<PAGE>

                                                                 EXHIBIT 10.10

                       ASSIGNMENT OF PARTNERSHIP INTEREST


     THIS ASSIGNMENT OF PARTNERSHIP INTEREST dated as of June 2, 1993 (herein
called the "Assignment") is by CORNERSTONE NATURAL GAS COMPANY, a Delaware
corporation (herein called "Assignor"), in favor of ENDEVCO PIPELINE COMPANY, a
Delaware corporation (herein called "Assignee").

                                  RECITALS:

     A.   Assignor and Western Natural Gas Company, a Delaware corporation
("Western"), entered into that certain General Partnership Agreement dated as of
March 7, 1989, as amended by Amendment No. 1 dated September 28, 1989 (as so
amended, the "Partnership Agreement"), creating Mountain Creek Joint Venture
("Mountain Creek").

     B.   On December 27, 1990, December 27, 1991, and December 31, 1992,
Western assigned its interest in Mountain Creek, the Partnership to Wesnat
Pipeline Company, a Delaware corporation ("Wesnat"), pursuant to that certain
Assignment, and Wesnat currently holds such interests and has the right to
consent to the assignments contemplated hereby.

     C.   Assignor and Western entered into that certain Natural Gas
Transportation Agreement dated February 3, 1989 (the "Transportation Agreement")
with Texas Utilities Fuel Company, a Texas corporation ("TUFCO"), and the
obligations under the Transportation Agreement have been performed by Mountain
Creek.

     D.   Mountain Creek entered into that certain Permanent Loan Agreement
dated September 28, 1989 (as from time to time amended, the "Loan Agreement")
with Chrysler Capital Corporation ("Chrysler"), pursuant to which Assignor, as a
general partner in Mountain Creek, became liable for the indebtedness of
Mountain Creek to Chrysler.

     E.   On June 1, 1992, Chrysler assigned to General Electric Capital
Corporation ("GECC") the Note (as such term is defined in the Loan Agreement),
as well as the Loan Agreement and the other Loan Papers referred to therein.

     F.   Assignor desires to assign to Assignee all of its rights and interests
in, to and under the Partnership Agreement and the Transportation Agreement, and
Western, Wesnat, Mountain Creek and GECC have each agreed to consent to such
assignment on the terms and conditions provided herein.

     NOW, THEREFORE, in consideration of the premises and the mutual agreements
herein contained, the parties hereto hereby act and agree as follows:

<PAGE>

                                  ARTICLE I

                                 ASSIGNMENT

     Section 1.1.  ASSIGNMENT.  Assignor, for and in consideration of the sum of
Ten Dollars ($10.00) and for other good and valuable consideration to Assignor
paid by Assignee, the receipt and sufficiency of which are hereby acknowledged,
does by these presents SELL, TRANSFER, CONVEY, ASSIGN, ENDORSE, SET OVER AND
DELIVER unto Assignee the following:

     (a)  The general partnership interest of Assignor in Mountain Creek,
          together with all rights, benefits, remedies and privileges of
          Assignor under the Partnership Agreement and all of the interest of
          Assignor in the Transportation Agreement (all of the foregoing are
          herein collectively called the "Assigned Interest"); and

     (b)  All rights, titles, interests, privileges, claims, demands, and
          equities existing or to exist in connection with the Assigned
          Interest.

TO HAVE AND TO HOLD the Assigned Interest, together and along with all such
rights, titles, interests, privileges, claims, demands and equities, now or
hereafter had by Assignor in connection therewith, unto Assignee forever.

     Section 1.2.  REPRESENTATIONS AND WARRANTIES OF ASSIGNOR.  The assignment
made hereby is made without recourse and without warranty or representation of
any nature whatsoever, express or implied, except Assignor does warrant that it
is the owner and holder of the Assigned Interest and has the full right, power
and authority to transfer the Assigned Interest free of any lawful claim of any
person or entity.

     Section 1.3.  ASSUMPTION BY ASSIGNEE.  Assignee hereby agrees to accept the
Assigned Interest and to assume and perform all of the obligations and duties
that Assignor has or will have under the Partnership Agreement.

