WO2016044278A1

WO2016044278A1 - Modular, retrievable valve packs for blowout preventer multiplexer controls

Info

Publication number: WO2016044278A1
Application number: PCT/US2015/050193
Authority: WO
Inventors: Christopher Lance KALINEC; Chad Eric YATES
Original assignee: Hydril USA Distribution LLC
Priority date: 2014-09-15
Filing date: 2015-09-15
Publication date: 2016-03-24
Also published as: US20160076331A1

Abstract

Retrievable sub-pods (400) for use in a blowout preventer (BOP) stack (100), including systems of sub-pods (400) and methods for use, are disclosed. The sub-pod (400) includes a valve assembly (300), wherein the valve assembly (300) includes a solenoid (302), an electronics interface (304), and a directional control valve (308). The sub-pod (400) further includes a modular valve pack (310) and a controller (410), wherein the modular valve pack (310) comprises a manifold (312), and wherein the manifold (312) is operable to support the valve assembly (300) within the manifold (312).

Description

MODULAR, RETRIEVABLE VALVE PACKS FOR BLOWOUT PREVENTER

MULTIPLEXER CONTROLS

RELATED PATENT APPLICATIONS

[0001] This application is a non-provisional application and claims priority to and the benefit of U.S. Provisional Patent Application No. 62/050,617, filed on September 15, 2014, incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field

[0002] The field of invention relates generally to blowout preventer (BOP) equipment for use in oil and gas production, and specifically to BOP multiplexer (MUX) control systems.

2. Description of the Related Art

[0003] BOP systems are hydraulic systems, used to prevent blowouts from subsea oil and gas wells. BOP equipment typically includes a set of two or more redundant control systems with separate hydraulic pathways to operate a specified BOP function. The redundant control systems are commonly referred to as blue and yellow control pods. Tn known systems, a communications and power cable sends information and electrical power to an actuator with a specific address. The actuator in turn moves a hydraulic valve, thereby opening fluid to a series of other valves/piping to control a portion of the BOP.

[0004] At times, the hydraulic elements in each of these redundant systems may fail to operate as intended, and necessitate that the control system switch master controls from one pod to the other. At this point, the drilling operator loses redundancy in the system, because there is no functioning back-up pod. As a result, the operator may be required to suspend operations and pull the BOP stack from the sea floor for costly downtime and repairs.

[0005] One problem with creating redundancy in hydraulic systems is that hydraulic systems are typically hard-plumbed, and are not capable of being readily re-configured or repaired. Due to size and weight constraints, functionality of the control system has been limited in the industry to only the necessary functions, and internal hydraulic redundancy has not been built in to existing systems. [0006] Previous methods for addressing system redundancy include having multiple back-up systems. Remotely operated vehicles (ROV's) and acoustic control systems have been used as back-ups; they, however, require a different controls interface and often lead to a degradation in system performance. Thus, they are often a method of last resort

SUMMARY

[0007J As disclosed, the present invention includes systems of and methods for using modular "sub-pods" to decentralize one or more BOP stack control pods. The present technology consists of a modular, retrievable valve pack array that comprises one or more sub-pods. As used herein, a sub-pod is a unit consisting of any number of desired control functions. The sub-pod(s) can be located anywhere on a BOP stack or lower marine riser package (LMRP), and can include a hydraulic supply, communications, and an electrical power supply to function. Any number of sub-pods can be used, individually or together, to provide redundancy to a control system.

[0008] In one example embodiment, the technology works by configuring an array of valves into discrete manifold modules. These discrete modules can then be combined and arranged to control any number of valves.

[0009] The technology described herein provides solutions to problems faced by known systems having one or more centralized control pods. For example, some known systems require excessive tubing running from one central hub throughout the BOP to various valves and other components. The present technology solves this problem by integrating the valve assembly (e.g., solenoids, pilot stage, and directional control valves) into valve manifolds, and allowing the modules to be split into sub-pods that have a sufficient level of on board components to function with just power, communications and hydraulic fluid being supplied. The present technology also reduces or eliminates problems associated with water hammer. Water hammer is associated with pilot stage plumbing issues, regulator chatter, and instability. Elimination of these problems can be accomplished by replacing current regulators with closed-loop, controlled, electro-hydraulic mechanisms located at each sub- pod.

