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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q

(Mark One)

For the quarterly period ended June 30, 2013

OR

For the transition period from                          to                         

Commission file number: 1-7414

NORTHWEST PIPELINE LLC

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (801) 583-8800

NORTHWEST PIPELINE GP

(Former name, former address and former fiscal year, if changed since last report)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  þ    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  þ    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer,” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  þ

THE REGISTRANT MEETS THE CONDITIONS SET FORTH IN GENERAL INSTRUCTION (H)(1)(a) AND (b) OF FORM 10-Q AND IS THEREFORE FILING THIS FORM 10-Q WITH THE REDUCED DISCLOSURE FORMAT.

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NORTHWEST PIPELINE LLC

FORM 10-Q

INDEX

Forward Looking Statements

Certain matters contained in this report include “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. These forward-looking statements relate to anticipated financial performance, management’s plans and objectives for future operations, business prospects, outcome of regulatory proceedings, market conditions, and other matters. We make these forward-looking statements in reliance on the safe harbor protections provided under the Private Securities Litigation Reform Act of 1995.

All statements, other than statements of historical facts, included in this report that address activities, events or developments that we expect, believe or anticipate will exist or may occur in the future are forward-looking statements. Forward-looking statements can be identified by various forms of words such as “anticipates,” “believes,” “seeks,” “could,” “may,” “should,” “continues,” “estimates,” “expects,” “assumes,” “forecasts,” “intends,” “might,” “goals,” “objectives,” “targets,” “planned,” “potential,” “projects,” “scheduled,” “will,” “guidance,” “outlook,” “in service date,” or other similar expressions. These statements are based on management’s beliefs and assumptions and on information currently available to management and include, among others, statements regarding:

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Forward-looking statements are based on numerous assumptions, uncertainties, and risks that could cause future events or results to be materially different from those stated or implied in this report. Many of the factors that will determine these results are beyond our ability to control or predict. Specific factors that could cause actual results to differ from results contemplated by the forward-looking statements include, among others, the following:

Given the uncertainties and risk factors that could cause our actual results to differ materially from those contained in any forward-looking statement, we caution investors not to unduly rely on our forward-looking statements. We disclaim any obligations to and do not intend to update the above list or to announce publicly the result of any revisions to any of the forward-looking statements to reflect future events or developments.

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In addition to causing our actual results to differ, the factors listed above and referred to below may cause our intentions to change from those statements of intention set forth in this report. Such changes in our intentions may also cause our results to differ. We may change our intentions, at any time and without notice, based upon changes in such factors, our assumptions, or otherwise.

Because forward-looking statements involve risks and uncertainties, we caution that there are important factors, in addition to those listed above, that may cause actual results to differ materially from those contained in the forward-looking statements. For a detailed discussion of those factors, see Part I, Item 1A. Risk Factors in our Annual Report on Form 10-K for the year ended December 31, 2012.

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PART I – FINANCIAL INFORMATION

ITEM 1. Financial Statements.

NORTHWEST PIPELINE GP

STATEMENT OF COMPREHENSIVE INCOME

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

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NORTHWEST PIPELINE GP

BALANCE SHEET

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

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NORTHWEST PIPELINE GP

BALANCE SHEET

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

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NORTHWEST PIPELINE GP

STATEMENT OF CASH FLOWS

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

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NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

1. BASIS OF PRESENTATION

In this report, Northwest Pipeline GP (Northwest) is at times referred to in the first person as “we”, “us” or “our.”

On June 30, 2013, Northwest was indirectly owned by Williams Partners L.P. (WPZ), a publicly traded Delaware limited partnership, which is consolidated by The Williams Companies, Inc. (Williams). At June 30, 2013, Williams holds an approximate 68 percent interest in WPZ, comprised of an approximate 66 percent limited partner interest and all of WPZ’s 2 percent general partner interest.

On July 1, 2013, Northwest converted from a Delaware general partnership to a Delaware limited liability company, Northwest Pipeline LLC (Company). Following the conversion, the sole member of the Company, Williams Partners Operating LLC, a Delaware limited liability company, executed a limited liability company operating agreement. In connection with the conversion of the Company to a limited liability company, the Amended and Restated Partnership Agreement of Northwest was terminated.

