blue Sky Laws...................................................................................  3.05(b)(i)
Business Combination Transaction................................................................  8.02(a)
Business Combination Transaction Proposal.......................................................  8.01(d)
business day....................................................................................  9.03(c)
Cancelable Shares...............................................................................  2.01(a)(i)
Certificate of Merger...........................................................................  1.02
Certificates....................................................................................  2.02(b)
Claim...........................................................................................  6.06(b)
Closing.........................................................................................  2.02
Closing Agreement...............................................................................  6.13(a)(ii)
COBRA Coverage..................................................................................  6.13(b)
Code............................................................................................  Recitals
Confidentiality Agreement.......................................................................  9.13
control.........................................................................................  9.03(d)
controlled by...................................................................................  9.03(d)
Delaware Law....................................................................................  Recitals
DOL.............................................................................................  6.13(n)
Dual Medical Plan...............................................................................  6.13(b)
Dual Unit Plan..................................................................................  6.13(j)
Effective Time..................................................................................  1.02
Employee Severance Plans........................................................................  6.13(h)
Environmental Claims............................................................................  3.14(b)(viii)
Environmental Laws..............................................................................  3.14(a)(ii)
Environmental Permits...........................................................................  3.14(b)(ii)
Exchange Act....................................................................................  3.05(b)(i)
Exchange Agent..................................................................................  2.02(a)
Exchange Fund...................................................................................  2.02(a)
Exchange Ratio..................................................................................  2.01(a)(i)
ERISA...........................................................................................  3.10
First Amendment.................................................................................  3.29(i)
401(k) Plan.....................................................................................  6.13(a)(i)
401(k) Plan Amendment...........................................................................  6.13(a)(i)
Governmental Authority..........................................................................  3.06
</TABLE>
 
                                      A-6
<PAGE>
<TABLE>
<CAPTION>
TERM                                                                                                 LOCATION
- ------------------------------------------------------------------------------------------------  ---------------
Group Insurance Plans...........................................................................  6.13(b)
<S>                                                                                               <C>
Hazardous Substances............................................................................  3.14(a)(i)
HSR Act.........................................................................................  3.05(b)(i)
IRS.............................................................................................  3.10(c)
Joint Bareboat Charter..........................................................................  3.29(vii)
Joint Operating Agreement.......................................................................  3.29(vi)
Laws............................................................................................  3.05(a)(y)(ii)
Long-Term Options...............................................................................  2.04(a)(i)
Material Adverse Effect.........................................................................  3.01
Material Contract...............................................................................  3.20
Merger..........................................................................................  Recitals
NASD............................................................................................  6.08(b)
1995 Balance Sheet..............................................................................  3.07(c)
Non-Employee Options............................................................................  2.04(b)
NYSE............................................................................................  2.02(E)
Officer Incentive Plan..........................................................................  6.13(g)
Order...........................................................................................  7.01(b)
Original Agreement..............................................................................  3.29(i)
PBGC............................................................................................  3.10(c)
person..........................................................................................  9.03(e)
Plans...........................................................................................  3.10(i)
Post-Employment Benefits........................................................................  3.10(k)(iv)
Pre-Merger Period...............................................................................  3.25(a)(i)
Prior Thrift Plan...............................................................................  6.13(a)(i)
Proxy Statement.................................................................................  6.01(a)
Registration Statement..........................................................................  6.01(a)
Retiree Medical Plan............................................................................  6.13(c)
Sale............................................................................................  3.25(b)
SD..............................................................................................  3.29(i)
SDD.............................................................................................  3.29(i)
SEC.............................................................................................  3.07(a)(ii)
Secretary.......................................................................................  1.02
SERP............................................................................................  6.13(d)(i)
Securities Act..................................................................................  3.05(b)(i)
Shareholders Agreement..........................................................................  3.29(i)
Specified Expenses..............................................................................  8.02(d)
Sime-Dual.......................................................................................  3.29(i)
Stockholder Plan................................................................................  3.25(b)
Subsidiary......................................................................................  3.01
subsidiary......................................................................................  9.03(f)
subsidiaries....................................................................................  9.03(f)
Surviving Corporation...........................................................................  1.01
Target..........................................................................................  Recitals
Target Affiliate................................................................................  6.14
Target Banker...................................................................................  3.16
Target Common Stock.............................................................................  2.01(a)(i)
Target Disclosure Schedule......................................................................  Article III
Target Drilling Rigs............................................................................  3.19
Target Employment Contracts.....................................................................  3.10(ii)
Target Options..................................................................................  2.04(h)
Target Permits..................................................................................  3.06
Target Preferred Stock..........................................................................  3.03
</TABLE>
 
                                      A-7
<PAGE>
<TABLE>
<CAPTION>
TERM                                                                                                 LOCATION
- ------------------------------------------------------------------------------------------------  ---------------
Target SEC Reports..............................................................................  3.07(a)
<S>                                                                                               <C>
Target's Stockholder Meeting....................................................................  6.02
Terminating Acquiror Breach.....................................................................  8.01(f)
Terminating Target Breach.......................................................................  8.01(f)
Third Party Provisions..........................................................................  9.05
Transactions....................................................................................  Recitals
Trust...........................................................................................  6.13(f)(i)
Trustee.........................................................................................  6.13(f) (i)
under common control............................................................................  9.03(d)
U.S. GAAP.......................................................................................  3.07(b)
Vessels.........................................................................................  3.28(a)
</TABLE>
 
                                      A-8
<PAGE>
    AGREEMENT AND PLAN OF MERGER, dated as of March 21, 1996 (this "AGREEMENT"),
by   and  among   ENSCO  International  Incorporated,   a  Delaware  corporation
("ACQUIROR"), DDC  Acquisition Company,  a Delaware  corporation and  a  direct,
wholly owned subsidiary of Acquiror ("ACQUIROR SUB"), and DUAL DRILLING COMPANY,
a Delaware corporation (the "TARGET").
 
    WHEREAS,  Acquiror Sub, upon the terms and subject to the conditions of this
Agreement and in accordance with the Delaware General Corporation Law ("DELAWARE
LAW"), will merge with and into the Target (the "MERGER");
 
    WHEREAS, the Board of  Directors of the Target  (i) has determined that  the
Merger  is in the best interests of the Target and its stockholders and approved
and  adopted   this  Agreement   and   the  transactions   contemplated   hereby
("TRANSACTIONS")  and (ii) has unanimously  recommended approval and adoption of
this Agreement and approval of the  Merger by, and directed that this  Agreement
and the Merger be submitted to a vote of, the stockholders of the Target;
 
    WHEREAS,  the  Boards  of  Directors  of  Acquiror  and  Acquiror  Sub  have
determined that the Merger  is in the best  interests of Acquiror, Acquiror  Sub
and  their stockholders and have unanimously approved and adopted this Agreement
and the Transactions; and
 
    WHEREAS, Acquiror  and  the  Target  intend that  the  Merger  constitute  a
tax-free  "reorganization"  within the  meaning of  Section 368(a)(1)(A)  of the
Internal Revenue Code of 1986 (the "CODE") by reason of Section 368(a)(2)(E)  of
the Code.
 
    NOW,  THEREFORE, in consideration of the  foregoing and the mutual covenants
and agreements  herein contained,  and  intending to  be legally  bound  hereby,
Acquiror, Acquiror Sub and the Target hereby agree as follows:
 
                                   ARTICLE I
                                   THE MERGER
 
    SECTION 1.01.  THE MERGER.  Upon the terms and subject to the conditions set
forth  in this Agreement, and in accordance  with Delaware Law, at the Effective
Time (as hereinafter defined),  Acquiror Sub shall be  merged with and into  the
Target.  As a result of the Merger, the separate corporate existence of Acquiror
Sub shall cease and  the Target shall continue  as the surviving corporation  of
the  Merger (the "SURVIVING CORPORATION"). The name of the Surviving Corporation
shall be Dual Holding Company.
 
    SECTION 1.02.  EFFECTIVE TIME; CLOSING.   As promptly as practicable and  in
no  event later than the first business day following the satisfaction or waiver
of the conditions set forth in Article VII (or such other date as may be  agreed
in  writing by each of  the parties hereto), the  parties hereto shall cause the
Merger to be consummated by filing a certificate of merger (the "CERTIFICATE  OF
MERGER")  with the Secretary of State of the State of Delaware (the "SECRETARY")
in such form as  is required by,  and executed in  accordance with the  relevant
provisions  of, Delaware Law. The term "EFFECTIVE  TIME" means the date and time
of the filing of  the Certificate of  Merger with the  Secretary (or such  later
time  as may be agreed in writing by each of the parties hereto and specified in
the Certificate of Merger). Immediately prior  to the filing of the  Certificate
of  Merger, a  closing (the "CLOSING")  will be held  at the offices  of Baker &
McKenzie, 2001 Ross Avenue, Suite 4500,  Dallas, Texas (or such other place  and
time as the parties may agree).
 
    SECTION  1.03.  EFFECT OF THE MERGER.   The effect of the Merger shall be as
provided in the applicable provisions of Delaware Law.
 
    SECTION 1.04.  CERTIFICATE OF INCORPORATION;  BYLAWS.  (a) At the  Effective
Time,  the Certificate of Incorporation of  the Target, as in effect immediately
prior to the Effective Time, shall be amended
 
                                      A-9
<PAGE>
as of the Effective  Time by operation  of this Agreement and  by virtue of  the
Merger  without  any further  action  by the  stockholders  or directors  of the
Surviving Corporation to read in its entirety as set forth in EXHIBIT A attached
hereto.
 
    (b) At  the  Effective  Time, the  Bylaws  of  Acquiror Sub,  as  in  effect
immediately  prior to the Effective  Time, shall be the  Bylaws of the Surviving
Corporation until  thereafter amended  as provided  by law,  the Certificate  of
Incorporation of the Surviving Corporation and such Bylaws.
 
    SECTION  1.05.   DIRECTORS  AND  OFFICERS.   The  directors of  Acquiror Sub
immediately prior to the  Effective Time shall be  the initial directors of  the
Surviving Corporation, each to hold office in accordance with the Certificate of
Incorporation  and  Bylaws of  the Surviving  Corporation  until a  successor is
elected or appointed and has qualified or until the earliest of such  director's
death,  resignation, removal or  disqualification, and the  officers of Acquiror
Sub immediately prior to the Effective Time shall be the initial officers of the
Surviving Corporation, in each case  until their respective successors are  duly
elected  or appointed and qualified,  or as otherwise provided  in the Bylaws of
the Surviving Corporation.
 
                                   ARTICLE II
               CONVERSION OF SECURITIES; EXCHANGE OF CERTIFICATES
 
    SECTION 2.01.  CONVERSION OF SECURITIES.   At the Effective Time, by  virtue
of  the Merger and without any action on the part of Acquiror Sub, the Target or
the holders of any of the following shares of capital stock:
 
        (a) Subject to the other provisions of this Section 2.01 and to  Section
    2.02:
 
           (i)  each  share of  common  stock, $0.01  par  value, of  the Target
       ("TARGET COMMON STOCK") issued and  outstanding immediately prior to  the
       Effective  Time (excluding  any shares  held by  the Target,  Acquiror or
       Acquiror Sub or any other direct  or indirect wholly owned subsidiary  of
       Acquiror  or the Target immediately prior  to the Merger (the "CANCELABLE
       SHARES")) shall be converted into the  right to receive .625 shares  (the
       "EXCHANGE  RATIO")  of common  stock, $0.10  par value  ("ACQUIROR COMMON
       STOCK"), of Acquiror. At  the Effective Time, all  such shares of  Target
       Common  Stock shall no  longer be outstanding  and automatically shall be
       canceled and cease to exist,  and each certificate previously  evidencing
       any  such  shares  shall  thereafter represent  the  right  to  receive a
       certificate representing the shares of  Acquiror Common Stock into  which
       such  shares of  Target Common  Stock were  converted in  the Merger. The
       holders of  certificates  previously  evidencing such  shares  of  Target
       Common  Stock outstanding immediately  prior to the  Effective Time shall
       cease to have  any rights with  respect to such  shares of Target  Common
       Stock  except  as  otherwise provided  herein  or by  Delaware  Law. Such
       certificates previously evidencing shares of Target Common Stock shall be
       exchanged for certificates representing  whole shares of Acquiror  Common
       Stock  issued  in  consideration  therefor  upon  the  surrender  of such
       certificates in  accordance  with  the provisions  of  Section  2.02.  No
       fractional  shares of Acquiror Common Stock shall be issued, and, in lieu
       thereof, a cash payment shall be made pursuant to Section 2.02(e);
 
           (ii) each Cancelable Share shall automatically be canceled and  cease
       to  exist, and no  consideration shall be  paid or payable  in respect of
       such shares; and
 
          (iii) each  share  of common  stock,  par  value $.01  per  share,  of
       Acquiror  Sub  ("ACQUIROR  SUB  COMMON  STOCK")  issued  and  outstanding
       immediately prior  to the  Effective  Time shall  be converted  into  and
       become  one validly issued, fully paid  and nonassessable share of common
       stock of the Surviving Corporation.
 
        (b) If between  the date of  this Agreement and  the Effective Time  the
    outstanding  shares of  Acquiror Common Stock  or Target  Common Stock shall
    have been changed into a different
 
                                      A-10
<PAGE>
    number of shares  or a  different class, by  reason of  any stock  dividend,
    reclassification, recapitalization, split, division, combination or exchange
    of  shares, the Exchange Ratio shall  be correspondingly adjusted to reflect
    such stock  dividend, reclassification,  recapitalization, split,  division,
    combination or exchange of shares.
 
    SECTION  2.02.   EXCHANGE OF  CERTIFICATES.   (a) Exchange  Agent. As  of or
before the  Effective  Time,  Acquiror  shall deposit,  or  shall  cause  to  be
deposited,  with a bank or trust company organized under the laws of, and having
an office in, the United States or any state thereof and designated by  Acquiror
and  approved by Target, which approval  shall not be unreasonably withheld (the
"EXCHANGE AGENT"), for  the benefit of  the holders of  shares of Target  Common
Stock,  for exchange  in accordance with  this Article II,  through the Exchange
Agent, certificates  representing  the whole  shares  of Acquiror  Common  Stock
issuable  pursuant to Section 2.01 in  exchange for outstanding shares of Target
Common Stock and cash in an amount sufficient to permit payment of cash  payable
in  lieu of fractional shares pursuant to Section 2.02(e) (such certificates for
shares of Acquiror Common  Stock, together with  any dividends or  distributions
with  respect thereto, and cash, being  hereinafter referred to as the "EXCHANGE
FUND"). The  Exchange Agent  shall, pursuant  to irrevocable  instructions  from
Acquiror,  deliver the Acquiror Common Stock  and cash contemplated to be issued
pursuant to Section 2.01 out of the Exchange Fund.
 
        (b)  EXCHANGE PROCEDURES.  As  soon as reasonably practicable after  the
    Effective  Time, Acquiror will  instruct the Exchange Agent  to mail to each
    holder of record of a certificate or certificates which immediately prior to
    the Effective  Time  evidenced outstanding  shares  of Target  Common  Stock
    (other  than  Cancelable  Shares)  (the  "CERTIFICATES")  (i)  a  letter  of
    transmittal and (ii) instructions for use in effecting the surrender of  the
    Certificates  in  exchange for  certificates  evidencing shares  of Acquiror
    Common Stock.  Upon  surrender of  a  Certificate for  cancellation  to  the
    Exchange  Agent together with such letter of transmittal, duly executed, and
    such  other  customary  documents  as  may  be  required  pursuant  to  such
    instructions, the holder of such Certificate shall be entitled to receive in
    exchange  therefor a certificate representing the  number of whole shares of
    Acquiror Common Stock which such holder has the right to receive in  respect
    of   the  shares  of  Target  Common  Stock  formerly  represented  by  such
    Certificate (after taking  into account  all shares of  Target Common  Stock
    then  held by such holder), together with  cash in lieu of fractional shares
    of Acquiror  Common Stock  to  which such  holder  is entitled  pursuant  to
    Section  2.02(e) and any dividends or  distributions to which such holder is
    entitled pursuant to  Section 2.02(c),  and the  Certificate so  surrendered
    shall   forthwith  be  canceled.  Subject   to  Section  2.02(h),  under  no
    circumstances will any holder  of a Certificate be  entitled to receive  any
    part  of the shares of Acquiror Common Stock into which the shares of Target
    Common Stock  were converted  in the  Merger until  such holder  shall  have
    surrendered  such Certificate.  In the event  of a transfer  of ownership of
    shares of  Target Common  Stock  which is  not  registered in  the  transfer
    records  of the Target, the shares of  Acquiror Common Stock into which such
    shares of Target Common Stock were converted in the Merger may be issued  in
    accordance  with  this  Article  II to  the  transferee  if  the Certificate
    evidencing such shares of Target Common  Stock is presented to the  Exchange
    Agent,  accompanied by  all documents required  to evidence  and effect such
    transfer and by evidence that any applicable stock transfer taxes have  been
    paid.   Until  surrendered  as  contemplated  by  this  Section  2.02,  each
    Certificate shall be deemed at any time after the Effective Time to evidence
    only the right to receive  upon such surrender the certificate  representing
    the number of whole shares of Acquiror Common Stock which the holder has the
    right  to receive in respect  of the shares of  Target Common Stock formerly
    represented by such  Certificate (after  taking into account  all shares  of
    Target Common Stock then held by such holder), together with cash in lieu of
    fractional  shares of Acquiror Common Stock to which such holder is entitled
    pursuant to Section 2.02(e) and any dividends or distributions to which such
    holder is entitled pursuant  to Section 2.02(c).  Acquiror agrees, from  and
    after  the Effective  Time, to  treat the  holders of  certificates formerly
    representing shares of Target  Common Stock as holding  of record the  whole
    number  of  shares  of Acquiror  Common  Stock  for purposes  of  voting and
    determinations of quorums for voting.
 
                                      A-11
<PAGE>
        (c)  DISTRIBUTIONS WITH RESPECT TO UNEXCHANGED SHARES OF ACQUIROR COMMON
    STOCK.   No dividends  or other  distributions declared  or made  after  the
    Effective  Time with  respect to  Acquiror Common  Stock with  a record date
    after the Effective Time  shall be paid to  the holder of any  unsurrendered
    Certificate  with respect to the shares  of Acquiror Common Stock into which
    such shares of Target Common Stock  were converted in the Merger, until  the
    holder  of such Certificate shall surrender such Certificate for exchange as
    provided herein.  Subject  to  the  effect  of  applicable  laws,  following
    surrender of any such Certificate, there shall be paid to the holder of such
    Certificate, in addition to the certificates representing shares of Acquiror
    Common  Stock  as  provided  in 2.02(b),  without  interest,  the  amount of
    dividends or other distributions with a record date after the Effective Time
    theretofore paid with respect to the  whole shares of Acquiror Common  Stock
    evidenced by such Certificate.
 
        (d)   NO FURTHER RIGHTS IN TARGET  COMMON STOCK.  All shares of Acquiror
    Common Stock delivered upon conversion of the shares of Target Common  Stock
    in  accordance  with the  terms  hereof (including  any  cash paid  or other
    distributions pursuant to Section 2.02(c) and  (e)) shall be deemed to  have
    been  issued in full satisfaction of all rights pertaining to such shares of
    Target Common Stock.
 
        (e)   NO  FRACTIONAL  SHARES.    No  certificates  or  scrip  evidencing
    fractional  shares  of  Acquiror  Common  Stock  shall  be  issued  upon the
    surrender for exchange of Certificates, but  in lieu thereof each holder  of
    shares  of Target Common Stock who would  otherwise be entitled to receive a
    fraction of a share of Acquiror  Common Stock, after aggregating all  shares
    of  Acquiror Common  Stock which  such holder  would be  entitled to receive
    under Section 2.01,  shall receive an  amount equal to  the Average  Trading
    Price  multiplied by  the fraction  of a share  of Acquiror  Common Stock to
    which such  holder  would  otherwise  be  entitled,  without  interest.  The
    "AVERAGE  TRADING PRICE" shall be the average  of the closing sale prices of
    the Acquiror Common Stock on the New York Stock Exchange ("NYSE") or, if not
    listed on the NYSE, any exchange on which the Acquiror Common Stock may then
    be principally listed  (as reported by  THE WALL STREET  JOURNAL or, if  not
    reported  thereby, by  another authoritative  source) over  the ten business
    days immediately preceding the Effective Time.
 
        (f)  TERMINATION  OF EXCHANGE FUND.   Any portion  of the Exchange  Fund
    which  remains undistributed to  the holders of Target  Common Stock for one
    year after the Effective Time shall  be delivered to Acquiror, upon  demand,
    and, subject to Section 2.02(g), any holders of Target Common Stock who have
    not  theretofore complied with this Article II shall thereafter look only to
    Acquiror for  the shares  of Acquiror  Common  Stock, any  cash in  lieu  of
    fractional  shares  of  Acquiror Common  Stock  and any  dividends  or other
    distributions to which they are entitled pursuant to this Section 2.02.
 
        (g)  NO LIABILITY.  Neither Acquiror nor the Surviving Corporation shall
    be liable to any holder of shares  of Target Common Stock for any shares  of
    Acquiror  Common Stock or  cash (or dividends  or distributions with respect
    thereto) delivered to a public official pursuant to any applicable abandoned
    property, escheat or similar law.
 
        (h)  LOST CERTIFICATES.  If any Certificate shall have been lost, stolen
    or destroyed, upon the  making of an  affidavit of that  fact by the  person
    claiming  such Certificate to be lost,  stolen or destroyed and, if required
    by the Surviving Corporation, the posting by  such person of a bond in  such
    reasonable  amount  as the  Surviving  Corporation may  direct  as indemnity
    against any  claim  that  may  be  made against  it  with  respect  to  such
    Certificate, the Exchange Agent will issue in exchange for such lost, stolen
    or  destroyed Certificate the shares of  Acquiror Common Stock, cash in lieu
    of fractional  shares of  Acquiror  Common Stock  and unpaid  dividends  and
    distributions  on  shares of  Acquiror Common  Stock deliverable  in respect
    thereof pursuant to this Agreement.
 
    SECTION 2.03.   STOCK  TRANSFER BOOKS.   At  the Effective  Time, the  stock
transfer  books of  the Target  shall be  closed and  there shall  be no further
registration of transfers  of shares of  Target Common Stock  thereafter on  the
records  of  the  Target.  At  or after  the  Effective  Time,  any Certificates
 
                                      A-12
<PAGE>
presented to the Exchange  Agent or Acquiror for  any reason shall be  converted
into  the right  to receive  shares of  Acquiror Common  Stock, cash  in lieu of
fractional  shares  of  Acquiror  Common  Stock  and  any  dividends  or   other
distributions to which they are entitled pursuant to Section 2.02.
 
    SECTION  2.04.  STOCK  OPTIONS.  (a)  Pursuant to the  Dual Drilling Company
1993 Long-Term Incentive  Plan, all outstanding  options issued thereunder  (the
"LONG-TERM   OPTIONS")  shall   be  surrendered   as  of   the  Effective  Time,
automatically and without any action on the part of the holder thereof, and  the
Surviving  Corporation  shall  within  three  (3)  Business  Days  following the
Effective Time, deliver to each holder of  a Long-Term Option a number of  whole
shares  of ENSCO Common Stock, and cash  in lieu of fractional shares thereof as
provided in  Section 2.02(e),  equal  to (A)  the excess,  if  any, of  (i)  the
Exchange  Ratio over  (ii) a  fraction, the numerator  of which  is the exercise
price under such Long-Term Option, and  the denominator of which is the  average
of the closing prices of Acquiror Common Stock on the NYSE for the five Business
Days  immediately preceding the Effective Time,  (B) multiplied by the number of
shares covered by such Long-Term Option.
 
    (b) On or before the Effective Time, the Target will endeavor to enter  into
one  or more agreements  with the holders  of all outstanding  options under the
Dual Drilling Company Non-Employee Director Stock Option Plan (the "NON-EMPLOYEE
OPTIONS" and,  together  with  the Long-Term  Options,  the  "TARGET  OPTIONS"),
pursuant  to which such holders shall surrender all such Non-Employee Options to
the Target no later than two business days prior to the Effective Time. Pursuant
to each such agreement  and as consideration for  such surrender, the  Surviving
Corporation  shall within three  (3) Business Days  following the Effective Time
deliver to the holder  of any Non-Employee  Option a number  of whole shares  of
ENSCO Common Stock, and cash in lieu of fractional shares thereof as provided in
Section  2.02(e), equal to  (A) the excess,  if any, of  (i) the Exchange Ration
over (ii) a fraction, the  numerator of which is  the exercise price under  such
Non-Employee  Option, and the denominator of which average of the closing prices
of Acquiror Common  Stock on  the NYSE for  the five  Business Days  immediately
preceding  the Effective Time, (B) multiplied by the number of shares covered by
such Non-Employee Option.
 
    (c) In performing its obligations pursuant to this Section 2.04, the  Target
shall  fully comply with the  terms and conditions of  the Dual Drilling Company
1993 Long-Term  Incentive  Plan  and  the  Dual  Drilling  Company  Non-Employee
Director Stock Option Plan.
 
                                  ARTICLE III
                  REPRESENTATIONS AND WARRANTIES OF THE TARGET
 
    Except  as set forth in the Disclosure  Schedule delivered by the Target and
signed by the Target and Acquiror for identification prior to the execution  and
delivery  of  this Agreement  (the  "TARGET DISCLOSURE  SCHEDULE"),  which shall
identify exceptions by specific section references, the Target hereby represents
and warrants to Acquiror and Acquiror Sub that:
 
    SECTION 3.01.  ORGANIZATION AND QUALIFICATION; SUBSIDIARIES.  The Target  is
a  corporation,  and  each  subsidiary  of  the  Target  (a  "Subsidiary")  is a
corporation or limited partnership (or in the case of the Sime-Dual, a Malaysian
company), in each  case duly organized,  validly existing and  in good  standing
under  the laws of  the jurisdiction of  its organization and  has the requisite
corporate or  partnership power  and authority  to own,  lease and  operate  its
properties and to carry on its business as it is now being conducted. The Target
and  each Subsidiary are duly qualified or  licensed as a foreign corporation or
limited  partnership  to  do  business,  and  are  in  good  standing,  in  each
jurisdiction  where the character of the properties owned, leased or operated by
them or  the nature  of their  business makes  such qualification  or  licensing
necessary,  except for such failures to be  so qualified or licensed and in good
standing that  would not,  individually or  in the  aggregate, have  a  Material
Adverse  Effect. As used  in this Agreement, the  term "MATERIAL ADVERSE EFFECT"
means with respect to any person, any change or effect that is or is  reasonably
likely  to be materially adverse to the financial condition, business or results
of operations of such person and its  subsidiaries, taken as a whole. As of  the
date  hereof, a  true and  correct list of  all Subsidiaries,  together with the
jurisdiction of organization of each
 
                                      A-13
<PAGE>
Subsidiary and the percentage of the  outstanding capital stock or other  equity
interests  of each Subsidiary owned by the Company and each other Subsidiary, is
set forth in Section 3.01 of the Target Disclosure Schedule. Except as disclosed
in Section 3.01 of the Target Disclosure Schedule, the Target does not  directly
or indirectly own any equity or similar interest in, or any interest convertible
into  or exchangeable or exercisable for any  equity or similar interest in, any
corporation, partnership, joint venture or other business association or entity.
 
    SECTION 3.02.   CERTIFICATE OF  INCORPORATION AND  BYLAWS.   The Target  has
heretofore  furnished or made available to  Acquiror a complete and correct copy
of the  Certificate of  Incorporation and  Bylaws or  equivalent  organizational
documents,  each as amended to date, of  the Target and each Subsidiary. Neither
the Target  nor  any  Subsidiary  is  in  violation  of  any  provision  of  its
Certificate of Incorporation, Bylaws or equivalent organizational documents.
 
    SECTION  3.03.  CAPITALIZATION.  The  authorized capital stock of the Target
consists of 50,000,000 shares  of Target Common Stock  and 10,000,000 shares  of
preferred  stock, $.01 par value ("TARGET  PREFERRED STOCK"). As of December 31,
1995 (a) 15,765,713 shares of Target  Common Stock were issued and  outstanding,
all of which are validly issued, fully paid and nonassessable and not subject to
preemptive  rights, (b) 41,499  shares of Target  Common Stock were  held in the
treasury of the Target or held by the Subsidiaries, and (c) 1,059,000 shares  of
Target  Common Stock  were issuable pursuant  to outstanding  Target Options. No
shares of Target Preferred Stock are issued and outstanding, and, except as  set
forth  in Section 3.01 of  the Target Disclosure Schedule,  no shares of capital
stock of, or other equity interests in,  the Target or any Subsidiary have  been
acquired  by the Target or any Subsidiary since December 31, 1995. Except as set
forth in Section 3.03 of  the Target Disclosure Schedule  and in the Target  SEC
Reports  (as herein  defined), there are  no options, warrants  or other rights,
agreements, arrangements or commitments of any character relating to the  issued
or  unissued capital stock of,  or other equity interests  in, the Target or any
Subsidiary obligating the Target or any  Subsidiary to issue or sell any  shares
of capital stock of, or other equity interests in, the Target or any Subsidiary.
Between  December 31, 1995 and  the date of this  Agreement, no shares of Target
Common Stock have been issued by the Target, except pursuant to the exercise  of
the  stock options described above that were outstanding on December 31, 1995 in
each case in accordance with their respective terms. All shares of Target Common
Stock subject  to  issuance  as  aforesaid,  upon  issuance  on  the  terms  and
conditions  specified in  the instruments pursuant  to which  they are issuable,
will be duly authorized, validly issued, fully paid and nonassessable. There are
no outstanding  contractual  obligations of  the  Target or  any  Subsidiary  to
repurchase, redeem or otherwise acquire any shares of Target Common Stock or any
capital stock of, or any equity interest in, any Subsidiary. Except as described
in  Section 3.03  of the Target  Disclosure Schedule, each  outstanding share of
capital stock  of,  or  other  equity  interest  in,  each  Subsidiary  is  duly
authorized,  validly issued, fully paid and nonassessable and each such share or
interest owned by  the Target or  another Subsidiary  is free and  clear of  all
security  interests, liens, claims,  pledges, options, rights  of first refusal,
agreements, limitations  on  the  Target's or  such  other  Subsidiary's  voting
rights, charges and other encumbrances of any nature whatsoever.
 
    SECTION  3.04.  AUTHORITY  RELATIVE TO THIS  AGREEMENT.  The  Target has all
necessary corporate power and  authority to execute  and deliver this  Agreement
and,  with  respect  to the  Merger,  upon  the approval  and  adoption  of this
Agreement by the  Target's stockholders  in accordance with  this Agreement  and
Delaware  Law,  to  perform  its obligations  hereunder  and  to  consummate the
Transactions. The execution and delivery of this Agreement by the Target and the
consummation by  the Target  of  the Transactions  have  been duly  and  validly
authorized  by all necessary corporate action and no other corporate proceedings
on the  part of  the Target  are necessary  to authorize  this Agreement  or  to
consummate  the  Transactions  (other  than, with  respect  to  the  Merger, the
approval and adoption of this Agreement by the holders of a majority of the then
outstanding shares of Target Common Stock  and the filing and recordation of  an
appropriate  Certificate of  Merger with the  Secretary as  required by Delaware
Law). This Agreement  has been duly  and validly executed  and delivered by  the
Target
 
                                      A-14
<PAGE>
and, assuming the due authorization, execution and delivery of this Agreement by
Acquiror  and Acquiror Sub, constitutes a legal, valid and binding obligation of
the Target, enforceable against the Target in accordance with its terms.
 
    SECTION 3.05.    NO  CONFLICT;  REQUIRED FILINGS  AND  CONSENTS.    (a)  The
execution  and  delivery  of  this  Agreement by  the  Target  do  not,  and the
performance of  this Agreement  by the  Target will  not, subject  to, (x)  with
respect  to the  Merger, obtaining the  requisite approval and  adoption of this
Agreement by the  Target's stockholders  in accordance with  this Agreement  and
Delaware  Law,  and (y)  obtaining the  consents, approvals,  authorizations and
permits and making the filings described in Section 3.05(b) and Section  3.05(b)
of  the Target Disclosure Schedule, (i) conflict with or violate the Certificate
of Incorporation, Bylaws or equivalent organizational documents of the Target or
any Subsidiary, (ii) conflict  with or violate any  domestic (federal, state  or
local)   or   foreign  law,   rule,  regulation,   order,  judgment   or  decree
(collectively, "LAWS") applicable to  the Target or any  Subsidiary or by  which
any  property or asset of the Target or  any Subsidiary is bound or affected, or
(iii) except  as specified  in  Section 3.05(a)(iii)  of the  Target  Disclosure
Schedule,  result in any  breach of or  constitute a default  (or an event which
with notice or lapse of time or both  would become a default) under, or give  to
others   any  right  of  termination,   unilateral  amendment,  acceleration  or
cancellation of, or  give to  others any right  to invalidate  or terminate  any
purchase  or other right to acquire property under, or result in the creation of
a lien  or other  encumbrance on  any property  or asset  of the  Target or  any
Subsidiary  or require  the consent  of any third  party pursuant  to, any note,
bond,  mortgage,  indenture,  contract,   agreement,  lease,  license,   permit,
franchise  or  other  instrument  or  obligation  to  which  the  Target  or any
Subsidiary is a party or by which  the Target or any Subsidiary or any  property
or  asset of the Target or any Subsidiary  is bound or affected, except for such
conflicts, violations,  breaches, defaults,  rights,  liens and  consents  which
individually  or in  the aggregate  would not reasonably  be expected  to have a
Material Adverse Effect on Target.
 