[0010] The present technology provides additional advantages, including: (1) modularity and scalability - for example, any number of sub-pods may be used, and functions can be added as required; (2) reduction in valve quantity - for example, the sub-pod valves can be configured to operate more than one function by utilizing a master spool valve to toggle circuits, or using 4-way 2-position control valves; and (3) redundancy - for example, any number of sub-pods may be added without affecting the central control, power and communication hardware.

[0011] Therefore, disclosed herein is a retrievable sub-pod for use in a blowout preventer (BOP) stack. The retrievable sub-pod includes a valve assembly, wherein the valve assembly comprises a solenoid, an electronics interface, and a directional control valve; a modular valve pack, wherein the modular valve pack comprises a manifold, and wherein the manifold is operable to support the valve assembly within the manifold; and a controller, wherein the controller is operable to control a state of the valve assembly for carrying out functions in the BOP stack, wherein the sub-pod can be docked subsea within the BOP stack, and wherein the sub-pod can be removed from the BOP stack subsea and replaced in the BOP stack subsea by an alternative sub-pod.

[0012] In some embodiments, the valve assembly comprises a pilot stage. In other embodiments, the sub-pod comprises more than one modular valve pack, the modular valve packs being stacked and separable subsea. Still other embodiments include a connection interface for connecting the retrievable sub-pod with hydraulic fluid, electronic communications, and power provided from the BOP stack. In certain embodiments, the connection interface includes an electronic communications connection selected from the group consisting of: a controller area network vehicle bus (CANbus) and a Modbus. Still in yet other embodiments, the sub-pod further includes an electro-hydraulic (EH) closed-loop controlled regulator.

[0013] Certain embodiments further include a hydraulic connection wedge and plumbing operable to dock the sub-pod to the BOP stack and to distribute hydraulic fluid to components in the BOP stack. Still in other embodiments, pressure surrounding the solenoid is controlled by a pressure control device selected from the group consisting of: a pressure compensated dielectric fluid and a pressure-controlled enclosure.

[0014] Further disclosed herein is a decentralized BOP stack system. The system includes a lower marine riser package (LMRP) portion; a lower stack portion, wherein the LMRP portion is disposed above the lower stack portion; at least two retrievable sub-pods, wherein each retrievable sub-pod independently comprises: a valve assembly; a modular valve pack, wherein the modular valve pack comprises a manifold, and wherein the manifold is operable to support the valve assembly within the manifold; and a controller, wherein the controller is operable to control a state of the valve assembly for carrying out functions in the BOP stack system, wherein each sub-pod can be docked subsea within the BOP stack system, and wherein each sub-pod can be removed from the BOP stack system subsea and replaced in the BOP stack system subsea by an alternative sub-pod.

[0015] In certain embodiments of the system, the at least two retrievable sub-pods are disposed within the LMRP portion of the BOP stack system. In some embodiments, at least one sub-pod is disposed within the lower stack portion of the BOP stack system. Still in other embodiments, the at least two retrievable sub-pods are stacked together. In some embodiments, an introduction of the at least two retrievable sub-pods to the system reduces a required amount of plumbing to operate the decentralized BOP stack system relative to the system without the at least two retrievable sub-pods. In certain embodiments, the valve assembly comprises a solenoid, an electronics interface, and a directional control valve.

[0016] Further disclosed herein is a method for decentralizing a BOP control pod in a BOP stack, the method comprising the steps of: integrating at least one sub-pod into the BOP stack, the at least one sub-pod connected to BOP stack hydraulics, communications, and a power supply; modularizing the functions of the BOP control pod by assigning functions of the BOP control pod to the sub-pod; and reducing an amount of tubing disposed in the BOP stack.

[0017] In some embodiments of the method, the step of integrating at least one sub-pod into the BOP stack occurs subsea. In other embodiments of the method, the steps include retrieving the at least one sub-pod subsea from the BOP stack; and replacing the at least one sub-pod subsea with an alternative sub-pod. Still in other embodiments of the method, the steps further include the step of reducing a height of the BOP control pod.

[0018] In some embodiments of the method, the at least one sub-pod comprises: a valve assembly, wherein the valve assembly comprises a solenoid, an electronics interface, and a directional control valve; a modular valve pack, wherein the modular valve pack comprises a manifold, and wherein the manifold is operable to support the valve assembly within the manifold; and a controller, wherein the controller is operable to control a state of the valve assembly for carrying out functions in the BOP stack, wherein the sub-pod can be docked subsea within the BOP stack, and wherein the sub-pod can be removed from the BOP stack subsea and replaced in the BOP stack subsea by an alternative sub-pod.