General

The accompanying interim financial statements do not include all the notes in our annual financial statements, and therefore, should be read in conjunction with the financial statements and notes thereto in our 2012 Annual Report on Form 10-K. The accompanying unaudited financial statements include all adjustments both normal recurring and others which, in the opinion of our management, are necessary to present fairly our interim financial statements.

The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.

Certain prior period amounts reported within Total operating expenses in the Statement of Comprehensive Income have been reclassified to conform to the current presentation. For the three and six months ended June 30, 2012, the effect of the correction increased Operation and maintenance costs $0.7 million and $1.2 million, respectively; increased General and administrative expenses $0.1 million and $0.4 million, respectively; and decreased Taxes, other than income taxes $0.8 million and $1.6 million, respectively; with no net impact on Total operating expenses, Operating Income, or Net Income.

2. CONTINGENT LIABILITIES AND COMMITMENTS

Environmental Matters

We are subject to the National Environmental Policy Act and other federal and state legislation regulating the environmental aspects of our business. Except as discussed below, our management believes that we are in substantial compliance with existing environmental requirements. Environmental expenditures are expensed or capitalized depending on their future economic benefit and potential for rate recovery. We believe that, with respect to any expenditures required to meet applicable standards and regulations, the Federal Energy Regulatory Commission (FERC) would grant the requisite rate relief so that substantially all of such expenditures would be permitted to be recovered through rates. We believe that compliance with applicable environmental requirements is not likely to have a material adverse effect upon our financial position or results of operations.

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NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

Beginning in the mid-1980s, we evaluated many of our facilities for the presence of toxic and hazardous substances to determine to what extent, if any, remediation might be necessary. We identified polychlorinated biphenyl (PCB) contamination in air compressor systems, soils and related properties at certain compressor station sites. Similarly, we identified hydrocarbon impacts at these facilities due to the former use of earthen pits and mercury contamination at certain natural gas metering sites. The PCBs were remediated pursuant to a Consent Decree with the U.S. Environmental Protection Agency (EPA) in the late 1980s, and we conducted a voluntary clean-up of the hydrocarbon and mercury impacts in the early 1990s. In 2005, the Washington Department of Ecology required us to re-evaluate our previous mercury clean-ups in Washington. Currently, we are conducting assessment and remediation activities for mercury and other constituents to bring the sites up to Washington’s current environmental standards. At June 30, 2013, we had accrued liabilities totaling approximately $7.5 million for these costs which are expected to be incurred through 2017. We are conducting environmental assessments and implementing a variety of remedial measures that may result in increases or decreases in the total estimated costs.

In March 2008, the EPA promulgated a new, lower National Ambient Air Quality Standard (NAAQS) for ground-level ozone. However, in September 2009, the EPA announced it would reconsider the 2008 NAAQS for ground-level ozone to ensure that the standards were clearly grounded in science, and were protective of both public health and the environment. As a result, the EPA delayed designation of new eight-hour ozone non-attainment areas under the 2008 standards until the reconsideration was complete. In January 2010, the EPA proposed to further reduce the ground-level ozone NAAQS from the March 2008 levels. In September 2011, the EPA announced that it was proceeding with required actions to implement the 2008 ozone standard and area designations. In May 2012, the EPA completed designation of new eight-hour ozone non-attainment areas. Based on the published designations, no Northwest facilities are located within the non-attainment areas. At this time, it is unknown whether future state regulatory actions associated with implementation of the 2008 ozone standard will impact our operations and increase the cost of additions to property, plant and equipment. Until any additional state regulatory actions are proposed, we are unable to estimate the cost of additions that may be required to meet any such new regulation.

Additionally, in August 2010, the EPA promulgated National Emission Standards for hazardous air pollutants (NESHAP) regulations that will impact our operations. The emission control additions required to comply with the hazardous air pollutant regulations are estimated to include capital costs in the range of $500 thousand to $1 million through 2013, the compliance date.