    (b)  Except  for  notification  relating  to  environmental  agencies,   the
execution  and  delivery  of  this  Agreement by  the  Target  do  not,  and the
performance of  this Agreement  by the  Target will  not, require  any  consent,
approval,  authorization or  permit of, or  filing with or  notification to, any
governmental or regulatory authority, domestic, foreign or supranational, except
(i) pursuant to the Securities Exchange  Act of 1934, as amended (the  "EXCHANGE
ACT"),  the Securities  Act of  1933, as  amended (the  "SECURITIES ACT"), state
securities or "blue sky" laws ("BLUE SKY LAWS"), the Hart-Scott-Rodino Antitrust
Improvements Act of 1976, as amended (the "HSR ACT"), the United States Maritime
Administration ("MARAD") and the  rules and regulations promulgated  thereunder,
and  filing and  recordation of  an appropriate  Certificate of  Merger with the
Secretary as required by Delaware Law,  (ii) as specified in Section 3.05(b)  of
the  Target Disclosure Schedule and (iii) where failure to obtain such consents,
approvals, authorizations or permits, or to make such filings or  notifications,
would  not prevent or delay consummation of the Merger, or otherwise prevent the
Target from timely performing its obligations under this Agreement.
 
    SECTION 3.06.  PERMITS; COMPLIANCE.  Except as disclosed in Section 3.06  of
the  Target Disclosure Schedule, each  of the Target and  the Subsidiaries is in
possession  of  all  franchises,  grants,  authorizations,  licenses,   permits,
easements,  variances, exceptions, consents,  certificates, approvals and orders
of any  United  States (federal,  state  or  local) or  foreign  government,  or
governmental,  regulatory or  administrative authority, agency  or commission or
court of  competent jurisdiction  ("GOVERNMENTAL AUTHORITY")  necessary for  the
Target or any Subsidiary to own, lease and operate its properties or to carry on
its business as it is now being conducted, except for those which the failure to
possess  would not  individually or in  the aggregate reasonably  be expected to
have a Material Adverse Effect on Target  (the "TARGET PERMITS") and, as of  the
date  hereof, no  suspension or  cancellation of  any of  the Target  Permits is
pending or, to the knowledge of  the Target, threatened. Except as disclosed  in
Section  3.06  of the  Target Disclosure  Schedule, neither  the Target  nor any
Subsidiary is in  conflict with, or  in default  or violation of,  or, with  the
giving    of    notice    or    the   passage    of    time,    would    be   in
 
                                      A-15
<PAGE>
conflict with, or  in default or  violation of,  (i) any Law  applicable to  the
Target  or any Subsidiary or by which any property or asset of the Target or any
Subsidiary is bound or affected, or (ii) any of the Target Permits.
 
    SECTION 3.07.  SEC FILINGS; FINANCIAL STATEMENTS.  (a) The Target has  filed
all  forms, reports and documents required to be filed by it with the Securities
and Exchange Commission (collectively, the "TARGET SEC REPORTS"). The Target SEC
Reports (i)  were prepared  in  all material  respects  in accordance  with  the
requirements of the Securities Act and the Exchange Act, as the case may be, and
the  rules and regulations  thereunder and (ii)  did not, at  the time they were
filed (or at the effective date thereof in the case of registration statements),
contain any untrue statement of a material fact or omit to state a material fact
required to be stated therein or necessary in order to make the statements  made
therein,  in the  light of  the circumstances  under which  they were  made, not
misleading. No Subsidiary  is currently  required to  file any  form, report  or
other document with the Securities and Exchange Commission ("SEC").
 
    (b)  Each of  the financial statements  (including, in each  case, any notes
thereto) contained in  the Target SEC  Reports was prepared  in accordance  with
United  States generally accepted accounting  principles applied on a consistent
basis ("U.S. GAAP") throughout the periods indicated (except as may be indicated
in the  notes  thereto  and  except  that  financial  statements  included  with
quarterly  reports  on Form  10-Q do  not contain  all U.S.  GAAP notes  to such
financial statements) and  each fairly  presented in all  material respects  the
financial  position, results of  operations and changes  in stockholders' equity
and cash flows  of the Target  as at the  respective dates thereof  and for  the
respective  periods  indicated  therein  (subject,  in  the  case  of  unaudited
statements, to normal and recurring year-end adjustments which were not and  are
not  expected,  individually or  in the  aggregate, to  have a  Material Adverse
Effect).
 
    (c) Except (i) to the  extent set forth on the  balance sheet of the  Target
and  the consolidated Subsidiaries as at  December 31, 1995, including the notes
thereto (the "1995 BALANCE SHEET"), (ii) as set forth in Section 3.07(c) of  the
Target  Disclosure Schedule or (iii) as disclosed in any SEC Report filed by the
Target after December 31,  1995, neither the Target  nor any Subsidiary has  any
liability  or obligation of any nature (whether accrued, absolute, contingent or
otherwise) which would be required to be reflected on a balance sheet, or in the
notes thereto, prepared in accordance with U.S. GAAP, except for liabilities and
obligations incurred in  the ordinary  course of business  consistent with  past
practice  since  December 31,  1995,  which would  not,  individually or  in the
aggregate, reasonably be expected to have a Material Adverse Effect on Target.
 
    (d) The Target  has heretofore  furnished to Acquiror  complete and  correct
copies  of all amendments and modifications (if any) that have not been filed by
the Target with the SEC to all agreements, documents and other instruments  that
previously  had been filed by  the Target as exhibits  to the Target SEC Reports
and are currently in effect.
 
    SECTION 3.08.   ABSENCE OF CERTAIN  CHANGES OR EVENTS.   Since December  31,
1995,  except  as  contemplated by,  or  disclosed pursuant  to,  this Agreement
including Section 3.08 of  the Target Disclosure Schedule,  or disclosed in  any
Target  SEC Report filed since  December 31, 1995 and prior  to the date of this
Agreement, each of the  Target and the Subsidiaries  has conducted its  business
only  in the ordinary course and in  a manner consistent with past practice and,
since December 31, 1995, there has not been (a) any amendment or other change to
the Certificate of  Incorporation or Bylaws  or other equivalent  organizational
documents  of  the Target  or any  Subsidiary, (b)  any issuance,  sale, pledge,
disposal, grant, encumbrance,  or authorization of  the issuance, sale,  pledge,
disposition,  grant or encumbrance  by the Target  or any Subsidiary  of (i) any
shares  of  their  capital  stock  of  any  class,  or  any  options,  warrants,
convertible securities or other rights of any kind to acquire any shares of such
capital  stock, or any other  ownership interest (including, without limitation,
any phantom interest), of the Target or any Subsidiary (except for the  issuance
of  shares of capital  stock issuable pursuant to  Target Options outstanding on
January 25, 1996), or (ii) any of their assets other than in the ordinary course
of business consistent with past  practice, (c) any declaration, setting  aside,
making or payment
 
                                      A-16
<PAGE>
of  any  dividend or  other distribution,  payable in  cash, stock,  property or
otherwise, with respect  to any  of their  capital stock  by the  Target or  any
Subsidiary,  (d)  any reclassification,  combination, split  or division  by the
Target or any Subsidiary of any  of their capital stock or redemption,  purchase
or  other acquisition, directly or indirectly, of  any of their capital stock or
securities or obligations  convertible into or  exchangeable or exercisable  for
such  capital  stock, (e)  any commitment  or  incurrence by  the Target  or any
Subsidiary  of  any  capital  expenditure   in  excess  of  $500,000,  (f)   any
mobilization  of, or any agreement entered into  by the Target or any Subsidiary
which would provide for the mobilization of, any drilling rig to any area of the
world other than such area  in which such drilling  rig was located on  December
31, 1995, (g) any drilling contract entered into by the Target or any Subsidiary
with  a rate fixed for a  period in excess of six  months, (h) any incurrence of
any indebtedness  for borrowed  money  or issuance  of  any debt  securities  or
assumption,  guarantee or endorsement,  or otherwise becoming  responsible as an
accommodation, for the  obligations of  any person, or  making of  any loans  or
advances,  (i) acquisition by  the Target or  any Subsidiary (including, without
limitation, by merger, consolidation or acquisition  of stock or assets) of  any
interest  in any  corporation, partnership,  other business  organization or any
division thereof or  any assets,  other than the  acquisition of  assets in  the
ordinary  course of business consistent with  past practice, (j) any contract or
agreement (other than those covered by  subparagraph (g) above) entered into  or
amended by the Target or any Subsidiary material to their businesses, results of
operations  or financial condition, (k) any increase in the compensation payable
or to become payable to any  director, officer or other employee, or  consultant
or  advisor, of the Target or any Subsidiary, or grant of any bonus to, or grant
of any severance or termination pay to, or any employment or severance agreement
entered into with,  any director, officer  or other employee,  or consultant  or
advisor,  of the Target or any Subsidiary or any collective bargaining agreement
entered into or amended,  (l) any bonus,  profit sharing, thrift,  compensation,
stock  option, restricted  stock, pension, retirement,  deferred compensation or
other plan, trust or fund established, adopted, entered into or amended for  the
benefit  of any  director, officer or  class of  employees of the  Target or any
Subsidiary, (m) any settlement or compromise by the Target or any Subsidiary  of
any  pending or threatened litigation which would reasonably be expected to have
a Material Adverse Effect  on Target or which  relates to the Transactions,  (n)
any  event or events (whether  or not covered by  insurance), individually or in
the aggregate, having a Material Adverse Effect, or (o) any change by the Target
or any Subsidiary in its accounting methods, principles or practices.
 
    SECTION 3.09.  ABSENCE OF LITIGATION.  Section 3.09 of the Target Disclosure
Schedule  sets  forth  each  instance  in  which  any  of  the  Target  and  the
Subsidiaries  (i)  is subject  to any  outstanding injunction,  judgment, order,
decree, ruling, or charge or (ii) is a party or, to the knowledge of the  Target
and  the Subsidiaries,  is threatened to  be made  a party to  any action, suit,
proceeding,  hearing,  or  investigation  of,   in,  or  before  any  court   or
quasi-judicial or administrative agency of any federal, state, local, or foreign
jurisdiction or before any arbitrator, which has not otherwise been disclosed in
the Target SEC Reports and which would reasonably be expected to have a Material
Adverse Effect on Target. Neither the Target nor any Subsidiary nor any property
or  asset of the  Target or any Subsidiary  is in violation  of any order, writ,
judgment, injunction, decree, determination or award.
 
    SECTION 3.10.  EMPLOYEE BENEFIT MATTERS.   (a) Set forth in Section  3.10(a)
of  the Target Disclosure Schedule  is a true, complete  and correct list of (i)
all "employee  benefit  plans"  as  defined in  Section  3(3)  of  the  Employee
Retirement  Income Security  Act of 1974,  as amended ("ERISA"),  whether or not
subject to ERISA,  and any  other employee profit-sharing,  bonus, incentive  or
deferred  compensation,  welfare, pension,  retirement,  termination, retention,
change of control,  stock option,  stock appreciation,  stock purchase,  phantom
stock  or  other equity-based,  performance, group  insurance or  other employee
benefit plan,  retiree  benefit  or  compensation  plan,  program,  arrangement,
agreement, policy, practice or understanding, whether written or unwritten, that
provides or may provide benefits or compensation with respect to any employee or
former   employee  employed  or   formerly  employed  by   the  Target,  or  the
beneficiaries or dependents of any such  employee or former employee, or to  any
director,  officer, stockholder or  consultant of the Target  or under which any
such individual is  or may become  eligible to participate  or derive a  benefit
(excluding social security,
 
                                      A-17
<PAGE>
Medicare,  Medicaid, national health care or any similar or analogous program or
plan sponsored by a foreign or  domestic governmental entity) and either (A)  is
or  has  been maintained  or established  by the  Target or  any other  trade or
business, whether or  not incorporated, which,  together with the  Target is  or
would  have been at any date of determination occurring within the preceding six
years treated as  a single employer  under Section  414 of the  Code or  Section
4001(b)(1) of ERISA (such other trades and businesses collectively, the "Related
Persons"), or (B) to which the Target or any Related Person contributes or is or
has been obligated or required to contribute or, to the Target's knowledge, with
respect  to which  the Target or  any Related  Person may have  any liability or
obligation (collectively, the "PLANS") which are not disclosed in the Target SEC
Reports and (ii) all written contracts and agreements relating to employment and
all severance agreements with any of the directors, officers or employees of the
Target or  the Subsidiaries  (other than,  in each  case, any  such contract  or
agreement  that is terminable  by the Target  or any Subsidiary  at will without
penalty or other adverse consequence) (the "TARGET EMPLOYMENT CONTRACTS")  which
are  not disclosed  in the Target  SEC Reports.  Except as set  forth in Section
3.10(a) of the Target Disclosure Schedule and the Target SEC Reports and  except
for  any liabilities arising out of any defects  that are or will be the subject
of the  procedures or  submissions  described in  Section 6.13(a),  6.13(m)  and
6.13(n),  to the knowledge of  the Target and the  Related Persons, there are no
material liabilities,  including  fines  and  penalties of  the  Target  or  any
Subsidiary,  with respect  to any plans,  arrangements or practices  of the type
described in the preceding sentence  that were terminated or discontinued  prior
to the date of this Agreement and previously maintained or contributed to by the
Target  or any  Related Person,  or to  which the  Target or  any Related Person
previously had an obligation to contribute.
 
    (b) Section 3.10(b) of the Target Disclosure Schedule sets forth the name of
each officer or employee of the Target or any of the Subsidiaries with a current
annual base compensation greater than $100,000 and the annual base  compensation
applicable to each such officer or employee.
 
    (c)  The Target  previously has delivered  to Acquiror  complete and correct
copies of  each of  the Plans  and Target  Employment Contracts,  including  all
amendments  thereto, and any  other documents or  other instruments applying and
relating  thereto  that   are  reasonably  requested   by  Acquiror,   including
descriptions  of all unwritten Plans;  all trust agreements, insurance contracts
or other  funding  arrangements;  the  three most  recent  actuarial  and  trust
reports; the three most recently filed Forms 5500 and all schedules thereto; the
most  recent determination letter from the Internal Revenue Service ("IRS"); any
documents submitted to the IRS concerning a pending request for a  determination
letter;  current summary plan descriptions; all material communications received
from or sent to  the IRS, the Pension  Benefit Guaranty Corporation ("PBGC")  or
the  Department of  Labor during the  three-year period  preceding the Effective
Time (including  a written  description of  any oral  communication) that  could
reasonably be expected to result in material liability or obligation on the part
of  the Target or any Subsidiary or  disqualification of any Plan intended to be
qualified under Section 401(a) of the Code; and all amendments and modifications
to any such document.
 
    (d) Except  as  set  forth  in Section  3.10(d)  of  the  Target  Disclosure
Schedule,  (i) each of the  Plans and Target Employment  Contracts is being, and
has been,  maintained, operated  and administered  in all  material respects  in
accordance  with  its  respective  terms,  (ii) each  of  the  Plans  and Target
Employment Contracts  has  been maintained,  operated  and administered  in  all
material  respects in  compliance with  all applicable  laws, including  but not
limited to the Age Discrimination in Employment Act, as amended, Title X of  the
Consolidated  Omnibus Budget Reconciliation  Act of 1986,  as amended, ERISA and
the Code, and (iii) no material  liability or obligation has been incurred  (and
is  unsatisfied) or is expected  to be incurred by  the Target or any Subsidiary
(either directly  or indirectly,  including as  a result  of an  indemnification
obligation,  but excluding the penalties that are or will be payable pursuant to
Section 6.13(a), (m) and (n)) under or pursuant to any applicable law, including
Titles I  and IV  of ERISA  and the  penalty, excise  tax or  joint and  several
liability provisions of the Code relating to employee benefit plans.
 
    (e)  The Target and the  Related Persons have not  within the past six years
had an obligation to contribute to a qualified "defined benefit plan" as defined
in Section 3(35) of ERISA and covered by
 
                                      A-18
<PAGE>
Part 3  of Title  I of  ERISA, a  pension plan  subject to  the minimum  funding
standards   of  Section  302  of  ERISA  or  Section  412  of  the  Code,  or  a
"multiemployer plan" as  defined in  Section 3(37) of  and covered  by ERISA  or
Section  414(f) of the Code or a  "multiple employer plan" within the meaning of
Section 210(a) of and covered by ERISA or Section 413(c) of the Code. Except  as
set  forth in Section 3.10(e)  of Target Disclosure Schedule,  no other trade or
business is,  or, at  any time  within the  past six  years, has  been  treated,
together  with the Target  and the Related  Persons, as a  single employer under
Section 414 of the Code or Section 4001 of ERISA.
 
    (f) Except with respect to those defects which are or will be the subject of
the procedures and submissions described in Section 6.13(a), each Plan  intended
to be qualified under Section 401(a) of the Code, and the trust (if any) forming
a part thereof, has received a favorable determination letter from the IRS as to
its  qualification under  the Code  and to  the effect  that each  such trust is
exempt from taxation  under Section  501(a) of the  Code or  an application  for
determination  under Section 401(a)  of the Code  has been submitted  to the IRS
prior to  the  expiration  of  the applicable  remedial  amendment  period,  all
amendments  necessary to maintain qualification of each such Plan have been made
within the time  allowed by  the Code  and ERISA and,  to the  knowledge of  the
Target  and all Related Persons, no event  has occurred or condition exists that
could  adversely   affect  such   determination  or   pending  application   for
determination  of qualification  or tax-exempt status,  including any compliance
problems resulting from failures to follow  the terms of the plan documents  for
any  Plan, and  no such  determination has been  revoked and  no application for
determination has been denied, nor has any  Plan been amended since the date  of
its most recent determination letter or application therefor in any respect that
would adversely affect its qualification or materially increase its costs.
 
    (g)  Except  as  set  forth  in Section  3.10(g)  of  the  Target Disclosure
Schedule, to the  knowledge of the  Target and all  Related Persons, there  have
been  no prohibited transactions as  defined in Section 406  of ERISA or Section
4975 of the  Code or  breaches of  any of  the duties  imposed on  "fiduciaries"
(within  the meaning  of Section 3(21)  of ERISA)  by ERISA with  respect to the
Plans that could result in the Target or any Subsidiary becoming liable directly
or indirectly (by indemnification or  otherwise) for any material liability  for
any excise tax, penalty or other liability under ERISA or the Code.
 
    (h) Except as set forth in Section 3.10(h) of the Target Disclosure Schedule
and  except with respect to  liabilities arising out of  any defects that are or
will be  the subject  of  the procedures  or  submissions described  in  Section
6.13(a),  6.13(m)  and 6.13(n),  there are  no  actions, suits,  arbitrations or
claims (other than routine claims for benefits), pending or, to the knowledge of
the Target or any Related Person, threatened, with respect to any Plan or Target
Employment Contract, any trust which is a part of any Plan or Target  Employment
Contract,  any  trustee,  fiduciary, custodian,  administrator  or  other person
holding or controlling assets of any Plan or Target Employment Contract, and, to
the knowledge of the Target and all Related Persons, no basis to anticipate  any
such  action, suit, arbitration  or claim exists (other  than routine claims for
benefits), and  there are  no investigations  or audits  of any  Plan or  Target
Employment  Contract by any  governmental authority currently  pending and there
have been  no  such investigations  or  audits  that have  been  concluded  that
resulted  in any liability of the Target or any Related Person that has not been
fully discharged. Except with respect to the procedure and submission  described
in  Section 6.13(a) hereof, no  closing agreement with the  IRS is being, or has
been, negotiated with respect to any Plan, and no Plan has been submitted to the
IRS pursuant  to the  Voluntary Compliance  Resolution Program  as described  in
Revenue Procedures 92-89 and 93-36.
 
    (i)  Except as set forth in Section  3.10(i), there are no unpaid or overdue
(i) insurance  premiums required  to be  paid with  respect to,  (ii)  benefits,
expenses,  and other  amounts due  and payable  under, and  (iii) contributions,
transfers or payments  required to  be made to,  any Plan  or Target  Employment
Contract  have  been made  on or  before their  due dates.  With respect  to any
insurance policy  providing  funding  for  benefits under  any  Plan  or  Target
Employment  Contract (i)  there is  no material liability  of the  Target or any
Related Person in the nature of a retroactive or retrospective rate  adjustment,
loss  sharing arrangement,  or other actual  or contingent  liability, nor would
there be any such liability if
 
                                      A-19
<PAGE>
such insurance policy was terminated on the  date hereof except as set forth  in
Section  3.10(i) of the Target Disclosure Schedule, and (ii) to the knowledge of
the Target and all Related Persons, no insurance company issuing any such policy
is in receivership,  conservatorship, liquidation or  similar proceeding and  no
such proceedings with respect to any insurer are imminent.
 
    (j)   With respect to any Plan that is an "employee welfare benefit plan" as
defined in Section 3(1) of ERISA, except as disclosed in Section 3.10(j) of  the
Target  Disclosure Schedule, (i)  no such Plan  is unfunded or  funded through a
"welfare benefit fund", as such term is  defined in Section 419(e) of the  Code,
(ii) to the knowledge of the Target and all Related Persons, each such Plan that
is  a "group health plan", as such term  is defined in Section 5000(b)(1) of the
Code, is in compliance in all material respects with the applicable requirements
of Section 4980B(f) of the  Code, and (iii) to the  knowledge of the Target  and
all  Related Persons, each such Plan  (including any such Plan covering retirees
or former employees) may be amended  or terminated subject to the provisions  of
any  applicable collective  bargaining agreement, without  material liability to
the Target and the Subsidiaries.
 
    (k) Section 3.10(k) of  the Target Disclosure  Schedule contains a  separate
identification  of each Plan and Target Employment Contract other than those set
forth in  Section  3.10(l)  of  the Target  Disclosure  Schedule  that  provides
benefits,  including,  without  limitation, death  or  medical  benefits, beyond
termination of employment or retirement other than (i) coverage mandated by law,
(ii) death  or retirement  benefits  under any  qualified Plan,  (iii)  deferred
compensation  benefits  fully  reflected  on  the  1995  Balance  Sheet  or (iv)
benefits, the full cost  of which is  borne by the  employee (or the  employee's
beneficiary)  (the "POST-EMPLOYMENT  BENEFITS"); such  balance sheets accurately
reflect  the  liabilities  relating  to  the  Post-Employment  Benefits  and  an
actuarial study of the Post-Employment Benefits has been delivered to Acquiror.
 
    (l)  Except  as  set  forth  in Section  3.10(l)  of  the  Target Disclosure
Schedule, the execution, delivery  and performance of  this Agreement will  not,
solely  in  and of  itself  and without  regard  to any  subsequent  events, (i)
constitute an  event under  any Plan  or Target  Employment Contract  that  will
result  in any payment (whether of severance pay or otherwise) becoming due from
the Target or  any Related Person  to any present  or former officer,  employee,
director, stockholder or consultant (or dependents), (ii) accelerate the time of
payment or vesting, or increase the amount of compensation due to any present or
former  officer, employee, director, stockholder or  consultant of the Target or
any Related  Person,  or  (iii)  constitute  a  "deemed  severance"  or  "deemed
termination"  under  any  Plan  or  Target  Employment  Contract  or  under  any
applicable law.
 
    (m) Neither  the  Target  nor  any Related  Person  has  any  obligation  in
connection  with  any Plan  pursuant  to the  terms  of a  collective bargaining
agreement.
 
    (n) The  Target  and  all  Related  Persons  have  made  or  will  make  all
contributions required under GAAP to be made by the Target or any Related Person
under  each  Plan and  Target Employment  Contract for  all periods  through and
including the Effective Time or adequate accruals therefor have been or will  be
provided.
 
    (o) Except as otherwise provided in Section 3.10(o) of the Target Disclosure
Schedule  and except with respect to the submissions and filings contemplated by
Section 6.13(a), 6.13(m) and  6.13(n) hereof, all  returns, reports and  filings
required  by any governmental  agency or which  must be furnished  to any person
with respect to  each of  the Plans and  Target Employment  Contracts have  been
timely  filed or furnished.  The Target and all  Subsidiaries shall cooperate in
full with Acquiror  with any  reasonably necessary action  to ensure  compliance
with  any federal  or state  law applicable to  the Plans  and Target Employment
Contracts, whether such action occurs prior to, on, or after the Effective Time.
 
    (p) Except as set forth in Section 3.10(p) of the Target Disclosure Schedule
and except as provided  in Section 6.13(a), the  Target and the Related  Persons
have  not agreed or committed  (orally or in writing)  to make any amendments to
any Plan or Target Employment Contract not already
 
                                      A-20
<PAGE>
embodied  in the documents comprising the Plans and Target Employment Contracts,
other than any  amendments required  by law, or  to establish  or implement  any
other employee or retiree benefit or compensation arrangement.
 
    (q)  To the knowledge of Target and  all Related Persons, the Target and the
Subsidiaries have not  incurred any liability  under, and have  complied in  all
respects  with, the  Worker Adjustment Retraining  Notification Act  and, to the
knowledge of Target, no fact or event  exists that could give rise to  liability
under  such act, except for such  occurrences, noncompliances and liabilities as
would not, individually or in the aggregate, have a Material Adverse Effect.
 
    SECTION 3.11.  LABOR MATTERS.   Neither the Target  nor any Subsidiary is  a
party  to  any collective  bargaining agreement  or  other labor  union contract
applicable to persons employed by the Target or any Subsidiary.
 
    SECTION 3.12.  INTELLECTUAL  PROPERTY.  Target and  the Subsidiaries own  or
have  valid,  binding  and enforceable  rights  to use  each  patent, invention,
industrial model, process, design and all registrations and applications for any
of the foregoing used, employed  or exploited in the  business of the Target  or
any Subsidiary without any known conflict with the rights of others.
 
    SECTION  3.13.  TAXES.  (a) The Target and each of the Subsidiaries have (i)
filed all federal, state, local and foreign tax returns required to be filed  by
them  prior to the date  of this Agreement (taking  into account extensions) and
all of such returns were  true and correct in  all material respects when  filed
and  in compliance with applicable law, (ii)  paid or accrued all taxes shown to
be due on such returns and paid all applicable ad valorem and value added  taxes
as  are due and (iii) paid or accrued all taxes for which a notice of assessment
or collection has  been received  (other than  amounts being  contested in  good
faith  by appropriate proceedings),  except in the  case of clause  (i), (ii) or
(iii) for any such filings, payments  or accruals which would not,  individually
or  in the  aggregate, have a  Material Adverse  Effect. Except as  set forth in
Section 3.13(a) of the Target Disclosure Schedule, neither the Internal  Revenue
Service  nor any  other federal,  state, local  or foreign  taxing authority has
asserted any  claim for  taxes,  or to  the best  knowledge  of the  Target,  is
threatening to assert any claims for taxes, which claims, individually or in the
aggregate,  could have a Material Adverse Effect.  The Target has open years for
federal, state  and foreign  income tax  returns only  as set  forth in  Section
3.13(a)  of the Target Disclosure Schedule.  The Target and each Subsidiary have
withheld or collected and paid over to the appropriate governmental  authorities
(or  are properly  holding for  such payment)  all taxes  required by  law to be
withheld or collected, except  for amounts which would  not, individually or  in
the  aggregate,  have a  Material  Adverse Effect.  Neither  the Target  nor any
Subsidiary has made an election under Section  341(f) of the Code. There are  no
liens  for taxes  upon the assets  of the  Target or any  Subsidiary (other than
liens for taxes that are not yet due  or that are being contested in good  faith
by  appropriate proceedings), except for liens  which would not, individually or
in the aggregate,  have a Material  Adverse Effect. Target  and each  Subsidiary
have  complied  with all  federal,  state, local  and  foreign tax  laws  in all
material respects.  Except  as  disclosed  in  Section  3.13(a)  of  the  Target
Disclosure  Schedule, neither the Target nor any Subsidiary (i) has executed any
waiver to extend the time for assessment of any federal, state, local or foreign
tax, or  (ii) filed,  or has  pending, any  request or  application for  ruling,
whether federal, state, local or foreign.
 
    (b)  Neither the Target nor  any Subsidiary has taken  or agreed to take any
action  that  would  prevent  the  Merger  from  constituting  a  reorganization
qualifying under the provisions of Section 368(a) of the Code.
 
    (c)  The 1995 Balance  Sheet includes appropriate  reserves for all federal,
state, local and foreign  taxes and other liabilities  incurred as of such  date
but not yet payable.
 
    (d)  The net operating  losses and other carryovers  available to the Target
and each Subsidiary as of the date  hereof are described in Section 3.13 of  the
Target Disclosure Schedule and as of the date
 
                                      A-21
<PAGE>
hereof the ability of Target and each Subsidiary to use such carryovers will not
have  been affected by  Section 382, 383 or  384 of the Code  or by the separate
return limitation year or consolidated return change of ownership limitations of
Treas. Regs. Section 1.1502-21 or 1.1502.22.
 
    (e) Neither  Target or  any Subsidiary  is  a U.  S. real  property  holding
company under Section 897 of the Code.
 
    SECTION  3.14.  ENVIRONMENTAL MATTERS.   (a) For purposes of this Agreement,
the  following  terms  shall  have   the  following  meanings:  (i)   "HAZARDOUS
SUBSTANCES"  means  (A)  those  substances defined  in  or  regulated  under the
following federal statutes and their state counterparts, as each may be  amended
from  time  to time,  and all  regulations  thereunder: the  Hazardous Materials
Transportation  Act,   the  Resource   Conservation   and  Recovery   Act,   the
Comprehensive  Environmental Response, Compensation and Liability Act, the Clean
Water Act,  the Safe  Drinking Water  Act, the  Atomic Energy  Act, the  Federal
Insecticide,  Fungicide, and Rodenticide  Act, the Toxic  Substances Control Act
and the Clean  Air Act;  (B) petroleum  and petroleum  products, byproducts  and
breakdown  products including crude  oil and any  fractions thereof; (C) natural
gas, synthetic gas, and any mixtures thereof; (D) polychlorinated biphenyls; (E)
any other chemicals, materials  or substances defined or  regulated as toxic  or
hazardous  or as a pollutant  or contaminant or as  a waste under any applicable
Environmental Law; and (F) any substance with respect to which a federal,  state
or  local agency requires environmental  investigation, monitoring, reporting or
remediation; and (ii) "ENVIRONMENTAL LAWS" means any federal, state, foreign, or
local law, rule or regulation,  now or hereafter in  effect and as amended,  and
any judicial or administrative interpretation thereof, including any judicial or
administrative  order,  consent decree  or  judgment, relating  to  pollution or
protection of the  environment, health, safety  or natural resources,  including
without  limitation, those  relating to (A)  releases or  threatened releases of
Hazardous Substances or  materials containing  Hazardous Substances  or (B)  the
manufacture,  handling,  transport,  use,  treatment,  storage  or  disposal  of
Hazardous Substances or materials containing Hazardous Substances.
 
    (b) Except as described in Section 3.14 of the Target Disclosure Schedule or
as would not individually or in the  aggregate result in or be likely to  result
in any fine, tax, assessment, penalty, loss, cost, damage, liability, expense or
other  payment related thereto  in excess of  $250,000: (i) the  Target and each
Subsidiary are and  have been  in compliance with  all applicable  Environmental
Laws;  (ii) the Target and each Subsidiary have obtained all permits, approvals,
identification numbers,  licenses or  other  authorizations required  under  any
applicable Environmental Laws ("ENVIRONMENTAL PERMITS") and are and have been in
compliance  with  their  requirements;  (iii)  such  Environmental  Permits  are
transferable to the  Surviving Corporation  pursuant to the  Merger without  the
consent  of  any  Governmental  Authority;  (iv)  there  are  no  underground or
aboveground storage tanks or any surface impoundments, septic tanks, pits, sumps
or lagoons in which Hazardous Substances are being or have been treated,  stored
or  disposed of  on any owned  or leased real  property or on  any real property
formerly owned, leased or  occupied by the Target  or any Subsidiary; (v)  there
is,  to the  best knowledge  of the  Target, no  asbestos or asbestos-containing
material on  any  owned or  leased  real  property in  violation  of  applicable
Environmental  Laws; (vi)  the Target  and the  Subsidiaries have  not released,
discharged or  disposed of  Hazardous Substances  on any  owned or  leased  real
property  or on  any real  property formerly  owned, leased  or occupied  by the
Target or any  Subsidiary in an  amount requiring remediation  and none of  such
property  is contaminated with  any Hazardous Substances  in an amount requiring
remediation; (vii) other than routine operational matters neither the Target nor
any of the Subsidiaries is  undertaking, and neither the  Target nor any of  the
Subsidiaries  has  completed, any  investigation  or assessment  or  remedial or
response action  relating to  any  such release,  discharge  or disposal  of  or
contamination  with  Hazardous Substances  at any  site, location  or operation,
either voluntarily or pursuant to the order of any Governmental Authority or the
requirements of any Environmental  Law; (viii) there are  no pending or, to  the
knowledge of Target, past or threatened actions, suits, demands, demand letters,
claims,  liens, notices of non-compliance or  violation, notices of liability or
potential liability,  investigations,  proceedings, consent  orders  or  consent
 
                                      A-22
<PAGE>
agreements  relating in any way to Environmental Laws, any Environmental Permits
or any Hazardous Substances ("ENVIRONMENTAL  CLAIMS") against the Target or  any
Subsidiary  or any of  their property, and  there are no  circumstances that can
reasonably be  expected to  form  the basis  of  any such  Environmental  Claim,
including  without  limitation with  respect to  any off-site  disposal location
presently or formerly used  by the Target  or any Subsidiaries  or any of  their
predecessors;  and (ix)  the Target and  each Subsidiary have  satisfied and are
currently  in  compliance   with  all   financial  responsibility   requirements
applicable  to their operations and imposed by  the U.S. Coast Guard or Minerals
Management Service pursuant to the Oil Pollution Act of 1990, as amended, or  by
any  other governmental  authority under  any other  Environmental Law,  and the
Target and the Subsidiaries have not  received any notice of noncompliance  with
any such financial responsibility requirements.
 