Still in other embodiments of the method, the steps include operating the controller to translate CANbus data from a central computer to solenoid functions. in certain embodiments, the method includes the step of stacking multiple sub-pods disposed within the BOP stack.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other features, aspects, and advantages of the present disclosure are better understood with regard to the follo wing Detailed Description of the Preferred Embodiments, appended Claims, and accompanying Figures.

[0020] FIG. 1 is a representative system overview of a BOP stack.

[0021] FIG. 2 is a representative diagram of a decentralized sub-pod system in one embodiment of the present disclosure.

[0022] FIGS. 3A and 3B are side view and front view schematics, respectively, showing valve assemblies in one embodiment of the present disclosure.

[0023] FIG. 4 is a schematic showing modular valve packs, which make up an exemplary sub-pod of the present disclosure.

[0024] FIG. 5 is a front-view schematic showing a pilot valve pack used with a sub-plate mounted (SPM) valve pack.

[0025] FIG. 6 is a schematic circuit diagram for the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The Specification, which includes the Summary, Brief Description of the Drawings and the Detailed Description of the Preferred Embodiments, and the appended Claims refer to particular features (including process or method steps) of the disclosure. Those of skill in the art understand that the invention includes all possible combinations and uses of particular features described in the Specificatioa Those of skill in the art understand that the disclosure is not limited to or by the description of embodiments given in the Specification. The inventive subject matter is not restricted except only in the spirit of the Specification and appended Claims.

[0027] Those of skill in the art also understand that the terminology used for describing particular embodiments does not limit the scope or breadth of the disclosure. In interpreting the Specification and appended Claims, all terms should be interpreted in the broadest possible manner consistent with the context of each term. All technical and scientific terms used in the Specification and appended Claims have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs unless defined otherwise.

[0028] As used in the Specification and appended Claims, the singular forms "a," "an," and "the" include plural references unless the context clearly indicates otherwise. The verb "comprises" and its conjugated forms should be interpreted as referring to elements, components or steps in a non-exclusive manner. The referenced elements, components or steps may be present, utilized or combined with other elements, components or steps not expressly referenced. The verb "couple" and its conjugated forms means to complete any type of required junction, including electrical, mechanical or fluid, to form a singular object from two or more previously non-joined objects. If a first device couples to a second device, the connection can occur either directly or through a common connector. "Optionally" and its various forms means that the subsequently described event or circumstance may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0029] The present invention relates to control systems and related methods for components of a subsea blow-out preventer (BOP). Typically, such control systems are hydraulic systems, and include a set of two or more redundant control systems with separate hydraulic pathways to operate a specified BOP function. The redundant control systems are commonly referred to as blue and yellow control pods. In known systems, a communications and power cable sends information and electrical power to an actuator with a specific address. The actuator in turn moves a hydraulic valve, thereby opening fluid to a series of other valves/piping to control a portion of the BOP and/or the BOP supporting equipment

[0030] Referring first to FIG. 1, a representative system overview of a BOP stack is shown. In FIG. 1, a BOP stack 100 is pictured, which includes a lower marine riser package (LMRP) 102 and a lower stack 104. LMRP 102 includes an annular 106, a blue control pod 108, and a yellow control pod 110. A hotline 112, a blue conduit 114, and a yellow conduit 120 proceed downwardly from a riser 122 into LMRP 102 and through a conduit manifold 124 to control pods 108, 110. A blue power and communications line 116 and a yellow power and communications line 118 proceed to control pods 108, 110, respectively. An LMRP connector 126 connects LMRP 102 to lower stack 104. Hydraulically activated wedges 128 and 130 are disposed to suspend connectable hoses or pipes 132, which can be connected to shuttle panels, such as shuttle panel 134.

[0031] Lower stack 104 includes shuttle panel 134, and further includes a casing shear ram BOP 136, a blind shear ram BOP 138, a first pipe ram 140, and a second pipe ram 142. BOP stack 100 is disposed above a wellhead connection 144. Lower stack 104 further includes optional stack-mounted accumulators 146 containing a necessary amount of hydraulic fluid to operate certain functions within BOP stack 100.

