SC 13D/A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 22)*

American Midstream Partners, LP

(Name of Issuer)

Common Units Representing Limited Partner Interests

(Title of Class of Securities)

02752P 100

(CUSIP Number)

Christine Miller

Magnolia Infrastructure Partners, LLC

c/o ArcLight Capital Partners

200 Clarendon Street, 55th Floor

Boston, MA 02117

(617) 531-6338

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

September 27, 2018

(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 Rule 13d-1(e), Rule 13d-1(f) or 13d-1(g), check the following box:  ☐

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

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.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person American Midstream GP, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 1,349,609 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 1,349,609 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 1,349,609 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 2.5% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 1,349,609 common units of the Issuer (“Common Units”) held by American Midstream GP, LLC, the Issuer’s general partner.

Note 2: Based on 52,981,070 Common Units outstanding as of August 6, 2018.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person AMID GP Holdings, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 1,349,609 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 1,349,609 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 1,349,609 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 2.5% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 1,349,609 common units of the Issuer (“Common Units”) held by American Midstream GP, LLC, which is approximately 23% owned by AMID GP Holdings, LLC.

Note 2: Based on 52,981,070 Common Units outstanding as of August 6, 2018.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person High Point Infrastructure Partners, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 10,276,518 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 10,276,518 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 10,276,518 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 16.6% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-l Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”) and 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point.

Note 2: Based on 61,907,979 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018 and (b) 8,926,909 Common Units issuable upon the conversion of Series A-l Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person Magnolia Infrastructure Partners, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 14,719,998 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 14,719,998 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 14,719,998 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 22.3% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point, and 618,921 Common Units held by Magnolia.

Note 2: Based on 65,732,538 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-l Units outstanding and (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person Magnolia Infrastructure Holdings, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 34,215,962 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 34,215,962 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 34,215,962 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 45.3% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, and 618,921 Common Units held by Magnolia.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person ArcLight Energy Partners Fund V, L.P.
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 37,069,444 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 37,069,444 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 37,069,444 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 49.1% (See Note 2)
14.	Type of Reporting Person PN

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person ArcLight PEF GP V, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 37,069,444 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 37,069,444 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 37,069,444 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 49.1% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person ArcLight Capital Holdings, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 37,069,444 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 37,069,444 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 37,069,444 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 49.1% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person ArcLight Capital Partners, LLC
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 37,069,444 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 37,069,444 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 37,069,444 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 49.1% (See Note 2)
14.	Type of Reporting Person OO (Limited Liability Company)

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

CUSIP No. 02752P 100 SCHEDULE 13D

1.	Name of Reporting Person Daniel R. Revers
2.	Check the Appropriate Box if a Member of a Group (a)  ☐        (b)  ☒
3.	SEC Use Only
4.	Source of Funds WC/OO
5.	Check if Disclosure of Legal Proceedings is Required Pursuant to Item 2(d) or 2(e)  ☐
6.	Citizenship or Place of Organization Delaware

Number of Shares Beneficially  Owned by Each Reporting Person With	7.	Sole Voting Power 0
	8.	Shared Voting Power 37,069,444 (See Note 1)
	9.	Sole Dispositive Power 0
	10.	Shared Dispositive Power 37,069,444 (See Note 1)

11.	Aggregate Amount Beneficially Owned by Each Reporting Person 37,069,444 (See Note 1)
12.	Check if the Aggregate Amount in Row 11 Excludes Certain Shares  ☐
13.	Percent of Class Represented by Amount in Row 11 49.1% (See Note 2)
14.	Type of Reporting Person IN

Note 1: Represents 7,707,571 Series A-l Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), convertible into 8,926,909 common units of the Issuer (“Common Units”), which are indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 3,302,158 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia, convertible into 3,824,559 Common Units, 9,241,642 Series C Convertible Preferred Units (“Series C Units”) held by Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”), convertible into 9,742,539 Common Units, 9,753,425 Common Units held by Magnolia Holdings, 1,349,609 Common Units held by American Midstream GP, LLC, which is approximately 77% owned by High Point and approximately 23% owned by AMID GP Holdings, LLC, which is approximately 93% owned by Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,853,482 Common Units held by Busbar II, LLC, taking into account the transactions discussed in Item 3.

Note 2: Based on 75,475,077 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 52,981,070 Common Units outstanding as of August 6, 2018, (b) 8,926,909 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,824,559 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 9,742,539 Common Units issuable upon the conversion of Series C Units outstanding.