    (c)  The Target and the Subsidiaries  have provided Acquiror or Acquiror Sub
with copies of any environmental reports, studies or analyses in its  possession
or under its control relating to owned or leased real property or the operations
of the Target or the Subsidiaries.
 
    SECTION  3.15.  OPINION OF  FINANCIAL ADVISOR.  The  Target has received the
written opinion of Simmons & Company International ("TARGET BANKER") on the date
of this Agreement to the effect that the consideration to be paid by Acquiror in
the Merger is fair from a financial  point of view to the Target's  stockholders
as  of the date  thereof. A copy  of the Target  Banker engagement letter, dated
October 17, 1995, has previously been delivered to Acquiror.
 
    SECTION 3.16.   VOTE REQUIRED.   The affirmative  vote of the  holders of  a
majority  of the then outstanding shares of Target Common Stock is the only vote
of the holders of any class or  series of capital stock of the Target  necessary
to approve the Merger.
 
    SECTION  3.17.  BROKERS.  No broker, finder or investment banker (other than
Target Banker) is entitled to any brokerage, finder's or other fee or commission
in connection with the Transactions based upon arrangements made by or on behalf
of the Target or any Subsidiary. The Target has heretofore furnished to Acquiror
a correct copy of all agreements  between the Target and Target Banker  pursuant
to   which  such  firm  would  be  entitled  to  any  payment  relating  to  the
Transactions.  The  total  fee  due  to  Target  Banker  as  a  result  of   the
Transactions, including expenses, shall not exceed $3,000,000.
 
    SECTION 3.18.  TANGIBLE PROPERTY.  The Target and its Subsidiaries have good
and  marketable title to, or  a valid leasehold interest  in, the properties and
assets used by them  or shown on  the 1995 Balance Sheet  or acquired after  the
date  thereof, other than all drilling  rigs owned, leased, chartered or managed
by the Target or any Subsidiary on the date hereof (the "TARGET DRILLING RIGS"),
are free and clear of any mortgage, pledge, lien, encumbrance, charge, or  other
security  interest, other than (a) mechanic's, materialmen's, and similar liens,
(b) liens for taxes not  yet due and payable or  for taxes that the taxpayer  is
contesting  in good  faith through  appropriate proceedings,  (c) purchase money
liens and liens securing rental  payments under capital lease arrangements,  and
(d)  liens and encumbrances identified and  reflected on the 1995 Balance Sheet,
except for properties and assets disposed of in the ordinary course of business.
 
    SECTION 3.19.  BAREBOAT  CHARTER.  Target has  exercised in accordance  with
the  terms of that certain Bareboat Charter  dated March 15, 1991 between Balboa
Marine Limited Partnership and Dual Offshore,  Ltd., as amended, its option  for
the  First Option  Term (as  defined therein)  and the  First Option  Term shall
expire on September 4, 1996. Neither Mosvold Shipping AS nor Dual Invest AS  nor
any  of either of their affiliates (other  than Target and the Subsidiaries) has
any right, title or interest (including profits interest) in or with respect  to
such bareboat charter agreement.
 
    SECTION  3.20.  MATERIAL  CONTRACTS.  Section 3.20  of the Target Disclosure
Schedule lists each contract which (i) is required by its terms or is  currently
expected  to result in the payment or receipt by the Target or any Subsidiary of
more than $500,000 and which is not terminable by the Target without the payment
of any penalty or fine on not  more than three months' notice, or (ii)  contains
any  terms or  provisions which restrict,  or would restrict  if terminated, the
ability of the Target or any
 
                                      A-23
<PAGE>
Subsidiary from  the  operation,  charter,  leasing  or  operation  of  offshore
drilling  rigs in any geographic area of  the world without the consent or joint
participation of,  or  payment  of any  kind  to,  a third  party  (a  "MATERIAL
CONTRACT")  to  which  the Target  or  any  Subsidiary is  a  party,  other than
contracts which have been filed  as an exhibit to  or have been incorporated  by
reference  in any Target SEC Report. Each Material Contract is in full force and
effect and,  to the  knowledge of  Target, is  enforceable against  the  parties
thereto  (other than the Target  or any such Subsidiary)  in accordance with its
terms and no condition or state of facts exists that, with notice or the passage
of time, or both, would constitute a default by the Target or any Subsidiary or,
to the  best  knowledge of  the  Target, any  third  party under  such  Material
Contracts, except for such defaults which individually or in the aggregate would
not  reasonably be  expected to  have a Material  Adverse Effect  on Target. The
Target or the applicable Subsidiary has  duly complied in all material  respects
with the provisions of each Material Contract to which it is a party.
 
    SECTION  3.21.  PARACHUTE PAYMENTS.  Except  as disclosed in Section 3.21 of
the Target  Disclosure  Schedule, neither  the  Target nor  any  Subsidiary  has
entered  into  any  agreement that  would  result  in the  making  of "parachute
payments," as defined in  Section 280G of  the Code, to any  person and none  of
such  agreements requires the Target or  any Subsidiary to gross-up or otherwise
pay the amount of any taxes due in respect of such "parachute payments."
 
    SECTION 3.22.    CERTAIN  BUSINESS  PRACTICES.   As  of  the  date  of  this
Agreement, neither the Target nor any Subsidiary, nor any director, officer, or,
to  the knowledge  of the  Target, any agent  or employee  of the  Target or any
Subsidiary  has  (i)   used  any  funds   for  unlawful  contributions,   gifts,
entertainment  or other unlawful  expenses relating to  political activity, (ii)
made any  unlawful  payment  to  foreign or  domestic  government  officials  or
employees  or to foreign or domestic  political parties or campaigns or violated
any provision of the  Foreign Corrupt Practices Act  of 1977, as amended,  (iii)
made  any other  unlawful payment,  or (iv)  violated any  of the  provisions of
Section 999  of the  Code or  Section 8  of the  Export Administration  Act,  as
amended.
 
    SECTION 3.23  REAL PROPERTY AND LEASES.
 
    (a)  Section  3.23 of  the Target  Disclosure  Schedule lists  and describes
briefly all real property that any of the Target and each Subsidiary owns.  With
respect  to  each such  parcel of  owned real  property and  except as  noted in
Section 3.23 of  the Target Disclosure  Schedule: (i) the  identified owner  has
good  and marketable title to the parcel of real property, free and clear of any
liens or  encumbrances, easement,  covenant, or  other restriction,  except  for
installments  of  special assessments  not  yet delinquent,  recorded easements,
covenants, and other restrictions, and utility easements, building restrictions,
zoning restrictions,  and other  easements and  restrictions existing  generally
with respect to properties of a similar character which do not affect materially
and  adversely the  current use,  occupancy, or  value, or  the marketability of
title, of the  property subject thereto;  (ii) there are  no leases,  subleases,
licenses,  concessions, or  other agreements, written  or oral,  granting to any
party or parties the right of use or  occupancy of any portion of the parcel  of
real  property; and (iii)  there are no  outstanding options or  rights of first
refusal to purchase, lease or occupy the parcel of real property, or any portion
thereof or interest therein.
 
    (b) Section  3.23 of  the  Target Disclosure  Schedule lists  and  describes
briefly  all real  property leased  or subleased  to any  of the  Target and any
Subsidiary. With respect to each lease and sublease (i) the lease or sublease is
legal, valid, binding, enforceable, and in full force and effect in all material
respects; (ii) to the knowledge of Target, no party to the lease or sublease  is
in  material breach or default, and no  event has occurred which, with notice or
lapse of  time,  would  constitute  a  material  breach  or  default  or  permit
termination, modification, or acceleration thereunder; (iii) to the knowledge of
Target,  no party to the lease or sublease has repudiated any material provision
thereof; (iv) there are  no material disputes,  oral agreements, or  forbearance
programs  in effect as to the  lease or sublease; (v) none  of the Target or any
Subsidiary has assigned, transferred, conveyed,  mortgaged, deeded in trust,  or
encumbered  any  interest  in  the  leasehold  or  subleasehold;  and  (vi)  all
facilities leased  or  subleased  thereunder  have  received  all  approvals  of
governmental authorities (including
 
                                      A-24
<PAGE>
material  licenses  and  permits)  required  in  connection  with  the operation
thereof, and have  been operated  and maintained in  accordance with  applicable
laws, rules, and regulations in all material respects.
 
    SECTION  3.24.  INSURANCE.   Section 3.24 of  the Target Disclosure Schedule
sets forth  a  list  of  each insurance  policy  (including  policies  providing
property,  casualty, liability, and workers'  compensation coverage and bond and
surety arrangements) to which  any of the  Target or any  Subsidiary has been  a
party,  a named insured,  or otherwise the  beneficiary of coverage  at any time
within the  past  three  years.  With respect  to  each  such  insurance  policy
designated as "current": (i) the policy is in full force and effect; (ii) Target
has  not received  notice from  any insurance carrier  of the  intention of such
carrier to discontinue any such policy; (iii)  neither any of the Target or  any
Subsidiary  nor, to the knowledge of Target, any other party to the policy is in
breach or default  (including with  respect to the  payment of  premiums or  the
giving of notices), and no event has occurred which, with notice or the lapse of
time,  would  constitute  such  a  breach  or  default,  or  permit termination,
modification, or acceleration, under the policy; and (iv) no party to the policy
has repudiated  any provision  thereof. Section  3.24 of  the Target  Disclosure
Schedule  lists any self-insurance arrangements affecting  any of the Target and
the Subsidiaries.  All  material  assets  and  risks  of  the  Target  and  each
Subsidiary  are covered by  valid and currently  effective insurance policies in
such types and amounts as are consistent with customary practices and  standards
of companies engaged in businesses and operations similar to those of the Target
or such Subsidiary.
 
    SECTION 3.25.  ACCOUNTING AND TAX MATTERS.
 
    (a)  Except  as  set  forth  in Section  3.25(a)  of  the  Target Disclosure
Schedule, the Target has no  knowledge of any plan or  intention on the part  of
the  Target's stockholders (a "STOCKHOLDER PLAN") to engage in a sale, exchange,
transfer, distribution  (including,  without  limitation, a  distribution  by  a
partnership  to its partners  or by a corporation  to its stockholders), pledge,
disposition or any other transaction which results in a reduction in the risk of
ownership or a direct or indirect disposition  (a "SALE") of a number of  shares
of  Acquiror  Common Stock  to be  issued  to such  stockholders in  the Merger,
sufficient to reduce  the Target's  stockholders' ownership  of Acquiror  Common
Stock  to a number  of shares having an  aggregate fair market  value, as of the
Effective Time of the Merger, of less than fifty percent (50%) of the  aggregate
fair market value, immediately prior to the Merger, of all outstanding shares of
Target  Common Stock.  For purposes of  this paragraph, shares  of Target Common
Stock (i) with respect to which  a Target stockholder receives consideration  in
the Merger other than Acquiror Common Stock (including, without limitation, cash
received in lieu of fractional shares of Acquiror Common Stock) and/or (ii) with
respect  to which  a Sale occurs  prior to  and in contemplation  of the Merger,
shall be  considered shares  of outstanding  Target Common  Stock exchanged  for
Acquiror  Common  Stock  in  the  Merger and  then  disposed  of  pursuant  to a
Stockholder Plan.
 
    (b) Neither  the Target  nor  any Subsidiary  is  an investment  company  as
defined in Section 368(a)(2)(F)(iii) and (iv) of the Code.
 
    (c)  As of the  Effective Time, the fair  market value of  the assets of the
Target and the  Subsidiaries will exceed  the sum of  its liabilities, plus  the
amount of other liabilities, if any, to which its assets are subject.
 
    (d)  Neither the Target  nor any Subsidiary  is under the  jurisdiction of a
court in a Title 11 or similar  case within the meaning of Section  368(a)(3)(A)
of the Code.
 
    SECTION  3.26.  BOARD RECOMMENDATION.  At  a meeting duly called and held in
compliance with  Delaware  Law,  the  Board  of  Directors  of  the  Target  has
unanimously   adopted  a  resolution  (i)  approving  the  Merger,  based  on  a
determination that the Merger offers the best value reasonably available to  the
stockholders  of Target and is in the best interests of such Target stockholders
and (ii)  approving  and  adopting  this  Agreement  and  the  Transactions  and
recommending approval and adoption of this Agreement and the Transactions by the
stockholders of the Target.
 
                                      A-25
<PAGE>
    SECTION  3.27.  CHANGE IN  CONTROL.  Except as set  forth in Section 3.27 of
the Target Disclosure Schedule, neither the Target nor any Subsidiary is a party
to any  contract,  agreement  or  understanding  which  contains  a  "change  in
control,"  "potential change  in control"  or similar  provision. Except  as set
forth in Section 3.27 of the Target Disclosure Schedule, the consummation of the
Transactions will not  (either alone or  upon the occurrence  of any  additional
acts  or events) result in  any payment (whether of  severance pay or otherwise)
becoming due from the Target or any Subsidiary to any person.
 
    SECTION 3.28.    TARGET DRILLING  RIGS.   (a)  Section  3.28 of  the  Target
Disclosure  Schedule  sets forth  a list  of  the Target  Drilling Rigs  and, if
applicable, the name of the nation  under which each drilling rig is  documented
and  flagged and indicates all drilling rigs that  are laid up or being held for
sale on the date  hereof. With respect  to the owned  Target Drilling Rigs,  the
Target  or a Subsidiary has good title to each such drilling rig, free and clear
of all  mortgages,  pledges, liens,  encumbrances,  charges, or  other  security
interests  except  for such  as  are disclosed  in  Section 3.28  of  the Target
Disclosure Schedule and in the Target SEC Reports.
 
    (b) With respect to each Target Drilling Rig that is operated by the  Target
or  any Subsidiary under lease or charter, (i) the Target or such Subsidiary has
a valid right to  charter or a  valid leasehold interest  in such drilling  rig;
(ii)  such charter agreement or lease is  in full force and effect in accordance
with its terms;  (iii) all rents,  charter payments and  other similar  monetary
amounts that have become due and payable thereunder have been paid in full; (iv)
no  waiver, indulgence  or postponement of  the obligations  thereunder has been
granted by the other party thereof; (v) there exists no material default (or  an
event  that, with notice  or lapse of  time or both  would constitute a material
default) on the part of the Target, or to the knowledge of the Target, any other
person under such charter agreement or lease; (vi) the Target or such Subsidiary
has not violated any of the terms or conditions under any such charter agreement
or lease and to the knowledge of the Target there are no conditions or covenants
to be observed or performed by any  other party under such charter agreement  or
lease  that have not  been observed or  performed in all  material respects; and
(vii) the transactions described in this Agreement will not constitute a default
under or cause  for termination  or modification of  any terms  of such  charter
agreement or lease.
 
    (c)  Section 3.28 of the  Target Disclosure Schedule contains  a list of all
leases or charters providing for  the use by the Target  or any Subsidiary of  a
Target  Drilling Rig. Complete and correct copies  of each lease or charter have
been delivered to Acquiror.
 
    (d) With respect to each Target Drilling Rig: (i) if applicable, such Target
Drilling Rig  is lawfully  documented under  the flag  of the  nation listed  on
Section  3.28 of  the Target Disclosure  Schedule for such  Target Drilling Rig;
(ii) if  applicable, such  Target Drilling  Rig is  afloat and  in  satisfactory
operating  condition for charter  hire; (iii) such Target  Drilling Rig holds in
full force and effect  all certificates, licenses,  permits and rights  required
for  operation in the manner drilling rigs of its kind are being operated in the
geographical area in which such Target Drilling Rig is presently being  operated
which  the  failure to  hold would  reasonably  be expected  to have  a Material
Adverse Effect on  Target; (iv) no  event has occurred  and no condition  exists
that  would  endanger  the  maintenance of  the  classification  of  such Target
Drilling Rig, (v) such Target  Drilling Rig which is  a jack-up drilling rig  is
considered  by the American Bureau of Shipping to be in class as a Maltese Cross
A1 Self-Elevating  Drilling Unit  and free  of any  recommendations and  average
damages  affecting class, and  (vi) there exists  no outstanding requirements or
recommendations resulting from any inspections  by, or rules or regulations  of,
the   U.S.  Coast  Guard,  the  U.S.   Minerals  Management  Service,  the  U.S.
Occupational Safety and Health Administration, or any nation under which any  of
the  Target Drilling Rigs are flagged which would reasonably be expected to have
a Material Adverse Effect on Target.
 
    (e) Section 3.28 of  the Target Disclosure Schedule  contains a list of  the
geographical  location in which each Target Drilling Rig is being operated as of
the date hereof. Each Target Drilling Rig can be
 
                                      A-26
<PAGE>
removed from such  geographical location  and transported to  the United  States
without  the payment, liability or imposition  of any material tax, duty, impost
or other payment of any kind to any foreign governmental authority.
 
    SECTION 3.29.   SIME-DUAL SDN BHD.   Section 3.29  of the Target  Disclosure
Schedule  lists  all agreements  in  respect of  Sime-Dual  Drilling Sdn  Bhd, a
Malaysian company ("SIME-DUAL").  Target represents  and warrants  that (i)  the
Shareholders Agreement dated October 24, 1994 by and among SD Holdings Berhad, a
Malaysian  company ("SD"), Target  and Sime-Dual (the  "ORIGINAL AGREEMENT"), as
amended pursuant to that certain  First Amendment to the Shareholders  Agreement
dated  June 5, 1995  by and among SD,  Sime Darby Drilling  Sdn Bhd, a Malaysian
company ("SDD"), Target, DAI and Sime-Dual (the "FIRST AMENDMENT") (the Original
Agreement  and  the  First  Amendment  are  collectively  referred  to  as,  the
"SHAREHOLDERS  AGREEMENT") has not been amended (other than the First Amendment)
and remains in full force  and effect and to the  knowledge of the Target  there
exists  no Event of Default (as defined in the Shareholders Agreement), or facts
or events  which  with  the passage  of  time  or the  giving  or  notice  would
constitute  an Event of Default, and no waivers of performance have been granted
by Target or DAI of any of SD's or SDD's obligations thereunder, (ii) neither  a
Dual  Triggering Event or a  SD Triggering Event (as  those terms are defined in
the Shareholders Agreement) has  occurred, (iii) there  have been no  guaranties
given  by either Target or, to the knowledge  of the Target, SDD as described in
Sub-Clause 6.3 of the First Amendment, (iv) there have been no notices  provided
by  SDD or SD to trigger the put option described in Clause 19.5 of the Original
Agreement, (v) the Joint Operating Agreement dated June 5, 1995 by and among SDD
and DAI (the "JOINT  OPERATING AGREEMENT") has not  been amended and remains  in
full  force  and effect  and  to the  knowledge of  the  Target there  exists no
material defaults, or  facts or events  which with  the passage of  time or  the
giving  or  notice  would  constitute  a material  default,  and  no  waivers of
performance have  been  granted  by Target  or  DAI  of any  of  SD's  or  SDD's
obligations  thereunder, (vi) no Sale Notice  (as defined in the Joint Operating
Agreement) has been given by  DAI or SDD, and  (vii) the Joint Bareboat  Charter
Agreement  dated June 5,  1995 by and  among SDD, DAI  and Sime-Dual (the "JOINT
BAREBOAT CHARTER") has not been amended and remains in full force and effect and
to the knowledge of the  Target there exists no  material defaults, or facts  or
events which with the passage of time or the giving or notice would constitute a
material  default, and no waivers of  performance have been granted by Sime-Dual
of any of SDD's or DAI's obligations thereunder.
 
    SECTION 3.30.    ACCOUNTS  RECEIVABLE.    All  of  the  accounts  receivable
reflected  in the 1995  Balance Sheet or  created thereafter to  the best of the
Target's knowledge are valid receivables subject to no setoffs or counterclaims,
are current and collectible and will be collected in accordance with their terms
at their recorded amounts, subject only to  the reserve for bad debts set  forth
in  the 1995 Balance  Sheet as adjusted for  operations and transactions through
the Effective Time in accordance with the past custom and practice of the Target
and the Subsidiaries.
 
                                   ARTICLE IV
          REPRESENTATIONS AND WARRANTIES OF ACQUIROR AND ACQUIROR SUB
 
    Except as set forth in the Disclosure Schedules delivered by Acquiror to the
Target and signed  by the Target  and Acquiror for  identification prior to  the
execution  and delivery of this Agreement (the "ACQUIROR DISCLOSURE SCHEDULES"),
which shall identify  exceptions by  specific section  references, Acquiror  and
Acquiror  Sub hereby, jointly and severally, represent and warrant to the Target
that:
 
    SECTION 4.01.    CORPORATE ORGANIZATION  AND  QUALIFICATION.   Acquiror  and
Acquiror  Sub  are corporations  duly organized,  validly  existing and  in good
standing under the laws of the jurisdiction of their incorporation and have  the
requisite corporate power and authority and all necessary governmental approvals
to  own, lease  and operate  their properties and  to carry  on their respective
businesses as they  are now being  conducted, except where  the failure to  have
such  power, authority and governmental approvals  would not, individually or in
the aggregate, have  a Material Adverse  Effect. Acquiror and  Acquiror Sub  are
duly  qualified or licensed as a foreign  corporation to do business, and are in
good
 
                                      A-27
<PAGE>
standing, in  each jurisdiction  where the  character of  the properties  owned,
leased  or operated by them  or the nature of  their respective businesses makes
such qualification or  licensing necessary, except  for such failures  to be  so
qualified  or licensed and in good standing as would not, individually or in the
aggregate, have a Material Adverse Effect.
 
    SECTION 4.02.    CERTIFICATE OF  INCORPORATION  AND BYLAWS.    Acquiror  has
heretofore furnished or made available to the Target a complete and correct copy
of  the Certificate of Incorporation and Bylaws of Acquiror, and the Certificate
of Incorporation and Bylaws  of Acquiror Sub, each  as amended to date.  Neither
Acquiror nor Acquiror Sub is in violation of any provision of its Certificate of
Incorporation or Bylaws.
 
    SECTION  4.03.   CAPITALIZATION.   As  of  the date  of this  Agreement, the
authorized capital stock of Acquiror consists of 125,000,000 shares of  Acquiror
Common  Stock, 5,000,000 shares  of First Preferred Stock,  $1.00 par value, and
15,000,000 shares of Serial Preferred Stock, $1.00 par value, 1,250,000 of which
have  been  designated  as  Series   A  Junior  Participating  Preferred   Stock
(collectively,  "ACQUIROR  PREFERRED  STOCK").  As  of  December  31,  1995, (a)
60,605,772 shares of Acquiror Common Stock  were issued and outstanding, all  of
which were validly issued, fully paid and nonassessable, (b) 6,285,448 shares of
Acquiror  Common  Stock were  held in  the treasury  of Acquiror,  (c) 1,120,850
shares of Acquiror Common  Stock were reserved for  future issuance pursuant  to
outstanding  stock options granted pursuant to Acquiror's stock option plan, and
(d) no  shares of  Acquiror  Preferred Stock  were outstanding.  The  authorized
capital  stock of Acquiror Sub consists of  10,000 shares of Acquiror Sub Common
Stock, of which, as of the date  of this Agreement, 1,000 shares are issued  and
outstanding  and held by  Acquiror. Except as contemplated  by this Agreement or
that certain Rights Agreement (herein so  called) dated February 21, 1995 or  as
set  forth in Section 4.03  of the Acquiror Disclosure  Schedule or the Acquiror
SEC Reports, as of the date of this Agreement, there are no options, warrants or
other rights, agreements, arrangements or commitments of any character  relating
to  the  issued or  unissued  capital stock  of  Acquiror or  any  subsidiary of
Acquiror, including Acquiror Sub ("ACQUIROR SUBSIDIARY"), obligating Acquiror or
any Acquiror Subsidiary  to issue or  sell any  shares of capital  stock of,  or
other equity interests in, Acquiror or any Acquiror Subsidiary. Between December
31, 1995 and the date of this Agreement, no shares of Acquiror Common Stock have
been  issued  by Acquiror,  except pursuant  to the  options, warrants  or other
rights, agreements, arrangements and commitments described in this Section  4.03
or Section 4.03 of the Acquiror Disclosure Schedule, in each case, in accordance
with their respective terms. There are no outstanding contractual obligations of
Acquiror  or any Acquiror Subsidiary to  repurchase, redeem or otherwise acquire
any shares of  Acquiror Common Stock,  or any  capital stock of,  or any  equity
interests in, any Acquiror Subsidiary. The shares of Acquiror Common Stock to be
issued  pursuant to  the Merger will  be duly authorized,  validly issued, fully
paid and nonassessable and not subject to preemptive rights created by  statute,
Acquiror's  Certificate of  Incorporation or  Bylaws or  any agreement  to which
Acquiror is a party or by which Acquiror is bound and will, when issued,  except
with  respect to shares to be issued to  Dual Invest AS, be registered under the
Securities Act and the Exchange Act  and registered or exempt from  registration
under applicable Blue Sky Laws.
 
    SECTION  4.04.  AUTHORITY RELATIVE TO THIS  AGREEMENT.  Each of Acquiror and
Acquiror Sub has  all necessary  corporate power  and authority  to execute  and
deliver  this  Agreement  and,  with  respect  to  the  Merger,  to  perform its
obligations hereunder  and to  consummate the  Transactions. The  execution  and
delivery  of this Agreement by Acquiror and Acquiror Sub and the consummation by
Acquiror and  Acquiror  Sub of  the  Transactions  have been  duly  and  validly
authorized  by all necessary corporate action and no other corporate proceedings
on the  part  of  Acquiror or  Acquiror  Sub  are necessary  to  authorize  this
Agreement  or to  consummate the Transactions  (other than, with  respect to the
issuance of Acquiror Common Stock pursuant  to the Merger, the applicable  rules
and  regulations  of  NYSE, and  with  respect  to the  Merger,  the  filing and
recordation of  an  appropriate Certificate  of  Merger with  the  Secretary  as
required by Delaware Law). This Agreement has been duly and validly executed and
delivered  by Acquiror  and Acquiror  Sub and,  assuming the  due authorization,
execution
 
                                      A-28
<PAGE>
and delivery of  this Agreement by  the Target, constitutes  a legal, valid  and
binding obligation of each of Acquiror and Acquiror Sub enforceable against each
of Acquiror and Acquiror Sub in accordance with its terms.
 
    SECTION  4.05.    NO  CONFLICT;  REQUIRED FILINGS  AND  CONSENTS.    (a) The
execution and delivery of  this Agreement by Acquiror  and Acquiror Sub do  not,
and  the performance of  this Agreement by  Acquiror and Acquiror  Sub will not,
subject to obtaining  the consents,  approvals, authorizations  and permits  and
making  the  filings described  in Section  4.05(b) and  Section 4.05(b)  of the
Acquiror Disclosure Schedule, (i)  conflict with or  violate the Certificate  of
Incorporation  or Bylaws  of either  Acquiror or  any Acquiror  Subsidiary, (ii)
conflict with  or  violate  any  Law applicable  to  Acquiror  or  any  Acquiror
Subsidiary  or  by which  any  property or  asset  of any  of  them is  bound or
affected, or (iii) result in any breach of or constitute a default (or an  event
which  with notice or  lapse of time or  both would become  a default) under, or
give  to  others   any  rights  of   termination,  amendment,  acceleration   or
cancellation of, or result in the creation of a lien or other encumbrance on any
property  or asset of Acquiror or any Acquiror Subsidiary or require the consent
of any third party pursuant to,  any note, bond, mortgage, indenture,  contract,
agreement,  lease, license, permit, franchise  or other instrument or obligation
to which Acquiror or any Acquiror Subsidiary is a party or by which Acquiror  or
any  Acquiror Subsidiary  or any property  or asset of  any of them  is bound or
affected, except for any such conflicts, violations, breaches, defaults or other
occurrences which would not, individually or  in the aggregate, have a  Material
Adverse  Effect on  Acquiror or  prevent Acquiror  and Acquiror  Sub from timely
performing their respective  obligations under this  Agreement and  consummating
the Transactions.
 
    (b)  The execution and  delivery of this Agreement  by Acquiror and Acquiror
Sub do not, and the performance of  this Agreement by Acquiror and Acquiror  Sub
will  not, require any consent, approval,  authorization or permit of, or filing
with or notification to, any  governmental or regulatory authority, domestic  or
foreign,  except (i) pursuant to the Exchange  Act, the Securities Act, Blue Sky
Laws and the HSR Act and filing and recordation of an appropriate Certificate of
Merger with the  Secretary as  required by Delaware  Law, (ii)  as specified  in
Section  4.05(b) of the Acquiror Disclosure Schedule, and (iii) where failure to
obtain such  consents, approvals,  authorizations or  permits, or  to make  such
filings  or notifications, would not have  a Material Adverse Effect on Acquiror
and would not prevent  or delay consummation of  the Transactions, or  otherwise
prevent  Acquiror or Acquiror  Sub from performing  their respective obligations
under this Agreement.
 
    SECTION 4.06.  SEC  FILINGS; FINANCIAL STATEMENTS.   (a) Acquiror has  filed
all  forms, reports and documents required to be  filed by it with the SEC since
December 31, 1992 (collectively, the  "ACQUIROR SEC REPORTS"). The Acquiror  SEC
Reports  (i)  were prepared  in  all material  respects  in accordance  with the
requirements of the Securities Act and the Exchange Act, as the case may be, and
the rules and regulations  thereunder and (ii)  did not, at  the time they  were
filed (or at the effective date thereof in the case of registration statements),
contain any untrue statement of a material fact or omit to state a material fact
required  to be stated therein or necessary in order to make the statements made
therein, in  the light  of the  circumstances under  which they  were made,  not
misleading.  No  Acquiror Subsidiary  is currently  required  to file  any form,
report or other document with the SEC under Section 12 of the Exchange Act.
 
    (b) Each of the consolidated financial statements (including, in each  case,
any  notes  thereto)  contained in  the  Acquiror  SEC Reports  was  prepared in
accordance with U.S.  GAAP throughout the  periods indicated (except  as may  be
indicated  in the  notes thereto and  except that  financial statements included
with interim  reports do  not contain  all  U.S. GAAP  notes to  such  financial
statements)  and each fairly presented in all material respects the consolidated
financial position, results  of operations and  changes in stockholders'  equity
and  cash  flows  of  Acquiror  and  its  consolidated  subsidiaries  as  at the
respective dates  thereof  and  for the  respective  periods  indicated  therein
(subject,  in the case of unaudited statements, to normal and recurring year-end
adjustments which  were  not  and  are not  expected,  individually  or  in  the
aggregate, to have a Material Adverse Effect on Acquiror).
 
                                      A-29
<PAGE>
    SECTION  4.07.  ABSENCE  OF CERTAIN CHANGES  OR EVENTS.   Since December 31,
1995, except  as  contemplated by,  or  disclosed pursuant  to,  this  Agreement
including  Section 4.07 of the Acquiror Disclosure Schedule, or disclosed in any
Acquiror SEC Report filed since December 31, 1995 and prior to the date of  this
Agreement,  Acquiror and each Acquiror Subsidiary has conducted their businesses
only in the ordinary course and in  a manner consistent with past practice  and,
since  December 31, 1995, there has not been (a) any event or events (whether or
not covered by insurance), individually or  in the aggregate, having a  Material
Adverse  Effect  on  Acquiror,  (b)  any  material  change  by  Acquiror  in its
accounting methods, principles or  practices, (c) any entry  by Acquiror or  any
Acquiror  Subsidiary into any commitment or  transaction material to Acquiror or
any  Acquiror  Subsidiary,  except  in  the  ordinary  course  of  business  and
consistent  with past practice, (d) any declaration, setting aside or payment of
any dividend or distribution in respect of any capital stock of Acquiror or  any
redemption,  purchase or other acquisition of any of its securities or (e) other
than pursuant to Acquiror's benefit plans,  any increase in or establishment  of
any  bonus,  insurance, severance,  deferred compensation,  pension, retirement,
profit sharing, stock  option, stock  purchase or other  employee benefit  plan,
except in the ordinary course of business consistent with past practice.
 
    (b)  As of the date hereof and the Effective Time, except for obligations or
liabilities incurred in  connection with its  incorporation or organization  and
the  Transactions  and except  for this  Agreement and  any other  agreements or
arrangements contemplated by this Agreement, Acquiror  Sub has not and will  not
have  incurred, directly or indirectly, through any subsidiary or affiliate, any
obligations or liabilities or engaged in any business activities of any type  or
kind whatsoever or entered into any agreements or arrangements with any person.
 
    SECTION  4.08.  BROKERS.  No broker, finder or investment banker is entitled
to any brokerage,  finder's or other  fee or commission  in connection with  the
Transactions  based upon arrangements  made by or  on behalf of  Acquiror or any
Acquiror Subsidiary.
 
