Exhibit 99.11

March 11, 2019

Blackstone Infrastructure Advisors L.L.C.

345 Park Avenue

New York, NY 10154

Attention: John G. Finley

Email: john.finley@blackstone.com

Ladies and Gentlemen:

Reference is made to that certain Purchase Agreement (“Purchase Agreement”), dated as of January 30, 2019, by and among Tallgrass Energy Holdings, LLC, a Delaware limited liability company (“Holdings”), the other sellers named therein (together with Holdings, each a “Seller” and collectively, the “Sellers”), Prairie GP Acquiror LLC, a Delaware limited liability company (“GP Acquiror”), Prairie ECI Acquiror LP, a Delaware limited partnership (“Up-C Acquiror 1”), Prairie VCOC Acquiror LP, a Delaware limited partnership (“Up-C Acquiror 2” and, together with Up-C Acquiror 1, “Up-C Acquirors”), and Prairie Non-ECI Acquiror LP, a Delaware limited partnership (“Class A Acquiror” and, together with GP Acquiror and Up-C Acquirors, each an “Acquiror” and collectively, the “Acquirors”), and David G. Dehaemers, Jr., John T. Raymond and Frank J. Loverro, in their respective capacities as Seller Representatives (each, a “Seller Representative” and collectively, the “Seller Representatives”). Each capitalized term used herein but not defined shall have the meaning ascribed to such term in the Purchase Agreement.

The purpose of this letter agreement (this “Side Letter”) is to confirm the understanding of the Sellers and the Acquirors with respect to certain matters related to the Purchase Agreement. Accordingly, in consideration of the mutual agreements contained herein, the parties hereby agree as follows:

1. Moler Trust. The William R. Moler Revocable Trust, under trust agreement dated August 27, 2013 (“Moler Trust”) will not sell any of its Class A Shares under the Purchase Agreement and is no longer a Party to, or bound by the terms and obligations of, the Purchase Agreement for any purpose. Accordingly, all references to the “Moler Trust” in the Purchase Agreement (including the exhibits thereto) are hereby omitted.

2. DGD Class A Shares. The David G. Dehaemers, Jr. Revocable Trust, a revocable trust under trust agreement dated April 26, 2006 (“DGD Trust”), will not sell any of its Class A Shares under the Purchase Agreement and all references to the “DGD Class A Shares” in the Purchase Agreement are hereby omitted. DGD Trust shall remain a Seller under the Purchase Agreement with respect to its Up-C Interests as set forth on Schedule 2.1 hereto.

3. Seller Notes. Promissory Notes will not be issued by Class A Acquiror, Up-C Acquiror 1 or Blackstone Infrastructure Partners V L.P. to certain of the Sellers at Closing, or otherwise in connection with the Purchase Agreement. Accordingly, all references to “Seller Notes” and “Exhibit D” in the Purchase Agreement are hereby omitted.

4. Purchase Price and Other Schedules.

a. Schedule 2.1 to the Purchase Agreement is hereby replaced in its entirety with the new Schedule 2.1 attached hereto;

b. The reference in the fifth recital to the Purchase Agreement to “26,339,528” TE Units, shall be changed to “21,751,018”;

c. The reference in Section 2.1 of the Purchase Agreement to an aggregate Purchase Price of “$3,272,860,010”, shall be changed to “$3,213,161,149”;

d. Schedule A-1 to the Purchase Agreement is hereby replaced in its entirety with the new Schedule A-1 attached hereto; and

e. Schedule 6.12(b) to the Purchase Agreement is hereby replaced in its entirety with the new Schedule 6.12(b) attached hereto.

5. Wires. Notwithstanding anything to the contrary in the Purchase Agreement, each Seller acknowledges and agrees that the payments at the Closing made pursuant to the funds flow attached hereto as Exhibit E satisfy the Acquirors’ payment obligations under Section 2.1 of the Purchase Agreement.

