<PAGE>


                                                                    EXHIBIT 10.5


                           FOURTH AMENDMENT AND WAIVER

     This Fourth Amendment and Waiver (this "Amendment") dated as of December
21, 2005, is by and among Willbros Group, Inc., a Republic of Panama corporation
(herein referred to as the "Company") and the Designated Subsidiaries; the
financial institutions parties hereto which are Banks under the Credit Agreement
(as defined below); and Calyon New York Branch (formerly known as Credit
Lyonnais New York Branch), as administrative agent for the Banks (in such
capacity, the "Agent"), and amends the Amended and Restated Credit Agreement
dated as of March 12, 2004, as previously amended by the First Amendment and
Waiver dated as of August 6, 2004, the Second Amendment and Waiver dated as of
July 19, 2005, and the Third Amendment and Waiver dated as of November 23, 2005
(the "Third Amendment"), among the Company, the Designated Subsidiaries from
time to time (WGI and such Designated Subsidiaries collectively, the "Obligors"
and individually, an "Obligor"); the several financial institutions from time to
time parties thereto as Banks, Calyon New York Branch (formerly known as Credit
Lyonnais New York Branch), as a Bank, as Issuing Bank (as defined herein), as
Agent, and as Lead Arranger and Book Runner; and CIBC, Inc., as Syndication
Agent (as previously amended and modified, the "Credit Agreement").

                             PRELIMINARY STATEMENTS

     Whereas, the Company intends to issue new Convertible Notes before December
31, 2005 on terms reasonably satisfactory to the Agent, in connection with which
Willbros U.S.A., Inc. will guarantee the Company's obligations under such
Convertible Notes pursuant to an unsecured guaranty;

     WHEREAS, after the issuance of such Convertible Notes, the Company will be
unable to comply with its covenant in Section 9.11(a) of the Credit Agreement to
maintain a ratio of Financial Indebtedness to Total Capitalization of no more
than 0.35 to 1.00, and its covenant in Section 9.13 of the Credit Agreement to
maintain a Fixed Charge Coverage Ratio of 3.00 to 1.00 for the fiscal quarter
ending June 30, 2006 and for each quarter thereafter, and has requested the
Banks to amend such covenants;

     WHEREAS, the Company failed to comply with its covenant in Section 9.11(c)
to not permit its ratio of Senior Secured Financial Indebtedness to Adjusted
EBITDA as of the fiscal quarter ending September 30, 2005 to exceed 1.00 to 1.00
(the "Senior Secured Financial Indebtedness Default") and has requested the
Banks to waive such default;

     WHEREAS, pursuant to the terms of the Third Amendment, the Banks previously
agreed to temporarily waive any Default or Event of Default that would occur
during the period through March 31, 2006 as a result of the Company's failure to
maintain a minimum cash balance of $15,000,000 (the "Prior Waiver");

     WHEREAS, the Banks have determined to limit the Prior Waiver to the period
up to the date upon which the Total Commitment is permanently reduced by the Net
Cash Proceeds from the Convertible Notes issuance and to reinstate from such
date the Company's obligation to maintain a minimum cash balance of $15,000,000
in accordance with Section 4.8 of the Credit Agreement; and

<PAGE>

     WHEREAS, the Banks and the Obligors have agreed to such amendments and
waiver, subject to the terms and conditions set forth in this Amendment;

                                    AGREEMENT

     NOW THEREFORE, in consideration of the premises and the mutual covenants
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto, intending to
be legally bound, hereby agree as follows. Capitalized terms used but not
otherwise defined herein shall have the meanings assigned such terms in the
Credit Agreement.

     Section 1. Waiver. The Banks hereby agree to waive the Senior Secured
Financial Indebtedness Default for the quarter ending September 30, 2005. The
express waiver set forth in this section is the only waiver provided by the
Agent and the Banks pursuant to this Amendment, and all other rights and
remedies of the Agent and the Banks under the Credit Agreement remain unchanged.

     Section 2. Amendments of Credit Agreement. Subject to the satisfaction of
the conditions described in Section 4 hereof, the Credit Agreement is hereby
amended as follows:

          (a) The defined term "Change of Control" in Section 1.1 of the Credit
     Agreement is hereby amended as of the Amendment Effective Date by adding
     the following clause to the end thereof: "or (c) the occurrence of any
     Fundamental Change as such term is defined in each indenture governing
     Convertible Notes".

          (b) Section 9.11(a) of the Credit Agreement is hereby amended as of
     September 30, 2005 by restating such subsection in its entirety as follows:

               (a) The Company shall not permit the ratio of (i) the Financial
          Indebtedness of the Company minus 100% of unrestricted cash and Cash
          Equivalents of the Company on the Company's balance sheet (excluding
          any such cash and Cash Equivalents in the Cash Collateral Account) to
          (ii) Total Capitalization minus 100% of unrestricted cash and Cash
          Equivalents of the Company on the Company's balance sheet (excluding
          any such cash and Cash Equivalents in the Cash Collateral Account), at
          the end of any fiscal quarter to exceed 0.50 to 1.00.