On January 22, 2010, the EPA set a new one-hour nitrogen dioxide (NO₂) NAAQS. The effective date of the new NO₂ standard was April 12, 2010. This standard is subject to challenges in federal court. On January 20, 2012, the EPA determined pursuant to available information that no area in the country is violating the 2010 NO₂ NAAQS, and thus, designated all areas of the country as “unclassifiable/attainment.” Also, at that time, the EPA noted its plan to deploy an expanded NO₂ monitoring network beginning in 2013. However, on October 5, 2012, the EPA proposed a graduated implementation of the monitoring network between January 1, 2014 and January 1, 2017. Once three years of data is collected from the new monitoring network, the EPA will reassess attainment status with the one-hour NO₂ NAAQS. Until that time, the EPA or states may require ambient air quality modeling on a case by case basis to demonstrate compliance with the NO₂ standard. Because we are unable to predict the outcome of the EPA’s or states’ future assessment using the new monitoring network, we are unable to estimate the cost of additions that may be required to meet this regulation.

Other Matters

Various other proceedings are pending against us and are considered incidental to our operations.

Summary

We estimate that for all matters for which we are able to reasonably estimate a range of loss, including those noted above and others that are not individually significant, our aggregate reasonably possible losses beyond amounts accrued for all of our contingent liabilities are immaterial to our expected

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NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

future annual results of operations, liquidity and financial position. These calculations have been made without consideration of any potential recovery from third-parties. We have disclosed all significant matters for which we are unable to reasonably estimate a range of possible loss.

3. DEBT AND FINANCING ARRANGEMENT

Credit Facility

Total letter of credit capacity available to WPZ under the $2.4 billion credit facility is $1.3 billion. At June 30, 2013, no letters of credit have been issued and no loans are outstanding under the credit facility. We may borrow up to $400 million under the credit facility to the extent not otherwise utilized by WPZ and Transcontinental Gas Pipe Line Company, LLC (Transco). At June 30, 2013, the full $400 million under the credit facility was available to us.

On July 31, 2013, WPZ amended the $2.4 billion credit facility to increase the aggregate commitments to $2.5 billion and extend the maturity date to July 31, 2018. The amended credit facility may also, under certain conditions, be increased by up to an additional $500 million. We may borrow up to $500 million under the amended credit facility to the extent not otherwise utilized by WPZ and Transco.

WPZ participates in a commercial paper program, and WPZ management considers amounts outstanding under this program to be a reduction of available capacity under the credit facility. At June 30, 2013, WPZ had $710 million in outstanding commercial paper.

4. FINANCIAL INSTRUMENTS

Fair Value of Financial Instruments

The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that value:

Cash and advances to affiliate—The carrying amounts approximate fair value because of the short-term nature of these instruments.

Long-term debt—The disclosed fair value of our long-term debt, which we consider as a level 2 measurement, is determined by a market approach using broker quoted indicative period-end bond prices. The quoted prices are based on observable transactions in less active markets for our debt or similar instruments. The carrying amount and estimated fair value of our long-term debt, including current maturities, were $694.1 million and $817.4 million, respectively, at June 30, 2013, and $694.0 million and $840.2 million, respectively, at December 31, 2012.

5. TRANSACTIONS WITH AFFILIATES

We are a participant in WPZ’s cash management program. At June 30, 2013 and December 31, 2012, our advances to WPZ totaled approximately $112.2 million and $29.3 million, respectively. These advances are represented by demand notes and are classified as Current Assets in the accompanying Balance Sheet. The interest rate on these intercompany demand notes is based upon the daily overnight investment rate paid on WPZ’s excess cash at the end of each month, which was approximately 0.01 percent at June 30, 2013. The interest income from these advances was minimal during the six months ended June 30, 2013 and June 30, 2012. Such interest income is included in Other (Income) and Other Expenses – Miscellaneous other (income) expenses, net on the accompanying Statement of Comprehensive Income.