    SECTION 4.09.   TAXES.  (a)  Acquiror and each  Acquiror Subsidiary has  (i)
filed  all federal, state, local and foreign tax returns required to be filed by
them prior to the  date of this Agreement  (taking into account extensions)  and
all  of such returns were  true and correct in  all material respects when filed
and in compliance with applicable law, (ii)  paid or accrued all taxes shown  to
be  due on such returns and paid all applicable ad valorem and value added taxes
as are due and (iii) paid or accrued all taxes for which a notice of  assessment
or  collection has  been received  (other than  amounts being  contested in good
faith by appropriate  proceedings), except in  the case of  clause (i), (ii)  or
(iii)  for any such filings, payments  or accruals which would not, individually
or in the  aggregate, have a  Material Adverse  Effect. Except as  set forth  in
Section  4.09(a)  of  the  Acquiror Disclosure  Schedule,  neither  the Internal
Revenue Service nor any other federal, state, local or foreign taxing  authority
has  asserted any  claim for  taxes, or  to the  best knowledge  of Acquiror, is
threatening to assert any claims for taxes, which claims, individually or in the
aggregate, could have  a Material Adverse  Effect. Acquiror has  open years  for
federal,  state and  foreign income  tax returns  only as  set forth  in Section
4.09(a) of  the  Acquiror Disclosure  Schedule,  and neither  Acquiror  nor  any
Acquiror  Subsidiary  (i)  has  executed  any  waiver  to  extend  the  time for
assessment of any federal, state,  local or foreign tax,  or (ii) filed, or  has
pending, any request or application for ruling, whether federal, state, local or
foreign.  Acquiror and each  Acquiror Subsidiary have  withheld or collected and
paid over to the appropriate  governmental authorities (or are properly  holding
for  such payment) all taxes required by law to be withheld or collected, except
for amounts that would  not, individually or in  the aggregate, have a  Material
Adverse  Effect.  Neither  Acquiror  nor any  Acquiror  Subsidiary  has  made an
election under Section 341(f) of the Code. There are no liens for taxes upon the
assets of Acquiror or any Acquiror  Subsidiary (other than liens for taxes  that
are  not  yet due  or  that are  being contested  in  good faith  by appropriate
proceedings),  except  for  liens  which  would  not,  individually  or  in  the
aggregate, have a Material Adverse Effect. Acquiror and each Acquiror Subsidiary
have  complied with all federal, state, local  and foreign tax laws except where
such failure would not result in a Material Adverse Effect.
 
                                      A-30
<PAGE>
    (b) Neither Acquiror nor any Acquiror Subsidiary has taken or agreed to take
any action  that would  prevent the  Merger from  constituting a  reorganization
qualifying under the provisions of Section 368(a) of the Code.
 
    (c)  The financial  statements included  in the  Acquiror SEC  Reports as of
December 31, 1995 includes  appropriate reserves for  all federal, state,  local
and  foreign taxes and  other liabilities incurred  as of such  date but not yet
payable.
 
    (d) The net operating losses and other carryovers available to Acquiror  and
each  Acquiror Subsidiary as of the date hereof are described in Section 4.09 of
the Acquiror Disclosure Schedule or the Acquiror SEC Reports and as of the  date
hereof  the  ability  of  Acquiror  and each  Acquiror  Subsidiary  to  use such
carryovers will not have been affected by  Section 382, 383, or 384 of the  Code
or  by  the separate  return limitation  year or  consolidated return  charge of
ownership limitations of Treas. Regs. Section 1.1502-21 or 1.1502-22.
 
    (e) Acquiror is not a U.S.  real property holding company under Section  897
of the Code.
 
    (f) Neither Acquiror nor Acquiror Sub is an investment company as defined in
Section 368(a)(2)(F)(iii) and (iv) of the Code.
 
    (g)  Neither Acquiror nor any Acquiror  Subsidiary is under the jurisdiction
of a  court  in  Title  11  or  similar  case  within  the  meaning  of  Section
368(a)(3)(A) of the Code.
 
    (h)  Neither Acquiror or  any Acquiror Subsidiary holds  stock of the Target
and will not hold stock of the Target prior to the Merger.
 
                                   ARTICLE V
                     CONDUCT OF BUSINESS PENDING THE MERGER
 
    SECTION 5.01.  CONDUCT OF  BUSINESS BY THE TARGET  PENDING THE MERGER.   The
Target  covenants and agrees  that, between the  date of this  Agreement and the
Effective Time, except  as set forth  in Section 5.01  of the Target  Disclosure
Schedule  or as  contemplated by any  other provision of  this Agreement, unless
Acquiror shall  otherwise  agree  in  writing  (which  agreement  shall  not  be
unreasonably  withheld), (i)  the business  of the  Target and  the Subsidiaries
shall be conducted only in, and the  Target and the Subsidiaries shall not  take
any  action  except in,  the ordinary  course of  business consistent  with past
practice and in accordance with all  applicable laws, (ii) the Target shall  use
all   reasonable  efforts   to  preserve   substantially  intact   its  business
organization, to keep available the services of the current officers,  employees
and  consultants of the Target and the  Subsidiaries and to preserve the current
relationships of the Target and  the Subsidiaries with customers, suppliers  and
other  persons with which the Target  or any Subsidiary has significant business
relations, (iii) the  Target shall not  and shall not  permit any Subsidiary  to
engage  in any practice, take  any action, or enter  into any transaction of the
sort described in Section  3.08 above, and (iv)  the Target and each  Subsidiary
shall  cause to be maintained in full force and effect, and without modification
or amendment, or any lapse of coverage under, all insurance polices described in
Section 3.24 of the Target Disclosure Schedule.
 
    SECTION 5.02.  CONDUCT OF BUSINESS BY ACQUIROR PENDING THE MERGER.  Acquiror
covenants and agrees that, between the date of this Agreement and the  Effective
Time, except as set forth in Section 5.02 of the Acquiror Disclosure Schedule or
as  contemplated by  any other  provision of  this Agreement,  unless the Target
shall otherwise  agree in  writing  (which agreement  will not  be  unreasonably
withheld),  (i) the businesses  of Acquiror and Acquiror  Sub shall be conducted
only in, and the Acquiror shall not,  and shall cause Acquiror Sub not to,  take
any  action  except in,  the ordinary  course of  business consistent  with past
practice and in accordance with all applicable Laws, and (ii) Acquiror will  not
(a)  amend or otherwise  change its Certificate of  Incorporation or Bylaws, (b)
declare, set aside, make or pay  any dividend or other distribution, payable  in
cash, stock, property or otherwise, with
 
                                      A-31
<PAGE>
respect to any of its capital stock, or (c) reclassify, combine, split or divide
its  capital  stock  or  redeem,  purchase  or  otherwise  acquire,  directly or
indirectly, any of its  capital stock or  securities or obligations  convertible
into or exchangeable for such capital stock.
 
                                   ARTICLE VI
                             ADDITIONAL AGREEMENTS
 
    SECTION  6.01.  REGISTRATION STATEMENT; PROXY STATEMENT.  (a) As promptly as
practicable after the execution  of this Agreement,  Acquiror shall prepare  and
file  with  the SEC  a registration  statement  on Form  S-4 (together  with all
amendments thereto, the "REGISTRATION  STATEMENT") including therein a  combined
proxy  statement  to be  sent  to the  stockholders  of the  Target  (the "PROXY
STATEMENT") and  Prospectus,  in  connection with  the  registration  under  the
Securities  Act  of the  shares of  Acquiror Common  Stock to  be issued  to the
stockholders of the Target pursuant to the Merger. Acquiror and the Target  each
shall  use all reasonable efforts to  cause the Registration Statement to become
effective as promptly as  practicable, and, prior to  the effective date of  the
Registration Statement, Acquiror shall take all or any action required under any
applicable  federal or state securities laws  in connection with the issuance of
shares of Acquiror Common Stock pursuant to  the Merger. Each of the Target  and
Acquiror shall pay its own expenses incurred in connection with the Registration
Statement,  Proxy Statement  and the Target's  Stockholders' Meeting, including,
without limitation,  the fees  and disbursements  of their  respective  counsel,
accountants  and other representatives, except that the Target and Acquiror each
shall pay one-half of any printing, filing and other fees and expenses  incurred
in connection therewith. The Target shall furnish all information concerning the
Target  as Acquiror may  reasonably request in connection  with such actions and
the preparation of the Registration  Statement and Proxy Statement. As  promptly
as practicable after the Registration Statement shall have become effective, the
Target  shall mail the Proxy Statement  to its stockholders. The Proxy Statement
shall include the  unanimous recommendation  of the  Board of  Directors of  the
Target in favor of the Merger.
 
    No  amendment  or  supplement to  the  Proxy Statement  or  the Registration
Statement will be made  by Acquiror or  the Target without  the approval of  the
other  party, which shall not be  unreasonably withheld. Acquiror and the Target
each will advise the  other, promptly after it  receives notice thereof, of  the
time  when the Registration Statement has  become effective or any supplement or
amendment has been filed, the issuance of any stop order, the suspension of  the
qualification  of  the Acquiror  Common Stock  issuable  in connection  with the
Merger for offering or sale in any  jurisdiction, or any request by the SEC  for
amendment  of  the Proxy  Statement or  the  Registration Statement  or comments
thereon and responses thereto or requests by the SEC for additional information.
 
    Acquiror shall promptly prepare and submit to the NYSE a listing application
covering the shares of Acquiror Common  Stock issuable in the Merger, and  shall
use its reasonable best efforts to obtain, prior to the Effective Time, approval
for  the listing of  such Acquiror Common  Stock, subject to  official notice of
issuance, and the  Target shall  cooperate with  Acquiror with  respect to  such
listing.
 
    (b)  Acquiror represents, warrants and  agrees that the information supplied
by Acquiror for inclusion in the Registration Statement and the Proxy  Statement
shall  not, at  (i) the time  the Registration Statement  is declared effective,
(ii) the  time the  Proxy  Statement (or  any  amendment thereof  or  supplement
thereto)  is first mailed to  the stockholders of the  Target, (iii) the time of
the  Target's  Stockholder  Meeting  (as  hereinafter  defined),  and  (iv)  the
Effective  Time, contain any statement  which, at such time  and in light of the
circumstances under which it is made, is false or misleading with respect to any
material fact, or omit to state any material fact required to be stated therein,
or necessary in order to make the statements therein not false or misleading. If
at any time prior to  the Effective Time any  event or circumstance relating  to
Acquiror  or  Acquiror Subsidiary,  or their  respective officers  or directors,
should be discovered by Acquiror which should be set forth in an amendment or  a
supplement  to  the Registration  Statement or  Proxy Statement,  Acquiror shall
promptly inform the Target. Notwithstanding the foregoing, Acquiror and Acquiror
Sub make no representation or warranty with respect to any information  supplied
by the Target or any of its
 
                                      A-32
<PAGE>
representatives  which  is  contained  in the  Registration  Statement  or Proxy
Statement. All documents that Acquiror is responsible for filing with the SEC in
connection with the transactions contemplated herein will comply as to form  and
substance  in  all  material aspects  with  the applicable  requirements  of the
Securities Act  and the  rules and  regulations promulgated  thereunder and  the
Exchange Act and the rules and regulations promulgated thereunder.
 
    (c) The Target represents, warrants and agrees that the information supplied
by  the  Target  for  inclusion  in the  Registration  Statement  and  the Proxy
Statement shall not,  at (i)  the time  the Registration  Statement is  declared
effective,  (ii)  the time  the  Proxy Statement  (or  any amendment  thereof or
supplement thereto) is first mailed to the stockholders of the Target, (iii) the
time of the Target's Stockholder Meeting,  and (iv) the Effective Time,  contain
any  statement which, at such time and in light of the circumstances under which
it is made, is false or misleading with respect to any material fact, or omit to
state any material fact required to be stated therein, or necessary in order  to
make the statements therein not false or misleading. If at any time prior to the
Effective Time any event or circumstance relating to the Target, or its officers
or directors, should be discovered by the Target which should be set forth in an
amendment  or a supplement to the Registration Statement or Proxy Statement, the
Target shall promptly inform Acquiror. Notwithstanding the foregoing, the Target
makes no representation or warranty with respect to any information supplied  by
Acquiror  or Acquiror  Sub or any  of their representatives  in the Registration
Statement or Proxy Statement. All documents  that the Target is responsible  for
filing with the SEC in connection with the transactions contemplated herein will
comply  as to form  and substance in  all material respects  with the applicable
requirements of the  Securities Act  and the rules  and regulations  promulgated
thereunder  and  the  Exchange Act  and  the rules  and  regulations promulgated
thereunder.
 
    (d) The Target, Acquiror  and Acquiror Sub each  hereby (i) consents to  the
use  of its name and, on behalf of its subsidiaries and affiliates, the names of
such subsidiaries and affiliates  and to the  inclusion of financial  statements
and  business  information  relating  to such  party  and  its  subsidiaries and
affiliates (in each case, to the extent required by applicable securities  laws)
in  the Registration Statement and  the Proxy Statement; (ii)  agrees to use all
reasonable efforts  to  obtain the  written  consent  of any  person  or  entity
retained  by it which may be required to be named (as an expert or otherwise) in
the  Registration  Statement  or  the  Proxy  Statement;  and  (iii)  agrees  to
cooperate,  and agrees to  use all reasonable efforts  to cause its subsidiaries
and  affiliates  to  cooperate,  with  any  legal  counsel,  investment  banker,
accountant  or  other agent  or representative  retained by  any of  the parties
specified in clause (i) above in connection with the preparation of any and  all
information  required,  as  determined  after  consultation  with  each  party's
counsel, to  be disclosed  by  applicable securities  laws in  the  Registration
Statement or the Proxy Statement.
 
    SECTION 6.02.  TARGET STOCKHOLDER MEETING.  The Target shall call and hold a
meeting  of its stockholders (the "TARGET'S STOCKHOLDER MEETING") as promptly as
practicable for the purpose  of voting upon the  approval of this Agreement  and
the Merger, and the Target shall use all commercially reasonable efforts to hold
the  Target's Stockholder Meeting as soon as practicable after the date on which
the  Registration  Statement  becomes  effective.  The  Target  shall  use   all
commercially  reasonable  efforts to  solicit from  its stockholders  proxies in
favor of the approval of this Agreement and the Merger and shall take all  other
action  reasonably  necessary or  advisable  to secure  the  vote or  consent of
stockholders required  by  Delaware  Law to  obtain  such  approvals  (including
unanimously recommending such approval).
 
    SECTION  6.03.   APPROPRIATE  ACTION; CONSENTS;  FILINGS.   (a)  The Target,
Acquiror and Acquiror Sub shall use their best efforts to (i) take, or cause  to
be  taken,  all appropriate  action, and  do, or  cause to  be done,  all things
necessary, proper or advisable under applicable  Law or required to be taken  by
any  Governmental Authority  or otherwise to  consummate and  make effective the
Transactions as  promptly  as practicable,  (ii)  obtain from  any  Governmental
Authorities  any consents, licenses, permits, waivers, approvals, authorizations
or orders required to be  obtained or made by Acquiror  or the Target or any  of
their  subsidiaries in connection with the authorization, execution and delivery
of this Agreement and the  consummation of the Transactions, including,  without
limitation, the Merger, and
 
                                      A-33
<PAGE>
(iii)  as promptly  as practicable, make  all necessary  filings, and thereafter
make any other  required submissions,  with respect  to this  Agreement and  the
Merger required under (A) the Securities Act and the Exchange Act, and any other
applicable  federal or state  securities Laws, (B) the  rules and regulations of
the NYSE, (C) Delaware Law, (D) the HSR Act and any related governmental request
thereunder, and (E)  any other applicable  Law; PROVIDED that  Acquiror and  the
Target shall cooperate with each other in connection with the making of all such
filings,  including providing  copies of  all such  documents to  the non-filing
party and  its  advisors  prior  to filing  and,  if  requested,  accepting  all
reasonable  additions, deletions  or changes suggested  in connection therewith.
The Target and  Acquiror shall use  reasonable best efforts  to furnish to  each
other  all information required for  any application or other  filing to be made
pursuant to  the rules  and regulations  of any  applicable Law  (including  all
information  required to be included in the Proxy Statement and the Registration
Statement) in connection with the transactions contemplated by this Agreement.
 
    (b) (i) Each of  Acquiror and the  Target shall give  (or shall cause  their
respective  subsidiaries to  give) any  notices to  third parties,  and use, and
cause their  respective  subsidiaries  to  use,  their  commercially  reasonable
efforts to obtain any third party consents (including those set forth in Section
3.05(a)(iii)),  (A) necessary to  consummate the Transactions,  (B) disclosed or
required to  be disclosed  in the  Target Disclosure  Schedule or  the  Acquiror
Disclosure  Schedule or (C)  required to prevent a  Material Adverse Effect from
occurring prior to or after the Effective Time.
 
        (ii) In the event that Acquiror or  the Target shall fail to obtain  any
    third  party consent described in subsection  (b)(i) above, it shall use its
    best efforts, and shall  take any such actions  reasonably requested by  the
    other  party, to minimize  any adverse effect upon  the Target and Acquiror,
    their respective subsidiaries, and their respective businesses resulting, or
    which could reasonably be expected to result after the Effective Time,  from
    the failure to obtain such consent.
 
       (iii) The Target agrees to cooperate with Acquiror prior to the Effective
    Time  to  clarify  the terms  of  any  agreements which  the  Target  or any
    Subsidiary is  currently a  party which  might restrict  the Target  or  any
    Subsidiary from the conduct of the operation, charter or leasing of drilling
    rigs  in any  area of the  world so that  from and after  the Effective Time
    Acquiror and the  Target will obtain  the benefits of  the current  practice
    under  such  agreements  whereby Acquiror  and  the  Target will  not  be so
    restricted in their operations.
 
    (c) From the  date of this  Agreement until the  Effective Time, each  party
shall  promptly notify the other party of  any pending, or to the best knowledge
of the first party, threatened, action, proceeding or investigation by or before
any Governmental  Authority  or any  other  person (i)  challenging  or  seeking
material  damages in connection with the Merger  or the conversion of the Target
Common Stock into Acquiror Common Stock  pursuant to the Merger or (ii)  seeking
to  restrain or prohibit the  consummation of the Merger  or otherwise limit the
right of Acquiror or, to the knowledge of such first party, Acquiror  Subsidiary
to  own or operate all or any portion of the businesses or assets of the Target,
which in either case is reasonably likely  to have a Material Adverse Effect  on
the Target prior to the Effective Time, or a Material Adverse Effect on Acquiror
and  the Acquiror Sub (including the  Surviving Corporation) after the Effective
Time.
 
    SECTION 6.04.   ACCESS  TO  INFORMATION; CONFIDENTIALITY.   Subject  to  the
Confidentiality  Agreement (as hereinafter defined), from the date hereof to the
Effective Time, Acquiror and the Target  will each provide to the other,  during
normal  business hours and upon reasonable notice, access to all information and
documents which the other may reasonably request regarding the business, assets,
liabilities, employees  and  other  aspects  of  the  other  party,  other  than
information  and documents that in the opinion of such other party's counsel may
not be disclosed under applicable Law.
 
    SECTION 6.05.   NO  SOLICITATION OF  TRANSACTIONS.   The Target  shall  not,
directly  or  indirectly, negotiate  with any  person  other than  Acquiror with
respect to the  acquisition of the  Target or  the shares of  the Target  Common
Stock  owned by Dual Invest AS  and it will not, and  will not permit any of its
officers, directors,  employees, agents  or representatives  (including  without
limitation,  investment  bankers,  attorneys and  accountants)  to  (i) initiate
contact with, (ii) make, solicit or encourage any
 
                                      A-34
<PAGE>
inquiries or proposals, (iii) enter into, or participate in, any discussions  or
negotiations  with, (iv) disclose,  directly or indirectly,  any information not
customarily disclosed concerning the  business and properties  of the Target  or
any  Subsidiary  to or  (v) afford  any access  to  any of  the Target's  or any
Subsidiary's properties, books and records to any person in connection with  any
possible proposal relating to (a) the disposition of their respective businesses
or  substantially  all or  their assets,(b)  the acquisition  of equity  or debt
securities of  the Target  including equity  or debt  securities owned  by  Dual
Invest  AS,  or (iii)  the merger,  share exchange  or business  combination, or
similar acquisition transaction of  or involving Target  or any Subsidiary  with
any  person other  than Acquiror; PROVIDED,  HOWEVER, that  nothing contained in
this Section  6.05 shall  prohibit the  Board of  Directors of  the Target  (the
"BOARD") from taking and disclosing to the stockholders of the Target a position
in  accordance with Rules 14d-9 and 14e-2 under the Exchange Act with respect to
a tender offer or an exchange offer  for share of Target Common Stock  commenced
by  a third party. The Target shall  notify Acquiror promptly if any proposal or
offer, or any inquiry or contact with  any person with respect thereto, is  made
and  shall, in any  such notice to  Acquiror, indicate in  reasonable detail the
identity of the person making such  proposal, offer, inquiry or contact and  the
terms  and conditions  of such proposal,  offer, inquiry or  contact. The Target
agrees not to  release any  third party  from, or  waive any  provision of,  any
confidentiality  or standstill  agreement to  which the  Target is  a party. The
Target  immediately  shall  cease  and  cause  to  be  terminated  all  existing
discussions  or negotiations with any  parties conducted heretofore with respect
to any of the foregoing.
 
    SECTION  6.06.    DIRECTORS'  AND   OFFICERS'  INDEMNIFICATION.    (a)   The
Certificate  of  Incorporation and  Bylaws  of the  Surviving  Corporation shall
contain provisions no less  favorable with respect  to indemnification than  are
set  forth in Article VIII  of the Bylaws of  the Target, which provisions shall
not be amended, repealed  or otherwise modified  for a period  of six (6)  years
from  the Effective Time  in any manner  that would affect  adversely the rights
thereunder of  individuals who  at any  time prior  to the  Effective Time  were
directors,  officers  or employees  of the  Target or  any of  its Subsidiaries,
unless such modification shall be required by Delaware Law.
 
    (b) From and  after the Effective  Time and for  a period of  six (6)  years
thereafter,  the Surviving Corporation shall indemnify, defend and hold harmless
each person who  is now,  or has  been at any  time prior  to the  date of  this
Agreement  or who becomes prior to the Effective Time, an officer or director of
Target or  any of  the Subsidiaries,  or an  employee of  Target or  any of  the
Subsidiaries  who acts as a fiduciary under any employee benefit plans of Target
or any of the Subsidiaries (collectively, the "INDEMNIFIED PARTIES") against all
losses,  expenses  (including  reasonable  attorneys'  fees),  claims,  damages,
liabilities  or amounts that are paid in settlement of, with the approval of the
Surviving Corporation (which  approval shall not  unreasonably be withheld),  or
otherwise  in connection  with, any  threatened or  actual claim,  action, suit,
proceeding or investigation (a "CLAIM"), based in whole or in part on or arising
in whole or in part  out of the fact that  the Indemnified Party (or the  Person
controlled  by the Indemnified Party)  is or was a  director, officer or such an
employee of  Target or  any of  the Subsidiaries  and pertaining  to any  matter
existing  or arising out  of actions or  omissions occurring at  or prior to the
Effective Time (including,  without limitation,  any Claim arising  out of  this
Agreement  or any of the transactions  contemplated hereby), whether asserted or
claimed prior to, at or  after the Effective Time, in  each case to the  fullest
extent permitted under Delaware Law, and shall pay any expenses, as incurred, in
advance  of  the final  disposition of  any  such action  or proceeding  to each
Indemnified Party to the  fullest extent permitted  under Delaware Law.  Without
limiting  the foregoing, in the  event any such claim  is brought against any of
the Indemnified  Parties,  (i)  such  Indemnified  Parties  may  retain  counsel
(including  local counsel)  satisfactory to them  and which  shall be reasonably
satisfactory to Acquiror and the Surviving Corporation shall pay all  reasonable
fees  and expenses of  such counsel for  such Indemnified Parties;  and (ii) the
Surviving Corporation shall use all reasonable efforts to assist in the  defense
of  any such Claim, provided that the  Surviving Corporation shall not be liable
for any settlement effected without its written consent, which consent, however,
shall not be  unreasonably withheld. The  Indemnified Parties as  a group  shall
retain only one law firm (plus appropriate local counsel) to represent them with
respect to each such Claim unless there is, as
 
                                      A-35
<PAGE>
determined  by counsel to the Indemnified Parties, under applicable standards of
professional conduct, a conflict or a reasonable likelihood of a conflict on any
significant issue between the positions of  any two or more Indemnified  Parties
at the expense of the Surviving Corporation.
 
    (c)  For a period of  six (6) years after  the Effective Time, the Surviving
Corporation shall  cause to  be maintained  in effect  the current  policies  of
directors'  and  officers'  liability  insurance maintained  by  Target  and the
Subsidiaries covering all of the individuals currently covered thereby (provided
that the Surviving Corporation may substitute therefor policies of at least  the
same  coverage and  amounts containing  terms and  conditions which  are no less
advantageous to such officers and directors) with respect to claims arising from
facts or events which occurred before the Effective Time; provided, however, the
Surviving Corporation shall not be required  to pay premiums for such  insurance
in excess of $275,000 in the aggregate.
 
    SECTION  6.07.  OBLIGATIONS OF ACQUIROR SUB.  Acquiror shall take all action
necessary to cause Acquiror Sub to perform its obligations under this  Agreement
and to consummate the Merger on the terms and subject to conditions set forth in
this Agreement.
 
    SECTION  6.08.   PUBLIC ANNOUNCEMENTS.   (a)  Acquiror and  the Target shall
consult with each other before issuing any press release or otherwise making any
public statements with respect  to this Agreement or  any Transaction and  shall
not issue any such press release or make any such public statement prior to such
consultation  and (b) prior to the Effective Time, the Target will not issue any
other press  release  or otherwise  make  any public  statements  regarding  its
business,  except as may  be required by  Law or any  listing agreement with the
National Association  of Securities  Dealers,  Inc. (the  "NASD") to  which  the
Target is a party.
 
    SECTION  6.09.  DELIVERY OF SEC DOCUMENTS.   Each of the Target and Acquiror
shall promptly  deliver to  the other  true and  correct copies  of any  report,
statement  or  schedule  filed with  the  SEC  subsequent to  the  date  of this
Agreement.
 
    SECTION 6.10.  ENVIRONMENTAL  ASSESSMENT.  The  Target agrees that  Acquiror
may  perform or have performed on its  behalf an environmental assessment of its
owned or leased real property. The  Target will give Acquiror and the  officers,
directors, employees, agents, consultants and representatives of Acquiror access
to  the owned or  leased real property, including  without limitation, access to
enter upon and investigate and collect air, surface water, groundwater and  soil
samples,  in  order to  conduct the  environmental  assessment. The  Target will
cooperate with Acquiror  in connection with  such assessment, including  without
limitation  scheduling site visits as necessary to complete the assessment prior
to the Effective Time. The environmental assessment conducted by Acquiror or  on
Acquiror's  behalf shall  be satisfactory to  Acquiror in its  sole and absolute
discretion.
 
    SECTION 6.11.   NOTIFICATION  OF CERTAIN  MATTERS.   The Target  shall  give
prompt  notice to Acquiror, and Acquiror shall give prompt notice to the Target,
of (i)  the occurrence,  or  non-occurrence, of  any  event the  occurrence,  or
non-occurrence, of which would be likely to cause any representation or warranty
contained  in this Agreement to be untrue  or inaccurate and (ii) any failure of
the Target, Acquiror  or Acquiror Sub,  as the case  may be, to  comply with  or
satisfy any covenant, condition or agreement to be complied with or satisfied by
it  hereunder; PROVIDED,  HOWEVER, that the  delivery of any  notice pursuant to
this Section 6.11  shall not limit  or otherwise affect  the remedies  available
hereunder to the party receiving such notice.
 
    SECTION  6.12.   FURTHER ACTION.   At any time  and from time  to time, each
party to this  Agreement agrees,  subject to the  terms and  conditions of  this
Agreement, to take such actions and to execute and deliver such documents as may
be  necessary  to effectuate  the  purposes of  this  Agreement at  the earliest
practicable time.
 
    SECTION 6.13.   EMPLOYEE BENEFITS.   (a) On  or before March  31, 1996,  the
Target  shall (i) amend the Dual  Drilling Company Employees Tax Deferred/Thrift
Savings Plan and Trust (the "401(K) PLAN") to (A) provide for the limitation  on
compensation that may be considered under the
 
                                      A-36
<PAGE>
401(k) Plan imposed by Section 401(a)(17) of the Code, as amended by the Omnibus
Budget Reconciliation Act of 1993, Publ. L. No. 103-66 effective January 1, 1994
and  (B) amend the 401(k) Plan to provide clarification that the 401(k) Plan, as
amended and restated effective  January 1, 1991,  also constitutes an  amendment
and restatement of the separate plan documents governing the 401(k) Plan and the
Dual  Drilling Company Employees Thrift Plan and  Trust that was merged into the
401(k) Plan effective January  1, 1991 (the "PRIOR  THRIFT PLAN") to the  extent
required  for both the 401(k) Plan and Prior Thrift Plan to constitute qualified
plans within the meaning  of Sections 401(a)  and 501(a) of  the Code for  their
plan  years 1987  through 1990 (collectively  the "401(K)  PLAN AMENDMENT"), and
(ii) file the 401(k) Plan Amendment with the Dallas Key District of the Internal
Revenue Service for the  purpose of entering into  a closing agreement with  the
Internal  Revenue Service regarding  the 401(k) Plan  Amendment pursuant to Rev.
Proc 94-16, 1994-1 C.B. 576, as modified by the Memorandum from Internal Revenue
Service Assistant Commissioner (Employee  Plans and Exempt Organizations)  James
McGovern to Field Offices Outlining Procedures for Processing Employee Plans not
Amended  Timely Under  Tax Reform  Act of 1986,  Released October  26, 1995 (the
"CLOSING AGREEMENT"). Target shall timely pay any monetary sanction assessed  by
the  Internal Revenue Service pursuant to  the Closing Agreement. Subject to the
Target taking  the  action required  by  the  two preceding  sentences  and  the
Internal Revenue Service entering into the Closing Agreement, Acquiror agrees to
(i)  assume sponsorship of the  401(k) Plan as of  the Effective Time, (ii) upon
entering into the  Closing Agreement, file  the 401(k) Plan  Amendment with  the
Internal  Revenue Service  for the purpose  of obtaining  a determination letter
from the Internal Revenue Service that both the 401(k) Plan and the Prior Thrift
Plan are qualified within the meaning of Sections 401(a) and 501(a) of the  Code
retroactive  to the 1987 plan  year of each plan and  (iii) upon issuance by the
Internal Revenue Service  of a  favorable determination letter  as described  in
clause  (ii) of this sentence, merge the 401(k) Plan into the ENSCO Savings Plan
as soon as  administratively practicable thereafter.  Amounts payable under  the
401(k)  Plan to employees whose employment is terminated for any reason prior to
December 31, 1996  shall be  paid promptly after  any such  termination and  not
after  the end  of the  1996 plan  year if  requested by  the employee  and such
payment is permitted by the terms of the 401(k) Plan.
 
    (b) Effective  as of  the  Effective Time,  Acquiror  agrees to  assume  or,
alternatively,  cause  the Surviving  Corporation to  assume, the  Dual Drilling
Company Employee  Health  Benefit  Plan  (the "DUAL  MEDICAL  PLAN"),  the  Dual
Drilling  Company Group Life Insurance Plan, the Dual Drilling Company Long-Term
Disability Plan, the Dual Drilling Company Group Travel Accident Insurance Plan,
the Voluntary Personal  Accident Insurance Plan,  the Long-Term Disability  Plan
for  Third Country Nationals,  and the Premium  Conversion Cafeteria Plan unless
any such plan has been terminated by the Target as directed by Acquiror prior to
the Effective Time (collectively, the "GROUP INSURANCE PLANS"). Acquiror  agrees
to  provide to  former employees  of the Target  and its  Subsidiaries under the
plans continued  or to  be  established by  Acquiror continuation  group  health
coverage  under  Section 4980B  of the  Code and  Sections 601  to 608  of ERISA
("COBRA COVERAGE") for any individuals  receiving COBRA Coverage under the  Dual
Medical  Plan as of the  Effective Time and for any  employees of the Target and
its Subsidiaries and their  dependents who become eligible  for and elect  COBRA
Coverage  as  a  result  of  the  Transactions  contemplated  by  the Agreement;
provided, however, nothing  contained herein shall  obligate Acquiror to  extend
COBRA Coverage beyond its normal expiration period. As soon as practicable after
the  Effective Time, the  Target shall transfer  to Acquiror or,  if directed by
Acquiror, to  the Surviving  Corporation, any  assets, including  any  insurance
policies,  held by the Target or a Subsidiary supporting the payment of benefits
under the Group Insurance Plans. Acquiror and the Target agree that Acquiror may
direct the Target to take such action as Acquiror deems appropriate to terminate
any Group  Insurance Plan  conditioned upon  the  occurrence of  and as  of  the
Effective Time, liquidate the assets of any trust or funding arrangement for any
such plan and settle all claims for benefits under any such plan.
 
