Archrock Inc. (AROC): Carlson Capital Increase Its Stake; Plans to Go Activist

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Archrock Inc.

(Name of Issuer)

Common Stock, $0.01 par value per share

(Title of Class of Securities)

03957W106

(CUSIP Number)

Carlson Capital, L.P.

Attn: Shahla Ali

2100
McKinney Avenue

Dallas, TX 75201

(214) 932-9600

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

May 4, 2016

(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 Rule 13d-1(g), check the following box.  x

The information required in 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 (the 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).

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This statement on Schedule 13D (this Schedule 13D) relates to shares of
common stock, par value $0.01 per share (the Common Shares), of Archrock Inc., a Delaware corporation (the Issuer). The Issuers principal executive offices are located at 16666 Northchase Drive, Houston, Texas 77060.

(a) This Schedule 13D is filed by: (i) Double Black Diamond Offshore Ltd.,
a Cayman Islands exempted company (DOF); (ii) Black Diamond Relative Value Offshore Ltd., a Cayman Islands exempted company (ROF); (iii) Black Diamond Relative Value Cayman, L.P., a Cayman Islands exempted limited
partnership (RVC); (iv) Black Diamond Offshore Ltd., a Cayman Islands exempted company (OFF); (v) Black Diamond SRI Offshore Ltd., a Cayman Islands exempted company (SRI); (vi) Black Diamond Energy
L/S Offshore Ltd., a Cayman Islands exempted company (ENO); (vii) Black Diamond Thematic Offshore Ltd., a Cayman Islands exempted company (TOF and, together with DOF, ROF, RVC, OFF, SRI and ENO, the Funds);
(viii) Worldwide Transactions Ltd., a British Virgin Islands limited corporation (the Managed Account); (ix) Carlson Capital, L.P., a Delaware limited partnership (Carlson Capital); (x) Asgard Investment Corp.
II, a Delaware corporation and the general partner of Carlson Capital (Asgard II); (xi) Asgard Investment Corp., a Delaware corporation and the sole stockholder of Asgard II (Asgard I); and (xii) Clint D. Carlson,
an individual (Mr. Carlson and, together with the Funds, the Managed Account, Carlson Capital, Asgard II and Asgard I, the Reporting Persons). The name, citizenship, present principal occupation or employment and business
address of each director and executive officer of Asgard I and Asgard II are set forth in Appendix A attached hereto.

(b) The principal business address
of each of the Reporting Persons is 2100 McKinney Avenue, Suite 1800, Dallas, TX 75201.

(c) The principal business of the Funds and the Managed Account
is to invest in securities. The principal business of Carlson Capital is to serve as the investment manager to the Funds and to certain managed accounts, including the Managed Account. The principal business of Asgard II is serving as the general
partner of Carlson Capital. The principal business of Asgard I is serving as the sole stockholder of Asgard II. The principal occupation of Mr. Carlson is serving as President of Asgard II, Asgard I and Carlson Capital.

(d) None of the Reporting Persons or persons listed on Appendix A has, during the last five years, been convicted in a criminal proceeding (excluding traffic
violations or similar misdemeanors).

(e) None of the Reporting Persons or persons listed on Appendix A has, during the last five years, been a party to a
civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceedings was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject
to, federal or state securities laws or finding any violation with respect to such laws.

(f) Mr. Carlson is a citizen of the United States. The
places of organization of each of the other Reporting Persons are listed in paragraph (a) of this Item 2.

The Reporting Persons used approximately $108,408,100
(including brokerage commissions) in the aggregate to purchase the Common Shares reported in this Schedule 13D.

The source of the funds used to acquire
the Common Shares reported herein is the working capital of the Funds and the Managed Account and margin borrowings described in the following sentence. Such Common Shares are held by the Reporting Persons in commingled margin accounts, which may
extend margin credit to the Reporting Persons from time to time, subject to applicable federal margin

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regulations, stock exchange rules and credit policies. In such instances, the positions held in the margin account are pledged as collateral security for the repayment of debit balances in the
account. The margin accounts bear interest at a rate based upon the brokers call rate from time to time in effect. Because other securities are held in the margin accounts, it is not possible to determine the amounts, if any, of margin used to
purchase the Common Shares reported herein.

(a)-(j) The Reporting Persons and their representatives intend, from time
to time, to engage in discussions and correspondence with management and the Issuers Board of Directors (the Board) regarding, among other things, the Issuers business, management and strategic alternatives and
direction. The Reporting Persons believe that the Issuers Common Shares trade at a substantial discount to the Issuers intrinsic value and represent an attractive investment opportunity. The Reporting Persons intend to have
discussions and correspondence with the Issuers management and the Board to discuss ways in which this undervaluation can be rectified, including among other things, reducing the Issuers expenses, altering the Companys capital
allocation and other changes to the Issuers corporate strategy, including changes to the composition of the Board.

