Patent Publication Number: US-9422782-B2

Title: Control pod for blowout preventer system

Description:
BACKGROUND 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     In order to meet consumer and industrial demand for natural resources, companies often invest significant amounts of time and money in finding and extracting oil, natural gas, and other subterranean resources from the earth. Particularly, once a desired subterranean resource such as oil or natural gas is discovered, drilling and production systems are often employed to access and extract the resource. These systems may be located onshore or offshore depending on the location of a desired resource. Further, such systems generally include a wellhead assembly through which the resource is accessed or extracted. These wellhead assemblies may include a wide variety of components, such as various casings, valves, fluid conduits, and the like, that control drilling or extraction operations. 
     Subsea wellhead assemblies typically include control pods that operate hydraulic components and manage flow through the assemblies. The control pods may route hydraulic control fluid to and from blowout preventers and valves of the assemblies via hydraulic control tubing, for instance. When a particular hydraulic function is to be performed (e.g., closing a ram of a blowout preventer), a control pod valve associated with the hydraulic function opens to supply control fluid to the component responsible for carrying out the hydraulic function (e.g., a piston of the blowout preventer). To provide redundancy, American Petroleum Institute Specification 16D (API Spec 16D) requires a subsea wellhead assembly to include two subsea control pods for controlling hydraulic components and the industry has built subsea control systems in this manner (with two control pods) for over forty years. This redundant control ensures that failure of a single control pod of a control system does not result in losing the ability to control the hydraulic components of the subsea stack. But such a failure of a single control pod causes the system to no longer comply with API Spec 16D, often leading an operator to shutdown drilling or other wellhead assembly operations until the malfunctioning control pod can be recovered to the surface and repaired. In the case of deep water operations, such recovery and repair can often take days and may cost an operator millions of dollars in lost revenue. Consequently, there is a need to increase the reliability of subsea control systems to reduce downtime and costs of operation. 
     SUMMARY 
     Certain aspects of some embodiments disclosed herein are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below. 
     Embodiments of the present disclosure generally relate to a subsea control system that includes control pods for operating components of a blowout preventer apparatus. The control pods in some instances are installed on a lower marine riser package that can be connected to a blowout preventer stack. A control pod in accordance with one embodiment includes a stack stinger that facilitates connection of the control pod to hydraulic components of the blowout preventer stack. The control pod can also include valves for routing control fluid to the hydraulic components of the blowout preventer stack and a control pod frame having a bottom plate with a central aperture, with the valves mounted within the control pod frame. The stack stinger extends through the central aperture of the bottom plate of the control pod frame and facilitates communication of control fluid from the valves to the hydraulic components of the blowout preventer stack through the stack stinger. Further, in at least one embodiment, the control pod does not include a riser stinger that facilitates communication of control fluid to additional hydraulic components of a lower marine riser package. 
     Various refinements of the features noted above may exist in relation to various aspects of the present embodiments. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. Again, the brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of some embodiments without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of certain embodiments will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  generally depicts a subsea system for accessing or extracting a resource, such as oil or natural gas, via a well in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a block diagram of various components of the stack equipment of  FIG. 1  in accordance with one embodiment; 
         FIG. 3  is a front perspective view of a lower marine riser package having three control pods in accordance with one embodiment of the present disclosure; 
         FIG. 4  is a rear perspective view of the lower marine riser package of  FIG. 3 ; 
         FIG. 5  is a top plan view of the lower marine riser package of  FIGS. 3 and 4 ; 
         FIG. 6  is a front perspective view of one control pod of the lower marine riser package of  FIGS. 3-5  having a stinger in accordance with one embodiment of the present disclosure; 
         FIG. 7  is a rear perspective view of the control pod of  FIG. 6 ; 
         FIG. 8  is another perspective view of the control pod of  FIGS. 6 and 7 ; 
         FIG. 9  is a perspective view of the stinger of the control pod depicted in  FIGS. 6-8 ; 
         FIGS. 10 and 11  are block diagrams generally depicting hydraulic components controlled by a control pod and the extension of the stinger to mate with an adapter of a lower blowout preventer stack in accordance with one embodiment; and 
         FIGS. 12-14  are block diagrams depicting various configurations of control cables for routing instructions to the control pods of a blowout preventer system in accordance with several embodiments. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, any use of “top,” “bottom,” “above,” “below,” other directional terms, and variations of these terms is made for convenience, but does not require any particular orientation of the components. 
     Turning now to the present figures, a system  10  is illustrated in  FIG. 1  in accordance with one embodiment. Notably, the system  10  (e.g., a drilling system or a production system) facilitates accessing or extraction of a resource, such as oil or natural gas, from a well  12 . As depicted, the system  10  is a subsea system that includes surface equipment  14 , riser equipment  16 , and stack equipment  18 , for accessing or extracting the resource from the well  12  via a wellhead  20 . In one subsea drilling application, the surface equipment  14  is mounted to a drilling rig above the surface of the water, the stack equipment  18  (i.e., a wellhead assembly) is coupled to the wellhead  20  near the sea floor, and the riser equipment  16  connects the stack equipment  18  to the surface equipment  14 . 