    (c)  Effective as  of the  Effective Time,  the Target  shall take  all such
action necessary (i) to amend the terms of the post-retirement medical  coverage
provided  by the Target under the Dual Medical Plan (the "RETIREE MEDICAL PLAN")
to  provide  that  no  employee  of  the  Target  or  any  Subsidiary,  or   any
 
                                      A-37
<PAGE>
beneficiary   or  dependent  of  any  such  employee,  may  become  entitled  to
post-retirement medical  coverage  under  the  Retiree Medical  Plan  due  to  a
retirement  or other termination of employment after the Effective Time and (ii)
to terminate  the Retiree  Medical Plan.  Effective as  of the  Effective  Time,
Acquiror  agrees  to provide  post-retirement medical  coverage under  the ENSCO
Medical Plan to any former  employee of the Target or  any Subsidiary who as  of
the  Effective  Time is  receiving  post-retirement medical  coverage  under the
Retiree Medical Plan, provided that such  coverage under the ENSCO Medical  Plan
shall be on the same terms and conditions and shall provide the same benefits as
currently  available to  retired former  employees of  Acquiror under  the ENSCO
Medical Plan, but without regard to the age and service provisions of the  ENSCO
Medical  Plan  that determine  initial  eligibility for  post-retirement medical
coverage and without regard to  any preexisting condition limitations  contained
in  the ENSCO Medical Plan. Any employee of  the Target or any Subsidiary who is
not entitled to post-retirement medical coverage under the Retiree Medical  Plan
as  of the Effective Time will  be eligible for post-retirement medical coverage
under the ENSCO Medical Plan only if the terms and conditions for such  coverage
under the ENSCO Medical Plan are satisfied.
 
    (d) Effective as of the day before the Effective Time, the Target shall take
all  such action  necessary (i)  to amend  the provisions  of the  Dual Drilling
Company Supplemental  Executive  Retirement  Plan (the  "SERP")  to  freeze  all
benefits  under the SERP as  of the day before the  Effective Time and to permit
Acquiror or the Surviving Corporation, as successor to the Target's rights under
the SERP, to distribute  the determined benefit under  the SERP payable to  each
participant  covered by the SERP in one  lump sum payment in the sole discretion
of Acquiror at any time  from and after January 1,  1997, (ii) to determine  the
benefits payable thereunder to each participant, and (iii) to terminate the SERP
effective as of the day before the Effective Time.
 
    (e) Effective as of the day before the Effective Time, the Target shall take
all  such action  necessary (i)  to amend  the provisions  of the  Dual Drilling
Company Benefit Restoration Plan (the "BENEFIT RESTORATION PLAN") to freeze  all
benefits  under the  Benefit Restoration  Plan, (ii)  to determine  the benefits
payable thereunder  to each  participant,  and (iii)  to terminate  the  Benefit
Restoration Plan effective as of the day before the Effective Time.
 
    (f)  To facilitate the payment of benefits from the Benefit Restoration Plan
and, if directed  by Acquiror to  facilitate the payment  of benefits under  the
SERP,  the Target shall take all such action necessary (i) to provide a schedule
of benefits payable under the Benefit  Restoration Plan and, if applicable,  the
SERP,  to the Trust Company of Texas  in its capacity as trustee (the "TRUSTEE")
of the Umbrella Trust evidenced by that certain Trust Agreement dated October 1,
1994 by  and between  Dual Drilling  Company  and Trust  Company of  Texas  (the
"TRUST"),  (ii) to  direct the Trustee  to make  payment from the  assets of the
Trust of all benefits  payable under the Benefit  Restoration Plan or the  SERP,
and  (iii) to cause the Trustee to liquidate  the assets of the Trust, except as
otherwise determined by Acquiror, provided that such actions are consistent with
the terms of the SERP, Benefit Restoration Plan and Trust, each as amended.
 
    (g) Effective as of the day before the Effective Time, the Target shall take
all such  action  necessary  to  freeze  all  benefits  under  the  Discontinued
Executive   and  Manager  Team  Incentive  Program  ("TEAM  INCENTIVE  PROGRAM")
maintained by  the Target  and Acquiror  agrees to  assume the  benefit  payment
obligations  of the  Target under  the Team Incentive  Plan, and  after all such
benefit payments have  been made,  the Team  Incentive Program  shall be  deemed
terminated  and Acquiror shall take such other action as it deems appropriate to
accomplish such termination.
 
    (h) Effective  as of  the Effective  Time, Acquiror  shall assume  the  Dual
Drilling  Company Severance  Pay Plan  for Office  Employees, the  Dual Drilling
Company Severance Pay Plan for Key Operating and Support Staff Employees and the
Dual Drilling  Company Severance  Pay  Plan for  Key Operating  and  Engineering
Managers  (collectively, the "EMPLOYEE SEVERANCE PLANS"), provided that Acquiror
shall not  be  obligated  to  continue the  Employee  Severance  Plans  for  any
specified period of time.
 
                                      A-38
<PAGE>
    (i) Effective as of the day before the Effective Time, the Target shall take
all  such action necessary  to freeze all benefits  under the Employee Incentive
Plan, the Safety Bonus Plan, and the Pro-Performance Bonus Plan for  Toolpushers
maintained  by  the Target  and Acquiror  agrees to  assume the  benefit payment
obligations of the Target under  such plans and cause  such payments to be  made
after  the Effective Time, and after all such payments have been made, the plans
shall be deemed terminated and Acquiror shall take such other action as it deems
appropriate to accomplish such termination.
 
    (j)  Acquiror agrees to assume the benefit payment obligations of the Target
under the Dual  Special Performance  Unit Plan,  effective August  21, 1995  (as
amended, the "DUAL UNIT PLAN"), maintained by the Target and cause such payments
to be made, and after all such payments have been made, the Dual Unit Plan shall
be  deemed terminated  and Acquiror  shall take  such other  action as  it deems
appropriate to accomplish  such termination.  Target agrees  that the  aggregate
amount of the Performance Bonus Pool (as defined in the Dual Unit Plan) plus the
amount payable to David W. Skarke as a performance bonus calculated based on the
amount  of the Performance Bonus Pool  pursuant to that certain letter agreement
dated October 2, 1995 (the "Letter  Agreement") shall be $2,000,000. The  Target
shall  take all required actions prior to  the Effective Time to amend the terms
of the Dual Unit  Plan and the  Letter Agreement so  that the aggregate  amounts
payable thereunder shall be $2,000,000.
 
    (k)  The Target agrees to terminate the Annual Incentive Plan as of the date
of the  commencement of  such  plan and  accordingly  no benefits  shall  accrue
thereunder.
 
    (l)  Nothing in this Section 6.13 is intended or shall be construed to limit
Acquiror's right to  modify, change,  terminate or  otherwise alter  any of  the
provisions of, or benefits provided under, any new plans or plans established by
Acquiror  or  the plans  described  in this  Section  6.13 that  are  assumed by
Acquiror or  the Surviving  Corporation,  or to  create  any vested  rights  for
participants in the benefits provided under such plans.
 
    (m)  Prior to the  Effective Time, the  Target shall file  with the Internal
Revenue Service, all Annual Reports, Form  5500, and the applicable Schedule  F,
required  to be  filed under Section  6039(d) of  the Code, with  respect to the
Premium Conversion Cafeteria Plan for each plan year since the inception of  the
Premium Conversion Cafeteria Plan.
 
    (n)  Prior  to  the  Effective  Time,  the  Target  shall,  pursuant  to the
requirements of the Delinquent Filer  Voluntary Compliance Program described  in
the  Department of Labor ("DOL")  Notice published in 59  Fed. Reg. 20873 (April
27, 1995), as a condition of relief from the annual reporting requirements,  (i)
elect  to file the  one-time statement required under  DOL Reg. 2520-104-23 with
respect to (A) the  SERP and (B) the  Target Employment Contracts applicable  to
L.H.  Dick Robertson, W. Allen Parks and  Dudley M. Haralson, (ii) file with the
DOL, the first page of the Form  5500, Annual Report, with the applicable  items
completed,  (iii) pay to the  DOL the Two Thousand  Five Hundred Dollar ($2,500)
penalty applicable to such  Form 5500 filings,  and (iv) file  with the DOL  the
applicable one-time statements required with respect to the SERP and such Target
Employment Contracts.
 
    (o)  All  of the  employment agreements  listed on  Schedule 6.13(o)  of the
Target Disclosure Schedule will be terminated by the Target as of the  Effective
Time.  Notwithstanding the foregoing, Acquiror may offer employment prior to the
Effective Time to persons who have  such employment agreements and the terms  of
such  employment, if  accepted by  such employee of  the Target,  may affect the
Target's obligations to the affected employee.
 
    SECTION 6.14.  AFFILIATES; ACCOUNTING AND  TAX TREATMENT.  (a) Section  6.14
of the Target Disclosure Schedule lists the names and addresses of those persons
who  are, in the Target's reasonable judgment, "affiliates" of the Target within
the meaning of Rule 145 under  the Securities Act (each, a "TARGET  AFFILIATE").
The  Target shall  use all commercially  reasonable efforts  to obtain Affiliate
Agreements in the form of EXHIBIT B hereto ("AFFILIATE AGREEMENTS") from (i)  at
least  30 days prior to the Effective  Time, each of the officers, directors and
stockholders (other than Dual Invest AS) of the
 
                                      A-39
<PAGE>
Target specified in Section 6.14 of the Target Disclosure Schedule and (ii)  any
person  who may be deemed to have become  an affiliate of the Target (under Rule
145 under the Securities Act) after the  date of this Agreement and on or  prior
to the Effective Time as soon as practicable after the date on which such person
attains  such status. Each party hereto shall  use its best efforts to cause the
Merger to qualify, and shall not take  any actions which would (or fail to  take
any actions the failure of which would) prevent the Merger from qualifying, as a
reorganization  qualifying under the  provisions of Section  368(a) of the Code,
including, without limitation, that Acquiror  agrees that it will cause  Target,
in  its new capacity as  a subsidiary of Acquiror,  to (i) continue the Target's
historic business and (ii)  use a significant portion  of its historic  business
assets in such business.
 
    (b)  The  Target  shall  provide  to Acquiror  for  inclusion  in  the Proxy
Statement a written opinion from Akin, Gump, Strauss, Hauer & Feld, L.L.P. dated
as of the date that the Proxy  Statement is first mailed to stockholders of  the
Target to the effect that (i) the Merger will be treated for U.S. federal income
tax  purposes as a  reorganization within the  meaning of Section  368(a) of the
Code; (ii) Acquiror, Acquiror Sub  and the Target will each  be a party to  that
reorganization  within the meaning of Section 368(b)  of the Code; and (iii) the
stockholders of the Target shall not recognize any gain or loss for U.S. federal
income tax purposes as  a result of  the Merger, other than  to the extent  such
stockholders receive cash in lieu of fractional shares.
 
    SECTION  6.15.   CERTAIN  EMPLOYEES.   The  Target  agrees to  terminate the
employment of  any  executive officer  (as  that term  is  defined in  Rule  405
promulgated  under the  Exchange Act)  of the  Target identified  by Acquiror at
least one day prior to the Effective Time. Such termination shall not  adversely
affect  any  Long-Term  Options,  severance  or  other  benefits  such executive
officers shall be entitled to receive.
 
                                  ARTICLE VII
                            CONDITIONS TO THE MERGER
 
    SECTION 7.01.  CONDITIONS TO THE OBLIGATIONS OF EACH PARTY.  The obligations
of the Target, Acquiror and Acquiror Sub to consummate the Merger are subject to
the satisfaction of the following conditions:
 
        (a) this Agreement and the  Transactions contemplated hereby shall  have
    been approved and adopted by the affirmative vote of the stockholders of the
    Target  in  accordance with  Delaware Law  and  the Target's  Certificate of
    Incorporation and Bylaws and the rules of the NASD;
 
        (b) no Governmental Authority  shall have enacted, issued,  promulgated,
    enforced  or entered any  order, executive order,  stay, decree, judgment or
    injunction (each an  "ORDER") or  statute, rule  or regulation  which is  in
    effect  and which has the  effect of making the  Merger illegal or otherwise
    prohibiting consummation of the Merger;
 
        (c) the Registration Statement shall  have been declared effective,  and
    no  stop order  suspending the  effectiveness of  the Registration Statement
    shall be in effect;
 
        (d) Acquiror and the Target shall  have received from the NYSE  evidence
    that the shares of Acquiror Common Stock to be issued to the stockholders of
    the  Target in the Merger shall be  listed on the NYSE immediately following
    the Effective Time; and
 
        (e) any applicable  waiting period  under the  HSR Act  relating to  the
    Merger shall have expired or been terminated.
 
    SECTION  7.02.    CONDITIONS TO  THE  OBLIGATIONS OF  ACQUIROR  AND ACQUIROR
SUB.  The obligations of Acquiror and Acquiror Sub to consummate the Merger  are
subject to the satisfaction of the following further conditions:
 
        (a) the Target shall have performed or complied in all material respects
    with all agreements and covenants required by this Agreement to be performed
    or complied with by it at or prior to the
 
                                      A-40
<PAGE>
    Effective  Time and each of the representations and warranties of the Target
    contained in  this Agreement  shall  be true  and  correct in  all  material
    respects  as of the Effective Time as though made on and as of the Effective
    Time, except that those representations and warranties which address matters
    only as of a particular date shall  remain true and correct as of such  date
    and  Acquiror shall have  received a certificate of  an executive officer of
    the Target to that effect;
 
        (b) Acquiror  shall have  received from  each Target  Affiliate and  any
    other  person who may  be deemed to  have become an  affiliate of the Target
    (under Rule 145 under the Securities  Act) after the date of this  Agreement
    and at or prior to the Effective Time a signed Affiliate Agreement;
 
        (c)  since the date of this Agreement, no material adverse change in the
    financial condition, results of operations or business of the Target and the
    Subsidiaries, taken as a whole, shall have occurred, and neither the  Target
    nor  any  Subsidiary shall  have suffered  any  damage, destruction  or loss
    materially affecting  the  business or  properties  of the  Target  and  the
    Subsidiaries, taken as a whole;
 
        (d)  the Target  and each Subsidiary  shall have  delivered to Acquiror,
    each dated as of a date not earlier than thirty days prior to the  Effective
    Time,   (i)   copies  of   the  certificates   of  incorporation   or  other
    organizational documents, including all amendments thereto, certified by the
    appropriate government official, of the Target and each Subsidiary, (ii)  to
    the  extent issued by  such jurisdiction, certificates  from the appropriate
    governmental official to the  effect that Target and  each Subsidiary is  in
    good  standing in such jurisdiction and listing all organizational documents
    of Target and each Subsidiary  on file, (iii) to  the extent issued by  such
    jurisdiction,  a certificate  from the appropriate  governmental official in
    each jurisdiction in which the Target and each Subsidiary is qualified to do
    business to  the  effect  that such  member  is  in good  standing  in  such
    jurisdiction,  (iv) to the extent  issued by such jurisdiction, certificates
    indicating all taxes are current for  the Target and each Subsidiary in  its
    jurisdiction  of organization and each jurisdiction  in which such member is
    qualified to do  or conducting business,  (v) to the  extent issued by  such
    jurisdiction, certificates from the appropriate governmental official to the
    effect  that  each Target  Drilling  Rig is  documented  and flagged  in the
    jurisdiction described in  Section 3.28 of  the Target Disclosure  Schedule,
    and  (vi) Confirmation  of Class  Certificates from  the American  Bureau of
    Shipping indicating that each of the Target Drilling Rigs that is a  jack-up
    drilling rig is in class and free of any recommendations; and
 
        (e) Acquiror shall have received from Akin, Gump, Strauss, Hauer & Feld,
    L.L.P.,  a written opinion dated as of  the date of the Closing covering the
    matters set  forth on  EXHIBIT C  hereto in  form and  substance  reasonably
    acceptable to counsel for Acquiror.
 
    SECTION 7.03.  CONDITIONS TO THE OBLIGATIONS OF THE TARGET.  The obligations
of  the  Target  to  consummate  the  Merger  are  subject  are  subject  to the
satisfaction of the following further conditions:
 
        (a) Acquiror and Acquiror  Sub shall have performed  or complied in  all
    material  respects  with  all  agreements  and  covenants  required  by this
    Agreement to  be performed  or complied  with by  them at  or prior  to  the
    Effective  Time and each  of the representations  and warranties of Acquiror
    and Acquiror Sub contained  in this Agreement shall  be true and correct  in
    all  material respects as of the Effective Time, as though made on and as of
    the Effective Time, except that  those representations and warranties  which
    address  matters only as of a particular  date shall remain true and correct
    as of such  date and  the Target  shall have  received a  certificate of  an
    executive officer of Acquiror to that effect;
 
        (b)  since the date of this Agreement, no material adverse change in the
    financial condition, results of operations  or business of Acquiror and  the
    Acquiror  Subsidiaries, taken as a whole,  shall have occurred, and Acquiror
    and  the  Acquiror  Subsidiaries  shall   not  have  suffered  any   damage,
    destruction  or  loss materially  affecting  the business  or  properties of
    Acquiror and the Acquiror Subsidiaries, taken as a whole; and
 
                                      A-41
<PAGE>
        (c) the  Target shall  have received  from Baker  & McKenzie  a  written
    opinion  dated as of the date of  the Closing covering the matters set forth
    on EXHIBIT D hereto in form  and substance reasonably acceptable to  counsel
    for the Target.
 
                                  ARTICLE VIII
                       TERMINATION, AMENDMENT AND WAIVER
 
    SECTION 8.01.  TERMINATION.  This Agreement may be terminated and the Merger
and  the other Transactions may be abandoned  at any time prior to the Effective
Time, notwithstanding any requisite approval and adoption of this Agreement  and
the Transactions, as follows:
 
        (a) by mutual written consent duly authorized by the Boards of Directors
    of each of Acquiror, Acquiror Sub and the Target;
 
        (b)  by either Acquiror or the Target,  if either (i) the Effective Time
    shall not have occurred on or before July 31, 1996; PROVIDED, HOWEVER,  that
    the  right to terminate this Agreement  under this Section 8.01(b) shall not
    be available to any party whose failure to fulfill any obligation under this
    Agreement has  been  the  cause of,  or  resulted  in, the  failure  of  the
    Effective  Time to occur on or before such  date; or (ii) there shall be any
    Order which is final  and nonappealable preventing  the consummation of  the
    Merger,  except if the party relying on such Order has not complied with its
    obligations under Section 6.03(a);
 
        (c) by Acquiror, if a tender offer or exchange offer for 50% or more  of
    the  outstanding shares of capital stock of the Target is commenced, and the
    Board of Directors of the Target fails to recommend against the stockholders
    of the Target  tendering their  shares into  such tender  offer or  exchange
    offer;
 
        (d)  by Acquiror, if the stockholders of the Target shall have failed to
    approve and adopt  this Agreement, the  Merger and other  Transactions at  a
    meeting duly convened therefor;
 
        (e) by Acquiror, upon a breach of any representation, warranty, covenant
    or  agreement on the part  of the Target set forth  in this Agreement, or if
    any representation or warranty  of the Target shall  have become untrue,  in
    either  case such that the conditions set forth in Section 7.02(a) would not
    be satisfied (a  "TERMINATING TARGET BREACH");  PROVIDED, HOWEVER, that,  if
    such Terminating Target Breach is curable by the Target through the exercise
    of its best efforts and for so long as the Target continues to exercise such
    best  efforts, Acquiror may not terminate  this Agreement under this Section
    8.01(e); or
 
        (f) by the Target, upon breach of any representation, warranty, covenant
    or agreement on the part of Acquiror set forth in this Agreement, or if  any
    representation  or warranty of Acquiror shall  have become untrue, in either
    case such  that  the conditions  set  forth in  Section  7.03 would  not  be
    satisfied  ("TERMINATING ACQUIROR BREACH"); PROVIDED, HOWEVER, that, if such
    Terminating Acquiror Breach is curable by Acquiror through best efforts  and
    for  so long as Acquiror continues to exercise such best efforts, the Target
    may not terminate this Agreement under this Section 8.01(f).
 
    SECTION 8.02.  FEES AND EXPENSES.   (a) The Target shall pay Acquiror a  fee
(an  "Alternative Proposal Fee") of $5,000,000, which amount is inclusive of all
of Specified Expenses (as hereinafter defined), if:
 
        (i) this Agreement is terminated pursuant to Section 8.01(c); or
 
        (ii) this  Agreement is  terminated  pursuant to  Section 8.01(d)  as  a
    result  of the  failure of  the stockholders  of the  Target to  approve the
    Merger and a Business Combination Transaction Proposal shall have been  made
    prior  to  such termination,  and  any Business  Combination  Transaction is
    thereafter consummated within 12 months of such termination.
 
                                      A-42
<PAGE>
    As used herein, the term  "BUSINESS COMBINATION TRANSACTION" shall mean  any
of  the following  involving the  Target: (1)  any merger,  consolidation, share
exchange, business  combination or  other similar  transaction (other  than  the
Transactions);  (2)  any sale,  lease, exchange,  transfer or  other disposition
(other than a pledge or mortgage) of 25% or more of the assets of the Target and
the Subsidiaries,  taken  as a  whole,  in a  single  transaction or  series  of
transactions;  or (3) the acquisition  by a person or  entity or any "group" (as
such term is defined under Section 13(d)  of the Exchange Act and the rules  and
regulations thereunder) of beneficial ownership of 33 1/3% or more of the shares
of Target Common Stock, whether by tender offer, exchange offer or otherwise.
 
    (b)  Acquiror shall be  entitled to receive its  Specified Expenses (but not
the Alternative Proposal Fee) in immediately  available funds in the event  that
this Agreement is terminated pursuant to Section 8.01(b) (subject to the proviso
thereof)  or Section 8.01(e). Target shall  be entitled to receive its Specified
Expenses in immediately  available funds  in the  event that  this Agreement  is
terminated pursuant to Section 8.01(f).
 
    (c) In the event of termination of this Agreement and the abandonment of the
Merger  pursuant to  Section 8.01, all  obligations of the  parties hereto shall
terminate except the obligations  of the parties pursuant  to this Section  8.02
and Sections 8.03, 8.04, 9.02, 9.03, 9.04, 9.05, 9.06, 9.07, 9.08, 9.10 and 9.11
and  pursuant to the Confidentiality Agreement. No termination of this Agreement
pursuant to  Section  8.01(e)  or  8.01(f) shall  prejudice  the  ability  of  a
non-breaching  party from seeking damages from any other party for any breach of
this Agreement, including, without limitation, attorneys' fees and the right  to
pursue  any  remedy  at law  or  in  equity. Notwithstanding  the  foregoing, if
Acquiror is required to file suit to seek the Alternative Proposal Fee or either
Acquiror or the Target is required to file suit to seek its Specified  Expenses,
and  it ultimately succeeds on the merits, it shall be entitled to all expenses,
including attorneys' fees, which it has  incurred in enforcing its rights  under
this Section 8.02.
 
    (d)  As used herein,  "SPECIFIED EXPENSES" means  all out-of-pocket expenses
and fees actually incurred or accrued by a Person or on its behalf in connection
with the Transactions  prior to  the termination of  this Agreement  (including,
without  limitation, all fees and expenses of counsel, financial advisors, banks
or other  entities  providing financing  to  such Person  (including  financing,
commitment and other fees payable thereto), accountants, environmental and other
experts  and consultants to such Person and its affiliates, and all printing and
advertising expenses)  and  in  connection with  the  negotiation,  preparation,
execution, performance and termination of this Agreement, the structuring of the
Transactions,  any agreements  relating thereto  and any  filings to  be made in
connection therewith.
 
    (e) The  Target agrees  that from  and  after the  Effective Time  it  shall
reimburse  Acquiror for all out-of-pocket expenses and fees actually incurred or
accrued by Acquiror in connection with the Transaction.
 
    (f) Except as  set forth in  this Section  and Section 6.01,  all costs  and
expenses  incurred in connection with this  Agreement and the Transactions shall
be paid by the party incurring such expenses, whether or not any Transaction  is
consummated.
 
    SECTION  8.03.   AMENDMENT.   This Agreement may  be amended  by the parties
hereto by action taken by or on  behalf of their respective Boards of  Directors
at  any time  prior to  the Effective Time;  PROVIDED, HOWEVER,  that, after the
approval and adoption of this Agreement and the Transactions by the stockholders
of the Target, no amendment may be  made which would violate Delaware Law.  This
Agreement  may not be amended  except by an instrument  in writing signed by the
parties hereto.
 
    SECTION 8.04.  WAIVER.  At any  time prior to the Effective Time, any  party
hereto  may (a) extend the  time for the performance  of any obligation or other
act of any other party hereto,  (b) waive any inaccuracy in the  representations
and warranties contained herein or in any document
 
                                      A-43
<PAGE>
delivered  pursuant  hereto  and  (c) waive  compliance  with  any  agreement or
condition contained herein. Any such extension or waiver shall be valid only  if
set forth in an instrument in writing signed by the party or parties to be bound
thereby.
 
                                   ARTICLE IX
                               GENERAL PROVISIONS
 
    SECTION   9.01.      NON-SURVIVAL   OF   REPRESENTATIONS,   WARRANTIES   AND
AGREEMENTS.  The  representations, warranties and  agreements in this  Agreement
and  any certificate delivered pursuant hereto  by any person shall terminate at
the Effective Time, except that  the agreements set forth  in Articles I and  II
and Sections 6.06 and 6.07 shall survive the Effective Time indefinitely.
 
    SECTION  9.02.  NOTICES.   All notices, requests,  claims, demands and other
communications hereunder shall be  in writing and shall  be given (and shall  be
deemed  to have been duly  given upon receipt) by  delivery in person, by cable,
facsimile, telegram  or  telex  or  by registered  or  certified  mail  (postage
prepaid,  return receipt requested)  to the respective  parties at the following
addresses (or at  such other  address for  a party as  shall be  specified in  a
notice given in accordance with this Section 9.02):
 
<TABLE>
<S>                                        <C>
if to Acquiror or Acquiror Sub:            with a copy to:
ENSCO International Incorporated           Daniel W. Rabun
1445 Ross Avenue, Suite 2700               Baker & McKenzie
Dallas, Texas 75202                        2001 Ross Avenue
Attention: C. Christopher Gaut             4500 Trammell Crow Center
Facsimile: 214/855-0300                    Dallas, Texas 75201
                                           Facsimile: 214/978-3096/99
 
if to the Target:                          with a copy to:
DUAL DRILLING COMPANY                      David S. Peterman
5956 Sherry Lane, Suite 1500               Akin, Gump, Strauss, Hauer & Feld, L.L.P.
Dallas, Texas 75225                        7900 Pennzoil Place -- South Tower
Attention: David W. Skarke                 711 Louisiana Street
Facsimile: 214/373-0533                    Houston, Texas 77002
                                           Facsimile: 713/236-0823
</TABLE>
 
    SECTION  9.03.   CERTAIN DEFINITIONS.   For purposes of  this Agreement, the
term:
 
        (a) "AFFILIATE" of a  specified person means a  person who, directly  or
    indirectly,  through one or more intermediaries, controls, is controlled by,
    or is under common control with, such specified person;
 
        (b) "BENEFICIAL OWNER"  with respect to  any shares means  a person  who
    shall  be deemed to  be the beneficial  owner of such  shares (i) which such
    person or any of its  affiliates or associates (as  such term is defined  in
    Rule  12b-2 promulgated under the  Exchange Act) beneficially owns, directly
    or indirectly, (ii) which such person or any of its affiliates or associates
    has, directly or indirectly, (A) the right to acquire (whether such right is
    exercisable immediately or subject only to the passage of time), pursuant to
    any  agreement,  arrangement  or  understanding  or  upon  the  exercise  of
    consideration rights, exchange rights, warrants or options, or otherwise, or
    (B)   the  right  to   vote  pursuant  to   any  agreement,  arrangement  or
    understanding, (iii) which are  beneficially owned, directly or  indirectly,
    by  any other  persons with  whom such  person or  any of  its affiliates or
    associates or any person with whom such  person or any of its affiliates  or
    associates  has any agreement, arrangement  or understanding for the purpose
    of acquiring,  holding, voting  or disposing  of any  such shares,  or  (iv)
    pursuant  to Section 13(d) of the Exchange  Act and any rules or regulations
    promulgated thereunder;
 
                                      A-44
<PAGE>
        (c)  "BUSINESS DAY" means any day on  which the principal offices of the
    SEC in Washington,  D.C. are  open to  accept filings,  or, in  the case  of
    determining  a date when any payment is due,  any day on which banks are not
    required or authorized to close in the New York, New York;
 
        (d) "CONTROL" (including  the terms  "CONTROLLED BY"  and "UNDER  COMMON
    CONTROL WITH") means the possession, directly or indirectly or as trustee or
    executor,  of the power to  direct or cause the  direction of the management
    and  policies  of  a  person,  whether  through  the  ownership  of   voting
    securities,  as trustee  or executor, by  contract or  credit arrangement or
    otherwise;
 
        (e) "PERSON"  means  an individual,  corporation,  partnership,  limited
    partnership, syndicate, person (including, without limitation, a "PERSON" as
    defined  in Section  13(d)(3) of  the Exchange  Act), trust,  association or
    entity or government, political subdivision, agency or instrumentality of  a
    government; and
 
        (f)  "SUBSIDIARY" or "SUBSIDIARIES" of any person means any corporation,
    partnership, joint  venture  or other  legal  entity of  which  such  person
    (either alone or through or together with any other subsidiary), owns or has
    rights to acquire, directly or indirectly, more than 50% (or 49% in the case
    of  Sime-Dual) of the stock  or other equity interests  the holders of which
    are generally entitled to vote for the election of the board of directors or
    other governing body of such corporation or other legal entity.
 
    SECTION 9.04.    SEVERABILITY.   If  any term  or  other provision  of  this
Agreement is invalid, illegal or incapable of being enforced by any rule of Law,
or  public policy, all  other conditions and provisions  of this Agreement shall
nevertheless remain in full force  and effect so long  as the economic or  legal
substance  of the Transactions is not  affected in any manner materially adverse
to any  party. Upon  such determination  that  any term  or other  provision  is
invalid,  illegal  or  incapable of  being  enforced, the  parties  hereto shall
negotiate in good faith to  modify this Agreement so  as to effect the  original
intent  of the parties as closely as possible in a mutually acceptable manner in
order that the  Transactions be  consummated as originally  contemplated to  the
fullest extent possible.
 
    SECTION  9.05.  ASSIGNMENT; BINDING EFFECT; BENEFIT.  Neither this Agreement
nor any of the rights, interests  or obligations hereunder shall be assigned  by
any of the parties hereto (whether by operation of law or otherwise) without the
prior  written consent of the other  parties. Subject to the preceding sentence,
this Agreement shall  be binding  upon and  shall inure  to the  benefit of  the
parties  hereto  and their  respective  successors and  assigns. Notwithstanding
anything contained in this Agreement to the contrary, except for the  provisions
of  Article II and  Sections 6.06 (collectively,  the "THIRD PARTY PROVISIONS"),
nothing in this Agreement,  expressed or implied, is  intended to confer on  any
person  other than the parties hereto or their respective successors and assigns
any rights, remedies,  obligations or  liabilities under  or by  reason of  this
Agreement.
 
    SECTION  9.06.  INCORPORATION OF SCHEDULES.   The Target Disclosure Schedule
and  the  Acquiror  Disclosure  Schedule  referred  to  herein  and  signed  for
identification  by the parties hereto are  hereby incorporated herein and made a
part hereof for all purposes as if fully set forth herein.
 
    SECTION 9.07.    SPECIFIC  PERFORMANCE.    The  parties  hereto  agree  that
irreparable  damage would occur in the event any provision of this Agreement was
not performed in accordance with the terms hereof and that the parties shall  be
entitled  to specific performance of the terms  hereof, in addition to any other
remedy at law or equity.
 
    SECTION 9.08.  GOVERNING  LAW.  EXCEPT  TO THE EXTENT  THAT DELAWARE LAW  IS
MANDATORILY  APPLICABLE TO THE MERGER AND THE  RIGHTS OF THE STOCKHOLDERS OF THE
TARGET AND ACQUIROR SUB, THIS AGREEMENT  SHALL BE GOVERNED BY, AND CONSTRUED  IN
ACCORDANCE  WITH, THE LAWS OF THE STATE OF  TEXAS WITHOUT REGARD TO THE RULES OF
CONFLICTS OF LAW THEREOF. ALL ACTIONS AND PROCEEDINGS ARISING OUT OF OR RELATING
TO THIS AGREEMENT SHALL BE HEARD AND DETERMINED IN ANY COURT SITTING IN THE CITY
OF DALLAS, TEXAS.
 
                                      A-45
<PAGE>
    SECTION 9.09.    HEADINGS.    The descriptive  headings  contained  in  this
Agreement are included for convenience of reference only and shall not affect in
any way the meaning or interpretation of this Agreement.
 
    SECTION  9.10.  COUNTERPARTS.  This  Agreement may be executed and delivered
(including by facsimile transmission)  in one or more  counterparts, and by  the
different  parties hereto in separate counterparts,  each of which when executed
and delivered shall be deemed to be an original but all of which taken  together
shall constitute one and the same agreement.
 
    SECTION  9.11.   WAIVER OF  JURY TRIAL.   Each  of Acquiror,  the Target and
Acquiror Sub hereby irrevocably waives all right to trial by jury in any action,
proceeding or  counterclaim  (whether  based on  contract,  tort  or  otherwise)
arising  out of or  relating to this  Agreement or the  actions of Acquiror, the
Target or  Acquiror  Sub in  the  negotiation, administration,  performance  and
enforcement thereof.
 