This Amendment No. 22 to Schedule 13D amends and supplements the Statement on Schedule 13D filed on May 3, 2013, as amended by Amendment No. 1 filed on November 14, 2014, Amendment No. 2 filed on February 17, 2015, Amendment No. 3 filed on April 3, 2015, Amendment No. 4 filed on May 15, 2015, Amendment No. 5 filed on August 18, 2015, Amendment No. 6 filed on August 19, 2015, Amendment No. 7 filed on September 17, 2015, Amendment No. 8 filed on November 18, 2015, Amendment No. 9 filed on December 22, 2015, Amendment No. 10 filed on January 6, 2016, Amendment No. 11 filed on January 11, 2016, Amendment No. 12 filed on February 1, 2016, Amendment No. 13 filed on February 24, 2016, Amendment No. 14 filed on April 27, 2016, Amendment No. 15 filed on May 18, 2016, Amendment No. 16 filed on November 3, 2016, Amendment No. 17 filed on December 6, 2016, Amendment No. 18 filed on March 8, 2017, Amendment No. 19 filed on August 18, 2017, Amendment No. 20 filed on October 12, 2017 and Amendment No. 21 filed on August 20, 2018 (as amended, this “Schedule 13D”), filed with respect to the common units representing limited partner interests (“Common Units”) of American Midstream Partners, LP (the “Issuer”).

ITEM 4. Purpose of Transaction.

The fourth paragraph of Item 4 is hereby amended and restated in its entirety as follows:

Consistent with its investment purpose, each 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 Units in the open market, in privately negotiated transactions or otherwise, or dispose of or seek to dispose of all or a portion of Units now owned or hereafter acquired. In addition, any 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 such 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. Each 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.

Furthermore, Item 4 is hereby amended and supplemented by adding the following immediately after the last paragraph:

On September 27, 2018, Magnolia Infrastructure Holdings, LLC (“Magnolia Holdings”) delivered a non-binding offer (the “Offer Letter”) to the board of directors of American Midstream GP, LLC (the “Issuer Board”) to acquire all of the issued and outstanding publicly held Common Units of the Issuer that are not directly owned by Magnolia Holdings or any of its affiliates in exchange for $6.10 in cash for each such Common Unit. The foregoing description of the Offer Letter does not purport to be complete and is qualified in its entirety by reference to the full text of the Offer Letter, which is filed as Exhibit 2 hereto and is incorporated by reference in its entirety into this Item 4.

There can be no assurance that any discussions that may occur between Magnolia Holdings and the Issuer with respect to the offer contained in the Offer Letter will result in the entry into a definitive agreement concerning a transaction or, if such a definitive agreement is reached, will result in the consummation of a transaction provided for in such definitive agreement. Discussions concerning a transaction may be terminated at any time and without prior notice. Entry into a definitive agreement concerning a transaction and the consummation of any such transaction is subject to a number of contingencies that are beyond the control of Magnolia Holdings, including the satisfactory completion of due diligence, the approval of the Issuer Board, the approval of a conflicts committee to be established by the Issuer Board, the approval by holders of a majority of the outstanding Common Units of the Issuer, and the satisfaction of any conditions to the consummation of a transaction set forth in any such definitive agreement.

Except as may be required by law, Magnolia Holdings does not intend to disclose developments with respect to the offer contained in the Offer Letter unless and until the Issuer Board and Magnolia Holdings have approved a specific transaction, if any, and Magnolia Holdings and the Issuer have then entered into a definitive agreement to effect such transaction.

ITEM 7. Material to be Filed as Exhibits.

See the Exhibit Index following the signature pages hereto.

SIGNATURES

After reasonable inquiry and to the best of each of the undersigned’s knowledge and belief, each of the undersigned hereby certifies that the information set forth in this statement is true, complete and correct.

Dated: September 28, 2018

AMERICAN MIDSTREAM GP, LLC

By its Class A Members:

AMID GP HOLDINGS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

and

HIGH POINT INFRASTRUCTURE PARTNERS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

AMID GP HOLDINGS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

HIGH POINT INFRASTRUCTURE PARTNERS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

MAGNOLIA INFRASTRUCTURE PARTNERS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

MAGNOLIA INFRASTRUCTURE HOLDINGS, LLC

/s/ Daniel R. Revers

Daniel R. Revers, President

ARCLIGHT ENERGY PARTNERS FUND V, L.P.
By:	ArcLight PEF GP V, LLC, its General Partner
By:	ArcLight Capital Holdings, LLC, its Manager
By:	ACHP II, L.P., its Managing Member
By:	ACH GP, LLC, its General Partner
/s/ Daniel R. Revers
Daniel R. Revers, Manager
ARCLIGHT PEF GP V, LLC
By:	ArcLight Capital Holdings, LLC, its Manager
By:	ACHP II, L.P., its Managing Member
By:	ACH GP, LLC, its General Partner
/s/ Daniel R. Revers
Daniel R. Revers, Manager
ARCLIGHT CAPITAL HOLDINGS, LLC
By:	ACHP II, L.P., its Managing Member
By:	ACH GP, LLC, its General Partner
/s/ Daniel R. Revers
Daniel R. Revers, Manager
ARCLIGHT CAPITAL PARTNERS, LLC
/s/ Daniel R. Revers
Daniel R. Revers, Managing Partner
/s/ Daniel R. Revers
Daniel R. Revers

EXHIBIT INDEX

Exhibit Number	Description
1.	Joint Filing Agreement (incorporated by reference to Exhibit 1 of Amendment No. 19 to Schedule 13D filed by the Reporting Persons on August 18, 2017).
2.	Offer Letter, dated September 27, 2018.