     As will be appreciated, the surface equipment  14  may include a variety of devices and systems, such as pumps, power supplies, cable and hose reels, control units, a diverter, a gimbal, a spider, and the like. Similarly, the riser equipment  16  may also include a variety of components, such as riser joints, flex joints, fill valves, control units, and a pressure-temperature transducer, to name but a few. The stack equipment  18 , in turn, may include a number of components, such as blowout preventers, that enable the control of fluid from the well  12 . 
     In one embodiment generally depicted in  FIG. 2 , the stack equipment  18  includes a lower marine riser package (LMRP)  22  coupled to a lower blowout preventer (BOP) stack  24 . The lower marine riser package  22  includes control pods  26  for controlling hydraulic components  28  and  30 . The components  28  and  30  perform various hydraulic functions on the stack equipment  18 , including controlling flow from the well  12  through the stack equipment  18 . In the depicted embodiment, the components  30  of the lower blowout preventer stack  24  include hydraulically controlled shear rams  32  and pipe rams  34  (of a ram-type blowout preventer). But it will be appreciated that the stack equipment  18  may include many hydraulic functions that would be performed by the hydraulic components  28  and  30 . By way of example, in various embodiments the hydraulic components  28  and  30  collectively include annular blowout preventers, other ram-type blowout preventers, and other valves to name but a few. The control pods  26  are connected to the components  28  and  30  by suitable conduits (e.g., control tubing or hoses). This allows the control pods  26  to route hydraulic control fluid to the components  28  and  30  to cause these components to perform their intended functions, such as closing the rams of a blowout preventer or opening a valve. 
     Because of the importance of the functions performed by hydraulic components of a wellhead assembly, it has become an industry standard to include two redundant control pods for controlling the hydraulic components of the wellhead assembly. These two redundant control pods are functionally identical (i.e., each of the control pods is capable of independently controlling the same hydraulic functions of the wellhead assembly), and the control pods are distinguishable from backup control systems different from the control pods, such as acoustical control systems, deadman&#39;s switches, and auto-shear systems that provide limited redundancies for only a certain subset of functions controlled by the control pods. 
     Although the control pods may be generally reliable, over time the control pods can fail and lead to shutdown of drilling operations until the source of the malfunction can be identified and repaired. As noted above, such a failure can lead to significant and costly downtime. Although the use of two control pods provides redundancy, it also increases the likelihood that at least one control pod will experience a failure condition that would lead an operator to stop drilling operations. As an example, if each of the two control pods of a blowout preventer system has a reliability rate of 99% over a given time period (i.e., a failure rate of 1%), the chance that at least one or the other of the two control pods would fail is almost twice as high (a system reliability rate of 98.01% and a failure rate of 1.99% over the given time period, wherein system reliability or failure is based on continued, proper functioning of two control pods). Given the costs of such failure, there has been a long-felt need in the industry to increase reliability of control pods and associated systems in a cost-efficient manner. Because the failure rate of a control pod depends on the failure rate of each component, past efforts at increasing reliability have been focused on increasing the reliability of the individual components of a control pod. But control pods include numerous valves and other components, and significantly increasing the reliability of these components can result in components that are greatly increased in size, that are made with more expensive materials or techniques, or both. And as reliability of the control pod depends on the reliability of all of its components, such an increase in size or cost can significantly impact the size and cost of the control pod. 
     Rather than following the trend of increasing efforts to wring out incremental improvements in the reliability of a control pod and its components, embodiments of the present disclosure instead include at least one extra control pod in addition to the typical two control pods. In some embodiments, the at least one extra control pod is functionally identical to the first two control pods (i.e., each of the three control pods controls all of the same hydraulic components). This added layer of redundancy will greatly impact reliability of a blowout preventer system, as the system could continue operations in accordance with API Spec 16D even upon the failure of one of the control pods (or, more generally in the case of a system having more than three control pods, the failure of N−2 control pods, where N is the total number of control pods). 
     The increased reliability of a blowout preventer system with three control pods may be better appreciated with further consideration of the example noted above, in which control pods have a reliability rate of 99% (and a failure rate of 1%) over a given time period. With the additional level of redundancy represented by a third control pod, the system can continue operating in accordance with API Spec 16D even if one of the control pods fails or otherwise malfunctions. As a result, such a blowout preventer system with three control pods would have a reliability rate of 99.9702% and a failure rate of 0.0298% over the given time period (again with system reliability or failure based on continued, proper functioning of two control pods in accordance with API Spec 16D). This represents a significant decrease in the system failure rate (over a 98.5% reduction in the failure rate) compared to the traditional two-pod system, and would substantially reduce costs associated with stoppage of drilling activities associated with malfunctioning systems. 
