UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934*

NuStar Energy L.P.

(Name of issuer)

Common Units Representing Limited Partner Interests

(Title of class of securities)

67058H102

(CUSIP number)

Robert L. Vitale, General Counsel

c/o EIG Management Company, LLC

600 New Hampshire Ave NW, Suite 1200

Washington, DC 20037

(202) 600-3388

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

April 30, 2020

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

2

This statement on Schedule 13D (this “Statement”) relates to the common units (the “Common Units”) representing limited partnership interests of NuStar Energy L.P., a Delaware limited partnership (the “Issuer”), with principal executive offices at 19003 IH-10 West, San Antonio, Texas 78257.

Pursuant to the Series D Cumulative Convertible Preferred Unit Purchase Agreement (the “Purchase Agreement”), dated as of June 26, 2018, by and among the Issuer, the Reporting Person and certain funds managed by EIG, the Reporting Person purchased 11,753,549 of the Issuer’s Series D Preferred Units (the “Preferred Units”) for a cash purchase price of $25.38 per Preferred Unit (the “Series D Unit Purchase Price”) on June 29, 2018 (the “Initial Issuance Date”). On July 13, 2018, pursuant to the terms of the Purchase Agreement, the Reporting Person purchased an additional 5,582,936 Preferred Units for a cash purchase price equal to the Series D Unit Purchase Price. The Preferred Units are convertible into Common Units at the election of the Reporting Person beginning on June 29, 2020 (the date that is the second anniversary of the Initial Issuance Date). Such Preferred Units are convertible into a number of Common Units equal to the quotient (the “Series D Conversion Ratio”) of (i) the sum of the Series D Unit Purchase Price and any unpaid distributions in respect of such Preferred Unit (calculated in accordance with the terms of the Issuer’s Eighth Amended and Restated Agreement of Limited Partnership, dated July 20, 2018 (the “LP Agreement”)) divided by (ii) the Series D Unit Purchase Price. The Preferred Units are entitled to vote on an as-converted basis with the Common Units.

The Reporting Person’s payment of its portion of the aggregate purchase price of the Preferred Units was funded by certain funds managed by EIG. The description of the Purchase Agreement, the Preferred Units and the LP Agreement in this Item 3 is not intended to be complete and is qualified in its entirety by the agreements that are filed as exhibits hereto and are incorporated by reference herein.

3

The information set forth in Items 3 and 6 of this Statement is incorporated by reference in its entirety into this Item 4.

The Reporting Person purchased the Preferred Units pursuant to the Purchase Agreement for investment purposes.

Although the Reporting Person does not have any specific plan or proposal to acquire, transfer or dispose of Common Units at the time of this filing, consistent with its investment purposes, the Reporting Person may, either directly or through one or more affiliates, from time to time or at any time and subject to price, market and general economic and fiscal conditions and other factors, acquire or seek to acquire additional Common Units in the open market, in privately negotiated transactions or otherwise, or dispose of or seek to dispose of all or a portion of such Common Units now owned or hereafter acquired. In addition, the Reporting Person may, either directly or through one or more affiliates, from time to time or at any time and subject to price, market and general economic and fiscal conditions and other factors, consolidate or seek to consolidate assets held by the Reporting Person and its affiliates, including acquiring assets owned by or selling assets to the Issuer, or make changes or seek to make changes to the capital structure of the Issuer. The Reporting Person reserves the right to change its intention with respect to any or all of the matters required to be disclosed in this Item 4.

The Reporting Person has not made a determination regarding a maximum or minimum number of Common Units or other securities of the Issuer that it may hold at any point in time.

Also, consistent with its investment intent and ongoing evaluation of its investment in the Issuer and alternatives to such investment, including a potential consolidation, acquisition or sale of assets or Common Units or changes to the Issuer’s capital structure, the Reporting Person may engage in communications with, without limitation, one or more unitholders of the Issuer or one or more officers of the Issuer regarding the Issuer. During the course of such communications, the Reporting Person may advocate or oppose one or more courses of action.

Except as set forth herein and below, or as would occur upon completion of any of the matters discussed herein, the Reporting Person has no present plans or proposals that would relate to or result in any of the matters set forth in clauses (a) through (j) of Item 4 of this Statement. Although the foregoing reflects activities presently contemplated by such person with respect to the Issuer, the foregoing is subject to change at any time.

4

The information set forth in Items 3 and 4 of this Statement is hereby incorporated by reference into this Item 6, as applicable.

The Issuer has entered into a Registration Rights Agreement with the Reporting Person and the other parties thereto (the “Registration Rights Agreement”), pursuant to which, among other things, the Issuer has granted the Reporting Person certain customary registration rights, including rights to require the Issuer to file and maintain the effectiveness of a registration statement with respect to the resale of the Common Units that are issuable upon the conversion of the Preferred Units, and under certain circumstances, to require the Issuer to initiate underwritten offerings for the Common Units that are issuable upon conversion of the Preferred Units.

The description of the Registration Rights Agreement in this Item 6 is not intended to be complete and is qualified in its entirety by the agreements that are filed as exhibits hereto and are incorporated by reference herein.

5

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this Statement is true, complete and correct.

Date: July 2, 2020

6