<PAGE>
                                                                   EXHIBIT 10.56

                             STOCK PLEDGE AGREEMENT

     THIS STOCK PLEDGE AGREEMENT (this "Agreement") is entered into on December
21, 2001, by and between IDS ENGINEERING MANAGEMENT, LC, a Texas company
("Pledgor"), and FLEET CAPITAL CORPORATION, a Rhode Island corporation, as Agent
for itself and the other financial institution(s) from time to time a party to
the Loan Agreement (as hereinafter defined) ("Pledgee").  All capitalized terms
used but not otherwise defined herein shall have the meanings ascribed to such
terms in the Loan Agreement.

          1.  For good and valuable consideration, the receipt and sufficiency
of which is hereby acknowledged, and as collateral security for and to secure
the prompt payment and performance in full of the Secured Obligations
(hereinafter defined), Pledgor hereby assigns to Pledgee and grants to Pledgee,
for the benefit of Lenders, a continuing security interest in all issued and
outstanding shares of capital stock (including, without limitation, all shares
of capital stock represented by the stock certificates identified on Schedule I
attached hereto) of Petrocon Engineering, Inc., a Texas corporation
("Borrower"), whether now or hereafter issued by Borrower, together with all
proceeds, products and increases thereof and substitutions and replacements
therefor (collectively, the "Collateral").

          As used in this Agreement, the term "Secured Obligations" shall mean
any and all (i) Obligations of Pledgor or Borrower to Pledgee and/or Lenders,
pursuant to that certain Second Amended and Restated Loan and Security
Agreement, dated as of the date hereof, by and among Pledgor, Borrower, IDS
Engineering, Inc., a Texas corporation, Thermaire, Inc., a Texas corporation,
Constant Power Manufacturing, Inc., a Texas corporation, Industrial Data
Systems, Inc., a Texas corporation, Triangle Engineers and Constructors, Inc., a
Texas corporation, Petrocon Systems, Inc., a Texas corporation, Petrocon
Technologies, Inc., a Texas corporation, Petrocon Engineering of Louisiana,
Inc., a Louisiana corporation, R.P.M. Engineering, Inc., a Louisiana
corporation, Alliance Engineering Associates, Inc., a Texas corporation,
Petrocon Construction Resources, Inc., a Texas corporation, Pledgee and Lenders
(as amended from time to time, the "Loan Agreement") and (ii) extensions,
renewals, modifications, increases and replacements of the foregoing.  The term
"Secured Obligations" shall include, without limitation, all unpaid accrued
interest thereon and all costs and expenses payable as hereinafter provided:
(i) whether now existing or hereafter incurred; (ii) whether direct, indirect,
primary, absolute, secondary, contingent, secured, unsecured, matured, or
unmatured; (iii) whether such indebtedness is from time to time reduced and
thereafter increased, or entirely extinguished and thereafter reincurred; (iv)
whether such indebtedness was originally contracted with Pledgee or with another
or others; (v) whether or not such indebtedness is evidenced by a negotiable or
nonnegotiable instrument or any other writing; and (vi) whether such
indebtedness is contracted by Pledgor or Borrower individually or jointly or
severally with another or others.

          2.  Pledgor represents and warrants that (i) Pledgor is the legal and
beneficial owner of the Collateral, free and clear of all liens and
encumbrances; (ii) there are no restrictions upon the transfer of any of the
Collateral, other than as may appear and may be referenced on the face of the
certificates, those arising under applicable state or federal securities laws
and those existing on the date hereof and disclosed to Pledgee in writing; (iii)
Pledgor owns 100% of the issued and

                                       1
<PAGE>

outstanding capital stock of Borrower; (iv) except as disclosed to Pledgee in
writing, there are no existing obligations to issue capital stock or securities
convertible into capital stock of Borrower.

          3.  In furtherance of Pledgee's security interest in the Collateral,
Pledgor agrees to deliver to Pledgee, on the date of this Agreement, the stock
certificates identified on Schedule I attached hereto, together with stock
powers duly executed in blank by Pledgor.

          4.  With respect to the Collateral, Pledgor hereby appoints Pledgee
its attorney-in-fact to arrange for the transfer of the Collateral on the books
of Borrower to the name of Pledgee during the continuance of any Event of
Default; provided, however, that Pledgee shall be under no obligation to do so.

          5.  During the term of this Agreement, provided no Event of Default
then exists, Pledgor shall have the right, where applicable, to vote the
Collateral on all corporate questions, and Pledgee shall, if necessary, execute
due and timely proxies in favor of Pledgor for this purpose.

          6.  While an Event of Default exists, Pledgee may exercise all of the
rights and privileges in connection with the Collateral to which a transferee
may be entitled as the record holder thereof, together with the rights and
privileges otherwise granted hereunder.  Pledgee shall be under no obligation to
exercise any of such rights or privileges.

          7.  If, with the consent of Pledgee, Pledgor shall substitute or
exchange other securities in place of those herein mentioned, all of the rights
and privileges of Pledgee and all of the obligations of Pledgor with respect to
the securities originally pledged or held as Collateral hereunder shall be
forthwith applicable to such substituted or exchanged securities.

          8.  While an Event of Default exists, Pledgee shall be authorized to
collect all dividends, interest payments, and other amounts (including amounts
received or receivable upon redemption or repurchase) that may be, or become,
due on any of the Collateral.  If Pledgor receives any such dividends, payments
or amounts after the occurrence and during the continuance of an Event of
Default, it shall immediately endorse and deliver the same to Pledgee in the
form received.  All such amounts which Pledgee receives and retains in
accordance with the terms of this paragraph 8 shall be applied to reduce the
principal amount outstanding on the Secured Obligations in the inverse order of
maturity.  Pledgee is, furthermore, authorized to give receipts in the name of
Pledgor for any amounts so received.  Pledgee shall be under no obligation to
collect any such amounts.