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We have no employees. Services necessary to operate our business are provided to us by Williams and certain affiliates of Williams. We reimburse Williams and its affiliates for all direct and indirect expenses incurred or payments made (including salary, bonus, incentive compensation, and benefits) in connection with these services. Employees of Williams also provide general administrative and management services to us, and we are charged for certain administrative expenses incurred by Williams. These charges are either directly identifiable or allocated to our assets. Direct charges are for goods and services provided by Williams at our request. Allocated charges are based on a three-factor formula, which considers revenues; property, plant, and equipment; and payroll. In management’s estimation, the allocation methodologies used are reasonable and result in a reasonable allocation to us of our costs of doing business incurred by Williams. We were billed $27.9 million and $54.3 million in the three and six months ended June 30, 2013, respectively, and $28.8 million and $55.0 million in the three and six months ended June 30, 2012, respectively, for these services. Such expenses are primarily included in General and administrative and Operation and maintenance expenses on the accompanying Statement of Comprehensive Income.

During the six months ended June 30, 2013 and 2012, we declared and paid equity distributions to our parent of $27.0 million and $73.0 million, respectively. During July 2013, we declared and paid equity distributions of $35.0 million to our parent.

During the six months ended June 30, 2012, we received contributions of $2.3 million from our parent to fund a portion of our expansion related expenditures for additions to property, plant, and equipment. No contributions were received from our parent during the six months ended June 30, 2013. In July 2013, our parent authorized an additional $1.9 million capital contribution to us to fund a portion of our expenditures for additions to property, plant and equipment.

We have entered into various other transactions with certain related parties, the amounts of which were not significant. These transactions and the above-described transactions are made on the basis of commercial relationships and prevailing market prices or general industry practices.

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Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations

GENERAL

The following discussion should be read in conjunction with the Financial Statements, Notes, and Management’s Discussion and Analysis contained in Items 7 and 8 of our 2012 Annual Report on Form 10-K and with the Financial Statements and Notes contained in this Form 10-Q.

RESULTS OF OPERATIONS

ANALYSIS OF FINANCIAL RESULTS

This analysis discusses financial results of our operations for the six-month periods ended June 30, 2013 and 2012. Variances due to changes in natural gas prices and transportation volumes have little impact on revenues, because under our rate design methodology, the majority of overall cost of service is recovered through firm capacity reservation charges in our transportation rates.

Our operating revenues increased $14.9 million, or 7 percent, as compared to the first six months of 2012. This increase is primarily attributed to new rates, which became effective January 1, 2013.

Transportation service and gas storage service accounted for 98 percent and 2 percent, respectively, of our operating revenues for the period ended June 30, 2013, and 97 percent and 3 percent, respectively, of our operating revenues for the period ended June 30, 2012.

Operating expenses decreased $1.0 million, or 1 percent, due primarily to lower employee incentive compensation expense of $2.2 million and lower labor costs of $1.0 million, partially offset by higher depreciation of $1.2 million, primarily attributed to property additions.

The increase in our net income for the first six months of 2013, as compared to the comparable period in 2012, is primarily due to the increase in operating revenues associated with our new rates.

CAPITAL EXPENDITURES

Our capital expenditures were $21.9 million and $47.1 million for the six months ended June 30, 2013 and 2012, respectively. Our capital expenditures estimate for 2013 is discussed in our 2012 Annual Report on Form 10-K. This estimate includes the following new capital project proposed by us:

South Seattle Lateral Delivery Expansion

We have executed an agreement with Puget Sound Energy to expand the South Seattle Lateral to provide an additional 64 MDth per day of capacity. The project is on schedule for a November 2013 in-service date, and is estimated to cost between $19 million and $21 million.