    SECTION  9.12.   ENTIRE AGREEMENT.   This  Agreement, the  Target Disclosure
Schedule, the Acquiror Disclosure Schedule, the confidentiality agreement, dated
November 2,  1995, (the  "Confidentiality Agreement"),  between the  Target  and
Acquiror,  the confidentiality  agreement, dated  February 5,  1996, between the
Target and Acquiror, and  any documents delivered by  the parties in  connection
herewith  constitute the entire agreement among  the parties with respect to the
subject matter  hereof and  supersede all  prior agreements  and  understandings
among  the parties with respect  thereto. No addition to  or modification of any
provision of this Agreement shall be  binding upon any party hereto unless  made
in writing and signed by all parties hereto.
 
                                      A-46
<PAGE>
    IN  WITNESS WHEREOF, Acquiror, Acquiror Sub  and the Target have caused this
Agreement to be executed as of the date first written above by their  respective
officers thereunto duly authorized.
 
                                          ENSCO INTERNATIONAL INCORPORATED
 
                                          By          /s/ CARL F. THORNE
 
                                             -----------------------------------
                                               Carl F. Thorne, Chairman of the
                                                          Board,
                                                President and Chief Executive
                                                         Officer
 
                                          DDC ACQUISITION COMPANY
 
                                          By       /s/ C. CHRISTOPHER GAUT
 
                                             -----------------------------------
                                               C. Christopher Gaut, President
 
                                          DUAL DRILLING COMPANY
 
                                          By         /s/ DAVID W. SKARKE
 
                                             -----------------------------------
                                                  David W. Skarke, Chairman
 
                                      A-47
<PAGE>
                                   EXHIBIT A
                          CERTIFICATE OF INCORPORATION
                                       OF
                            DDC ACQUISITION COMPANY
 
                                   ARTICLE I
 
    The name of the corporation is DDC Acquisition Company (the "Corporation").
 
                                   ARTICLE II
 
    The  address of the Corporation's registered office in the State of Delaware
is Corporation Trust Center, 1209 Orange Street, Wilmington, New Castle  County,
Delaware  19801.  The  name of  its  registered  agent at  such  address  is The
Corporation Trust Company.
 
                                  ARTICLE III
 
    The purpose of the Corporation  is to engage in  any lawful act or  activity
for which corporations may be organized under the General Corporation Law of the
State of Delaware.
 
                                   ARTICLE IV
 
    The  total number of  shares of stock  which the Corporation  shall have the
authority to issue is  Ten Thousand (10,000) shares  of Common Stock, par  value
$0.10 per share.
 
                                   ARTICLE V
 
    The name and mailing address of the incorporator is as follows:
 
<TABLE>
<CAPTION>
           NAME                  MAILING ADDRESS
- ---------------------------  ------------------------
<S>                          <C>
Albert G. McGrath, Jr.       2700 Fountain Place
                             1445 Ross Avenue
                             Dallas, Texas 75202
</TABLE>
 
                                   ARTICLE VI
 
    The  powers of  the incorporator  are to terminate  upon the  filing of this
certificate of incorporation, and  the name and mailing  address of the  persons
who are to serve as the board of directors until the first annual meeting of the
stockholders or until their successors are elected and qualified are as follows:
 
<TABLE>
<CAPTION>
     NAMES OF DIRECTORS            MAILING ADDRESS
- -----------------------------  ------------------------
<S>                            <C>
William S. Chadwick, Jr.       2700 Fountain Place
                               1445 Ross Avenue
                               Dallas, Texas 75202
C. Christopher Gaut            2700 Fountain Place
                               1445 Ross Avenue
                               Dallas, Texas 75202
H. E. Malone                   2700 Fountain Place
                               1445 Ross Avenue
                               Dallas, Texas 75202
</TABLE>
 
                                      A-48
<PAGE>
                                  ARTICLE VII
 
    Elections  of directors need not  be by written ballot  unless the bylaws of
the Corporation shall so provide.
 
                                  ARTICLE VIII
 
    The Board of Directors of the Corporation is expressly authorized to  adopt,
amend  or  repeal  bylaws of  the  Corporation,  but the  stockholders  may make
additional bylaws and may alter or repeal  any bylaw whether adopted by them  or
otherwise.
 
                                   ARTICLE IX
 
    No  contract or transaction between  the Corporation and one  or more of its
directors, officers, or stockholders or  between the Corporation and any  person
(as  used  herein "person"  means  other corporation,  partnership, association,
firm, trust, joint venture, political subdivision, or instrumentality) or  other
organization  in which one  or more of its  directors, officers, or stockholders
are directors, officers, or stockholders, or have a financial interest, shall be
void or  voidable solely  for this  reason, or  solely because  the director  or
officer  is present at or participates in  the meeting of the board or committee
which authorizes the  contract or transaction,  or solely because  his, her,  or
their  votes are counted for such purpose, if:  (i) the material facts as to his
or her  relationship or  interest and  as  to the  contract or  transaction  are
disclosed or are known to the board of directors or the committee, and the board
of  directors or committee in good  faith authorizes the contract or transaction
by the affirmative  votes of  a majority  of the  disinterested directors,  even
though  the disinterested directors be less than  a quorum; or (ii) the material
facts as  to his  or her  relationship or  interest and  as to  the contract  or
transaction  are disclosed  or are  known to  the stockholders  entitled to vote
thereon, and the contract or transaction is specifically approved in good  faith
by  vote of the stockholders; or (iii) the contract or transaction is fair as to
the Corporation as of the  time it is authorized,  approved, or ratified by  the
board  of  directors,  a  committee  thereof,  or  the  stockholders.  Common or
interested directors may be counted in determining the presence of a quorum at a
meeting of  the  board of  directors  or of  a  committee which  authorizes  the
contract or transaction.
 
                                   ARTICLE X
 
    The  Corporation shall indemnify any person who was, is, or is threatened to
be made a party to a proceeding  (as hereinafter defined) by reason of the  fact
that  he or she (i) is  or was a director or  officer of the Corporation or (ii)
while a director or officer of the Corporation, is or was serving at the request
of the  Corporation  as  a director,  officer,  partner,  venturer,  proprietor,
trustee,  employee, agent, or similar functionary of another foreign or domestic
corporation, partnership, joint  venture, sole  proprietorship, trust,  employee
benefit  plan, or  other enterprise, to  the fullest extent  permitted under the
Delaware General  Corporation  Law, as  the  same  exists or  may  hereafter  be
amended.  Such right  shall be  a contract right  and as  such shall  run to the
benefit of any director or  officer who is elected  and accepts the position  of
director  or officer  of the  Corporation or  elects to  continue to  serve as a
director or officer of the  Corporation while this Article  X is in effect.  Any
repeal  or amendment of this  Article X shall be  prospective only and shall not
limit the rights  of any  such director  or officer  or the  obligations of  the
Corporation with respect to any claim arising from or related to the services of
such  director or officer in  any of the foregoing  capacities prior to any such
repeal or amendment to this Article X. Such right shall include the right to  be
paid  by the Corporation  expenses incurred in defending  any such proceeding in
advance of  its final  disposition to  the maximum  extent permitted  under  the
Delaware  General  Corporation  Law, as  the  same  exists or  may  hereafter be
amended. If a claim for indemnification or advancement of expenses hereunder  is
not paid in full by the Corporation within sixty (60) days after a written claim
has  been received by the  Corporation, the claimant may  at any time thereafter
bring suit against the  Corporation to recover the  unpaid amount of the  claim,
and if successful in whole or in part, the claimant shall also be entitled to be
paid  the expenses of prosecuting such claim. It  shall be a defense to any such
action  that  such   indemnification  or   advancement  of   costs  of   defense
 
                                      A-49
<PAGE>
are  not permitted under the Delaware General Corporation Law, but the burden of
proving such defense  shall be on  the Corporation. Neither  the failure of  the
Corporation  (including  its  board  of  directors  or  any  committee  thereof,
independent legal counsel, or stockholders) to have made its determination prior
to the commencement of  such action that indemnification  of, or advancement  of
costs  of defense to,  the claimant is  permissible in the  circumstances nor an
actual determination by the Corporation (including its board of directors or any
committee  thereof,  independent  legal  counsel,  or  stockholders)  that  such
indemnification  or advancement  is not  permissible shall  be a  defense to the
action or create a presumption that  such indemnification or advancement is  not
permissible.  In  the  event  of the  death  of  any person  having  a  right of
indemnification under the foregoing  provisions, such right  shall inure to  the
benefit   of  his  or   her  heirs,  executors,   administrators,  and  personal
representatives. The rights conferred above shall not be exclusive of any  other
right  which any person may have or  hereafter acquire under any statute, bylaw,
resolution of stockholders or directors, agreement, or otherwise.
 
    The Corporation  may additionally  indemnify any  employee or  agent of  the
Corporation to the fullest extent permitted by law.
 
    As  used herein,  the term  "proceeding" means  any threatened,  pending, or
completed action, suit, or proceeding, whether civil, criminal,  administrative,
arbitrative,   or  investigative,  any  appeal  in  such  an  action,  suit,  or
proceeding, and any inquiry or investigation that could lead to such an  action,
suit, or proceeding.
 
                                   ARTICLE XI
 
    Whenever  a compromise or  arrangement is proposed  between this Corporation
and its creditors or any class of  them and/or between this Corporation and  its
stockholders  or any class  of them, any court  of equitable jurisdiction within
the State  of  Delaware  may, on  the  application  in a  summary  way  of  this
Corporation  or of any creditor or stockholder  thereof or on the application of
any receiver or receivers appointed for this Corporation under the provisions of
Section 291 of the General  Corporation Law of the State  of Delaware or on  the
application of trustees in dissolution or of any receiver or receivers appointed
for  this  Corporation  under  the  provisions of  Section  279  of  the General
Corporation Law of the State  of Delaware, order a  meeting of the creditors  or
class  of creditors, and/or of the stockholders or class of stockholders of this
Corporation, as the case may be, to be summoned in such manner as the said court
directs. If a  majority in  number representing  three-fourths in  value of  the
creditors  or  class  of  creditors,  and/or of  the  stockholders  or  class of
stockholders of this Corporation, as the case may be, agree to any compromise or
arrangement and to any reorganization of this Corporation as consequence of such
compromise or  arrangement, the  said  compromise or  arrangement and  the  said
reorganization  shall, if sanctioned by the  court to which the said application
has been made, be binding on all of the creditors or class of creditors,  and/or
on all of the stockholders or class of stockholders, of this Corporation, as the
case may be, and also on this Corporation.
 
                                  ARTICLE XII
 
    A  director  of  the  Corporation  shall not  be  personally  liable  to the
Corporation or its  stockholders for  monetary damages for  breach of  fiduciary
duty  as a director, except  for liability (i) for  any breach of the director's
duty of  loyalty  to the  Corporation  or its  stockholders,  (ii) for  acts  or
omissions not in good faith or which involve intentional misconduct or a knowing
violation  of law, (iii) under Section 174 of the General Corporation Law of the
State of Delaware, as the same exists  or hereafter may be amended, or (iv)  for
any transaction from which the director derived an improper personal benefit. If
the  General Corporation Law of the State  of Delaware is amended after the date
of filing of  this certificate  of incorporation to  authorize corporate  action
further  eliminating or limiting  the personal liability  of directors, then the
liability of a  director of the  Corporation, in addition  to the limitation  on
personal  liability  provided herein,  shall be  limited  to the  fullest extent
permitted by the amended General Corporation  Law of the State of Delaware.  Any
repeal or modification of this Article XII by
 
                                      A-50
<PAGE>
the  stockholders of  the Corporation shall  be prospective only,  and shall not
adversely affect any limitation on the  personal liability of a director of  the
Corporation existing at the time of such repeal or modification.
 
    The  undersigned incorporator under penalties of perjury hereby acknowledges
that the foregoing certificate of incorporation is his act and deed and that the
facts stated therein are true.
 
                                          --------------------------------------
                                                  Albert G. McGrath, Jr.
 
                                      A-51
<PAGE>
                                   EXHIBIT B
                              AFFILIATE AGREEMENT
 
ENSCO International Incorporated
1445 Ross Avenue, Suite 2700
Dallas, Texas 75202
 
Ladies and Gentlemen:
 
    I have been advised that as of the date of this letter I may be deemed to be
an "affiliate" of Dual Drilling Company, a Delaware corporation ("Dual"), as the
term "affiliate" is defined for purposes of  paragraphs (c) and (d) of Rule  145
of the rules and regulations (the "Rules and Regulations") of the Securities and
Exchange  Commission (the  "Commission") under  the Securities  Act of  1933, as
amended (the "Act"). Pursuant to the terms  of the Agreement and Plan of  Merger
dated   March       ,  1996  (the   "Agreement"),  between  ENSCO  International
Incorporated, a  Delaware corporation  ("ENSCO"),                 ,  a  Delaware
corporation  and a wholly owned subsidiary  of ENSCO ("Acquiror Sub"), and Dual,
Acquiror Sub will be merged with and into Dual (the "Merger").
 
    In connection with  the transactions  contemplated by the  Agreement, I  may
receive  shares of Common Stock, par value  $.10 per share, of ENSCO (the "ENSCO
Securities").
 
    I represent, warrant and covenant to ENSCO  that in the event I receive  any
ENSCO Securities as a result of the Merger:
 
        A.   I  shall not make  any sale,  transfer or other  disposition of the
    ENSCO Securities in violation of the Act or the Rules and Regulations.
 
        B.  I have  carefully read this letter  and the Agreement and  discussed
    the  requirements of such documents and other applicable limitations upon my
    ability to sell, transfer  or otherwise dispose of  the ENSCO Securities  to
    the extent I felt necessary with my counsel or counsel for Dual.
 
        C.   I have been advised that the  issuance of ENSCO Securities to me in
    connection with  the transactions  contemplated by  the Agreement  has  been
    registered  with the Commission under the Act on a Registration Statement on
    Form S-4. However,  I have also  been advised  that, since at  the time  the
    Merger  and the Agreement were  submitted for a vote  of the stockholders of
    Dual, I may be deemed to have been an affiliate of Dual and the distribution
    by me of the ENSCO Securities has  not been registered under the Act, I  may
    be prohibited from selling, transferring or otherwise disposing of the ENSCO
    Securities  issued to me in connection with the transactions contemplated by
    the Agreement unless (i) such sale,  transfer or other disposition has  been
    registered  under the Act, (ii) such  sale, transfer or other disposition is
    made in conformity  with Rule 145  promulgated by the  Commission under  the
    Act, or (iii) in the opinion of counsel reasonably acceptable to ENSCO, or a
    "no  action"  letter  obtained by  the  undersigned  from the  staff  of the
    Commission, such sale,  transfer or  other disposition  is otherwise  exempt
    from registration under the Act.
 
        D.  I understand that ENSCO is under no obligation to register the sale,
    transfer  or other disposition of the ENSCO Securities by me or on my behalf
    under the  Act or  to  take any  other action  necessary  in order  to  make
    compliance with an exemption from such registration available.
 
    It  is understood and agreed that prior to  any transfer of any of the ENSCO
Securities, I will give written notice to  ENSCO of my intention to effect  such
offer,  sale  or transfer,  describing  the proposed  transaction  in sufficient
detail  to  enable  ENSCO  and  its  counsel  to  determine  that  the  proposed
transaction will not violate the Act.
 
                                      A-52
<PAGE>
    Execution  of this letter should  not be considered an  admission on my part
that I am an  "affiliate" of Dual  as described in the  first paragraph of  this
letter  or as a waiver of any rights I may have to object to any claim that I am
such an affiliate on or after the date of this letter.
 
                                          Very truly yours,
                                          --------------------------------------
                                          Name:
 
Accepted this   day of May, 1996 by
ENSCO INTERNATIONAL INCORPORATED
By:
   ----------------------------------------------
   William S. Chadwick, Jr., Secretary
 
                                      A-53
<PAGE>
                                   EXHIBIT C
                    LEGAL OPINION FOR COUNSEL FOR THE TARGET
                        [COUNSEL FOR TARGET LETTERHEAD]
 
            , 1996
ENSCO International Incorporated
1445 Ross Avenue, Suite 2700
Dallas, Texas 57202
To whom it may concern:
 
    We have acted as  counsel to Dual Drilling  Company, a Delaware  corporation
(the  "Target"), in connection with the  execution and delivery of the Agreement
and Plan of Merger (the  "Agreement"), dated as of  March 21, 1996, among  ENSCO
International   Incorporated,   a   Delaware   corporation   ("Acquiror"),  Dual
Acquisition Company, a  Delaware corporation  and a  wholly-owned subsidiary  of
Acquiror  ("Acquiror Sub"), and the Target  providing for the merger of Acquiror
Sub with  and into  the Target  (the  "Merger"). This  opinion letter  is  being
furnished  to you  pursuant to Section  7.02 of the  Agreement. Unless otherwise
defined herein or the context hereof  otherwise requires, each term used  herein
with  its initial letter capitalized  has the meaning given  to such term in the
Agreement.
 
    We have made such legal and factual examinations and inquiries, including an
examination of the originals, or copies certified or otherwise identified to our
satisfaction, of such documents, corporate  records and other instruments as  we
have deemed necessary or appropriate for the purposes of this opinion, including
(i) the Agreement, (ii) the Certificate of Merger dated as of             , 1996
between  the Target and Acquiror Sub, and (iii) the Certificate of Incorporation
and  Bylaws,  or  equivalent  organizational  documents,  of  Target  and   each
Subsidiary.
 
    We have relied, to the extent we deem appropriate, upon oral advice of Staff
of  the Securities  and Exchange  Commission and,  as to  matters of  fact, upon
representations made by the Target in the Agreement, certificates of the  Target
or  its officers  and certificates or  other written statements  of officials of
jurisdictions having custody of documents respecting the corporate existence  or
good  standing  of  the Target.  We  have  assumed that  the  signatures  on all
documents examined by  us are  genuine, that all  documents submitted  to us  as
originals  are accurate and complete, and that  all documents submitted to us as
copies are true and correct copies of  the originals thereof. We are members  of
the  State Bar of Texas.  We call your attention to  the fact that, in rendering
our opinion, we are  expressing our views only  as to the laws  of the State  of
Texas and the federal laws of the United States of America.
 
    Based  upon the foregoing and subject  to the qualifications and limitations
set forth herein, we are of the opinion that:
 
    1.  The Target is a corporation,  and each U.S. Subsidiary is a  corporation
or  limited partnership,  in each case  duly organized, validly  existing and in
good standing under the laws of the jurisdiction of its organization and has the
requisite corporate or partnership power and authority to own, lease and operate
its properties and to carry  on its business as it  is now being conducted.  The
Target  and each  U.S. Subsidiary  are duly qualified  or licensed  as a foreign
corporation or limited partnership to do business, and are in good standing,  in
each  jurisdiction  where  the  character of  the  properties  owned,  leased or
operated by them  or the nature  of their business  makes such qualification  or
licensing necessary, except for such failures to be qualified or licensed and in
good  standing that would not, individually or in the aggregate, have a Material
Adverse Effect.
 
                                      A-54
<PAGE>
    2.  The Target  has all necessary corporate  power and authority to  execute
and  deliver  the Agreement  and, with  respect  to the  Merger, to  perform its
obligations thereunder and  to consummate  the Transactions.  The execution  and
delivery  of the Agreement by  the Target and the  consummation by the Target of
the Transactions  have  been  duly  and  validly  authorized  by  all  necessary
corporate  action and no other  corporate proceedings on the  part of the Target
are necessary  to authorize  the  Agreement or  to consummate  the  Transactions
(other  than the filing and recordation  of an appropriate Certificate of Merger
with the Secretary as required by Delaware Law).
 
    3.  The Agreement has  been duly and validly  executed and delivered by  the
Target  and,  assuming  the due  authorization,  execution and  delivery  of the
Agreement by Acquiror and Acquiror Sub,  constitutes a legal, valid and  binding
obligation  of the Target, enforceable against the Target in accordance with its
terms, except  as  may be  (1)  limited by  bankruptcy,  insolvency,  fraudulent
conveyance,  reorganization, moratorium, or other  laws affecting enforcement of
creditors' rights generally, and  (2) subject to  general principles of  equity,
regardless  of whether enforcement  is considered in  a proceeding at  law or in
equity.
 
    4.  The execution and  delivery of the Agreement by  the Target do not,  and
the  performance of the  Agreement by the  Target will not  (1) conflict with or
violate the Certificate  of Incorporation, Bylaws  or equivalent  organizational
documents  of the Target  or any U.S.  Subsidiary, (2) conflict  with or violate
Delaware Law or any Laws of the State of Texas or the United States of  America,
or to the best of our knowledge, any other Laws applicable to the Target or any,
U.S.  Subsidiary or  by which any  property or asset  of the Target  or any U.S.
Subsidiary is bound or affected, or (3) to the best of our knowledge, result  in
any breach of or constitute a default (or an event which with notice or lapse of
time  or both  would become  a default) under,  or give  to others  any right of
termination, amendment, acceleration or cancellation  of, or give to others  any
right to invalidate or terminate any purchase or other right to acquire property
under,  or result in the creation of a lien or other encumbrance on any property
or asset of the  Target or any  Subsidiary or require the  consent of any  third
party  pursuant to,  any note,  bond, mortgage,  indenture, contract, agreement,
lease, license, permit, franchise or other instrument or obligation to which the
Target or any Subsidiary is a party or by which the Target or any Subsidiary  or
any property or asset of the Target or any Subsidiary is bound or affected.
 
    5.   The execution and  delivery of the Agreement by  the Target do not, and
the performance of the  Agreement by the Target  will not, require any  consent,
approval,  authorization or  permit of, or  filing with or  notification to, any
domestic governmental or regulatory authority, except the filing and recordation
of an  appropriate Certificate  of  Merger with  the  Secretary as  required  by
Delaware Law.
 
    6.   To our knowledge, except as set forth in the Target Disclosure Schedule
(as may have  been updated),  no actions, suits  or proceedings  are pending  or
threatened  against Target  or any  Subsidiary seeking  to prevent  or delay the
transactions contemplated by the  Agreement or challenging any  of the terms  or
provisions of the Agreement or seeking material damages in connection therewith.
 
    7.   The authorized capital stock of  the Target and each U.S. Subsidiary is
as set forth in Section 3.03 of the Agreement.
 
    8.  Upon the issuance of a  Certificate of Merger by the Secretary of  State
of  the State of Delaware,  the Merger will become  effective in accordance with
the Agreement and  the Delaware Law,  and each issued  and outstanding share  of
Company  Common Stock (other than any  Cancelable Shares) will be converted into
the consideration provided in Section 2.01 of the Agreement.
 
    We have participated in conferences with officers and other  representatives
of  Acquiror,  Acquiror  Sub and  the  Target,  and the  representatives  of the
independent auditors of  Acquiror, Acquiror  Sub and  the Target,  at which  the
contents  of the  Registration Statement  on Form  S-4 (File  No. 333-[       ])
declared effective by  the SEC on  [                ], 1996, (the  "Registration
Statement"),  the Proxy Statement/Prospectus dated [            ], 1996 included
in the Registration Statement (the  "Proxy Statement") and related matters  were
discussed. Although we do not pass upon, and are not assuming any responsibility
for   and  have  not  independently   verified  the  accuracy,  completeness  or
 
                                      A-55
<PAGE>
fairness of the statements contained in the Registration Statement and the Proxy
Statement, on the basis of the foregoing  (relying as to materiality to a  large
extent upon statements and representations of officers and other representatives
of  the Target, Acquiror and Acquiror Sub),  no facts have come to our attention
which lead us  to believe that  the Registration Statement  (except for (i)  the
financial  statements  and related  schedules  contained therein,  including the
notes thereto  and the  independent auditors'  reports thereon,  (ii) the  other
financial and statistical data contained therein and (iii) the exhibits thereto,
as  to which we  do not comment), at  the time it  became effective contained an
untrue statement  of a  material fact  or  omitted to  state any  material  fact
required  to be stated therein  or necessary to make  the statements therein not
misleading, or that the Proxy Statement (except for (i) the financial statements
and related schedules  contained therein,  including the notes  thereto and  the
independent  auditors'  reports  thereon,  and  (ii)  the  other  financial  and
statistical data contained therein, as  to which we do  not comment), as of  the
date  thereof, contained any untrue  statement of a material  fact or omitted to
state a material  fact necessary  in order to  make statements  therein, in  the
light of the circumstances under which they were made, not misleading.
 
    The  preceding opinions and  "negative assurance" statements  are subject to
the following qualifications and limitations:
 
        A.  In connection with statements herein qualified by "to our knowledge"
    or as to matters that have "come to our attention," our examination has been
    limited to discussions with  the officers and  other representatives of  the
    Target and each Subsidiary by, and those statements refer only to what is in
    the  actual current consciousness  of, attorneys in  the Dallas, Houston and
    Washington D.C.  offices  of  this  Firm  who  have  been  involved  in  the
    representation  of the  Target and  each Subsidiary  in connection  with the
    transactions described in  the Agreement,  and we have  made no  independent
    investigations   as  to  the   accuracy  or  completeness   of  any  of  the
    representations, warranties,  data or  other information,  written or  oral,
    made or furnished by Acquiror or Acquiror Sub to us or to you.
 
        B.   This opinion letter  is limited in all respects  to the laws of the
    State of Texas, the  federal laws of  the United States  of America and  the
    General  Corporation  Law  of  the  State  of  Delaware,  and  we  assume no
    responsibility as to the applicability or  the effect of any other laws.  No
    opinion  is expressed herein with respect  to any laws, ordinances, statutes
    or regulations of  any county, city  or other political  subdivision of  the
    State of Texas.
 
        C.   The  opinions and  statements expressed  herein are  limited to the
    matters specifically addressed, and  no opinion or  statement is implied  or
    may be inferred beyond the matters so specifically addressed.
 
        D.   With respect to  the opinion expressed in  Paragraph 6, we have not
    conducted any search of any indexes,  dockets or other records of any  court
    or other Governmental Entity.
 
        E.   The opinions and statements expressed herein are rendered as of the
    time immediately preceding the  Effective Time, and  we hereby disclaim  any
    obligation  to  advise you  of,  or to  supplement  any of  our  opinions or
    statements because of, any changes in fact or laws which might affect any of
    those opinions or statements.
 
        F.  This opinion  letter is solely for  your benefit in connection  with
    the  transactions described  in the  Agreement and  may not  be relied upon,
    quoted or otherwise  used by any  other person  or entity or  for any  other
    purpose without our express written consent.
 
                                          Very truly yours,
 
                                          AKIN, GUMP, STRAUSS, HAUER & FELD,
                                          L.L.P.
 
                                      A-56
<PAGE>
                                   EXHIBIT D
                     LEGAL OPINION FOR COUNSEL FOR ACQUIROR
                       [COUNSEL FOR ACQUIROR LETTERHEAD]
 
            , 1996
DUAL DRILLING COMPANY
5956 Sherry Lane, Suite 1500
Dallas, Texas 75225
 
    We  have acted  as counsel to  ENSCO International  Incorporated, a Delaware
corporation ("Acquiror"), and Dual  Acquisition Company, a Delaware  corporation
and  a wholly-owned subsidiary of Acquiror  ("Acquiror Sub"), in connection with
the  execution  and  delivery  of  the   Agreement  and  Plan  of  Merger   (the
"Agreement"),  dated  as  of March  21,  1996,  among DUAL  DRILLING  COMPANY, a
Delaware corporation (the  "Target"), Acquiror and  Acquiror Sub, providing  for
the merger of Acquiror Sub with and into the Target (the "Merger"). This opinion
letter  is being  furnished to  you pursuant to  Section 7.03  of the Agreement.
Unless otherwise defined herein or  the context hereof otherwise requires,  each
term  used herein with its  initial letter capitalized has  the meaning given to
such term in the Agreement.
 
    We have examined  and are familiar  with originals or  copies, certified  or
otherwise  authenticated to our  satisfaction, of such  documents and records of
Acquiror and Acquiror Sub, and such statutes, regulations and instruments as  we
have  deemed necessary  or advisable  for the  purposes of  this opinion letter,
including, without limitation, (i) the Agreement, (ii) the Certificate of Merger
dated as of              ,  1996 between the Target and Acquiror Sub, (iii)  the
Certificate of Incorporation and Bylaws of Acquiror, and (iv) the Certificate of
Incorporation and Bylaws of Acquiror Sub.
 
    As  to certain facts  material to our  opinions herein, we  have assumed the
accuracy of the representations  of Acquiror and Acquiror  Sub in the  Agreement
and  of one or more  officers of Acquiror or Acquiror  Sub. In addition, we have
assumed that all signatures on all  documents presented to us are genuine,  that
all  documents submitted to us as originals  are accurate and complete, that all
documents submitted  to  us  as copies  are  true  and complete  copies  of  the
originals  thereof,  that  all  information submitted  to  us  was  accurate and
complete, and that all persons executing  and delivering originals or copies  of
documents  examined by us were competent  to execute and deliver such documents.
We have also assumed the due authorization, execution and delivery by the Target
of the Agreement and the  Certificate of Merger and  that the Agreement and  the
Certificate  of Merger  constitute legal, valid  and binding  obligations of the
Target enforceable against the Target in accordance with their terms.
 
    We have also considered applicable provisions of the Code and Department  of
Treasury  regulations promulgated under the Code (whether proposed, temporary or
final) now in effect (collectively, "Treasury Regulations"), pertinent  judicial
authorities   regarding  applicable   provisions  of   the  Code   and  Treasury
Regulations, interpretive rulings of the IRS and such other federal  tax-related
authorities as we have considered relevant.
 
                                      A-57
<PAGE>
    Based  upon the foregoing and subject  to the qualifications and limitations
set forth below, we are of the opinion that:
 
        1.  Each of Acquiror and  Acquiror Sub is a corporation duly  organized,
    validly  existing  and in  good  standing under  the  laws of  the  State of
    Delaware and has the requisite corporate  power and authority to own,  lease
    and operate its properties as now owned, leased and operated and to carry on
    its business as is now being conducted.
 
        2.   Each of Acquiror and Acquiror Sub has all necessary corporate power
    and authority to execute and deliver the Agreement and, with respect to  the
    Merger,   to  perform  its  obligations  hereunder  and  to  consummate  the
    Transactions. The execution and  delivery of the  Agreement by Acquiror  and
    Acquiror  Sub  and the  consummation  by Acquiror  and  Acquiror Sub  of the
    Transactions  have  been  duly  and  validly  authorized  by  all  necessary
    corporate  action and no other corporate proceedings on the part of Acquiror
    or Acquiror Sub are  necessary to authorize the  Agreement or to  consummate
    the  Transactions (other than  the filing and  recordation of an appropriate
    Certificate of Merger with the Secretary as required by Delaware Law).
 
        3.  The Agreement  has been duly and  validly executed and delivered  by
    Acquiror and Acquiror Sub and, assuming the due authorization, execution and
    delivery  of the  Agreement by  the Target,  constitutes a  legal, valid and
    binding obligation of each of Acquiror and Acquiror Sub enforceable  against
    each  of Acquiror and Acquiror  Sub in accordance with  its terms, except as
    may  be  (i)  limited  by  bankruptcy,  insolvency,  fraudulent  conveyance,
    reorganization,   moratorium,  or   other  laws   affecting  enforcement  of
    creditors' rights  generally,  and (ii)  subject  to general  principles  of
    equity,  regardless of whether enforcement is  considered in a proceeding at
    law or in equity.
 
        4.  The execution and delivery of the Agreement by Acquiror and Acquiror
    Sub do not, and  the performance of the  Agreement by Acquiror and  Acquiror
    Sub  will not (i) conflict with  or violate the Certificate of Incorporation
    or Bylaws of either Acquiror or any Acquiror Subsidiary, (ii) conflict  with
    or  violate any Law applicable to Acquiror  or any Acquiror Subsidiary or by
    which any property or asset  of any of them is  bound or affected, or  (iii)
    result  in any  breach of or  constitute a  default (or an  event which with
    notice or lapse of time  or both would become a  default) under, or give  to
    others  any rights  of termination, amendment,  acceleration or cancellation
    of, or result in the creation of a lien or other encumbrance on any property
    or asset of Acquiror  or any Acquiror Subsidiary  or require the consent  of
    any  third party pursuant to, any note, bond, mortgage, indenture, contract,
    agreement,  lease,  license,  permit,  franchise  or  other  instrument   or
    obligation  to which Acquiror  or any Acquiror  Subsidiary is a  party or by
    which Acquiror or any Acquiror Subsidiary or any property or asset of any of
    them is  bound  or  affected  and  which  is  identified  in  the  Officer's
    Certificate attached hereto as Exhibit A (the "Officer's Certificate").
 
        5.  The execution and delivery of the Agreement by Acquiror and Acquiror
    Sub  do not, and the performance of  this Agreement by Acquiror and Acquiror
    Sub will not, require any consent, approval, authorization or permit of,  or
    filing  with or notification  to, any governmental  or regulatory authority,
    domestic or foreign,  except pursuant to  the filing and  recordation of  an
    appropriate Certificate of Merger with the Secretary as required by Delaware
    Law.
 
        6.   Upon the  issuance of a  Certificate of Merger  by the Secretary of
    State of  the  State  of  Delaware, the  Merger  will  become  effective  in
    accordance  with the  Agreement and  the Delaware  Law, and  each issued and
    outstanding share of Company Common Stock (other than any Cancelable Shares)
    will be converted  into the consideration  provided in Section  2.01 of  the
    Agreement.
 
