13D Filing: Magnolia Infrastructure Holdings, LLC and American Midstream Partners LP (AMID)

Published on October 12, 2017 at 1:36 pm by Insider Monkey Staff in Hedge Funds

You can access the original SEC filing by clicking here.

Ownership Summary Table

Name	Shared Voting Power	Shared Dispositive Power	Aggregate Amount Owned Power	Percent of Class
S.S. or	13,721,405	13,721,405	13,721,405	21.6%
S.S. or	32,400,677	32,400,677	32,400,677	44.7%
S.S. or	9,577,433	9,577,433	9,577,433	15.6%
S.S. or	32,400,677	32,400,677	32,400,677	44.7%
S.S. or	1,349,609	1,349,609	1,349,609	2.6%
S.S. or	34,656,431	34,656,431	34,656,431	47.8%
S.S. or	34,656,431	34,656,431	34,656,431	47.8%
S.S. or	34,656,431	34,656,431	34,656,431	47.8%
S.S. or	34,656,431	34,656,431	34,656,431	47.8%
S.S. or	34,656,431	34,656,431	34,656,431	47.8%

Page 1 of 16 – SEC Filing

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 20)*

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)

October 2, 2017

(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 persons 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.

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Page 2 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 13,721,405 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 13,721,405 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 13,721,405 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 21.6% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 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, a wholly owned subsidiary of Magnolia Infrastructure Holdings, LLC (Magnolia Holdings), and 618,921 Common Units held by Magnolia, taking into account the transactions
discussed in Item 3.

Note 2: Based on 63,512,662 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding and (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding.

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Page 3 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 32,400,677 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 32,400,677 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 32,400,677 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 44.7% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held directly by
Magnolia Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 Common Units, 9,753,425 Common Units held directly 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, a wholly owned subsidiary of Magnolia Holdings, and 618,921 Common Units held by Magnolia, taking into account the transactions discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units outstanding as of
August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units issuable upon the
conversion of Series C Units outstanding.

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Page 4 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 9,577,433 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 9,577,433 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 9,577,433 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 15.6% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 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 and approximately 23% owned by AMID GP Holdings, LLC, a wholly owned subsidiary of Magnolia Holdings, taking into account the transactions discussed in Item 3.

Note 2: Based on 59,987,611 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units outstanding as of August
3, 2017 and (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding.

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Page 5 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 32,400,677 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 32,400,677 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 32,400,677 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 44.7% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held directly by
Magnolia Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 Common Units, 9,753,425 Common Units held directly 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, a wholly owned subsidiary of Magnolia Holdings, and 618,921 Common Units held by Magnolia, taking into account the transactions discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units outstanding as of August
3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units issuable upon the conversion of
Series C Units outstanding.

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Page 6 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 2.6% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 1,349,609 Common Units held by American Midstream GP, LLC, the Issuers general partner.

Note 2: Based on 51,759,787 Common Units outstanding as of August 3, 2017.

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Page 7 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 34,656,431 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 34,656,431 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 34,656,431 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 47.8% (See Note 2)
14.	Type of reporting person PN

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held by Magnolia
Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 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, a wholly owned subsidiary of Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,255,754 Common Units held by Busbar II, LLC, taking into account the transactions
discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units
issuable upon the conversion of Series C Units outstanding.

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Page 8 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 34,656,431 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 34,656,431 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 34,656,431 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 47.8% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held by Magnolia
Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 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, a wholly owned subsidiary of Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,255,754 Common Units held by Busbar II, LLC, taking into account the transactions
discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units
issuable upon the conversion of Series C Units outstanding.

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Page 9 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 34,656,431 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 34,656,431 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 34,656,431 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 47.8% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held by Magnolia
Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 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, a wholly owned subsidiary of Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,255,754 Common Units held by Busbar II, LLC, taking into account the transactions
discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units
issuable upon the conversion of Series C Units outstanding.

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Page 10 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 34,656,431 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 34,656,431 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 34,656,431 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 47.8% (See Note 2)
14.	Type of reporting person OO (Limited Liability Company)

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held by Magnolia
Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 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, a wholly owned subsidiary of Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,255,754 Common Units held by Busbar II, LLC, taking into account the transactions
discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units
issuable upon the conversion of Series C Units outstanding.