     One embodiment having such an arrangement with three control pods for controlling hydraulic functions of stack equipment  18  is depicted in  FIGS. 3-5  by way of example. In this embodiment, the lower marine riser package  22  includes not only a pair of redundant control pods  40  and  42  installed on a frame  38 , but also a third redundant control pod  44 . In other arrangements having only two control pods, one of the control pods is typically referred to as a “yellow” control pod while the other is referred to as a “blue” control pod. In the present embodiment, the control pods  40  and  42  may be referred to as yellow and blue pods, respectively, while the third control pod  44  could be referred to by any desired color, such as a “red” pod. In at least some embodiments, the control pods  40 ,  42 , and  44  are functionally identical in that each of the control pods is capable of controlling all of the hydraulic functions that can be controlled by the other control pods. The control pods  40 ,  42 , and  44  can control various numbers of hydraulic functions. In some embodiments, each of the control pods control from 48 to 144 hydraulic functions of the wellhead assembly, and in one embodiment each of the three control pods controls 120 hydraulic functions. In another embodiment, each of the three control pods controls 128 hydraulic functions. The three control pods  40 ,  42 , and  44  represent a blowout preventer control assembly that can be coupled as part of a wellhead assembly. In the presently depicted embodiment, the control assembly includes the lower marine riser package  22  on which the control pods are mounted, but the control pods could also be mounted to a wellhead assembly in some other manner. 
     The depicted lower marine riser package  22  includes a hydraulic component  28  in the form of a connector  46 . The connector  46  enables the lower marine riser package  22  to be landed on and then secured to the lower blowout preventer stack  24 . On an opposite end of the assembly, a riser adapter  48  enables connection of the lower marine riser package  22  to the riser equipment  16  described above. As depicted, the lower marine riser package  22  also includes a flex joint  50  that accommodates angular movement of riser joints of riser equipment  14  with respect to the lower marine riser package  22  (i.e., it accommodates relative motion of the surface equipment  14  with respect to the stack equipment  18 ). The lower marine riser package  26  also includes a hydraulic component  28  in the form of a hydraulically controlled annular blowout preventer  52 . And still further, the lower marine riser package  22  includes a kill line  54  ( FIG. 3 ) and a choke line  58  ( FIG. 4 ). These kill and choke lines  54  and  58  can be connected to the lower blowout preventer stack  24  by respective kill and choke connector assemblies  56  and  60 . 
     An example of one of the control pods installed on the lower marine riser package  22  of  FIGS. 3-5  is depicted in greater detail in  FIGS. 6-8 . Although the control pod depicted in these additional figures is denoted control pod  44 , it is noted that one or both of control pods  40  and  42  is identical to the control pod  44  in at least some embodiments. The control pod  44  includes a frame  72  with a lower section  68  and an upper section  70 . The lower section  68  includes numerous valves for controlling flow of hydraulic control fluid to hydraulic components of the wellhead assembly and the upper section  70  (which may also be referred to as a multiplexing section) includes a subsea electronics module  74  that controls operation of the valves of section  68  based on received command signals. In the depicted embodiment, the lower section  68  includes panels or sub-plates  80 , 82 , and  84  having sub-plate mounted valves  86 . 
     The valves  86  can be connected to the hydraulic components  28  and  30  to control operation of these components. In one embodiment, those valves  86  that control hydraulic components  30  of the lower blowout preventer stack  24  are connected to those components  30  by control tubing routed to a stinger  92  of the control pod  44 . And those valves  86  that control hydraulic components  28  of the lower marine riser package  22  are connected directly to their respective components  28  without being routed through a stinger. The stinger  92  of the present embodiment is a movable stinger that may be extended from and retracted into a shroud  94 . Extension of the stinger  92  from the shroud  94  enables connection of the hydraulic components  30  of the lower blowout preventer stack  24  to their respective control valves  86 . Accordingly, the stinger  92  may also be referred to as a stack stinger. This is in contrast to a riser stinger (not included in the presently depicted embodiment), which would facilitate connection of valves of a control pod to hydraulic components of a lower marine riser package. The shroud  94  protects the stinger  92  during installation of the control pod  44  on the lower marine riser package  22  and during landing of the lower marine riser package  22  on the lower blowout preventer stack  24 . 
