NAMES OF REPORTING PERSONS

William E. Macaulay

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

(a)  ¨     (b)  x    

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

United States of America

NUMBER OF  

SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON

WITH:  

SOLE VOTING POWER

0

SHARED VOTING POWER

52,384,345

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

52,384,345

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

52,384,345

CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

¨

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

12.8%*

TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

IN

NAMES OF REPORTING PERSONS

First Reserve GP XI, Inc.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

(a)  ¨     (b)  x    

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER OF  

SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON

WITH:  

SOLE VOTING POWER

0

SHARED VOTING POWER

52,384,345

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

52,384,345

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

52,384,345

CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

¨

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

12.8%*

TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

CO

NAMES OF REPORTING PERSONS

First Reserve GP XI, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

(a)  ¨     (b)  x    

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER OF  

SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON

WITH:  

SOLE VOTING POWER

0

SHARED VOTING POWER

52,384,345

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

52,384,345

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

52,384,345

CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

¨

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

12.8%*

TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

PN

NAMES OF REPORTING PERSONS

First Reserve Fund XI, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

(a)  ¨     (b)  x    

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER OF  

SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON

WITH:  

SOLE VOTING POWER

0

SHARED VOTING POWER

39,262,817

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

39,262,817

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

39,262,817

CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

¨

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

9.6%*

TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

PN

NAMES OF REPORTING PERSONS

FR XI Onshore AIV, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

(a)  ¨     (b)  x    

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER OF  

SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON

WITH:  

SOLE VOTING POWER

0

SHARED VOTING POWER

13,121,528

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

13,121,528

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

13,121,528

CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)

¨

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

3.2%*

TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

PN

Cobalt Center, 920 Memorial City Way, Suite 100, Houston, Texas 77024

This Schedule 13G is filed on behalf of each of the following entities (collectively, the “Reporting Persons”):

William E. Macaulay

First Reserve GP XI, Inc.

First Reserve GP XI, L.P.

First Reserve Fund XI, L.P.

FR XI Onshore AIV, L.P.

First Reserve Fund XI, L.P. (“Fund XI”) and FR XI Onshore AIV, L.P. (“Onshore AIV”) each directly holds the Shares listed under its respective name in Item 4 below. First Reserve GP XI, L.P. (“GP XI”) is the general partner of Fund XI and Onshore AIV, and First Reserve GP XI, Inc (“GP XI Inc.”) is the general partner of GP XI. Mr. Macaulay is a director of GP XI Inc. and has the right to appoint a majority of the board of directors of GP XI Inc. In such capacities, each of Mr. Macaulay, GP XI and GP XI Inc. may be deemed to share beneficial ownership of the Shares of the Issuer held by Fund XI and Onshore AIV.

One Lafayette Place, Third Floor, Greenwich, Connecticut 06830

Delaware — GP XI Inc., GP XI, Fund XI and Onshore AIV

United States —William E. Macaulay

Shares of Common Stock, $0.01 par value per share (“Shares”)

(e)

19075F 106

(a)

¨  Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

¨  Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

¨  Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

¨  Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).

(e)

¨  An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

¨  An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

¨  A parent holding company or control person in accordance with §240.13d-1(b)(1)(ii)(G);

(h)

¨  A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

¨  A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

¨  Group, in accordance with §240.13d-1(b)(1)(ii)(J).

As of the date hereof, the following Shares were beneficially owned by the Reporting Persons (all percentages of Shares reported in this statement on Amendment No. 3 to Schedule 13G (the “Schedule 13G”) have been calculated based on the Shares outstanding as of December 31, 2012, as reported in the Issuer’s Prospectus Supplement dated January 15, 2013 filed with the U.S. Securities and Exchange Commission on January 17, 2013). As of December 31, 2012, Fund XI directly held 46,757,960 Shares and Onshore AIV directly held 15,626,385 Shares.

William E. Macaulay

Amount beneficially owned:

Percent of class:

Sole voting power:

Shared voting power:

Sole dispositive power:

Shared dispositive power:

12.8

0

52,384,345

0

52,384,345

% 

First Reserve GP XI, Inc.

Amount beneficially owned:

Percent of class:

Sole voting power:

Shared voting power:

Sole dispositive power:

Shared dispositive power:

12.8

0

52,384,345

0

52,384,345

% 

First Reserve GP XI, L.P.

Amount beneficially owned:

Percent of class:

Sole voting power:

Shared voting power:

Sole dispositive power:

Shared dispositive power:

12.8

0

52,384,345

0

52,384,345

% 

First Reserve Fund XI, L.P.

Amount beneficially owned:

Percent of class:

Sole voting power:

Shared voting power:

Sole dispositive power:

Shared dispositive power:

9.6

0

39,262,817

0

39,262,817

% 

FR XI Onshore AIV, L.P.

Amount beneficially owned:

Percent of class:

Sole voting power:

Shared voting power:

Sole dispositive power:

Shared dispositive power:

3.2

0

13,121,528

0

13,121,528

% 

Not applicable.

Not applicable.

Not applicable.

Each of Fund XI and Onshore AIV is a party to (a) a Stockholders Agreement, dated as of December 15, 2009 (the “Stockholders Agreement”), by and among the Issuer, funds affiliated with Goldman, Sachs & Co., funds affiliated with Riverstone Holdings LLC and The Carlyle Group, and funds associated with KERN Partners Ltd. (collectively, the “Other Stockholders”) and (b) a Tag-Along Agreement, dated as of December 15, 2009 (the “Tag-Along Agreement”), by and among Fund XI, Onshore AIV and the Other Stockholders (or affiliates thereof).