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Page 11 of 16 – SEC Filing


CUSIP No. 02752P 100		SCHEDULE 13D


1.	Name of Reporting Person; S.S. or IRS Identification 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 box 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 34,656,431 (See Note 1)
		9.	Sole dispositive power 0
		10.	Shared dispositive power 34,656,431 (See Note 1)
11.	Aggregate amount beneficially owned by each reporting person 34,656,431 (See Note 1)
12.	Check box if the aggregate amount in Row 11 excludes certain shares ☐
13.	Percent of class represented by amount in Row 11 47.8% (See Note 2)
14.	Type of reporting person IN

Note 1: Represents 7,376,568 Series A-1 Convertible Preferred Units (Series A-1 Units) held by High Point
Infrastructure Partners, LLC (High Point), convertible into 8,227,824 common units of the Issuer (Common Units), which are indirectly owned by Magnolia Infrastructure Partners, LLC (Magnolia),
3,160,347 Series A-2 Convertible Preferred Units (Series A-2 Units) held by Magnolia, convertible into 3,525,051 Common Units, 8,792,205 Series C Convertible Preferred Units (Series C Units) held by Magnolia
Infrastructure Holdings, LLC (Magnolia Holdings), convertible into 8,925,847 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, a wholly owned subsidiary of Magnolia Holdings, 618,921 Common Units held by Magnolia and 2,255,754 Common Units held by Busbar II, LLC, taking into account the transactions
discussed in Item 3.

Note 2: Based on 72,438,509 Common Units outstanding, on an as-converted basis, which equals the sum of (a) 51,759,787 Common Units
outstanding as of August 3, 2017, (b) 8,227,824 Common Units issuable upon the conversion of Series A-1 Units outstanding, (c) 3,525,051 Common Units issuable upon the conversion of Series A-2 Units outstanding and (d) 8,925,847 Common Units
issuable upon the conversion of Series C Units outstanding.

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Page 12 of 16 – SEC Filing

This Amendment No. 20 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
and Amendment No. 19 filed on August 18, 2017 (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). Amendment No. 20 is being filed to report the disposition of certain securities of the Issuer by the reporting persons.

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Page 13 of 16 – SEC Filing

ITEM 3.

Source and Amount of Funds or Other Consideration.

Item 3 is hereby supplemented with respect to the
disposition of securities reported hereby as follows:

On October 2, 2017, pursuant to the terms of the Fifth Amended and Restated Agreement of Limited
Partnership, as amended (the Partnership Agreement), of American Midstream Partners, LP (AMID), AMID exercised its call right to repurchase all of the 2,333,333 outstanding Series D Convertible Preferred Units representing
limited partner interests in AMID (Series D Units) from Magnolia Infrastructure Holdings, LLC for approximately $37.0 million in cash. After the closing date of such redemption, which occurred on October 2, 2017, no Series D Units remain
outstanding.

The foregoing is a summary only and the terms and conditions of the Partnership Agreement are qualified in their entirety by reference to
the Partnership Agreement, which are incorporated by reference into this Schedule 13D as set forth in Item 7 wherever such references and descriptions appear.

ITEM 4.

Purpose of Transaction.

The information set forth or incorporated in Item 3 is incorporated herein by
reference.

ITEM 5.

Interest in Securities of the Issuer.

Item 5 is hereby amended as follows:

(a)-(c) The information contained on the cover pages to this Amendment No. 20 to Schedule 13D and the information set forth and incorporated by reference
in Item 3 are incorporated herein by reference.

ITEM 7.

Material to be Filed as Exhibits.

See the Exhibit Index following the signature pages hereto.

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Page 14 of 16 – SEC Filing

SIGNATURES

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.

Dated: October 11, 2017


AMERICAN MIDSTREAM GP, LLC

/s/ Eric T. Kalamaras
Eric T. Kalamaras
Senior Vice President and Chief Financial Officer

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

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

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Page 15 of 16 – SEC Filing


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

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Page 16 of 16 – SEC Filing

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.		Fifth Amended and Restated Agreement of Limited Partnership of American Midstream Partners, LP (the Issuer), dated as of April 25, 2016 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on April 29, 2016), as amended by Amendment No. 1, effective as of May 1, 2016 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on June 22, 2016), Amendment No. 2, effective as of October 31, 2016 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on November 4, 2016), Amendment No. 3, effective as of March 8, 2017 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on March 8, 2017), Amendment No. 4, dated May 25, 2017 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on May 31, 2017), Amendment No. 5 , dated July 14, 2017 and effective as of June 30, 2017 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on July 14, 2017) and Amendment No. 6, dated September 7, 2017 and effective August 31, 2017 (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Issuer on September 17, 2017).

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