[0032] Referring now to FIG. 2, a representative diagram of a decentralized sub-pod system is shown. Sub-pod system 200 has an LMRP portion 202 and a lower stack portion 204. A coupling 206 proceeds between LMRP portion 202 and lower stack portion 204. Coupling 206 can include any one of or any combination of electric communication connections, power connections, and hydraulic connections. LMRP portion 202 includes a first sub-pod 208 and a second sub-pod 210. More or fewer sub-pods can be disposed within LMRP portion 202. Sup-pods 208, 210 can replace components of a single pod, such as, for example, blue control pod 108 or yellow control pod 110 of FIG. 1. Sup-pod 208 is operably coupled to annular BOP 209, and sub-pod 210 is operably coupled to annular BOP 21 1. Sup-pod 208 controls operation of annular BOP 209, and sup-pod 210 is used to control annular BOP 211.

[0033] Lower stack portion 204 includes a sub-pod 212. Sub-pod 212 is in fluid communication with a casing shear ram BOP 236, a blind shear ram BOP 238, a first pipe ram 240, and a second pipe ram 242. More or fewer sub-pods and/or rams can be disposed within lower stack portion 204. Sub-pods 208, 210, and 212 can be controlled by centrally- located remote controls, such as, for example, a personal computer. Sub-pods 208, 210, and 212 advantageously decentralize a single control pod, such that the failure of any one component does not require the replacement of all components. For instance, Sub-pods 208, 210, and 212 are independently retrievable by a remotely operated vehicle (ROV), or similar means, and are independently replaceable and repairable, without replacing all of the sub- pods.

[0034] In the embodiment of FIG. 2, sub-pods 208, 210, 212 individually communicate with a central subsea electronics module, or SEM (not pictured), which in turn communicates with a user on the surface. Electrical connections can be wireless, wet-mate, or hard-wired to the surface. The power/communications (P/C) module in FIG. 2 receives instructions from the user on the surface, or other auxiliary inputs (e.g. an ROV), and via a chosen communications protocol (such as described below with regard to FIG. 4) instructs the appropriate sub-pod's controller, such as controller 410 shown in FIG. 4, to execute a commanded function. A controller, such as controller 410 shown in FIG. 4, translates the instructions into discrete output signals that will power a solenoid or other energy transducer required for the requested function. A sub-pod controller will also determine the required pressure for the requested function (e.g. blind-shear ram (BSR) close, annular BOP close, etc.), and send the appropriate output signal to a closed-loop controlled regulator, such as closed-loop controlled regulator 412.

[0035] Sub-pods 208, 210, and 212 include modular valve packs that can be scaled as required. They are located as required to minimize plumbing and/or achieve other layout goals within LMRP portion 202 and lower stack portion 204. Any number of sub-pods can be used in either LMRP portion 202 or lower stack portion 204 as is required for a number of customer functions and/or required redundancy. Sub-pods 208, 210, and 212 include common connection interfaces for hydraulics, electrical power, and communications.

[0036] For a new BOP stack, plumbing can be customized to suit the layout of the BOP stack with one or more sub-pods. In other words, a sub-pod would be placed where it optimally suits the individual BOP stack layout. For a retrofit of an existing BOP stack, the plumbing might be new from the sub-pods up to the shuttle valves, such as shuttle panel 134 in FIG. 1, but from there the existing plumbing in the BOP stack would be used.

[0037] Referring now to FIGS. 3A and 3B, side view and front view schematics are shown, respectively, providing valve assemblies in one embodiment of the present disclosure. In some embodiments of the present disclosure, three valve components are integrated into a single assembly. FIG. 3 A provides a side view of a valve assembly 300, which includes a solenoid 302, an electronics interface 304, an optional pilot stage 306, and a 3-way, 2- position directional control valve (DCV) 308. In existing systems, a 3 -way, 2-position DCV is a sub-plate mounted (SPM) valve. Solenoid 302 is in a pressure compensated dielectric fluid or a pressure controlled enclosure at about 1 atmosphere (arm). FIG. 3B provides a front view of modular valve pack 310. Manifold 312 can include any number of valves, such as valve assembly 300, in any configuration. For example, there can be two rows and five columns of valves, or there can be ten rows and nine columns of valves.

[0038] Solenoid 302, electronics interface 304, optional pilot stage 306, and 3-way, 2- position directional control valve (DCV) 308 are new components in the sub-pods relative to existing control pods. In some embodiments, the solenoids and pilots would be selected based on the requirements of a BOP system. Electronics interface 304, like in existing pods, houses certain electronics (such as wiring, solenoids, etc.) and can be an oil-filled pressure compensated enclosure, or an air or nitrogen filled enclosure that is structurally suitable for deep subsea environments and the external pressure loads.