Based on the above discussions
with the Issuer, the Reporting Persons may (i) have discussions regarding the Issuer with other stockholders, persons that may be interested in transactions with the Company, and other interested market and industry participants; (ii) make
additional proposals to the Issuer concerning its business, management, strategic alternatives and direction; (iii) acquire additional Common Shares and/or other equity, debt, notes or other securities, including but not limited to derivative
or other instruments that are based upon or relate to the value of the Common Shares or the Issuer (collectively, Securities) in the open market or otherwise; (iv) dispose of any or all of their Securities in the open market or
otherwise; (v) nominate or recommend candidates to serve on the Board; (vi) propose or consider any one or more of the actions described in subsections (a) through (j) of Item 4 of Schedule 13D; or (vii) change their
intention with respect to any and all matters referred to in this Item 4.

(a), (b) The Reporting Persons may be deemed to beneficially
own in the aggregate 9,258,249 Common Shares. Based upon a total of 69,626,230 Common Shares outstanding as of March 2, 2016, as reported in the Issuers Proxy Statement on Schedule 14A for the Issuers 2016 annual meeting of
stockholders, filed with the Securities and Exchange Commission (the SEC) on March 18, 2016, the Reporting Persons shares represent approximately 13.30% of the outstanding Common Shares.

Carlson Capital, Asgard II, Asgard I and Mr. Carlson may be deemed to beneficially own and have the power to vote and direct the disposition of
(i) the 5,498,738 Common Shares reported herein as owned by DOF, (ii) the 1,262,697 Common Shares reported herein as owned by ROF, (iii) the 237,432 Common Shares reported herein as owned by RVC, (iv) the 367,486 Common Shares
reported herein as owned by OFF, (v) the 31,700 Common Shares reported herein as owned by SRI, (vi) the 1,509,883 Common Shares reported herein as owned by TOF, (vii) the 306,409 Common Shares reported herein as owned by ENO, and
(viii) the 43,904 Common Shares reported herein as owned by the Managed Account.

DOF may be deemed to beneficially own and has the power to vote and
dispose the 5,498,738 Common Shares reported herein as owned by it, which shares represent approximately 7.90% of the outstanding Common Shares.

ROF may
be deemed to beneficially own and has the power to vote and dispose the 1,262,697 Common Shares reported herein as owned by it, which shares represent approximately 1.81% of the outstanding Common Shares.

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RVC may be deemed to beneficially own and has the power to vote and dispose the 237,432 Common Shares reported
herein as owned by it, which shares represent approximately 0.34% of the outstanding Common Shares.

OFF may be deemed to beneficially own and has the
power to vote and dispose the 367,486 Common Shares reported herein as owned by it, which shares represent approximately 0.53% of the outstanding Common Shares.

SRI may be deemed to beneficially own and has the power to vote and dispose the 31,700 Common Shares reported herein as owned by it, which shares represent
approximately 0.05% of the outstanding Common Shares.

TOF may be deemed to beneficially own and has the power to vote and dispose the 1,509,883 Common
Shares reported herein as owned by it, which shares represent approximately 2.17% of the outstanding Common Shares.

ENO may be deemed to beneficially own
and has the power to vote and dispose the 306,409 Common Shares reported herein as owned by it, which shares represent approximately 0.44% of the outstanding Common Shares.

The Managed Account may be deemed to beneficially own and has the power to vote and dispose the 43,904 Common Shares reported herein as owned by it, which
shares represent approximately 0.06% of the outstanding Common Shares.

(c) The Reporting Persons have not effected any transactions in the Common Shares
in the last 60 days.

(d) Except as set forth herein, no person other than the Reporting Persons is known to have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the sale of, Common Shares.

(e) Not applicable.

The
Reporting Persons are parties to an agreement with respect to the joint filing of this Schedule 13D and any amendments thereto. A copy of such agreement is attached hereto as Exhibit 1 and is incorporated by reference herein.

Except as set forth herein, there are no contracts, understandings or relationships among the Reporting Persons or between the Reporting Persons and any other
person with respect to the Common Shares.

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SIGNATURES

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

DATED: May 4, 2016

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Appendix A

DIRECTORS AND EXECUTIVE OFFICERS OF CERTAIN REPORTING PERSONS

The following sets forth the name, position and principal occupation of each director and executive officer of Asgard I and Asgard II,
respectively. Except as otherwise indicated, the business address of each director and officer is 2100 McKinney Avenue, Suite 1800, Dallas, TX 75201. To the best of the Reporting Persons knowledge, except as set forth in this Schedule 13D,
none of the directors or executive officers of Asgard I and Asgard II owns any Common Shares.

Asgard I

Asgard II