        7.   To our  knowledge, except as  set forth in  the Acquiror Disclosure
    Schedule (as may have  been updated), no actions,  suits or proceedings  are
    pending or threatened against Acquiror or
 
                                      A-58
<PAGE>
    Acquiror  Sub seeking to  prevent or delay  the transactions contemplated by
    the Agreement or challenging any of the terms or provisions of the Agreement
    or seeking material damages in connection therewith.
 
        8.  The authorized capital stock of Acquiror and Acquiror Sub is as  set
    forth in Section 4.03 of the Agreement.
 
        9.   The shares of Acquiror Common Stock to be issued in the Merger have
    been duly authorized and, when issued and delivered pursuant to the terms of
    the Agreement, will be validly  issued, fully paid, non-assessable and  free
    of preemptive rights.
 
    We  have participated in conferences with officers and other representatives
of Acquiror, Acquiror Sub and the Target, and representatives of the independent
auditors for the Target and Acquiror, at which the contents of the  Registration
Statement  on Form S-4  (File No. 333-       ) declared effective  by the SEC on
            , 1996 (the "Registration Statement"), the Proxy
Statement/Prospectus dated                 , 1996  included in the  Registration
Statement (the "Proxy Statement") and related matters were discussed. We are not
passing  upon, do not assume any responsibility  for and have not, and shall not
be deemed  to  have,  independently  verified  the  accuracy,  completeness,  or
fairness of the statements contained in the Registration Statement and the Proxy
Statement;  however, on the basis of our participation in the preparation of the
Proxy  Statement  and  the  Registration  Statement  and  our  participation  in
discussions  relating  to  the  contents  thereof, no  facts  have  come  to our
attention which lead us to believe that the information with respect to Acquiror
and Acquiror Sub contained in the Registration Statement or the Proxy  Statement
(except for the financial information, financial statements, financial schedules
and  other  financial or  statistical  data contained  therein,  as to  which we
express no opinion), on  the date such  Registration Statement became  effective
under  the Securities Act, on the date  such Proxy Statement was first mailed to
stockholders of the  Target, on  the date  of the  Target Stockholders'  Meeting
convened  to consider the Merger,  or on the date  hereof, contained or contains
any untrue statement of a material fact or omitted or omits to state a  material
fact  required to be stated therein or necessary to make the statements therein,
in light of the circumstances under which they were made, not misleading.
 
    The preceding opinions  and "negative assurance"  statements are subject  to
the following qualifications and limitations:
 
        A.  In connection with statements herein qualified by "to our knowledge"
    or as to matters that have "come to our attention," our examination has been
    limited  to  discussions  with  the officers  and  other  representatives of
    Acquiror and Acquiror Sub by, and those statements refer only to what is  in
    the  actual current consciousness  of, attorneys in  the Dallas, Chicago and
    Washington D.C.  offices  of  this  Firm  who  have  been  involved  in  the
    representation   of  Acquiror  and  Acquiror  Sub  in  connection  with  the
    transactions described in  the Agreement,  and we have  made no  independent
    investigations   as  to  the   accuracy  or  completeness   of  any  of  the
    representations, warranties,  data or  other information,  written or  oral,
    made or furnished by Acquiror or Acquiror Sub to us or to you.
 
        B.   This opinion letter  is limited in all respects  to the laws of the
    State of Texas, the  federal laws of  the United States  of America and  the
    General  Corporation  Law  of  the  State  of  Delaware,  and  we  assume no
    responsibility as to the applicability or  the effect of any other laws.  No
    opinion  is expressed herein with respect  to any laws, ordinances, statutes
    or regulations of  any county, city  or other political  subdivision of  the
    State of Texas.
 
        C.   The  opinions and  statements expressed  herein are  limited to the
    matters specifically addressed, and  no opinion or  statement is implied  or
    may be inferred beyond the matters so specifically addressed.
 
        D.   With respect to  the opinion expressed in  Paragraph 7, we have not
    conducted any search of any indexes,  dockets or other records of any  court
    or other Governmental Entity.
 
                                      A-59
<PAGE>
        E.   The opinions and statements expressed herein are rendered as of the
    time immediately preceding the  Effective Time, and  we hereby disclaim  any
    obligation  to  advise you  of,  or to  supplement  any of  our  opinions or
    statements because of, any changes in fact or law which might affect any  of
    those opinions or statements.
 
        F.   This opinion letter  is solely for your  benefit in connection with
    the transactions described  in the  Agreement and  may not  be relied  upon,
    quoted  or otherwise  used by any  other person  or entity or  for any other
    purpose without our express written consent.
 
                                          Very truly yours,
 
                                      A-60
<PAGE>
                                   APPENDIX B
                                FAIRNESS OPINION
 
                                      B-1
<PAGE>
March 21, 1996
Board of Directors
DUAL DRILLING COMPANY
5956 Sherry Lane, Suite 1500
Dallas, Texas 75225
Members of the Board:
 
    You   have  requested  the  opinion   of  Simmons  &  Company  International
("Simmons") as investment bankers as to the fairness, from a financial point  of
view,  to the holders of shares of common  stock, par value $0.01 per share (the
"Company Common  Stock"),  of  DUAL  DRILLING COMPANY  (the  "Company")  of  the
consideration  to be received by such stockholders in the proposed merger of the
Company with DDC Acquisition Company (the  "Sub"), a wholly owned subsidiary  of
ENSCO  International Incorporated ("ENSCO"), pursuant  to the Agreement and Plan
of Merger (the "Agreement") to  be executed by ENSCO,  the Sub, and the  Company
(the "Proposed Merger").
 
    As more specifically set forth in the Agreement, in the Proposed Merger each
issued  and outstanding share of the Company Common Stock will be converted into
0.625 of a  share of  common stock,  par value $0.10  per share,  of ENSCO  (the
"ENSCO Common Stock").
 
    Simmons, as a specialized energy-related investment banking firm, is engaged
in  the valuation of businesses and  their securities in connection with mergers
and acquisitions, the management and underwriting of sales of equity and debt to
the public, and private  placements of equity and  debt. Simmons has  previously
rendered investment banking services to the Company and ENSCO in connection with
a  number of transactions for which  Simmons received customary compensation and
may provide investment banking services to ENSCO in the future. In addition,  in
the  ordinary course of  business, Simmons may actively  trade the securities of
the Company and ENSCO for its own account and for the accounts of customers and,
accordingly, may at any time hold a long or short position in such securities.
 
    In connection with rendering its opinion, Simmons has reviewed and analyzed,
among other things, the  following: (i) the Letter  of Intent between ENSCO  and
the  Company dated January  25, 1996; (ii)  a draft of  the Agreement; (iii) the
financial statements and other information concerning the Company, including the
Annual Reports on Form 10-K  of the Company for  the three years ended  December
31,  1995, the Company's prospectus dated August 5, 1993 relating to the sale of
6.25 million  shares of  Company Common  Stock, the  Company's prospectus  dated
January  20, 1994 relating to  the sale of $100  million principal amount of its
senior subordinated notes, and  the Current Reports on  Form 8-K of the  Company
dated  January 25,  1996, August 5,  1995 and  June 5, 1995;  (iv) certain other
internal information, primarily financial in nature, concerning the business and
operations of the  Company furnished  by the  Company for  purposes of  Simmons'
analysis;  (v) certain publicly available information concerning the trading of,
and the trading  market for,  the Company  Common Stock;  (vi) certain  publicly
available  information concerning  ENSCO, including  the Annual  Reports on Form
10-K of ENSCO  for the  three years  ended December  31, 1995,  and the  Current
Reports on Form 8-K dated January 25, 1996, September 11, 1995, May 23, 1995 and
March 23, 1995; (vii) certain other internal information, primarily financial in
nature,  concerning the business and operations  of ENSCO furnished by ENSCO for
purposes of  Simmons' analysis;  (viii) certain  publicly available  information
concerning  the trading of, and the trading market for, ENSCO Common Stock; (ix)
certain publicly available information with  respect to certain other  companies
that  Simmons believes to be comparable to  the Company or ENSCO and the trading
markets for certain of  such other companies'  securities; (x) certain  publicly
available  information  concerning the  estimates  of the  future  operating and
financial performance  of  the  Company,  ENSCO  and  the  comparable  companies
prepared by industry analysts unaffiliated with either the Company or ENSCO; and
(xi)  certain publicly available information concerning  the nature and terms of
certain other transactions considered relevant to the inquiry. Simmons has  also
met with certain officers and employees of the
 
                                      B-2
<PAGE>
Company  and ENSCO to discuss  the foregoing, as well  as other matters believed
relevant to the inquiry. In addition,  Simmons has made such other analyses  and
examinations  as it considered  appropriate in expressing  its opinion contained
herein.
 
    In arriving at its opinion, Simmons has assumed and relied upon the accuracy
and completeness of all of the  financial and other information provided by  the
Company  and  ENSCO,  or  publicly  available,  including,  without  limitation,
information with respect to asset conditions, tax positions, liability  reserves
and  insurance coverages, and  has not attempted independently  to verify any of
such information. Simmons has not conducted a physical inspection of any of  the
assets,  properties or facilities of the Company  or ENSCO, nor has Simmons made
or obtained any  independent evaluations  or appraisal  of any  of such  assets,
properties  or facilities. In  addition, Simmons has  discussed the prospects of
the Company with certain  representatives of the Company  and has been  provided
with  certain financial projections  of the Company.  Simmons has also discussed
the prospects of ENSCO with certain representatives of ENSCO.
 
    In conducting its analysis and arriving at its opinion as expressed  herein,
Simmons has considered such financial and other factors as it deemed appropriate
under  the  circumstances  including,  among  others,  the  following:  (i)  the
historical and  current financial  position  and results  of operations  of  the
Company  and ENSCO; (ii) the business prospects  of the Company and ENSCO; (iii)
the historical and current market for the Company Common Stock, for ENSCO Common
Stock and for the  equity securities of certain  other companies believed to  be
comparable  to the Company or ENSCO;  (iv) the respective contributions in terms
of various financial measures of the Company and ENSCO to the combined  company,
and  the relative pro forma ownership of  ENSCO after the Proposed Merger by the
current holders of the Company Common Stock and ENSCO Common Stock; and (v)  the
nature and terms of certain other acquisition transactions that Simmons believes
to  be relevant. Simmons has  also taken into account  its assessment of general
economic, market and financial conditions and its experience in connection  with
similar  transactions  and  securities'  valuation  generally.  Simmons' opinion
necessarily is based upon conditions as they exist and can be evaluated on,  and
on  the information made available at, the  date hereof. The opinion rendered by
Simmons does not constitute an  opinion as to the  future value of ENSCO  Common
Stock  upon consummation  of the  Proposed Merger  or the  price at  which ENSCO
Common Stock will trade at any time.
 
    Simmons is acting as  financial advisor to the  Company in this  transaction
and will receive a fee for its services that is contingent upon the consummation
of the Proposed Merger.
 
    It  is understood that  this letter is  for the information  of the Board of
Directors of the  Company and  may not  be used  for any  other purpose  without
Simmons' prior written consent.
 
    Based  upon and  subject to  the foregoing,  Simmons is  of the  opinion, as
investment bankers, that the consideration to be received by the holders of  the
Company  Common Stock  in the  Proposed Merger  is fair  to such  holders from a
financial point of view.
 
                                                        Sincerely,
                                             SIMMONS & COMPANY INTERNATIONAL
                                                     /s/ BEN A. GUILL
                                          --------------------------------------
                                                       Ben A. Guill
                                                    MANAGING DIRECTOR
 
                                      B-3
<PAGE>
                                   APPENDIX C
                       DUAL SPECIAL PERFORMANCE UNIT PLAN
 
                                      C-1
<PAGE>
                       DUAL SPECIAL PERFORMANCE UNIT PLAN
 
    1.  PURPOSE.  Dual Drilling Company, a Delaware corporation (the "Company"),
hereby establishes this DUAL Special Performance Unit Plan (the "Plan") in order
to retain and reward key executives of the Company by granting them special cash
bonuses in the event of the sale of the Company during the term of the Plan. The
Plan  is intended to provide the Participants with the incentive to increase the
value of the business of the Company  by allowing them to participate in a  cash
bonus pool that is commensurate with the sale price of the Company.
 
    2.   ADMINISTRATION.   The Plan shall  be administered by  a committee which
shall be (i) appointed by the Board  of Directors of the Company (the  "Board"),
(ii)  constituted so as  to permit the Plan  to comply with  Rule 16(b)-3 of the
Securities Exchange Act of 1934 and  (iii) constituted such that all members  of
such  committee  are "outside  directors" as  defined in  Section 162(m)  of the
Internal Revenue  Code  of  1986  and  all  rules  and  regulations  promulgated
thereunder  (the  "Code"). Such  committee shall  be referred  to herein  as the
"Committee." The  Committee shall  administer the  Plan in  accordance with  its
terms  and  shall  settle  all disputes  arising  thereunder.  In  addition, the
Committee shall have  all authority  and duties prescribed  for a  "compensation
committee"  under Section 162(m) of the  Code. The Compensation Committee of the
Board is hereby appointed as  the Committee for purposes  of the Plan, to  serve
until such time as the Board appoints another committee.
 
    3.   PARTICIPATION.  The participants int he Plan (the "Participants") shall
consist of  the following  individual officers:  L.H. Robertson,  President  and
C.E.O.; W. Allen Parks, Executive Vice President and C.F.O.; Dudley M. Haralson,
Senior  Vice President; Robert C. McCoy,  Senior Vice President; Lewis W. Kreps,
Senior Vice President; William R. Dudark, Vice President; and Robert F.  Chrone,
Controller and Corporate Secretary.
 
    4.   PAYMENT  OF PERFORMANCE  BONUSES.   If a  Sale Transaction  (as defined
below) occurs  on  or prior  to  the end  of  the term  of  the Plan  and  if  a
Participate  is employed by the Company on such date, then the Participant shall
be paid his respective Applicable Percentage of the Performance Bonus Pool. Such
amount shall be paid  within thirty (30)  days after the  effective date of  the
Sale  Transaction and shall be payable in a  single cash lump sum amount to each
Participant.
 
    5.  DEFINITIONS.   For purposes of  the Plan, the  following terms have  the
indicated meanings when used herein:
 
        "Applicable   Percentage"  shall  mean,  as  to  each  Participant,  the
    percentage shown for such Participant on Schedule A attached hereto and made
    a part hereof for all purposes.
 
        "Performance  Bonus  Pool"  shall  mean  a  cash  amount  based  on  the
    Equivalent Share Price, determined as follows:
 
<TABLE>
<S>                                 <C>
IF THE EQUIVALENT SHARE PRICE IS:     THEN THE PERFORMANCE BONUS POOL AMOUNT IS:
At least $12.00 but less than       The product of (i) the excess of the
$13.00                               Equivalent Share Price over $10.00 multiplied
                                     by (ii) 100,000.
At least $13.00 but less than       The product of (i) the excess of the
$14.00                               Equivalent Share Price over $10.00 multiplied
                                     by (ii) 200,000.
At least $14.00 but less than       The product of (i) the excess of the
$15.00                               Equivalent Share Price over $10.00 multiplied
                                     by (ii) 300,000.
$15.00 and above                    The   product  of   (i)  the   excess  of  the
                                     Equivalent Share Price over $10.00 multiplied
                                     by (ii) 400,000.
</TABLE>
 
                                      C-2
<PAGE>
        Each equivalent share  price in the  above table shall  be adjusted  and
    increased  or  decreased,  as  the  case  may  be,  in  an  amount  that  is
    commensurate with any increases or decreases in the Equivalent Share Number,
    as provided herein.
 
        "Equivalent Share  Price"  shall  mean  a dollar  amount  which  is  the
    quotient  of (A) divided by (B) where "(A)" is the quotient of (i) the value
    of the consideration  (as reasonably  determined by the  Committee) paid  or
    given for the stock or assets of the Company sold or transferred pursuant to
    the Sale Transaction divided by (ii) the decimal representing the percentage
    of  the Common Stock  or assets of  the Company that  is sold or transferred
    pursuant to the Sale Transaction, and "(B)" is the Equivalent Share Number.
 
        "Equivalent Share Number" shall mean the number of outstanding shares of
    common stock of the Company on the  Effective Date of the Plan increased  or
    decreased  in the  discretion of the  Committee as necessary  to reflect any
    changes in the outstanding  Common Stock of the  Company by reason of  stock
    dividends,   stock  splits,   recapitalizations,  reorganizations,  mergers,
    consolidations,  combinations,  exchanges  or  other  relevant  changes   in
    capitalization occurring after the Effective Date of the Plan.
 
        "Sale  Transaction" shall mean any one  of the following events: (i) the
    merger, consolidation or other  reorganization of the  Company in which  the
    outstanding Common Stock of the Company is converted into or exchanged for a
    class  of  securities  of any  other  issuer  (except a  direct  or indirect
    subsidiary of the Company), cash or other property; and (ii) the sale, lease
    or exchange of all or substantially all of the assets of the Company to  any
    other  corporation or entity (except a  direct or indirect subsidiary of the
    Company).
 
    6.  APPROVAL  OF COMMITTEE.   By executing  this Plan  below, the  Committee
thereby  approves,  ratifies  and  establishes  the  Performance  Goals  and the
compensation payable with respect to such goals  that are set forth in the  Plan
for purposes of Code Section 162(m).
 
    7.   NO PROMISE  OF EMPLOYMENT.  Nothing  in this Plan  or any other writing
executed pursuant  to or  in connection  with  the Plan  shall be  construed  to
promise or guarantee future employment of any person.
 
    8.   WITHHOLDING.  The foregoing provisions of the Plan notwithstanding, the
Company shall withhold from any amounts payable under the Plan all amounts  that
are  required to be withheld under federal, state and local law, and all amounts
payable hereunder shall be paid net of such withheld amounts, if any.
 
    9.   NO TAX  GUARANTY.   Nothing in  this Plan  or in  any writing  executed
pursuant  to or  in connection with  the Plan  shall be construed  to promise or
guarantee any particular federal or state income or excise tax treatment of  any
amounts payable hereunder.
 
    10.  APPLICABLE LAW.  This Plan shall be governed by the law of the State of
Texas,  exclusive of the conflict of law provisions thereof. Venue for any court
action arising out of  this Plan or  the interpretation of  this Plan shall  lie
exclusively in the courts of the State of Texas.
 
    11.   BINDING  EFFECT.   This agreement  shall be  binding upon  the parties
hereto, and the assigns  of, and the  successors to, the  interest of any  party
hereto  in  and to  the Company  or the  business  or assets  of Company.  It is
specifically provided that  this Plan  shall apply to  and be  binding upon  the
transferee  of any interest in  the Company and any  interest in the business or
assets of the Company.
 
    12.  EFFECTIVE DATE.   The effective  date of the Plan  shall be August  21,
1995  (the  "Effective Plan"),  subject to  shareholder  approval. The  Plan or,
alternatively, its  material terms,  as per  the regulations  promulgated  under
Section  162(m)  of the  Code, shall  be  submitted to  the shareholders  of the
Company for their approval and adoption. The Plan shall not be effective and  no
amount  shall  be payable  hereunder,  unless and  until  the Plan  has  been so
approved and adopted at a meeting of the Company's shareholders.
 
                                      C-3
<PAGE>
    13.  TERM.  Unless extended in a manner as may be required by all applicable
law, the Plan  shall terminate on  August 20,  1997, and only  those bonuses  or
other  compensation shall be payable pursuant to the Plan with respect to a Sale
Transaction that is effective, or as to which a definitive binding agreement  is
in effect, on or before such date.
 
    14.   AMENDMENT.  The Plan may be  amended by the Board of Directors subject
to any shareholder approval as may be  required by Code Section 162(m) or  other
applicable law.
 
    IN  WITNESS  WHEREOF, the  Company and  the  Committee have  indicated their
approval of the Plan as of the Effective Date by signing in the spaces  provided
below.
 
<TABLE>
<S>                                          <C>        <C>
COMPANY:                                     DUAL DRILLING COMPANY,
                                             a Delaware corporation
 
                                             By:        /s/ L.H. Dick Robertson
                                                        ------------------------------------------
 
                                             Title: PRESIDENT AND CHIEF EXECUTIVE OFFICER
 
COMMITTEE:                                   COMPENSATION COMMITTEE OF THE BOARD OF DIRECTORS OF
                                             DUAL DRILLING COMPANY
 
                                             By:        /s/ Frank Junger
                                                        ------------------------------------------
 
                                             Title: CHAIRMAN OF THE COMPENSATION COMMITTEE
                                             ---------------------------------------------
</TABLE>
 
                                      C-4
<PAGE>
                                   SCHEDULE A
                                     TO THE
                       DUAL SPECIAL PERFORMANCE UNIT PLAN
 
<TABLE>
<S>                                                   <C>
L. H. Robertson.....................................  27.0%
W. A. Parks.........................................  15.6%
D. M. Haralson......................................  15.6%
R. C. McCoy.........................................  13.2%
L. W. Kreps.........................................  12.0%
W. R. Dudark........................................  12.0%
R. F. Chrone........................................  4.5%
</TABLE>
 
                                      C-5
<PAGE>
                                   APPENDIX D
                        PRINCIPAL STOCKHOLDER AGREEMENT
 
                                      D-1
<PAGE>
                                   AGREEMENT
 
    This  Agreement, dated as of March  21, 1996, is between ENSCO International
Incorporated, a Delaware corporation ("ENSCO"), and Dual Invest AS, a  Norwegian
corporation (the "Stockholder").
 
    WHEREAS, concurrently herewith, ENSCO, ENSCO Acquisition Company, a Delaware
corporation  and a wholly-owned  subsidiary of ENSCO  ("Acquiror Sub"), and DUAL
DRILLING COMPANY, a Delaware corporation  (the "Company"), are entering into  an
Agreement  and Plan  of Merger (the  "Merger Agreement";  capitalized terms used
without definition herein  having the  meanings ascribed thereto  in the  Merger
Agreement);
 
    WHEREAS,  the Stockholder  is the record  and beneficial  owner of 9,382,354
shares of Target Common Stock (the "Block Shares");
 
    WHEREAS, approval of the  Merger Agreement and the  Merger by the  Company's
stockholders is a condition to the consummation of the Merger; and
 
    WHEREAS, as a condition to its entering into the Merger Agreement, ENSCO has
required  that the Stockholder  agree, and the Stockholder  has agreed, to enter
into this Agreement.
 
    NOW THEREFORE, in consideration  of the foregoing  and the mutual  covenants
and agreements set forth herein, the parties hereto agree as follows:
 
    Section  1.   VOTING AGREEMENT AND  GRANT OF PROXY.   From the  date of this
Agreement until July 31, 1996:
 
    (a) The Stockholder hereby agrees that at any meeting of the stockholders of
the Company, however called,  and any action by  consent of the stockholders  of
the  Company, the Stockholder shall vote the  Block Shares, and any other voting
securities of the  Company, whether  issued heretofore or  hereafter, which  are
held  of record or beneficially  by the Stockholder, (i)  in favor of the Merger
and the Merger Agreement,  (ii) in favor  of adoption and  approval of the  Dual
Special  Performance  Unit  Plan,  effective  August  21,  1995,  as  amended as
contemplated by the  Merger Agreement, and  (iii) against any  proposal for  any
recapitalization,  merger  (other  than the  Merger),  sale of  assets  or other
business combination between the  Company and any person  or entity (other  than
ENSCO  or Acquiror Sub) or any other action or agreement that ENSCO notifies the
Stockholder in writing before any vote would result in a breach of any covenant,
representation or warranty or any other  obligation or agreement of the  Company
under the Merger Agreement or which could result in any of the conditions to the
Merger Agreement not being fulfilled.
 
    (b)  Except as provided  in this Section  1 and except  for transfers to the
Stockholder from an affiliate, the Stockholder hereby agrees that it shall  not,
and  shall not offer or agree to, sell, transfer, tender, assign, hypothecate or
otherwise dispose of, or  create or permit to  exist any pledge, lien,  security
interest,  mortgage, charge, claim, option,  proxy, voting restriction, right of
first refusal, limitation on disposition, or encumbrance of any kind on or  with
respect  to the Block Shares or other  voting securities of the Company, whether
issued heretofore or hereafter, which are held of record or beneficially by  the
Stockholder.
 
    (c) The Stockholder, by this Agreement, with respect to the Block Shares and
any  other  voting  securities  of the  Company,  whether  issued  heretofore or
hereafter, which are held of record  by the Stockholder, does hereby  constitute
and  appoint ENSCO and Acquiror  Sub, or any nominee  of ENSCO and Acquiror Sub,
with full power of substitution, from the date hereof to the earlier to occur of
July 31, 1996 or the Effective Time,  as its true and lawful attorney and  proxy
(its  "Proxy"),  for  and in  its  name, place  and  stead, to  demand  that the
Secretary of  the Company  call a  special meeting  of the  stockholders of  the
Company  for  the  purpose of  considering  any  actions related  to  the Merger
Agreement and to vote each of the  Block Shares and any other voting  securities
of the Company, whether issued heretofore or hereafter, which are held of record
by the Stockholder, at every annual,
 
                                      D-2
<PAGE>
special  or adjourned meeting of the  stockholders of the Company, including the
right to sign  its name (as  stockholder) to any  consent, certificate or  other
document  relating to  the Company  that the  law of  the State  of Delaware may
permit or require:
 
        (i) in favor of the Merger and the Merger Agreement;
 
        (ii) in favor of adoption and  approval of the Dual Special  Performance
    Unit  Plan, effective  August 21,  1995, as  amended as  contemplated by the
    Merger Agreement; and
 
       (iii) against any proposal for  any recapitalization, merger (other  than
    the  Merger),  sale  of assets  or  other business  combination  between the
    Company and any person or entity (other  than ENSCO or Acquiror Sub) or  any
    other  action or  agreement that ENSCO  notifies the  Stockholder in writing
    before any vote would result in a breach of any covenant, representation  or
    warranty  or  any other  obligation or  agreement of  the Company  under the
    Merger Agreement or  could result  in any of  the conditions  to the  Merger
    Agreement not being fulfilled.
 
THIS  PROXY AND POWER OF  ATTORNEY IS IRREVOCABLE AND  COUPLED WITH AN INTEREST.
The Stockholder  acknowledges  receipt  and  review of  a  copy  of  the  Merger
Agreement. The Stockholder hereby revokes all proxies heretofore made by it that
are inconsistent with this Section 1.
 
    (d) The Stockholder shall perform such further acts and execute such further
documents  and instruments as  may reasonably be  required to vest  in ENSCO and
Acquiror Sub the power to  carry out and give effect  to the provisions of  this
Agreement.
 
    (e)  The Company will  cause each certificate  of the Stockholder evidencing
the Block Shares outstanding during the period that this Section 1 is in  effect
to bear a legend in the following form:
 
    THE SHARES REPRESENTED BY THIS CERTIFICATE MAY NOT BE SOLD, EXCHANGED OR
    OTHERWISE TRANSFERRED OR DISPOSED OF EXCEPT IN COMPLIANCE WITH THE TERMS
    AND  CONDITIONS  OF AN  AGREEMENT DATED  MARCH  21, 1996,  AS IT  MAY BE
    AMENDED, AMONG DUAL DRILLING  COMPANY, ENSCO INTERNATIONAL  INCORPORATED
    AND  THE REGISTERED HOLDER  OF THIS CERTIFICATE,  A COPY OF  WHICH IS ON
    FILE AT THE PRINCIPAL EXECUTIVE OFFICES OF THE ISSUER.
 
Upon the expiration of the period during which this Section 1 is in effect or in
the  event  that  the  Block  Shares  otherwise  cease  to  be  subject  to  the
restrictions  on transfer set  forth in this Agreement,  the Company shall, upon
the written  request  of  the  Stockholder,  issue  to  the  Stockholder  a  new
certificate  evidencing such shares without the  legend required by this Section
1(e).
 
    (f) The Stockholder agrees that until July 31, 1996 it will not vote any  of
the Block Shares at any annual, special or adjourned meeting of the stockholders
of  the Company, including  the right to  sign its name  (as stockholder) to any
consent, certificate or other document relating  to the Company that the law  of
the  State of Delaware may permit or require, (i) to approve of the adoption and
approval of the Dual Special Performance Unit Plan, effective August 21, 1995 in
any manner except as contemplated by the Merger Agreement, or (ii) in any manner
that is intended, or  could reasonably be expected,  to impede, interfere  with,
delay, postpone, or materially adversely affect the transactions contemplated by
the Merger Agreement.
 
    Section  2.    SECURITIES ACT  COVENANTS  AND REPRESENTATIONS.    Subject to
ENSCO's obligations  under  Section  3.1,  the  Stockholder  hereby  agrees  and
represents to ENSCO as follows:
 
    (a)  The Stockholder has been  advised that the offer,  sale and delivery of
the Acquiror Common Stock to the Stockholder  pursuant to the Merger may not  be
registered  under  the  Securities  Act,  despite  ENSCO's  obligations  to  use
commercially reasonable efforts to effect such registration. The Stockholder has
been advised that if the offer, sale  and delivery of the Acquiror Common  Stock
to  the Stockholder  pursuant to  the Merger has  not been  registered under the
Securities Act, then such
 
                                      D-3
<PAGE>
shares (the "Merger Shares") may not be offered, sold, pledged, hypothecated  or
otherwise transferred unless subsequently registered under the Securities Act or
an  exemption from such registration is available. The Stockholder has also been
advised that even  if the sale  and delivery  to the stockholder  of the  Merger
Shares  is registered under the Securities Act, to the extent the Stockholder is
considered an "affiliate"  of the Company  at the time  the Merger Agreement  is
submitted  for a vote of the stockholders of the Company, any public offering or
sale by the Stockholder  of the Merger Shares  will, under current law,  require
either  (i)  the further  registration under  the Securities  Act of  the Merger
Shares,  which  ENSCO  is  obligated  under  Section  3.1  to  use  commercially
reasonable  efforts to effect, (ii) compliance  with Rule 145 promulgated by the
Securities and Exchange Commission (the  "Commission") under the Securities  Act
or  (iii) the availability of another exemption from such registration under the
Securities Act.
 
    (b) The Stockholder has read this Agreement and the Merger Agreement and has
discussed their requirements and other  applicable limitations upon its  ability
to  sell, transfer or otherwise dispose of  the Merger Shares, to the extent the
Stockholder believed necessary, with its counsel or counsel for the Company.
 
    (c) The Stockholder also understands that stop transfer instructions will be
given to ENSCO's transfer agents  with respect to the  Merger Shares and that  a
legend  will be placed on  the certificates for the  Merger Shares issued to the
Stockholder, to the extent the Stockholder  is considered an "affiliate" of  the
Company  at  the  time the  Merger  Agreement is  submitted  for a  vote  of the
stockholders of the Company.
 
    Section 3.  REGISTRATION OF MERGER SHARES.
 
    (a) ENSCO shall  use all commercially  reasonable efforts on  or before  the
Effective  Time  to effect  the  registration under  the  Securities Act,  on an
appropriate form, of the  transfer of the Merger  Shares to the Stockholder  and
the  Stockholder's subsequent  transfer of  such Merger  Shares to  the security
holders of  the Stockholder  in the  manner contemplated  on Exhibit  A and  the
resale  of the Merger Shares by the Stockholder or by B. Skaugen Shipping AS and
its affiliates (collectively, "Skaugen")(all  persons who receive Merger  Shares
and  whose resales are covered by this  Section 3(a) shall be referred to herein
as the "Selling  Stockholders") unless ENSCO  shall have provided  to Skaugen  a
no-action  letter,  or  opinion  of counsel  reasonably  acceptable  to Skaugen,
concluding that Skaugen will not be restricted in any way in its ability  absent
such  registration  to  resell  Merger  Shares  received  by  Skaugen  from  the
Stockholder. ENSCO  shall  be  required  to  file  only  one  such  registration
statement  and shall  keep such  registration continuously  effective until such
time as the Merger Shares have been disposed of by the Selling Stockholders  but
in  no event for  a period of  longer than twelve  months after the  date of the
Effective Time. For purposes of  this Section 3, "Registration Statement"  means
the  registration statement  covering the  Merger Shares  filed pursuant hereto,
including, to the extent applicable, the prospectus (the "Prospectus")  included
in  any such registration statement, all  amendments and supplements to any such
registration statement (including  post-effective amendments),  all exhibits  to
any  such registration statement  and all material  incorporated by reference in
any such registration statement.
 
