<PAGE>
                                                                     EXHIBIT 5.1

                          [VINSON & ELKINS LETTERHEAD]


May 23, 2002



Williams Energy Partners L.P.
One Williams Center
Tulsa, Oklahoma 74172

Ladies and Gentlemen:

         We have acted as counsel to Williams Energy Partners L.P., a Delaware
limited partnership (the "Partnership"), and Williams GP LLC, a Delaware limited
liability company and the general partner of the Partnership (the "General
Partner"), in connection with the registration under the Securities Act of 1933,
as amended (the "Securities Act") of the offering and sale of up to an aggregate
of 9,200,000 common units representing limited partner interests in the
Partnership (the "Common Units").

         As the basis for the opinion hereinafter expressed, we examined such
statutes, including the Delaware Uniform Revised Limited Partnership Act,
corporate records and documents, certificates of corporate and public officials,
and other instruments and documents as we deemed necessary or advisable for the
purposes of this opinion. In such examination, we assumed the authenticity of
all documents submitted to us as originals and the conformity with the original
documents of all documents submitted to us as copies.

         Based on the foregoing and on such legal considerations as we deem
relevant, we are of the opinion that:

     1.   The Partnership has been duly formed and is validly existing as a
          limited partnership under the Delaware Revised Uniform Limited
          Partnership Act.

     2.   The Common Units, when issued and paid for under the Partnership's
          Registration Statement on Form S-3 (the "Registration Statement"),
          relating to the Common Units, will be duly authorized, validly issued,
          fully paid and nonassessable.

         We hereby consent to the reference to us under the heading "Legal" in
the prospectus forming a part of the Registration Statement and to the filing of
this opinion as an exhibit to the Registration Statement, but we do not thereby
admit that we are within the class of persons whose consent is required under
the provisions 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.

                                        VINSON & ELKINS L.L.P.