[0039] The valves in modular valve pack 310 can be similar to valve assembly 300 shown in FIG. 3A, or can be of a different configuration. Modular valve packs, such as modular valve pack 310, can be stacked together to form any desired size "sub-pod," such as, for example, sub-pods 208, 210, and 212 shown in FIG. 2. As noted previously, any sub-pods are independently retrievable by an ROV or similar device. Similarly, in some embodiments the modular valve packs are independently retrievable by an ROV or similar device. Independently retrievable refers to detaching a single sub-pod from a BOP stack and raising it to the surface, optionally for repairs, while not removing other sub-pods on the BOP stack.

[0040] Referring now to FIG. 4, a schematic is provided showing the modular valve packs that make up an exemplary sub-pod of the present disclosure. A sub-pod 400 includes modular valve packs 402, 404, 406, and 408. Additionally, sub-pod 408 has controller 410 and electro-hydraulic (EH) closed-loop controlled regulator 412. In such a system, valve repair can be done by individual modular valve packs 402, 404, 406, and 408. In other words, modular valve packs 402, 404, 406, and 408 are stackable, unstackable, and retrievable subsea.

[0041] In this way, one defective modular valve pack can be removed from service and/or removed from sub-pod 400. While the defective modular valve pack is being serviced, either by an ROV with the modular valve pack remaining in the sub-pod or at the surface, spare modular valve packs, either already in the sub-pod or placed in the sub-pod, can be used to replace the functions of the defective modular valve pack. Ultimately the sub-pods can be designed to be retrievable subsea by a unit such as an ROV, as could individual modular valve packs 402, 404, 406, 406, controller 410 and closed-loop controlled regulator 412.

[0042] The present technology reduces or eliminates problems associated with water hammer. Water hammer is associated with pilot stage plumbing issues, regulator chatter, and instability. Elimination of these problems can be accomplished by replacing current regulators with closed-loop, controlled, electro-hydraulic mechanisms, such as EH closed- loop controlled regulator 412, located at each sub-pod.

[0043] In the embodiment of FIG. 4, sub-pod 400 has each of the following connections: at least one 1.5 inch hydraulic pipe connection, at least one electronic communications connection, and at least one power connection. The communications connection can be any one of or any combination of a controller area network vehicle bus (CANbus) and a Modbus. Controller 410 is used to translate CANbus data from a central computer to the solenoid functions, such as, for example, solenoid 302 in FIG. 3A. Controller 410 is also used control regulators to any desired output pressures.

[0044] Sub-pod 400 also includes a wedge 414 and plumbing 416, similar to hydraulically activated wedges 128, 130 and connectable hoses or pipes 132 shown in FIG. 1. Wedge 414 is a hydraulic connection interface used to dock sub-pod 400 and distribute hydraulic fluid to components in a BOP stack. Any other suitable hydraulic connection can also be used. Plumbing 416 distributes hydraulic fluid to various functions on the BOP stack, such as the rams, for example casing shear ram BOP 136, blind shear ram BOP 138, first pipe ram 140, and second pipe ram 142 shown in FIG. 1.

[0045] Referring now to FIG. 5, a front-view schematic is provided showing a pilot valve pack used with a sub-plate mounted (SPM) valve pack. A system 500 includes a pilot valve pack 502 and an SPM valve pack 504. As noted previously, FIG. 3A provides a side view of valve assembly 300, which includes solenoid 302, electronics interface 304, an optional pilot stage 306, and 3-way, 2-position directional control valve (DCV) 308. In some embodiments of the present disclosure, a system such as system 500 is replaced by, integrated with, or repackaged according to the embodiments of FIGS. 3 A and 3B. [0046] Such replacement, integration, or repackaging can be used to reduce the height of the system 500 (shown as distance "D" in FIG. 5), and can be used to modularize a BOP control pod stack. In some embodiments, the distance D can be about 92 inches, and modularization of the system 500 including pilot valve pack 502 and SPM valve pack 504 reduces the distance D to less than about 92 inches. Modularization allows for removal and replacement of one module, rather than pulling the entire stack for repair or replacement. Additionally, replacement, integration, or repackaging can be used to eliminate tubing which runs from pilot valve pack 502 to SPM valve pack 504 (shown and described in FIG. 6).

