ASSIGNMENT AGREEMENT

Exhibit 10.6

ASSIGNMENT AGREEMENT

This Assignment Agreement (“Assignment”), entered into by and among Northwest Pipeline Services LLC, a Delaware limited liability company (“Assignor”), and Williams WPC-I, LLC, a Delaware limited liability company (“Assignee”), is effective as of January 1, 2013 (the “Effective Date”).

RECITALS

A. WHEREAS, Northwest Pipeline GP, a Delaware general partnership, and Assignor are parties to an Administrative Services Agreement dated October 1, 2007 (the “Agreement”).

B. WHEREAS, pursuant to an organizational restructuring of The Williams Companies, Inc. and its affiliates, employees of Assignor ceased to be employees of Assignor and became employees solely of Assignee on the Effective Date.

Assignor and Assignee hereto agree as follows:

ASSIGNMENT AND ASSUMPTION

1. Assignment. Assignor hereby assigns to Assignee all rights, obligations, and other interests in and under the Agreement.

2. Assumption. Assignee shall assume and be responsible for all of Assignor’s rights, obligations, and other interests in and under the Agreement, including, but not limited to, any rights or obligations arising before the Effective Date.

IN WITNESS WHEREOF, the parties hereto have caused this Assignment to be executed by their duly authorized representatives on February 13, 2013.