6. Effect of Side Letter. Except as otherwise expressly provided herein, all of the respective terms and conditions of the Purchase Agreement remain unchanged and continue in full force and effect. In the case of any conflict between this Side Letter and the Purchase Agreement, the terms of this Side Letter will govern. This Side Letter shall be deemed to be in full force and effect from and after the execution of this Side Letter by the parties.

7. Governing Law. This Side Letter shall be governed by and construed in accordance with the laws of the State of Delaware, without regard to the principles of conflicts of laws thereof.

8. Counterparts. This Side Letter may be executed and delivered (including by facsimile or other electronic transmission) in one or more counterparts, and by the different parties hereto in separate counterparts, each of which when executed shall be deemed to be an original, but all of which taken together shall constitute one and the same agreement.

[Signature Pages Follow]

If the foregoing correctly sets forth your understanding of our agreement with respect to the matters above, please indicate your acceptance and approval below.

	Very truly yours,


	TALLGRASS ENERGY HOLDINGS, LLC


	By:	/s/ David G. Dehaemers, Jr.
	Name:	David G. Dehaemers, Jr.
	Title:	Chief Executive Officer and President


	TALLGRASS KC, LLC


	By:	/s/ David G. Dehaemers, Jr.
	Name:	David G. Dehaemers, Jr.
	Title:	Manager


	/s/ David G. Dehaemers, Jr.
	DAVID G. DEHAEMERS, JR., TRUSTEE OF THE DAVID G. DEHAEMERS, JR. REVOCABLE TRUST, UNDER TRUST AGREEMENT DATED APRIL 26, 2006


	TALLGRASS HOLDINGS, LLC

	By:	EMG Fund II Management, LP,
		its manager

	By:	EMG Fund II Management, LLC,
		its general partner


	By:	/s/ John T. Raymond
	Name:	John T. Raymond
	Title:	Chief Executive Officer

	KIA VIII (RUBICON), L.P.

	By:	KIA VIII (Rubicon) GP, L.P.,
		its general partner

	By:	Kelso GP VIII, LLC,
		its general partner


	By:	/s/ James J. Connors, II
	Name:	James J. Connors, II
	Title:	Managing Member


	KEP VI AIV (RUBICON), LLC


	By:	/s/ James J. Connors, II
	Name:	James J. Connors, II
	Title:	Managing Member

	HOBBS VENTURES LLC


	By:	/s/ Charles Scott Hobbs
	Name:	Charles Scott Hobbs
	Title:	Manager


	HOBBS FAMILY FOUNDATION


	By:	/s/ Charles Scott Hobbs
	Name:	Charles Scott Hobbs
	Title:	President


	WYLIE VENTURES, LLC


	By:	/s/ Forrest Wylie
	Name:	Forrest Wylie
	Title:	President


	/s/ Cathy Harry Grills
	CATHY HARRY GRILLS


	MATAGATOR FOUNDATION


	By:	/s/ Forrest Wylie
	Name:	Forrest Wylie
	Title:	Trustee

Acknowledged and agreed:

ACQUIRORS:

PRAIRIE GP ACQUIROR LLC
By: Prairie Non-ECI Acquiror LP, its sole member
By: BIP Holdings Manager L.L.C., its general partner


By:	/s/ Sean Klimzack
Name:	Sean Klimczak
Title:	Senior Managing Director


PRAIRIE ECI ACQUIROR LP
By:	BIP Holdings Manager L.L.C., its general partner


By:	/s/ Sean Klimzack
Name:	Sean Klimczak
Title:	Senior Managing Director


PRAIRIE VCOC ACQUIROR LP
By: BIP Holdings Manager L.L.C., its general partner


By:	/s/ Sean Klimzack
Name:	Sean Klimczak
Title:	Senior Managing Director


PRAIRIE NON-ECI ACQUIROR LP
By: BIP Holdings Manager L.L.C., its general partner

By:	/s/ Sean Klimzack
Name:	Sean Klimczak
Title:	Senior Managing Director

Acknowledged and agreed:


/s/ William R. Moler
WILLIAM R. MOLER, TRUSTEE OF THE WILLIAM R. MOLER REVOCABLE TRUST UNDER TRUST AGREEMENT DATED AUGUST 27, 2013