     As shown in  FIG. 9 , the stinger  92  includes a fluid distribution hub  100  connected to a plate  102 . In the depicted embodiment, the hub  100  includes four wedge-shaped elements with inlets  106  and outlets  108 . Those valves  86  that control hydraulic components  30  of the lower blowout preventer stack  24  may be coupled (e.g., with hydraulic control tubing) to the inlets  106 , which themselves are connected with the outlets  108  via internal conduits in the hub  100 . When the lower marine riser package  22  is landed on the lower blowout preventer stack  24 , the stingers  92  of the control pods  40 ,  42 , and  44  can be extended to mate with respective adapters (e.g., control pod bases) constructed to route control fluid from the outlets  108  to the hydraulic components  30  of the lower blowout preventer stack  24 . The outlets  108  are depicted as including recessed shoulders for receiving seals to inhibit leaking at the interface between the outlets  108  and the mating adapters that receive the stingers  92 . And in some embodiments, the wedge-shaped pieces of the hub  100  can be driven outwardly into engagement with the mating adapter to promote sealing engagement of the seals against the mating adapter. 
     An example of a control pod  26  having a stinger that can be extended to engage a mating adapter on a lower blowout preventer stack is depicted in  FIGS. 10 and 11 . As described above, components of the lower marine riser package  22  include control pods  26  and hydraulic components  28 , while the lower blowout preventer stack  24  includes hydraulic components  30 . And as shown in  FIGS. 10  and  11 , the lower blowout preventer stack  24  also includes at least one adapter  118  that receives the mating stinger  92  of the control pod  26 . Although  FIGS. 10 and 11  only depict a single control pod  26  and a single adapter  118  for the sake of explanation, it will be appreciated that the lower marine riser package  22  may include a greater number of control pods  26  (e.g., three control pods) and the system may include adapters  118  in sufficient number to receive the control pods. 
     In one embodiment, the valves  86  include lower blowout preventer stack valves  114  for controlling hydraulic components  30  and lower marine riser package valves  116  for controlling hydraulic components  28 . The valves  114  and  116  are controlled by instructions from the subsea electronics module  74 . In the embodiment generally depicted in  FIGS. 10 and 11 , the lower marine riser package valves  116  are coupled directly to the hydraulic components they control (e.g., by hydraulic control tubing) rather than being routed through a riser stinger. In contrast, the lower blowout preventer stack valves  114  are hydraulically coupled to the stinger  92  (e.g., also with hydraulic control tubing). The stinger  92  can be extended from the control pod  26  into the adapter  118 , as generally represented by the downward arrow next to the stinger  92  in  FIG. 11 . In the presently depicted embodiment, the lower blowout preventer stack valves  114  are not only hydraulically coupled to the stinger  92 , but they are also connected with the stinger  92  such that the valves  114  move with the stinger  92  as it is extended or retracted with respect to the control pod  26 . For example, the valves  114  may be installed on one or more panels coupled to move with the stinger  92 , while the valves  116  can be installed on one or more different panels that do not move with the stinger  92 . 
     Various ways of connecting the control pods  26  to a control unit  130  are generally depicted in  FIGS. 12-14  in accordance with certain embodiments. In a control system  128  of  FIG. 12 , for instance, each of the control pods  40 ,  42 , and  44  is connected to the control unit  130  by a respective cable  132 . The control unit  130  can include any suitable equipment (e.g., computers, human-machine interfaces, and networking equipment with appropriate software) for communicating instructions to the control pods  26 . The cables  132  enable command signals (i.e., control instructions) to be sent from the control unit  130  to the control pods  26  (e.g., to the subsea electronic modules  74  of the control pods). In at least some embodiments, the cables  132  are provided on cable reels. The command signals can be sent to the control pods  26  sequentially or redundant command signals can be sent simultaneously to the control pods  26 . In some embodiments, the control system can detect malfunctioning of one of the three control pods  26 . But because the system includes three control pods, drilling operations may continue in accordance with API Spec 16D using the two remaining, non-malfunctioning control pods  26 . 
     While each control pod  26  can be connected to its own cable  132  for receiving instructions, other arrangements could also be used in a given application. For example, the control system  136  of  FIG. 13  includes only two signal cables  138  for passing instructions from the control unit  130  to the control pods  26 . The two cables  138  can first be connected to two of the control pods  26  (here control pods  40  and  42 ). But either of the cables  138  could be disconnected from a control pod (a malfunctioning control pod, for instance) and then reattached to a new control pod, as generally represented by the dashed line  140  in  FIG. 13 . In some instances, this disconnecting and reattaching of the cable  138  could be performed (e.g., by a subsea remote operated vehicle) while the control pods  26  remain installed on the subsea wellhead assembly and while the subsea wellhead assembly remains installed at the subsea well. And as yet another example, the control system  144  of  FIG. 14  includes a pair of cables  146  connected at one end to the control unit  130 . But while one of the two cables  146  is routed through to a control pod  26  (here control pod  44 ), the other of the cables  146  is connected to a distribution point  148  (e.g., a multiplexer), with additional cables  150  connecting the distribution point  148  to the other control pods  26  (here control pods  40  and  42 ). 
     While the aspects of the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. But it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.