          (c) Section 9.13 of the Credit Agreement is hereby amended as of the
     Amendment Effective Date by restating such section in its entirety as
     follows:

          Fixed Charge Coverage Ratio. The Company shall not permit its Fixed
          Charge Coverage Ratio, calculated at the end of each fiscal quarter on
          and after June 30, 2006 for the four fiscal quarter period then ended,
          to be less than 2.50 to 1.00.

     Section 3. Additional Agreements. The Banks and the Obligors agree that the
following agreements shall apply as if fully incorporated into the terms of the
Credit Agreement:


                                       2

<PAGE>

          (a) No later than December 28, 2005, the Company shall provide to the
     Agent evidence satisfactory to the Agent that the Company has received Net
     Cash Proceeds of at least $30,000,000 from the sale of new Convertible
     Notes, and failure to provide such satisfactory evidence by such date shall
     automatically be deemed an Event of Default under the Credit Agreement, and
     the Agent may exercise, or shall exercise at the request of the Required
     Banks, any remedies provided under the terms of the Credit Agreement.

          (b) Notwithstanding anything to the contrary in the Third Amendment,
     from and after the date upon which the Company receives Net Cash Proceeds
     from the sale of new Convertible Notes, the obligation of the Company under
     Section 4.8 of the Credit Agreement to maintain a minimum cash balance of
     $15,000,000 shall be in full force and effect.

          (c) The Obligors shall provide within thirty (30) days of the date
     hereof, the following corporate documents:

               (i) (A) copies of the certificate or articles of incorporation or
          other equivalent organizational documents, including all amendments
          thereto, of each Obligor, certified as of a recent date by the
          Secretary of State of the state of its organization or the foreign
          equivalent thereof in each jurisdiction where such concept is
          relevant, or (B) a certificate of the Secretary or Assistant Secretary
          of such Obligor, certifying the full force and effect of the articles
          of incorporation or other equivalent organizational documents
          previously delivered to the Agent in connection with the Credit
          Agreement provided such Obligor makes available a copy of such
          articles of incorporation or other equivalent organizational document
          to any Bank upon request;

               (ii) a certificate of the Secretary, Assistant Secretary or other
          officer of each Obligor certifying (A)(1) that attached thereto is a
          true and complete copy of the bylaws or similar governing documents of
          such Obligor as currently in effect or (2) the full force and effect
          of the bylaws or similar governing documents of such Obligor
          previously delivered to the Agent in connection with the Credit
          Agreement, provided such Obligor makes available a copy of such bylaws
          or similar governing document to any Bank upon request, (B) that
          attached thereto is a true and complete copy of the resolutions duly
          adopted by the Board of Directors or similar governing body of such
          Obligor authorizing the execution, delivery and performance of this
          Amendment and all other Credit Documents to be executed and delivered
          by such Obligor after the date hereof, and (C) that such resolutions
          have not been modified, rescinded or amended and are in full force and
          effect; and

               (iii) a certificate of another officer as to the incumbency and
          specimen signature of the Secretary, Assistant Secretary or other
          officer executing the certificate pursuant to clause (ii) above.

     Section 4. Conditions to Effectiveness. This Amendment shall become
effective on the date upon which (the "Amendment Effective Date") the Agent has
confirmed (and has so notified the Company and the Banks) that it has determined


                                       3

<PAGE>

the terms and conditions of the new Convertible Notes to be satisfactory and
received the following:

          (a) counterparts hereof duly executed by the Obligors and the Required
     Banks;

          (b) a Certificate of the Company, signed on behalf of the Company by
     its President or Chief Financial Officer, dated as of the Amendment
     Effective Date, certifying that, after giving effect hereto, the following
     statements shall be true:

               (i) the representations and warranties of each Obligor contained
          in the Credit Documents are correct in all material respects on and as
          of such date (other than those representations and warranties that
          expressly relate solely to a specific earlier date, which shall remain
          correct as of such earlier date, or expressly relate to the Senior
          Secured Financial Indebtedness Default), as though made on and as of
          such date; and

               (ii) no event has occurred and is continuing, or is anticipated
          to occur, which constitutes (or would constitute when it occurs) a
          Default, or an Event of Default or both unless waived by this
          Amendment; and

          (c) a Certificate of the Company, signed by the Secretary, Assistant
     Secretary or other officer on behalf of the Company and the other Obligors,
     certifying as to the incumbency and specimen signature of each officer of
     each Obligor executing this Amendment and any other document delivered in
     connection herewith.

