<PAGE>
                                                                     EXHIBIT 5.1


                       [VINSON & ELKINS L.L.P. LETTERHEAD]




March 7, 2002



Williams Energy Partners L.P.
Williams GP Inc.
Williams OLP, L.P.
Williams Acquisition (DE) LLC
Williams NGL, LLC
Williams Pipelines Holdings, L.P.
Williams Terminals Holdings, L.P.
Williams Ammonia Pipeline, L.P.
Williams Fractionation Holdings, L.P.
One Williams Center
Tulsa, Oklahoma 74172


Ladies and Gentlemen:

         We have acted as counsel for Williams Energy Partners L.P., a Delaware
limited partnership (the "Partnership"), with respect to certain legal matters
in connection with the registration by the Partnership under the Securities Act
of 1933, as amended (the "Securities Act"), of the offer and sale by the
Partnership from time to time, pursuant to Rule 415 under the Securities Act, of
(i) units representing limited partner interests in the Partnership (the
"Units"), (ii) unsecured debt securities, in one or more series, consisting of
notes, debentures or other evidences of indebtedness (the "Debt Securities") and
(iii) guarantees (the "Guarantees") of such Debt Securities by Williams GP Inc.,
a Delaware corporation ("GP Inc."), Williams OLP, L.P., a Delaware limited
partnership ("Williams OLP"), Williams Acquisition (DE) LLC, a Delaware limited
liability company ("Acquisition LLC"), Williams NGL, LLC, a Delaware limited
liability company, ("WNGL LLC"), Williams Pipelines Holdings, L.P., a Delaware
limited partnership ("WPH LP"), Williams Terminals Holdings, L.P., a Delaware
limited partnership ("WTH LP"), Williams Ammonia Pipeline, L.P., a Delaware
limited partnership ("WAP LP") and Williams Fractionation Holdings, L.P.
("Holdings LP" and together with GP Inc., Williams OLP, Acquisition LLC, WNGL
LLC, WPH LP, WTH LP and WAP LP, the "Subsidiary Guarantors"). The Units, Debt
Securities and Guarantees are collectively referred to herein as the
"Securities." We have also participated in the preparation of the Prospectus
(the "Prospectus") contained in the Partnership's Registration Statement on Form
S-3 (the "Registration Statement") to which this opinion is an exhibit.
Capitalized terms not defined herein shall have the meanings ascribed to them in
the Prospectus.

         In rendering the opinions set forth below, we have examined and relied
upon (i) the Registration Statement, including the Prospectus; (ii) the Amended
and Restated Agreement of Limited Partnership of the Partnership; (iii) the
Certificate of Incorporation and Bylaws of GP Inc., (iv) the Amended and
Restated Agreement of Limited Partnership of Williams OLP; (v) the



<PAGE>
Page 2
March 7, 2002


Limited Liability Company Agreement of Acquisition LLC; (vi) the Limited
Liability Company Agreement of WNGL LLC; (vii) the Agreement of Limited
Partnership of WPH LP; (viii) the Agreement of Limited Partnership of WTH LP;
(ix) the Agreement of Limited Partnership of WAP LP; (x) the Agreement of
Limited Partnership of Holdings LP; (xi) the forms of Senior and Subordinated
Indenture filed as exhibits to the Registration Statement and (xii) such
certificates, statutes and other instruments and documents as we consider
appropriate for purposes of the opinions hereafter expressed.

         In connection with this opinion, we have assumed that (i) the
Registration Statement and any amendments thereto (including post-effective
amendments) will have become effective; (ii) a Prospectus Supplement will have
been prepared and filed with the Commission describing the Securities offered
thereby; (iii) all Securities will be issued and sold in compliance with
applicable federal and state securities laws and in the manner stated in the
Registration Statement and the appropriate Prospectus Supplement; (iv) a
definitive purchase, underwriting or similar agreement with respect to any
Securities offered will have been duly authorized and validly executed and
delivered by the Partnership and the other parties thereto; and (v) any
Securities issuable upon conversion, exchange or exercise of any Security being
offered will be duly authorized, created and, if appropriate, reserved for
issuance upon such conversion, exchange or exercise.

         Based upon and subject to the foregoing, we are of the opinion that:

         1.       With respect to the Units, when (i) the Partnership has taken
                  all necessary action to approve the issuance of such Units,
                  the terms of the offering and related matters and (ii) the
                  Units have been issued and delivered in accordance with the
                  terms of the applicable definitive purchase, underwriting or
                  similar agreement approved by the Partnership upon payment of
                  the consideration therefor provided for therein, then the
                  Units will be validly issued, fully paid and nonassessable.

         2.       With respect to the Debt Securities and the Guarantees, when
                  (i) the Indenture has been duly qualified under the Trust
                  Indenture Act of 1939, as amended; (ii) the Partnership and
                  the Subsidiary Guarantors have taken all necessary action to
                  approve the issuance and terms of such Debt Securities and
                  Guarantees, the terms of the offering thereof and related
                  matters; and (iii) such Debt Securities and Guarantees have
                  been duly executed, authenticated, issued and delivered in
                  accordance with the provisions of the Indenture and the
                  applicable definitive purchase, underwriting or similar
                  agreement approved by the Partnership and the Subsidiary
                  Guarantors upon payment of the consideration therefor provided
                  for therein, such Debt Securities and Guarantees will be
                  legally issued and will constitute valid and legally binding
                  obligations of the Partnership and the Subsidiary Guarantors,
                  respectively, enforceable against the Partnership and the
                  Subsidiary Guarantors in accordance with their terms, except
                  as such enforcement is subject to any applicable bankruptcy,
                  insolvency, reorganization, fraudulent conveyance or other law
                  relating to or affecting creditors' rights generally and
                  general principles of equity.



<PAGE>

Page 3
March 7, 2002


         The opinions expressed herein are qualified in the following respects:

         A.       We have assumed, without independent verification, that the
                  certificates for the Units will conform to the specimens
                  thereof examined by us and will have been duly countersigned
                  by a transfer agent and duly registered by a registrar of the
                  Units.

         B.       We have assumed that (i) each document submitted to us for
                  review is accurate and complete, each such document that is an
                  original is authentic, each such document that is a copy
                  conforms to an authentic original and all signatures on each
                  such document are genuine, and (ii) each certificate from
                  governmental officials reviewed by us is accurate, complete
                  and authentic, and all official public records are accurate
                  and complete.

         C.       This opinion is limited in all respects to federal laws, the
                  Delaware Revised Uniform Limited Partnership Act, the Delaware
                  General Corporation Law, the Delaware Limited Liability
                  Company Act and the Constitution of the State of Delaware, as
                  interpreted by the courts of the State of Delaware and of the
                  United States.

         We hereby consent to the references to this firm under the caption
"Legal" in the Prospectus and to the filing of this opinion as an Exhibit to the
Registration Statement. By giving such consent, we do not admit that we are
within the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations of the Securities and Exchange
Commission issued thereunder.

Very truly yours,



/s/ Vinson & Elkins L.L.P.