EX-99.2

Exhibit 2

Magnolia Infrastructure Holdings, LLC

c/o ArcLight Capital Partners, LLC

200 Clarendon Street, 55^th Floor

Boston, MA 02116

September 27, 2018

American Midstream Partners, LP

c/o American Midstream GP, LLC, its general partner

2103 CityWest Blvd.

Building #4, Suite 800

Houston, TX 77042

Attn: Members of the Board of Directors of American Midstream GP, LLC (the “Board”)

Re: Possible acquisition of all outstanding publicly held Common Units of

American Midstream Partners, LP

Ladies and Gentlemen:

Magnolia Infrastructure Holdings, LLC (“Magnolia”), a subsidiary of ArcLight Energy Partners Fund V, L.P., is pleased to submit this non-binding offer (this “Non-Binding Offer”) to American Midstream Partners, LP (“AMID”) regarding a possible transaction pursuant to which a subsidiary of Magnolia (“ArcLight SPV”) would acquire all of the outstanding Common Units of AMID not already owned by Magnolia and/or certain of its affiliates (the “Transaction”), as described in more detail below.

As you are aware, Magnolia and certain of its affiliates currently own, in the aggregate and on an as-converted basis, 37,069,444 AMID Common Units, representing approximately 49.1% of the total outstanding Common Units of AMID. In addition, Magnolia, together with certain of its affiliates, currently indirectly owns the 1.3% general partner interest in AMID through American Midstream GP, LLC, the general partner of AMID. Subject to the negotiation and execution of a definitive agreement containing terms and conditions customary for a transaction of the type set out in this letter, we are pleased to offer $6.10 in cash in exchange for each issued and outstanding publicly held Common Unit of AMID that is not directly owned by Magnolia and its affiliates.

1. Structure.

The Transaction would be structured as a merger between AMID and a subsidiary of ArcLight SPV, with AMID surviving the merger as a wholly owned subsidiary of ArcLight SPV.

2. Conflicts Committee.

We anticipate that the Board will desire to establish a Conflicts Committee of the Board (the “Conflicts Committee”) to evaluate and negotiate the proposed Transaction, and that the Conflicts Committee will engage independent advisors to assist in the evaluation of this Non-Binding Offer.

3. Key Assumptions.

The principal terms set out above are based on the following key assumptions:

(a) ArcLight SPV and AMID would enter into a definitive merger agreement or other similar agreement (the “Definitive Agreement”) that would set out all of the terms and conditions relevant to the Transaction.

(b) ArcLight SPV would obtain customary debt and equity financing on reasonable terms, which ArcLight SPV expects to procure on an expedited basis. ArcLight SPV intends to arrange for fully committed financing by signing.

(c) The Transaction would be subject to customary closing conditions, such as the expiration of an HSR waiting period and other customary regulatory approvals, a bring down of fundamental representations and warranties, absence of a material adverse effect and performance of covenants, among others.

(d) The Transaction would be approved by the Conflicts Committee and the ArcLight Investment Committee.

(e) The closing of the Transaction would be subject to approval by holders of a majority of the outstanding Common Units of AMID.

4. Internal Approvals.

The delivery of this Non-Binding Offer to AMID has been approved by the ArcLight Investment Committee.

5. Disclosures.

Tomorrow, Magnolia and certain of its affiliates intend to file an amended Schedule 13D as required under applicable securities laws and regulations and issue a press release, which such filing and press release will disclose this Non-Binding Offer.

6. Legal Effect.

This Non-Binding Offer is not intended to, and does not, constitute or create any legally binding obligations or liabilities on the part of Magnolia or any of its affiliates. A binding obligation of Magnolia (or any of its affiliates) to effect the Transaction shall be created only upon the execution and delivery by ArcLight SPV and AMID of a Definitive Agreement. Magnolia and its affiliates reserve their right to withdraw this Non-Binding Offer at any time, for any reason, at Magnolia’s sole discretion.

Magnolia is well positioned to negotiate and complete the Transaction, including obtaining any required financing, in an expeditious manner. After receipt of this offer and, if this Non-Binding Offer is acceptable to AMID, Magnolia and its affiliates are prepared to begin negotiating the Definitive Agreement and any related agreements with AMID in respect to the Transaction.

We look forward to receiving AMID’s response to this Non-Binding Offer and stand prepared to discuss our analysis of the potential Transaction with the Conflicts Committee and its advisors and any other representatives of AMID at AMID’s convenience.

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Sincerely,
Magnolia Infrastructure Holdings, LLC
By:	/s/ Daniel R. Revers
Name	Daniel R. Revers
Title:	President