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Item 4. Controls and Procedures

Our management, including our Senior Vice President—West and our Vice President and Chief Accounting Officer, does not expect that our disclosure controls and procedures (as defined in Rules 13a-15(e) and 15d-15(e) of the Securities Exchange Act) (Disclosure Controls) or our internal controls over financial reporting (Internal Controls) will prevent all errors and all fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and the benefits of controls must be considered relative to their costs. Because of the inherent limitations in all control systems, no evaluation of controls can provide absolute assurance that all control issues and instances of fraud, if any, within Northwest have been detected. These inherent limitations include the realities that judgments in decision-making can be faulty and that breakdowns can occur because of simple error or mistake. Additionally, controls can be circumvented by the individual acts of some persons, by collusion of two or more people, or by management override of the control. The design of any system of controls also is based in part upon certain assumptions about the likelihood of future events, and there can be no assurance that any design will succeed in achieving its stated goals under all potential future conditions. Because of the inherent limitations in a cost-effective control system, misstatements due to error or fraud may occur and not be detected. We monitor our Disclosure Controls and Internal Controls and make modifications as necessary; our intent in this regard is that the Disclosure Controls and Internal Controls will be modified as systems change and conditions warrant.

Evaluation of Disclosure Controls and Procedures

An evaluation of the effectiveness of the design and operation of our Disclosure Controls was performed as of the end of the period covered by this report. This evaluation was performed under the supervision and with the participation of our management, including our Senior Vice President—West and our Vice President and Chief Accounting Officer. Based upon that evaluation, our Senior Vice President—West and our Vice President and Chief Accounting Officer concluded that these Disclosure Controls are effective at a reasonable assurance level.

Second-Quarter 2013 Changes in Internal Controls

There have been no changes during the second quarter of 2013 that have materially affected, or are reasonably likely to materially affect, our Internal Controls.

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PART II. OTHER INFORMATION

Item 1. Legal Proceedings

The information called for by this item is provided in Note 2. Contingent Liabilities and Commitments, included in the Notes to Financial Statements included under Part 1, Item 1. Financial Statements of this Form 10-Q, which information is incorporated by reference into this item.

Item 5. Other Information

Entry Into a Material Definitive Agreement & Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant.

First Amended & Restated Credit Agreement

On July 31, 2013, Williams Partners L.P. (WPZ), Northwest Pipeline LLC (Northwest) and Transcontinental Gas Pipe Line Company, LLC (Transco and, together with WPZ and Northwest, collectively the “Borrowers”) entered into a First Amended & Restated Credit Agreement (the “Restated Credit Agreement”), with Citibank, N.A. (Citi), as administrative agent, and the lenders named therein. The Restated Credit Agreement amends and restates that certain Credit Agreement, dated as of June 3, 2011 (as amended prior to July 31, 2013, the “Existing Credit Agreement”), among the Borrowers, Citi, as administrative agent and the lenders named therein. Capitalized terms used in this Item 5 and not otherwise defined herein have the meaning given to them in the Restated Credit Agreement.

The Restated Credit Agreement increases the Aggregate Commitments available to the Borrowers by $100 million (the “Incremental Commitments”) and extends the Maturity Date to July 31, 2018. Additionally, the Restated Credit Agreement lowers, in certain cases, the applicable margin and commitment fees payable by each Borrower based on such Borrower’s senior unsecured debt ratings. The Incremental Commitments are increased Commitments from lenders named in the Existing Credit Agreement as well as a new Commitment from an institution party to the Restated Credit Agreement. After giving effect to the Restated Credit Amendment, the Borrowers may borrow, in the aggregate, up to $2.5 billion under the Restated Credit Agreement. Northwest and Transco are each subject to a $500 million borrowing sublimit. In addition, WPZ may request an increase of up to an additional $500 million in Commitments from either new lenders or increased Commitments from existing lenders named in the Restated Credit Agreement. However, at no time may the Aggregate Commitments under the Restated Credit Agreement exceed $3.0 billion.

The foregoing description of the Restated Credit Amendment does not purport to be complete and is qualified in its entirety by reference to the Restated Credit Amendment, a copy of which has been filed as Exhibit 10 to WPZ’s Quarterly Report on Form 10-Q, filed on July 31, 2013 (File No. 001-32599) and incorporated into this Item 5 by reference.

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The following instruments are included as exhibits to this report.

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: July 31, 2013

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EXHIBIT INDEX

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