    (b) In connection with ENSCO's registration obligations pursuant to  Section
3(a)  and, except as provided in  Section 3(b)(i), ENSCO shall keep continuously
effective the Registration Statement for the period of time provided in  Section
3(a),  to permit  the sale  of the  Merger Shares  pursuant to  the Registration
Statement in  accordance with  the intended  method or  methods of  distribution
thereof specified by the Stockholder in Section 3(a) above and in Exhibit A, and
shall:
 
        (i)   notify  the  Selling   Stockholders,  promptly  (A)   when  a  new
    Registration Statement, Prospectus or  supplement thereto or  post-effective
    amendment  has been filed, and, with respect to a new Registration Statement
    or post-effective amendment when it has become effective, (B) of any request
    by  the  Commission  for  amendments  or  supplements  to  any  Registration
    Statement  or Prospectus or for additional  information, (C) of the issuance
    by the Commission of any comments with respect to any filing and of any stop
    order suspending the effectiveness of any Registration
 
                                      D-4
<PAGE>
    Statement or the initiation of any proceedings for that purpose, (D) of  the
    receipt  by ENSCO of any notification with  respect to the suspension of the
    qualification of  the Merger  Shares for  sale in  any jurisdiction  or  the
    initiation  or threatening  of any proceeding  for such purpose,  (E) of the
    happening of any  event that makes  any statement made  in any  Registration
    Statement,  Prospectus  or any  document  incorporated therein  by reference
    untrue or  that requires  the  making of  any  changes in  any  Registration
    Statement,  Prospectus or any document  incorporated therein by reference in
    order to make  the statements  therein not  misleading, and  (F) of  ENSCO's
    determination  that a  post-effective amendment to  a Registration Statement
    would be appropriate;
 
        (ii) furnish  to  the  Selling Stockholders,  without  charge,  as  many
    conformed copies of the Registration Statement and any amendments thereto as
    may reasonably be requested by the Selling Stockholders;
 
       (iii) deliver to the Selling Stockholders, without charge, as many copies
    of  the  Prospectus  covering  the  Merger  Shares  and  any  amendments  or
    supplements thereto  as  the Stockholder  or  the Selling  Stockholders  may
    reasonably request;
 
       (iv)  register, qualify, or obtain  an exemption therefrom, in connection
    with the registration or qualification or exemption therefrom of the  Merger
    Shares  for offer and sale under the securities or blue sky laws of New York
    and, as the Stockholder reasonably requests, any other jurisdictions  within
    the  United States  and do  any and  all other  acts or  things necessary or
    advisable to  enable the  disposition in  such jurisdictions  of the  Merger
    Shares  covered by the Registration Statement; PROVIDED, HOWEVER, that ENSCO
    shall not be required to qualify as  a dealer in securities or as a  foreign
    corporation, or otherwise subject itself to taxation in connection with such
    activities,  or to execute  a general consent  to service of  process in any
    jurisdiction;
 
        (v) otherwise use its best efforts  to comply with all applicable  rules
    and  regulations of  the Commission  relating to  such registration  and the
    distribution of the securities being  offered, and make generally  available
    to  its securities holders  earning statements satisfying  the provisions of
    Section 11(a) of the Securities Act no  later than 90 days after the end  of
    any  12-month period (or 120 days if such period is a fiscal year) beginning
    with the  first month  of  the first  fiscal  quarter commencing  after  the
    effective  date  of such  Registration  Statement, which  earning statements
    shall cover such 12-month periods;
 
       (vi) in  no  event  later  than  ten  business  days  before  filing  any
    Registration  Statement or Prospectus, or any amendment or supplement (other
    than any amendment or supplement made solely as a result of incorporation by
    reference of documents) to  any thereof (or, in  the case of any  Prospectus
    supplement   or  post-effective  amendment  relating  to  a  proposed  shelf
    "draw-down," three Business Days before the filing thereof), furnish to  the
    Stockholder  copies  of  all  such documents  proposed  to  be  filed, which
    documents shall be subject to the reasonable review of the Stockholder;
 
       (vii)  promptly  after  the  filing  of  any  document  that  is  to   be
    incorporated  by  reference  into a  Registration  Statement  or Prospectus,
    provide copies of such document to the Stockholder; and
 
      (viii)  use  all  commercially  reasonable  efforts  to  take  all  action
    necessary   or  advisable  to   effect  such  registration   in  the  manner
    contemplated by this Agreement.
 
    (c) The Stockholder and each Selling Stockholder shall furnish to ENSCO such
information regarding the Stockholder and each Selling Stockholder and the  plan
of  distribution of the Merger Shares as  ENSCO may from time to time reasonably
request.
 
    (d) The Stockholder and each Selling Stockholder agrees that upon receipt of
any notice from ENSCO  of the happening  of any event of  the kind described  in
Sections  3(b)(i)(B), 3(b)(i)(C), 3(b)(i)(D), 3(b)(i)(E) or 3(b)(i)(F), it shall
forthwith  discontinue  disposition  of  the  Merger  Shares  pursuant  to   the
Prospectus  until (A) it is advised in  writing by ENSCO that a new Registration
Statement covering the offer of the Merger Shares has become effective under the
Securities Act or
 
                                      D-5
<PAGE>
(B) it receives copies of a supplemented or amended Prospectus, or (C) until  it
is  advised in writing by  ENSCO that the use of  the Prospectus may be resumed.
ENSCO shall promptly take  all such action as  may be necessary or  appropriate,
including,  without limitation, the filing of a new Registration Statement or an
amendment to the  then current Registration  Statement and/or the  filing of  an
amended  Prospectus, to limit the duration of any discontinuance with respect to
the disposition of the Merger Shares pursuant to this Section 3(d).
 
    (e) The Stockholder and each Selling Stockholder shall cooperate with  ENSCO
in  all reasonable respects in connection with the preparation and filing of the
Registration Statement; provided, however, that the Stockholder and each Selling
Stockholder shall not be  required to incur any  material out-of-pocket cost  or
expense when providing such cooperation.
 
    (f) The Stockholder and any Selling Stockholder agrees at any time beginning
not  earlier than  six (6)  months after  the Effective  Time not  to effect any
public sale  or distribution  of any  of ENSCO's  securities during  the  period
beginning  10  days  prior  to and  ending  90  days after,  the  closing  of an
underwritten offering of securities by ENSCO if the managing underwriter in such
offering determines the sale of the  Merger Shares would have an adverse  effect
on  an orderly public distribution of securities in the underwritten offering or
would have an  adverse effect  on the  price of  the securities  offered in  the
underwritten   offering;  provided  that  if  the  Stockholder  or  any  Selling
Stockholder is prevented from selling or distributing ENSCO's securities  during
any  period pursuant to this Section 3(f), then the registration provided for in
Section 3(a) shall be kept continuously effective for at least 90 days after the
end of any such period notwithstanding  the limitation provided in Section  3(a)
that  such registration shall in no event be required to be kept effective for a
period of longer than 12 months after the date of the Effective Time;
 
    (g) All expenses incident to ENSCO's performance of or compliance with  this
Agreement,  including without limitation all  registration and filing fees, fees
and expenses for compliance with securities or blue sky laws (including fees and
disbursements of ENSCO's counsel in  connection with blue sky qualifications  or
registrations  (or the obtaining of exemptions therefrom) of the Merger Shares),
printing expenses (including expenses  of printing Prospectuses), messenger  and
delivery   expenses,  internal  expenses  (including,  without  limitation,  all
salaries and  expenses  of  its  officers  and  employees  performing  legal  or
accounting  duties), fees and  disbursements of its  counsel and its independent
certified public accountants, fees and expenses of any special experts  retained
by ENSCO in connection with any registration hereunder, and fees and expenses of
other persons retained by ENSCO, but excluding fees and disbursements of counsel
retained  by  the Stockholder,  any fees  and expenses  of any  underwriters and
transfer taxes, if any, relating to the Merger Shares, shall be borne by ENSCO.
 
    (h) ENSCO shall indemnify and hold harmless, to the full extent permitted by
law, the Stockholder,  its officers, directors,  employees, representatives  and
agents,  and each Person who controls (within the meaning of the Securities Act)
the Stockholder, and each other Selling Stockholder against all losses,  claims,
damages,  liabilities and expenses (including  reasonable costs of investigation
and legal expenses) resulting from any  untrue or alleged untrue statement of  a
material  fact contained in any Registration Statement or any Prospectus, or any
amendment or supplement thereto, or any omission or alleged omission to state in
any thereof a material fact required to  be stated therein or necessary to  make
the  statements therein  not misleading, except  in each case  insofar, but only
insofar, as the  same arises  out of  or is based  upon an  untrue statement  or
alleged  untrue statement of a material fact  or an omission or alleged omission
to state a material fact  in such Registration Statement, Prospectus,  amendment
or  supplement, as  the case may  be, made  or omitted, as  the case  may be, in
reliance upon and in conformity with written information furnished to ENSCO by a
Selling Stockholder expressly for use therein.
 
    (i) The Stockholder and each Selling  Stockholder each with respect only  to
written  information  furnished  by  it  to  ENSCO  expressly  for  use  in  any
Registration Statement, any Prospectus, or  any amendment or supplement  thereto
shall  indemnify and hold harmless, to the  full extent permitted by law, ENSCO,
its officers, directors, employees, representatives and agents, and each  Person
who
 
                                      D-6
<PAGE>
controls  (within the meaning of the  Securities Act) ENSCO, against all losses,
claims,  damages,  liabilities  and  expenses  (including  reasonable  costs  of
investigation  and legal expenses)  resulting from any  untrue or alleged untrue
statement of  a material  fact  contained in  such Registration  Statement,  any
Prospectus,  or any amendment or supplement  thereto, or any omission or alleged
omission to state in any thereof a  material fact required to be stated  therein
or  necessary to make the statements therein  not misleading, as the same arises
out of or is  based upon an  untrue statement or alleged  untrue statement of  a
material  fact or an  omission or alleged  omission to state  a material fact in
such Registration Statement,  Prospectus, amendment or  supplement, as the  case
may  be, made or omitted, as the case may be, in reliance upon and in conformity
with such written information.
 
    (j)   Each party  entitled  to indemnification  under  this Section  3  (the
"Indemnified  Party")  shall  give  notice  to  the  party  required  to provide
indemnification (the "Indemnifying Party") promptly after such Indemnified Party
has actual knowledge of any claim as to which indemnity may be sought, and shall
permit the Indemnifying Party  to assume the  defense of any  such claim or  any
litigation  resulting  therefrom; PROVIDED,  that  counsel for  the Indemnifying
Party, who will conduct the defense of such claim or litigation, is approved  by
the  Indemnified  Party (whose  approval will  not  be unreasonably  withheld or
delayed); and PROVIDED, FURTHER,  that the failure of  any Indemnified Party  to
give  notice as provided herein shall not  relieve the Indemnifying Party of its
obligations except to  the extent that  its defense of  the claim or  litigation
involved is prejudiced by such failure. The Indemnified Party may participate in
such  defense at such party's expense;  PROVIDED, HOWEVER, that the Indemnifying
Party shall pay such expense if representation of such Indemnified Party by  the
counsel  retained by the Indemnifying Party would be inappropriate due to actual
or potential conflicts of interest between  the Indemnified Party and any  other
party  represented by such counsel in such proceeding. No Indemnifying Party, in
the defense of any such claim or  litigation, shall, except with the consent  of
each  Indemnified Party,  consent to  entry of  any judgment  or enter  into any
settlement that does not include as an unconditional term thereof the giving  by
the  claimant  or plaintiff  to such  Indemnified  Party of  a release  from all
liability in respect of any claim  or litigation, and no Indemnified Party  will
consent  to entry of any judgment or  settle any claim or litigation without the
prior written consent of  the Indemnifying Party.  Each Indemnified Party  shall
furnish  such information regarding himself or  itself and the claim in question
as the Indemnifying  Party may  reasonably request  and as  shall be  reasonably
required  in connection with the defense  of such claim and litigation resulting
therefrom.
 
    (k) If for  any reason the  indemnification provided for  in this Section  3
from  an Indemnifying Party is unavailable  to an Indemnified Party hereunder or
insufficient to hold  it harmless as  contemplated by this  Section 3, then  the
Indemnifying  Party,  in  lieu  of indemnifying  such  Indemnified  Party, shall
contribute to the amount paid or payable  by such Indemnified Party as a  result
of  such losses, claims, damages, liabilities  or expenses in such proportion as
is appropriate  to reflect  the  relative fault  of  an Indemnifying  Party  and
Indemnified  Party in connection with the  actions that resulted in such losses,
claims, damages,  liabilities  or  expenses,  as  well  as  any  other  relevant
equitable  considerations.  The relative  fault  of the  Indemnifying  Party and
Indemnified Party  shall be  determined  by reference  to, among  other  things,
whether any action in question, including any untrue or alleged untrue statement
of  a material fact, has been made by, or relates to information supplied by, an
Indemnifying Party  or  Indemnified Party,  and  the parties'  relative  intent,
knowledge,  access to  information and  opportunity to  correct or  prevent such
action. The amount paid or payable by a party as a result of the losses, claims,
damages, liabilities and expenses referred to above shall be deemed to  include,
subject to the limitations set forth in Section 3(j), any legal or other fees or
expenses  reasonably incurred by such party in connection with any investigation
or proceeding. The parties hereto agree that it would not be just and  equitable
if  contribution  pursuant to  this  Section 3(k)  were  determined by  pro rata
allocation or by any other  method of allocation that  does not take account  of
the equitable considerations referred to in the immediately preceding paragraph.
No  person guilty of fraudulent misrepresentation (within the meaning of Section
11(f) of the Securities Act) shall  be entitled to contribution from any  person
who was not guilty of such fraudulent misrepresentation.
 
                                      D-7
<PAGE>
    Section  4.  TAX REPRESENTATION.  The Stockholder represents and warrants to
ENSCO that, except as set forth on Exhibit  A hereto, it has no present plan  or
intention  to  sell, exchange,  transfer by  gift, or  otherwise dispose  of the
Merger Shares.
 
    Section 5.    HART-SCOTT-RODINO ANTITRUST  IMPROVEMENTS  ACT OF  1976.    In
connection  with  the Merger,  to  the extent  required  by applicable  law, the
Stockholder agrees  promptly and  timely to  file,  or cause  to be  filed,  all
notifications,  including responses to requests  for information, required to be
filed by it  by the HSR  Act. The  Stockholder agrees to  cooperate with  ENSCO,
Acquiror  Sub and the Company to the  extent necessary to permit them to prepare
their separate filings  and to  supply any  additional information  that may  be
submitted  to the Federal Trade Commission or the Department of Justice relating
to the transactions  contemplated by  the Merger Agreement  under the  antitrust
laws.
 
    Section  6.  DISCLOSURE.  The Stockholder  will consult with ENSCO and allow
ENSCO a reasonable time to consider and  comment on any press release or  public
disclosure  by the Stockholder of matters  related to this Agreement, the Merger
Agreement or the  Merger, except to  the extent required  by applicable law  and
based  on the advice of counsel. ENSCO  agrees to provide the Stockholder a copy
of any  press release  or public  disclosure  of any  matters relating  to  this
Agreement,  the  Merger Agreement  or the  Merger. ENSCO  will consult  with the
Stockholder and allow it a reasonable time to consider and comment on any  press
release  or public disclosure by ENSCO of matters related to this Agreement, the
Merger Agreement or the Merger, except to the extent required by applicable  law
and based on the advice of counsel.
 
    Section  7.  NO  SOLICITATION.  The  Stockholder agrees that  until July 31,
1996, it will not negotiate with any person other than ENSCO with respect to the
acquisition of the Company or the  Target Common Stock owned by the  Stockholder
and  it will not, and will not permit any of its officers, directors, employees,
agents or  representatives (including  without limitation,  investment  bankers,
attorneys  and accountants) to  (a) initiate contact with,  (b) make, solicit or
encourage any  inquiries or  proposals, (c)  enter into  or participate  in  any
discussions  or  negotiations with,  (d) disclose,  directly or  indirectly, any
information not customarily disclosed concerning the business and properties  of
the  Company  or Stockholder's  interest  in the  Company  under the  control of
Stockholder to or (e) afford any  access to the Company's properties, books  and
records  in its possession or under its control to any person in connection with
any possible proposal relating  to (i) the disposition  of the Company's or  the
Stockholder's  businesses or substantially all  of their respective assets, (ii)
the acquisition of equity or debt securities of the Company or the  Stockholder,
including  equity or debt securities in the Company owned by the Stockholder, or
(iii) the merger, share exchange or business combination, or similar acquisition
transaction of or involving the Company or the Stockholder with any person other
than ENSCO. Until July 31, 1996,  the Stockholder will immediately notify  ENSCO
orally,  and subsequently confirm in writing,  all the relevant details relating
to all  inquiries  and proposals  which  it may  receive  relating to  any  such
matters.  Until July 31, 1996, the Stockholder will not, and will not permit any
of its  representatives,  at any  time,  to enter  into  or participate  in  any
discussions  or  negotiations  regarding, or  accept,  any proposal  for  such a
transaction received by them from a third party or that a third party  expresses
a desire to communicate to it.
 
    Section  8.  FURTHER ASSURANCES.  Each  party shall execute and deliver such
additional instruments and other documents  and shall take such further  actions
as  may be necessary or appropriate to effectuate, carry out and comply with all
of their obligations under  this Agreement. Without  limiting the generality  of
the  foregoing, none  of the  parties hereto shall  enter into  any agreement or
arrangement (or alter, amend or terminate any existing agreement or arrangement)
if such action would materially impair  the ability of any party to  effectuate,
carry out or comply with all the terms of this Agreement. If requested by ENSCO,
the Stockholder and each Selling Stockholder agrees to execute a letter to ENSCO
representing  that  the Stockholder  and/or  Selling Stockholder  executing such
letter has  complied with  its obligations  hereunder  as of  the date  of  such
letter.
 
    Section  9.  REPRESENTATIONS AND WARRANTIES  OF ENSCO.  ENSCO represents and
warrants to the Stockholder  as follows: Each of  this Agreement and the  Merger
Agreement has been approved by the
 
                                      D-8
<PAGE>
Board  of Directors of ENSCO, and the  Merger Agreement has been approved by the
Board of Directors  of Acquiror  Sub and  by ENSCO  as the  sole stockholder  of
Acquiror  Sub, in each  case representing all necessary  corporate action on the
part of ENSCO and  Acquiror Sub (no  action by the  stockholders of ENSCO  being
required).  Each  of  this Agreement  and  the  Merger Agreement  has  been duly
executed and delivered by a duly authorized officer of ENSCO and, in the case of
the Merger  Agreement, Acquiror  Sub.  Each of  this  Agreement and  the  Merger
Agreement constitutes a valid and binding agreement of ENSCO and, in the case of
the  Merger Agreement, Acquiror Sub, enforceable  against ENSCO and, in the case
of the Merger Agreement, Acquiror Sub in accordance with its terms.
 
    Section 10.    REPRESENTATIONS  AND  WARRANTIES OF  THE  STOCKHOLDER.    The
Stockholder represents and warrants to ENSCO as follows: This Agreement has been
duly  executed and  delivered by the  Stockholder and constitutes  the valid and
binding agreement of  the Stockholder,  enforceable against  the Stockholder  in
accordance  with its terms. The  Block Shares are the  only voting securities of
the Company owned (beneficially or of record) by the Stockholder, and, except as
provided in  this Agreement,  the Block  Shares are  not subject  to any  voting
trust, voting agreement or similar arrangement whatsoever.
 
    Section  11.  EFFECTIVENESS AND TERMINATION.  It is a condition precedent to
the effectiveness of this  Agreement that the Merger  Agreement shall have  been
executed  and  delivered. In  the event  the Merger  Agreement is  terminated in
accordance with its terms, this  Agreement shall automatically terminate and  be
of  no further force or effect. Upon such termination, except for any rights any
party may have in respect  of any breach by any  other party of its  obligations
hereunder,  none  of the  parties hereto  shall have  any further  obligation or
liability hereunder.
 
    Section 12.  MISCELLANEOUS.
 
    (a)  NOTICES, ETC.  All  notices, requests, demands or other  communications
required  by or otherwise with respect to this Agreement shall be in writing and
shall be deemed to have been duly  given to any party when delivered  personally
(by  courier service or otherwise), when  delivered by telecopy and confirmed by
return telecopy, or seven days after  being mailed by first-class mail,  postage
prepaid, in each case to the applicable addresses set forth below:
 
<TABLE>
<S>                                        <C>
If to ENSCO:                               with a copy to:
ENSCO International Incorporated           Daniel W. Rabun, Esq.
2700 Fountain Place                        Baker & McKenzie
1445 Ross Avenue                           2001 Ross Avenue, Suite 4500
Dallas, TX 75202-2792                      Dallas, TX 75201
Attn: C. Christopher Gaut                  Telecopy: (214) 978-3099
Telecopy: (214) 855-0300
 
If to the Stockholder:                     with a copy to:
Dual Invest AS                             Martin B. McNamara
P. O. Box 1611                             Gibson, Dunn & Crutcher
Vika 0119                                  1717 Main Street, Suite 5400
Oslo, Norway                               Dallas, TX 75201-4605
</TABLE>
 
or to such other address as such party shall have designated by notice so given
to each other party.
 
    (b)   AMENDMENTS, WAIVERS, ETC.  This Agreement may not be amended, changed,
supplemented, waived or otherwise modified or terminated except by an instrument
in writing signed by ENSCO and the Stockholder.
 
                                      D-9
<PAGE>
    (c)  SUCCESSORS AND ASSIGNS.  This Agreement shall be binding upon and shall
inure to the benefit of and be  enforceable by the parties and their  respective
successors  and  assigns,  including  without  limitation  in  the  case  of any
corporate party hereto any corporate successor by merger or otherwise.
 
    (d)  ENTIRE  AGREEMENT.  This  Agreement embodies the  entire agreement  and
understanding  among  the  parties relating  to  the subject  matter  hereof and
supersedes all  prior agreements  and understandings  relating to  such  subject
matter.  There are  no representations, warranties  or covenants  by the parties
hereto relating to such subject matter  other than those expressly set forth  in
this  Agreement. Notwithstanding the foregoing, in no event shall this Agreement
or the execution and delivery of  the Merger Agreement affect the  Stockholder's
obligations  under Section 2 of that  certain letter agreement between ENSCO and
the Stockholder dated January 25, 1996.
 
    (e)  SEVERABILITY.  If any term of this Agreement or the application thereof
to any  party or  circumstance shall  be held  invalid or  unenforceable to  any
extent,  the remainder of this Agreement and the application of such term to the
other parties  or circumstances  shall  not be  affected  thereby and  shall  be
enforced  to the greatest  extent permitted by applicable  law, provided that in
such event the parties shall negotiate in  good faith in an attempt to agree  to
another  provision (in  lieu of the  term or  application held to  be invalid or
unenforceable) that  will  be valid  and  enforceable  and will  carry  out  the
parties' intentions hereunder.
 
    (f)   REMEDIES CUMULATIVE.   All rights, powers  and remedies provided under
this Agreement or  otherwise available  in respect hereof  at law  or in  equity
shall  be cumulative and not  alternative, and the exercise  or beginning of the
exercise of any  thereof by  any party shall  not preclude  the simultaneous  or
later exercise of any other such right, power or remedy by such party.
 
    (g)   NO  WAIVER.  The  failure of any  party hereto to  exercise any right,
power or remedy provided under this Agreement or otherwise available in  respect
hereof  at law  or in equity,  or to insist  upon compliance by  any other party
hereto with its obligations hereunder, and any custom or practice of the parties
at variance with the terms hereof, shall  not constitute a waiver by such  party
of  its right to exercise any such or  other right, power or remedy or to demand
such compliance.
 
    (h)  THIRD  PARTY BENEFICIARIES.   Except for the  Selling Stockholders  and
those  persons  for  whom indemnification  is  provided, this  Agreement  is not
intended to be for the benefit of and shall not be enforceable by any person  or
entity  which is not a party hereto.  The Selling Stockholders and those persons
for whom indemnification is provided, however, may enforce this Agreement.
 
    (i)   GOVERNING  LAW.   This  Agreement  is  governed by  and  construed  in
accordance with the laws of the State of Delaware (without regard to conflict of
laws principles).
 
    (j)  ARBITRATION.
 
        (i)  Any dispute arising  out of or  related to this  Agreement shall be
    finally settled by arbitration  in accordance with the  Rules of the  London
    Court  of International Arbitration  ("LCIA"), which rules  are deemed to be
    incorporated by reference  into this  clause. Unless  the parties  otherwise
    agree,  the arbitration  shall take  place in  London, England.  The parties
    hereby agree to exclude any right of application or appeal to the courts  of
    said  jurisdiction in  connection with  any question  of law  arising in the
    course of reference  out of  the award, in  particular the  right of  appeal
    under Section 1 of the Arbitration Act 1979 in relation to any award made by
    the  arbitrators and the right to apply to the High Court under Section 2 of
    the Arbitration  Act 1979  for  the determination  of  any question  of  law
    arising  in the course of any arbitration proceedings hereunder. Each of the
    parties shall appoint one arbitrator and the two so nominated shall in  turn
    choose  a third arbitrator. If the  arbitrators chosen by the parties cannot
    agree on the choice of the third arbitrator within a period of fourteen (14)
    days after their nomination, then the third arbitrator shall be appointed by
    the Court of the LCIA.
 
                                      D-10
<PAGE>
        (ii) The  arbitration  shall  be  conducted  in  the  English  language.
    Relevant  documents in other  languages shall be  translated into English if
    the arbitrators so direct. In arriving at their award, the arbitrators shall
    give effect  insofar as  possible to  the  desire of  the parties  that  the
    dispute  or  controversy  be  resolved in  accordance  with  good commercial
    practice and principles of fairness and equity, and shall make every  effort
    to  find a solution to  the dispute in the  provisions of this Agreement and
    shall give  full effect  to all  parts hereof.  In particular,  the  parties
    acknowledge  that money damages are not an adequate remedy for violations of
    Section 1 of this Agreement and ENSCO may, in its sole discretion, apply  to
    the arbitral tribunal for specific performance in order to enforce Section 1
    of this Agreement and, to the extent permitted by applicable law, each party
    waives any objection to the imposition of such relief.
 
       (iii)  To the extent a solution cannot be found in the provisions of this
    Agreement, the arbitrators shall apply the law of Delaware.
 
       (iv) The  parties  agree  that  after either  has  filed  a  Request  for
    Arbitration,  they shall, upon request, make discovery and disclosure of all
    written materials relevant to  the subject of  the dispute. The  arbitrators
    shall  make the final determination as to any discovery disputes between the
    parties. Examination  of witnesses  at the  hearings by  the parties,  their
    legal  counsel  and  by  the  arbitrators  shall  be  permitted.  A  written
    transcript of the hearing may be ordered by either of the parties at its own
    expense.
 
        (v) The  award of  a majority  of  the arbitrators  shall be  final  and
    binding  upon the parties, and shall be  the exclusive remedy of the parties
    for all claims, counterclaims,  issues or accountings  presented or pled  to
    the  arbitrators.  Any  award  (other than  specific  performance)  shall be
    granted and paid in  U.S. Dollars exclusive of  any deduction or offset  and
    shall  include interest from the date of  breach or other violations of this
    Agreement until the award  is fully paid, computed  at the prime  commercial
    lending  rate announced from time to time by Citibank, N.A., adjusted daily.
    The arbitrators shall have the authority to order that all or a part of  the
    legal  or other costs, fees and expenses  of a party, including fees paid to
    the LCIA and the arbitrators and the reasonable attorneys' fees, be paid  by
    another  party. Judgment upon the  award may be entered  in any court having
    jurisdiction. An application may  be made to any  such court for a  judicial
    acceptance of the award and an order for enforcement.
 
       (vi)  Nothing in  this section shall  be construed to  preclude any party
    from  seeking  provisional  remedies  at  any  stage  of  the   negotiation,
    conciliation  or  arbitration  proceedings,  including  but  not  limited to
    temporary restraining orders  or preliminary injunctions  from any court  of
    competent  jurisdiction  which such  party  in good  faith  deems reasonably
    necessary for the protection  of its rights.  Such preliminary relief  shall
    not be sought as a means of avoiding conciliation or arbitration.
 
    (l)   NAME,  CAPTIONS, GENDER.   The  name assigned  this Agreement  and the
section captions used herein are for convenience of reference only and shall not
affect the  interpretation  or construction  hereof.  Whenever the  context  may
require,  any  pronoun used  herein shall  include the  corresponding masculine,
feminine or neuter forms.
 
    (m)   COUNTERPARTS.    This Agreement  may  be  executed in  any  number  of
counterparts,  each of which shall be deemed to be an original, but all of which
together constitute an instrument. Each counterpart  may consist of a number  of
copies  each signed by  less that all,  but together signed  by all, the parties
hereto.
 
                                      D-11
<PAGE>
    IN WITNESS WHEREOF, the parties have duly executed this Agreement as of  the
date first above written.
 
                                          ENSCO INTERNATIONAL INCORPORATED
 
                                          By:       /s/ C. CHRISTOPHER GAUT
 
                                             -----------------------------------
                                             C. Christopher Gaut, Vice President
                                                             and
                                                   Chief Financial Officer
 
                                          DUAL INVEST AS
 
                                          By:        /s/ MAGNE KRISTIANSEN
                                             -----------------------------------
                                                 Magne Kristiansen, Managing
                                                           Director
 
The  Company by  execution of  this Agreement  acknowledges that  the execution,
delivery and performance of this  Agreement, as it may  be amended from time  to
time,  has been approved by its Board of Directors, and agrees to perform all of
its obligations under Section 1(e) of this Agreement.
 
                                          DUAL DRILLING COMPANY
 
                                          By:         /s/ DAVID W. SKARKE
 
                                             -----------------------------------
                                               David W. Skarke, Chairman
 
B. Skaugen Shipping AS by execution of  this Agreement agrees to perform all  of
its obligations as a Selling Stockholder under this Agreement.
 
                                          B. SKAUGEN SHIPPING AS
 
                                          By:         /s/ RICHARD ARNESEN
 
                                             -----------------------------------
                                                   Richard Arnesen, Managing
                                              Director
 
                                      D-12
<PAGE>
                                   EXHIBIT A
 
    (1) The Stockholder may dispose of the Block Shares in the following manner:
 
        (a)  Sale of the  Block Shares, from  time to time,  directly or through
    broker-dealers or underwriters who may act  solely as agents or may  acquire
    shares  of Common  Stock as  principals, in all  cases as  designated by the
    Stockholder.
 
        (b) Distributions  of  the  Block  Shares to  the  stockholders  of  the
    Stockholder, including Skaugen.
 
        (2)   Skaugen  may  dispose  of  the  Block  Shares  acquired  from  the
    Stockholder, from  time  to  time, directly  or  through  broker-dealers  or
    underwriters  who may act solely  as agents or may  acquire shares of Common
    Stock as principals, in all cases as designated by Skaugen.
 
                                      D-13
<PAGE>
                                REVOCABLE PROXY
            THIS PROXY IS BEING SOLICITED ON BEHALF OF THE BOARD OF
                       DIRECTORS OF DUAL DRILLING COMPANY
 
    The undersigned stockholder of DUAL DRILLING COMPANY, a Delaware corporation
(the "Company"), hereby appoint(s) W. Allen Parks and Robert F. Chrone, and each
of  them, each with  full power of substitution  and resubstitution, proxies and
attorneys-in-fact  of  the  undersigned,  with  all  of  the  powers  that   the
undersigned  would  possess if  personally present,  to attend  and act  for the
undersigned at the Special  Meeting of Stockholders  (the "Special Meeting")  of
the  Company to be  held on June 12,  1996 at Park City  Club, 5956 Sherry Lane,
Suite 1700, Dallas, Texas 75225 commencing at 10:00 a.m., Central time, and  any
and  all  adjournments  and  postponements thereof,  and  (without  limiting the
generality of the foregoing) in connection  therewith to vote and represent  all
of  the shares  of common  stock of  the Company  that the  undersigned would be
entitled to vote.
 
    Said proxies and attorneys, and each of  them, shall have all of the  powers
that  the undersigned  would have  if voting  in person.  The undersigned hereby
revokes any  other proxies  to vote  at  such meeting  and hereby  ratifies  and
confirms  all that said proxies and attorneys, and each of them, may lawfully do
by virtue hereof. Said proxies, without hereby limiting their several authority,
are specifically authorized to vote in accordance with their best judgment  with
respect  to all matters incident  to the conduct of  the Special Meeting and all
matters presented  at the  meeting  but which  are not  known  to the  Board  of
Directors of the Company at the time of the solicitation of this proxy.
 
    The  Board of Directors of  the Company recommends a  vote FOR the following
Proposals:
 
1.  Proposal to approve and adopt the Agreement and Plan of Merger, dated as  of
    March  21,  1996, among  ENSCO  International Incorporated,  DDC Acquisition
    Company and the Company, as amended.
 
            / /  FOR            / /  AGAINST            / /  ABSTAIN
 
2.  Proposal to approve the adoption of the DUAL Special Performance Unit Plan.
 
            / /  FOR            / /  AGAINST            / /  ABSTAIN
 
Each of the above named proxies present at the Special Meeting, either in person
    or by substitute,  shall have and  exercise all the  powers of said  proxies
    hereunder.
 
                  (CONTINUED AND TO BE SIGNED ON REVERSE SIDE)
<PAGE>
THIS  PROXY, WHEN PROPERLY EXECUTED, WILL BE VOTED AT THE SPECIAL MEETING OR ANY
ADJOURNMENT OR  POSTPONEMENT THEREOF  IN ACCORDANCE  WITH THE  INSTRUCTIONS  SET
FORTH  ABOVE OR, IN THE EVENT NO INSTRUCTIONS  ARE SET FORTH, THIS PROXY WILL BE
VOTED FOR THE PROPOSAL.
 
    The undersigned acknowledges  receipt of  a copy  of the  Notice of  Special
Meeting  and Prospectus/Proxy Statement (with  all Appendices and annexes) dated
May 9, 1996 relating to the Special Meeting.
 
<TABLE>
<S>                                                                  <C>
Date                                                                 Signature
 
Date
                                                                     Signature
                                                                     IMPORTANT: Please date this Proxy and sign exactly as your name
                                                                     appears above. If shares are held by joint tenants, both should
                                                                     sign.  When  signing  as  attorney,  executor,   administrator,
                                                                     trustee   or  guardian,  please  give   title  as  such.  If  a
                                                                     corporation,  please  sign  in  full  corporate  name  by   the
                                                                     president or other authorized officer. If a partnership, please
                                                                     sign in partnership name by an authorized person.
</TABLE>


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