<Page>
                                                                Exhibit 10.10(b)

"Pages where confidential treatment has been requested are marked 'Confidential
Treatment Requested.' The redacted material has been separately filed with the
Commission, and the appropriate section has been marked at the appropriate place
and in the margin with a star (*)."

                       'Confidential Treatment Requested'

                                SECOND AMENDMENT
                                       TO
                  MASTER NATURAL GAS LIQUIDS PURCHASE AGREEMENT

          This Amendment is executed this 22 day of March, 1999, to be effective
as of the Effective Date provided herein, by and between CHEVRON U.S.A. INC.
("Chevron") and DYNEGY MIDSTREAM SERVICES, LIMITED PARTNERSHIP ("Dynegy").

          WHEREAS, Chevron and Dynegy (formerly known as Warren Petroleum
Company, Limited Partnership) entered into that certain Master Natural Gas
Liquids Purchase Agreement, effective as of September 1, 1996 (the "Master NGL
Agreement"), as amended, under which Dynegy agreed to purchase, and Chevron
agreed to sell, all of Chevron's right, title and interest in certain natural
gas liquids ("NGLs"), as further described in the Master NGL Agreement; and

          WHEREAS, Chevron and Dynegy now wish to amend the Master NGL Agreement
as set forth herein;

          NOW THEREFORE, in consideration of the premises and other valuable
consideration, Chevron and Dynegy agree as follows:

          1. All references in the Master NGL Agreement to "Warren Petroleum
Company, Limited Partnership" shall hereafter be deemed to refer to Dynegy.

          2. Section 5.1 of the Master NGL Agreement is hereby deleted in its
entirety and replaced by the following new Section 5.1:

               5.1 Except as otherwise provided herein, WPC shall pay CUSA for
*         the NGLs purchased hereunder (i) a price equal to [REDACTED] as
          quoted by the Oil Price Information Service ("OPIS") for Mont
          Belvieu, Texas (Non-TET) for the Month in which NGLs are delivered
          to WPC, less Mont Belvieu T&F Costs, for the volumes of NGLs
          delivered to WPC at the applicable Delivery Point at the tailgate of
          the Plants listed in Exhibit "B" which is attached hereto and made a
*         part hereof, (ii) a price equal to [REDACTED] as quoted by OPIS for
          Group 140 (Conway, Kansas) for the Month in which NGLs are delivered
          to WPC, less Conway T&F Costs, for the volumes of NGLs delivered to
          WPC at the applicable Delivery Point at the tailgate of the Plants
          listed in Exhibit "C" which is attached hereto and made a part
*         hereof (iii) a price equal to [REDACTED] as quoted by OPIS for Mont
          Belvieu, Texas (Non-TET) for the Month in which NGLs are delivered
          to WPC, less Mont Belvieu T&F Costs, plus the T&F Costs paid by CUSA
          to a third Person to fractionate Raw NGL Mix into Fractionated NGLs
          for the volumes of NGLs delivered to WPC at the applicable Delivery
          Point at the tailgate of the Plants listed in Exhibit "D" which is
          attached hereto and made a part hereof, (iv)

<Page>

                        'Confidential Treatment Requested'

*         [REDACTED] percent of the Netback Price for the volumes of NGLs
          delivered to WPC at the applicable Delivery Point at the tailgate of
          the Plants listed in Exhibit "E" which is attached hereto and made a
          part hereof, and (v) the price for each NGL delivered to WPC at the
          Delivery Point at the tailgate of the Venice Fractionator as set
          forth in Exhibit "F" which is attached hereto and made a part
          hereof. WPC shall use every reasonable effort to obtain the highest
          Netback Price for NGLs. Notwithstanding the foregoing, it is
          understood and agreed that WPC's share of the Netback Price received
*         from the disposition of NGLs (i.e., [REDACTED] as set forth in (iv)
*         above), shall never be less than [REDACTED] per Gallon of each NGL
          delivered to WPC at the Delivery Point. The Parties recognize and
          acknowledge that during any given Month, NGLs delivered to WPC may
          be sold to WPC under different pricing scenarios as set forth in (i)
          through (iv) above, depending on the particular market for such NGLs
          as set forth in Exhibits "B", "C", "D" and "E".

          3. This Amendment is effective on and after February 1, 1999 (the
"Effective Date").

          4. All other provisions of the Master NGL Agreement shall remain in
full force and effect.

          IN WITNESS WHEREOF, Chevron and Dynegy have each caused their duly
authorized representative to execute this Amendment.


DYNEGY MIDSTREAM SERVICES,                  CHEVRON U.S.A. INC.
LIMITED PARTNERSHIP
By Dynegy Midstream G.P., Inc.,
its General Partner

By: /s/ Ron Logan                           By:    /s/ Larry D. Robison
Name:   Ron Logan                           Name:  Larry D. Robison
Title:  DIRECTOR SUPPLY AND                 Title: Manager, Transportation and
        TERM SALES                          Regulatory


                                      2