[0047] In certain embodiments, the height of one or more control pods is reduced as valve assemblies of the present disclosure, such as valve assembly 300 in FIG. 3 A, are much smaller compared to the existing equivalent, such as that shown in FIG. 5.

[0048] As discussed above, FIG. 3B provides a front view of modular valve pack 310. Manifold 312 can include any number of valves in any configuration. For example, there can be two rows and five columns of valves, or there can be ten rows and nine columns of valves.

[0049] Referring now to FIG. 6, a schematic circuit diagram is shown for the embodiment of FIG. 5. A circuit 600 includes a solenoid operated pilot valve 602 (also known as a shear seal valve) and an SPM valve 604. Referring also to FIG. 5, solenoid operated pilot valves, such as solenoid operated pilot valve 602, are present in pilot valve pack 502, and SPM valves, such as SPM valve 604, are present in SPM valve pack 504. Solenoid operated pilot valve 602 and SPM valve 604 are fluidly coupled by tubing 606. About four feet of tubing 606 runs between pilot valve pack 502 and SPM valve pack 504 for each function or SPM valve (about 96 functions in some embodiments of BOP control pods). The various functions are mostly actuators of various sizes.

[0050] In embodiments of the present disclosure, tubing between pilot valve packs and SPM valve packs, such as for example tubing 606 of FIG. 6 which is used to connect pilot valve pack 502 to SPM valve pack 504, is not required.

[0051] At an inlet 608, hydraulic fluid is supplied to solenoid operated pilot valve 602 at a pressure of about 3,000 psi. Solenoid operated pilot valve 602 has a vent 610 that can vent excess pressure to the surrounding ocean environment After passing through solenoid operated pilot valve 602, the hydraulic fluid passes through tubing 606. In some embodiments, the max flow rate of tubing 606 can be at least about 250 gallons per minute (gpm), but in other embodiments the max flow rate of tubing 606 can be at least about 500 gpm.

[0052] At an inlet 612, hydraulic fluid is supplied to SPM valve 604 at a pressure of between about 1,500 psi to about 5,000 psi. SPM valve 604 has a vent 614 that can vent excess pressure to the surrounding ocean environment SPM valve 604 can include valve sizes of about 0.5, 1.0, and 1.5 inches in various embodiments of circuit 600. In some embodiments of the present disclosure, solenoid operated pilot valve 602, tubing 606, and SPM valve 604 are combined into one integral package. For example, FIG. 3A provides a side view of valve assembly 300, which includes solenoid 302, electronics interface 304, optional pilot stage 306, and 3-way, 2-position directional control valve (DCV) 308. FIG. 3B provides a front view of modular valve pack 310.

[0053] In some embodiments, cartridge-style valves, such as that shown in FIG. 3A, are placed in modular, manifold arrays, such as that shown in FIGS. 3B and 4. Solenoid stages can be enclosed in oil-filled, pressure-balanced enclosures.

Claims

CLAIMS What is claimed is:

1. A retrievable sub-pod (400) for use in a blowout preventer (BOP) stack ( 100), the retrievable sub-pod (400) characterized by: a valve assembly (300), wherein the valve assembly (300) comprises a solenoid (302), an electronics interface (304), and a directional control valve (308); a modular valve pack (310), wherein the modular valve pack (310) comprises a manifold (312), and wherein the manifold (312) is operable to support the valve assembly (300) within the manifold (312); and a controller (410), wherein the controller (410) is operable to control a state of the valve assembly (300) for carrying out functions in the BOP stack (100), wherein the sub- pod (400) can be docked subsea within the BOP stack (100), and wherein the sub-pod (400) can be removed from the BOP stack (100) subsea and replaced in the BOP stack (100) subsea by an alternative sub-pod (208).

2. The retrievable sub-pod (400) of claim 1, wherein the valve assembly (300) comprises a pilot stage (306).

3. The retrievable sub-pod (400) of claim 1, wherein the sub-pod (400) comprises more than one modular valve pack (310), the modular valve packs being stacked and separable subsea.

4. The retrievable sub-pod (400) of claim 1, further comprising a connection interface for connecting the retrievable sub-pod (400) with hydraulic fluid, electronic communications, and power provided from the BOP stack.