     Section 1.4.  RATIFICATION BY ASSIGNEE, MOUNTAIN CREEK AND WESNAT.  Each of
Mountain Creek and Wesnat hereby consents to the assignment contemplated hereby,
and each of Mountain Creek, Wesnat and Assignee hereby ratifies and confirms the
Partnership Agreement, as modified by this Assignment, and agrees that on and
after the date hereof all references in the Partnership Agreement or any related
document to "Cornerstone," "Managing Partner" or similar references shall refer
to Assignee.

                                     ARTICLE II

                                    MISCELLANEOUS

     Section 2.1.  ADDRESSES.  For the purposes of this Assignment, the
addresses of the parties shall hereafter be as shown beside their signatures
hereto.


<PAGE>

     Section 2.2.  ADDITIONAL DOCUMENTS.  Assignor will from time to time, at
the request of Assignee and at no expense to Assignor, execute and deliver to
Assignee any documents deemed by Assignor to be reasonably necessary or
desirable to carry out more effectively the purposes of this Assignment,
including without limitation any assignments, properly completed (and
acknowledged when required).

     Section 2.3.  CHOICE OF LAW.  THIS ASSIGNMENT SHALL BE CONSTRUED AND
ENFORCED IN ACCORDANCE WITH AND GOVERNED BY THE LAWS OF THE STATE OF TEXAS
APPLICABLE TO CONTRACTS MADE AND TO BE PERFORMED ENTIRELY WITHIN SUCH STATE AND
THE LAWS OF THE UNITED STATES OF AMERICA.

     Section 2.4.  COUNTERPARTS.  This Assignment may be executed in several
counterparts, all of such counterparts together constituting one and the same
instrument.

     Section 2.5.  FINAL AGREEMENT; NO ORAL AGREEMENTS.  THIS WRITTEN ASSIGNMENT
REPRESENTS THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED
BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE
PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

     IN WITNESS WHEREOF, the parties hereto have each executed this Assignment
on the dates set forth in their respective acknowledgements below to be
effective for all purposes as of the date first written above.

"ASSIGNOR"

CORNERSTONE NATURAL GAS COMPANY
a Delaware corporation

By: /s/ JAMES W. BRYANT
    --------------------------
Name:  James W. Bryant
       -----------------------
Title: Chairman of the Board
       -----------------------

Address of Assignor is:
8080 N. Central Expressway
Twelfth Floor, L.B. 47
Dallas, Texas  75206


<PAGE>

"ASSIGNEE"

ENDEVCO PIPELINE COMPANY
a Delaware corporation

By: /s/ JAMES W. BRYANT
    --------------------------
Name:  James W. Bryant
       -----------------------
Title: Chairman of the Board
       -----------------------

Address of Assignee is:
8080 N. Central Expressway
Twelfth Floor, L.B. 47
Dallas, Texas  75206


     Each of the following, by its execution in the space provided below, hereby
consents to the Assignment and the transactions contemplated thereby.


WESNAT PIPELINE COMPANY
a Delaware corporation

By: /s/ TED C. PARKER, JR.
    --------------------------
Name:  Ted C. Parker, Jr.
       -----------------------
Title: President
       -----------------------

WESTERN NATURAL GAS COMPANY
a Delaware corporation


By: /s/ TED C. PARKER, JR.
    --------------------------
Name:  Ted C. Parker, Jr.
       -----------------------
Title: President
       -----------------------


<PAGE>

MOUNTAIN CREEK JOINT VENTURE
a Texas general partnership

By:  CORNERSTONE NATURAL GAS COMPANY

By: /s/ JAMES W. BRYANT
    --------------------------
Name:  James W. Bryant
       -----------------------
Title: Chairman of the Board
       -----------------------

By:  WESNAT PIPELINE COMPANY

By: /s/ TED C. PARKER, JR.
    --------------------------
Name:  Ted C. Parker, Jr.
       -----------------------
Title: President
       -----------------------


Address of Mountain Creek is:
8080 N. Central Expressway
Suite 1200, L.B. 47
Dallas, Texas  75206


GENERAL ELECTRIC CAPITAL CORPORATION
a New York corporation

By: /s/ JOYCE TAYLOR
    ------------------------
Name:  Joyce Taylor
       ---------------------
Title: RCA
       ---------------------