     Section 5. Amendment and Ratification. Upon the effectiveness hereof as
provided in the foregoing Section 3, this Amendment shall be deemed to be an
amendment and waiver to the Credit Agreement, and the Credit Agreement, as
modified hereby, is hereby ratified, approved and confirmed to be in full force
and effect in each and every respect. All references to the Credit Agreement in
any other document, instrument, agreement or writing shall hereafter be deemed
to refer to the Credit Agreement as modified hereby.

     SECTION 6. Governing Law. THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF
THE PARTIES HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND BE GOVERNED BY
THE LAWS OF THE STATE OF NEW YORK.

     Section 7. Descriptive Headings, Etc. The descriptive headings of the
several sections of this Amendment are inserted for convenience only and shall
not be deemed to affect the meaning or construction of any of the provisions
hereof.

     Section 8. Counterparts. This Amendment may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same agreement.

     Section 9. Waiver of Jury Trial. Each of the parties hereto hereby
irrevocably and unconditionally waives, to the fullest extent it may effectively
do so under applicable law, any


                                       4

<PAGE>

and all right to trial by jury in any action or proceeding arising out of or
relating to any Loan Document or the transactions contemplated hereby or
thereby.

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                                       5

<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their respective duly authorized officers as
of the Amendment Effective Date.

                                        OBLIGORS:

                                        WILLBROS GROUP, INC.


                                        By:
                                            ------------------------------------
                                        Name: Warren L. Williams
                                        Title: Senior Vice President, Chief
                                               Financial Officer and Treasurer


                                        WILLBROS U.S.A., INC.


                                        By:
                                            ------------------------------------
                                        Name: Warren L. Williams
                                        Title: Senior Vice President, Chief
                                               Financial Officer and Treasurer

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        WILLBROS INTERNATIONAL, INC.,
                                        WILLBROS WEST AFRICA, INC.,
                                        WILLBROS MIDDLE EAST, INC.
                                        WILLBROS MARINE ASSETS, INC.
                                        INTERNATIONAL PIPELINE EQUIPMENT, INC.,
                                        WILLBROS CONSTRUCTORS, INC.,
                                        WILLBROS OPERATING SERVICES, INC.,
                                        WILLBROS RPI, INC.,
                                        WILLBROS MSI CANADA INC. and
                                        WILLBROS PROCESS ELECTRIC AND CONTROL,
                                        INC.


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Treasurer

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        WILLBROS (NIGERIA) LIMITED and
                                        WILLBROS (OFFSHORE) NIGERIA LIMITED


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Authorized Representative


                                        CONSTRUCTORA CAMSA, C.A. and
                                        ESCA EQUIPMENT SERVICE C.A.


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Attorney-in-Fact


                                        WILLBROS ENERGY SERVICES COMPANY,
                                        WILLBROS ENGINEERS SLC, INC. (f/k/a
                                        Willbros Process Engineering Design,
                                        Inc.) and
                                        WILLBROS MT. WEST, INC. (individually
                                        and as successor by merger to
                                        Willbros Pacific Industrial Electric,
                                        Inc.)


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Vice President and Treasurer

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                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        WILLBROS TRANSANDINA S.A.


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Alternate Director


                                        WILLBROS ENGINEERS, INC.


                                        By:
                                            ------------------------------------
                                        Name: Gay Stanley Mayeux
                                        Title: Assistant Treasurer

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        AGENT:

                                        CALYON NEW YORK BRANCH (formerly known
                                        as Credit Lyonnais New York Branch)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        ISSUING BANK:

                                        CALYON NEW YORK BRANCH (formerly known
                                        as Credit Lyonnais New York Branch)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        LEAD ARRANGER AND BOOK RUNNER:

                                        CALYON NEW YORK BRANCH (formerly known
                                        as Credit Lyonnais New York Branch)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        SYNDICATION AGENT:

                                        CIBC INC.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        BANKS:

                                        CALYON NEW YORK BRANCH (formerly known
                                        as Credit Lyonnais New York Branch)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        ABU DHABI INTERNATIONAL BANK INC.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        ARAB BANKING CORPORATION (B.S.C.)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        THE BANK OF NOVA SCOTIA


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        BANK OF TEXAS, N.A.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        CIBC INC.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        COMMERCEBANK, N.A.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        COMPASS BANK


                                        By:
                                            ------------------------------------
                                        Name: Collis G. Sanders
                                        Title: Senior Vice President

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        JPMORGAN CHASE BANK


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        NATEXIS BANQUES POPULAIRES


                                        By:
                                            ------------------------------------
                                        Name: Tim Polvado
                                        Title: Vice President and Manager


                                        By:
                                            ------------------------------------
                                        Name: Renaud d'Herbes
                                        Title: Senior Vice President and
                                               Regional Manager

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        RZB FINANCE, LLC


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]

<PAGE>

                                        AMEGY BANK NATIONAL ASSOCIATION
                                        (formerly known as Southwest Bank of
                                        Texas, NA)


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------

                      [SIGNATURE PAGE TO FOURTH AMENDMENT]