5. The retrievable sub-pod (400) of claim 4, the connection interface comprising an electronic communications connection selected from the group consisting of: a controller area network vehicle bus (CANbus) and a Modbus.

6. The retrievable sub-pod (400) of claim 1, further comprising an electro-hydrauUc (EH) closed-loop controlled regulator (412).

7. The retrievable sub-pod (400) of claim 1, further comprising a hydraulic connection wedge (414) and plumbing (416) operable to dock the sub-pod (400) to the BOP stack (100) and to distribute hydraulic fluid to components in the BOP stack (100).

8. The retrievable sub-pod (400) of claim 1, wherein pressure surrounding the solenoid (302) is controlled by a pressure control device selected from the group consisting of: a pressure compensated dielectric fluid and a pressure-controlled enclosure.

9. A decentralized BOP stack system (200), the system characterized by: a lower marine riser package (LMRP) portion (202); a lower stack portion (204), wherein the LMRP portion (202) is disposed above the lower stack portion (204); at least two retrievable sub-pods (208, 210), wherein each retrievable sub-pod (208, 210) independently comprises: a valve assembly (300); a modular valve pack (310), wherein the modular valve pack (310) comprises a manifold (312), and wherein the manifold (312) is operable to support the valve assembly (300) within the manifold (312); and a controller (410), wherein the controller (410) is operable to control a state of the valve assembly (300) for carrying out functions in the BOP stack system (200), wherein each sub-pod (208, 210) can be docked subsea within the BOP stack system (200), and wherein each sub-pod (208, 210) can be removed from the BOP stack system (200) subsea and replaced in the BOP stack system (200) subsea by an alternative sub-pod (212).

10. The decentralized BOP stack system (200) of claim 9, wherein the at least two

retrievable sub-pods (208, 210) are disposed within the LMRP portion (202) of the BOP stack system (200).

11. The decentralized BOP stack system (200) of claim 9, wherein at least one sub-pod (212) is disposed within the lo wer stack portion (204) of the BOP stack system (200).

12. The decentralized BOP stack system (200) of claim 9, wherein the at least two retrievable sub-pods (208, 210) are stacked together.

13. The decentralized BOP stack system (200) of claim 9, wherein an introduction of the at least two retrievable sub-pods (208, 210) to the system (200) reduces a required amount of plumbing (606) to operate the decentralized BOP stack system (200) relative to the system without the at least two retrievable sub-pods (500).

14. The decentralized BOP stack system (200) of claim 9, wherein the valve assembly (300) comprises a solenoid (302), an electronics interface (304), and a directional control valve (308).

15. A method for decentralizing a BOP control pod (108, 110) in a BOP stack (100), the method characterized by the steps of: integrating at least one sub-pod (400) into the BOP stack (100), the at least one sub-pod (400) connected to BOP stack hydraulics, communications, and a power supply; modularizing the functions of the BOP control pod (108, 110) by assigning functions of the BOP control pod (108, 1 10) to the sub-pod (400); and reducing an amount of tubing (606) disposed in the BOP stack (100).

16. The method of claim 15, wherein the step of integrating at least one sub-pod (400) into the BOP stack (100) occurs subsea.

17. The method of claim 15, further comprising the steps of: retrieving the at least one sub-pod (400) subsea from the BOP stack (100); and replacing the at least one sub-pod (400) subsea with an alternative sub-pod (208).

18. The method of claim 15, further comprising the step of reducing a height of the BOP control pod (108, 110).

19. The method of claim 15, wherein the at least one sub-pod (400) comprises: a valve assembly (300), wherein the valve assembly (300) comprises a solenoid (302), an electronics interface (304), and a directional control valve (308); a modular valve pack (310), wherein the modular valve pack (310) comprises a manifold (312), and wherein the manifold (312) is operable to support the valve assembly (300) within the manifold (312); and a controller (410), wherein the controller (410) is operable to control a state of the valve assembly (300) for carrying out functions in the BOP stack (100), wherein the sub- pod (400) can be docked subsea within the BOP stack (100), and wherein the sub-pod (400) can be removed from the BOP stack (100) subsea and replaced in the BOP stack (100) subsea by an alternative sub-pod (208).

20. The method of claim 19, further comprising the step of operating the controller (410) to translate CANbus data from a central computer to solenoid functions.

21. The method according to claim 15, further comprising the step of stacking multiple sub-pods (208, 210) disposed within the BOP stack.