UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D
Under the Securities Exchange Act of 1934
(Amendment No. 4)*

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)
May 15, 2015
(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.

 

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, which is 95% owned by High Point.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
7,379,470 (See Note 1)	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
7,379,470 (See Note 1)	
11.		Aggregate amount beneficially owned by each reporting person
 
7,379,470 (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
 
24.5% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”) and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point.
Note 2: Based on 30.136.947 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015 and (c) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
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
 
1,301,282 (See Note 1)	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
1,301,282 (See Note 1)     	
11.		Aggregate amount beneficially owned by each reporting person
 
1,301,282 (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
 
5.4% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC, the Issuer’s general partner (the “General Partner”).
Note 2: Based on 24,058,759 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015 and (b) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
PN	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point. See Item 5.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point. See Item 5.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point. See Item 5.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  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
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
OO (Limited Liability Company)	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point. See Item 5.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

SCHEDULE 13D

CUSIP NO. 02752P 100			

  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)   x
 	
  3.		SEC use only
 	
  4.		Source of funds
 
WC    	
  5.		Check box if disclosure of legal proceedings is required pursuant to Item 2(d) or 2(e)
 
¨     	
  6.		Citizenship or place of organization
 
United States	
Number of
Shares
Beneficially
Owned by
Each
Reporting
Person
With:
 		  7. 		Sole voting power
 
0    	
		  8.		Shared voting power
 
8,554,981 (See Note 1)    	
		  9.		Sole dispositive power
 
0    	
		10.		Shared dispositive power
 
8,554,981 (See Note 1)    	
11.		Aggregate amount beneficially owned by each reporting person
 
8,554,981 (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
 
27.3% (See Note 2)	
14.		Type of reporting person
 
IN	
Note 1: Represents 6,078,188 Series A-1 Convertible Preferred Units (“Series A-1 Units”) held by High Point Infrastructure Partners, LLC (“High Point”), which is indirectly owned by Magnolia Infrastructure Partners, LLC (“Magnolia”), 1,175,511 Series A-2 Convertible Preferred Units (“Series A-2 Units”) held by Magnolia and 1,301,282 Series B Convertible Preferred Units (“Series B Units”) held by American Midstream GP, LLC (the “General Partner”), which is 95% owned by High Point. See Item 5.
Note 2: Based on 31,312,458 common units of the Issuer (“Common Units”) outstanding, on an as-converted basis, which equals the sum of (a) 22,757,477, the number of Common Units outstanding as of May 8, 2015, as reported in the Issuer’s Quarterly Report on Form 10-Q filed on May 11, 2015, (b) 6,078,188 Common Units issuable upon the conversion of Series A-1 Units outstanding as of May 15, 2015, (c) 1,175,511 Common Units issuable upon the conversion of Series A-2 Units outstanding as of May 15, 2015 and (d) 1,301,282 Common Units issuable upon the conversion of Series B Units outstanding as of May 15, 2015, taking into account the distributions discussed in Item 3.

This Amendment No. 4 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 and Amendment No. 3 filed on April 3, 2015 (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 3.	Source and Amount of Funds or Other Consideration.	
Item 3 is hereby supplemented as follows:
On May 15, 2015, the Issuer paid quarterly in-kind distributions on the Series A-1 Convertible Preferred Units held by High Point Infrastructure Partners, LLC (“HPIP”) in the amount of 168,839 Series A-1 Convertible Preferred Units, the Series A-2 Convertible Preferred Units held by Magnolia Infrastructure Partners, LLC (“Magnolia”) in the amount of 32,653 Series A-2 Convertible Preferred Units and the Series B Convertible Preferred Units held by American Midstream GP, LLC (the “General Partner”) in the amount of 23,510 Series B Convertible Preferred Units, in accordance with the terms and conditions of the Fourth Amended and Restated Agreement of Limited Partnership of the Issuer dated March 30, 2015.
 
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 Schedule 13D are incorporated herein by reference.
ArcLight Capital Holdings, LLC (“ArcLight Holdings”) is the sole manager and member of ArcLight Capital Partners, LLC (“ArcLight Partners”). ArcLight Partners is the investment adviser to ArcLight Energy Partners Fund V, L.P. (“Fund V” and together with ArcLight Holdings and ArcLight Partners, the “ArcLight Entities”). ArcLight Holdings is the manager of the general partner of Fund V. Mr. Revers is a manager of ArcLight Holdings and a managing partner of ArcLight Partners and has certain voting and dispositive rights as a member of ArcLight Partners’ investment committee. Fund V indirectly owns Magnolia and, through indirectly controlled subsidiaries, Fund V owns approximately 90% of the ownership interest in HPIP, which in turn owns 95% of the General Partner. As a result, the ArcLight Entities and Mr. Revers may be deemed to indirectly beneficially own the securities of the Issuer held by Magnolia, HPIP and the General Partner, but disclaim beneficial ownership except to the extent of their respective pecuniary interests therein.
(d)-(e) Not applicable.
 
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 my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.
Dated: May 19, 2015

AMERICAN MIDSTREAM GP, LLC	
		
		/s/ Daniel Campbell	
		Daniel Campbell	
		Chief Financial Officer	

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	

ARCLIGHT ENERGY PARTNERS FUND V, L.P.	
			
		By:		ArcLight PEF GP V, L.P.	
				its General Partner	

		By:		ArcLight Capital Holdings, LLC	
				its Manager	

/s/ Daniel R. Revers	
Daniel R. Revers, Managing Partner	

ARCLIGHT PEF GP V, LLC	
			
		By:		ArcLight Capital Holdings, LLC	
				its Manager	

		/s/ Daniel R. Revers	
		Daniel R. Revers, Managing Partner	

			
ARCLIGHT CAPITAL HOLDINGS, LLC	
		
		/s/ Daniel R. Revers	
		Daniel R. Revers, Managing Partner	

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. 3 to Schedule 13D filed by the reporting persons on April 2, 2015)