          9.  In the event that, during the term of this Agreement, subscription
warrants or any other rights or options shall be issued in connection with the
Collateral, such warrants, rights or options shall be immediately assigned, if
necessary, by Pledgor to Pledgee.  If any such warrants, rights or options are
exercised by Pledgor, all new  securities so acquired by Pledgor shall be
immediately assigned to Pledgee, shall become part of the Collateral and shall
be endorsed to, delivered to and held by Pledgee under the terms of this
Agreement in the same manner as the securities originally pledged.

                                       2
<PAGE>

          10.  In the event that, during the term of this Agreement, any share,
dividend, reclassification, readjustment or other change is declared or made in
the capital structure of Borrower, all new, substituted and additional shares,
or other securities, issued by reason of any such change shall become part of
the Collateral and shall be endorsed to, delivered to and held by Pledgee under
the terms of this Agreement in the same manner as the securities originally
pledged.

          11.  Pledgor authorizes Pledgee, without notice or demand, and without
affecting the liability of Pledgor hereunder, from time to time to:

          (A) hold security in addition to and other than the Collateral for the
payment of the Secured Obligations or any part thereof, and exchange, enforce,
waive and release any Collateral or any part thereof, or any other such
security, or part thereof;

          (B) release any of the endorsers or guarantors of the Secured
Obligations secured hereunder or any part thereof, or any other person
whomsoever liable for or on account of such Secured Obligations;

          (C) on the transfer of all or any part of the Secured Obligations
secured hereunder, Pledgee may assign all or any part of Pledgee's security
interest in the Collateral and shall be fully discharged thereafter from all
liability and responsibility with respect to the Collateral so transferred;
provided, however, that in no event shall Pledgee be liable for any act or
omission or negligent act or negligent omission with respect to the Collateral,
other than acts or omissions constituting gross negligence, willful misconduct,
tortious breach of contract or violation of law.  The transferee of the
Collateral shall be vested with the rights, powers and remedies of Pledgee
hereunder, and with respect to any Collateral not so transferred, Pledgee shall
retain all rights, powers and remedies hereby given; and

          (D) Pledgor hereby waives any right to require Pledgee to proceed
against Pledgor, Borrower  or any other person whomsoever, to proceed against or
exhaust any collateral or any other security held by Pledgee, or to pursue any
other remedy available to Pledgee.  Pledgor further waives any defense arising
by reason of any liability or other  defense of Pledgor or of any other person.
Pledgor shall have no right to require Pledgee to marshal collateral.

          12.  It shall not be necessary for Pledgee to inquire into the powers
of Pledgor or the officers, directors or agents acting or purporting to act on
behalf of Pledgor, and any obligations made or created in reliance on the
professed exercise of such powers shall be secured hereunder.

          13.  To the extent permitted by applicable law and in the Loan
Agreement, Pledgee shall be under no duty or obligation whatsoever to make or
give any presentments, demands for performance, notices of non-performance,
protests, notices of protest, or notices of dishonor in connection with the
Secured Obligations.

          14.  The occurrence of an "Event of Default" under the Loan Agreement
shall, at the option of Pledgee, constitute an "Event of Default" under this
Agreement.

                                       3
<PAGE>

          15.  While an Event of Default exists, Pledgee shall have the right to
exercise all rights and remedies afforded to it under the Loan Agreement and, in
this connection, the Pledgee may sell all or any part of the Collateral in such
manner as provided in the Loan Agreement.  Pledgee shall apply the proceeds of
any such sale or sales of the Collateral in the manner provided in the Loan
Agreement.

          16.  Upon the indefeasible repayment in full in cash of the Secured
Obligations, Pledgee will promptly, at Pledgor's expense, deliver all of the
Collateral to Pledgor along with all instruments of assignment executed in
connection therewith, and execute and deliver to Pledgor such documents as
Pledgor shall reasonably request to evidence Pledgee's release of Pledgee's
security interest hereunder.

          17.  THIS AGREEMENT SHALL BE INTERPRETED AND THE RIGHTS AND
LIABILITIES OF THE PARTIES HERETO DETERMINED IN ACCORDANCE WITH THE LOCAL LAW OF
THE STATE OF TEXAS EXCLUDING ANY CONFLICTS OF LAW RULE OR PRINCIPLE THAT MIGHT
OTHERWISE REFER CONSTRUCTION OR INTERPRETATION OF THIS AGREEMENT TO THE
SUBSTANTIVE LAW OF ANOTHER JURISDICTION.


                  [Remainder of Page Intentionally Left Blank]

                                       4
<PAGE>

          IN WITNESS WHEREOF, Pledgor and Pledgee have executed this Agreement
as of the date first above written.

                                 PLEDGOR:

                                 IDS ENGINEERING MANAGEMENT, LC


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

                                 PLEDGEE:

                                 FLEET CAPITAL CORPORATION, as Agent


                                 By:
                                    -----------------------------
                                    Dan A. Hughes
                                    Vice President
<PAGE>

                                   SCHEDULE I

            Capital Stock Pledged by IDS Engineering Management, LC

<TABLE>
<CAPTION>

    Name                    Certificate            Record Holder                 Number of
                               Number                                              Shares
<S>                         <C>             <C>                                  <C>
Petrocon Engineering, Inc.                  IDS Engineering Management, LC
 </TABLE>
<PAGE>

                                  SCHEDULE II
                                  -----------

                  Original Stock Certificates with Stock Power