Patent Publication Number: US-8122964-B2

Title: Subsea stack alignment method

Description:
BACKGROUND 
     1. Field of the Disclosure 
     Embodiments disclosed herein relate generally to joining subsea stack assemblies. In particular, embodiments disclosed herein relate to methods to design and assemble interchangeable subsea stack assemblies. 
     2. Background Art 
     Well control is an important aspect of oil and gas exploration. When drilling a well in, for example, oil and gas exploration applications, devices must be put in place to prevent injury to personnel and equipment associated with the drilling activities. One such well control device is known as a blowout preventer (BOP). 
     Blowout preventers are generally used to seal a wellbore. For example, drilling wells in oil or gas exploration involves penetrating a variety of subsurface geologic structures, or “layers.” Each layer generally comprises a specific geologic composition such as, for example, shale, sandstone, limestone, etc. Each layer may contain trapped fluids or gas at different formation pressures, and the formation pressures increase with increasing depth. The pressure in the wellbore is generally adjusted to at least balance the formation pressure by, for example, increasing a density of drilling mud in the wellbore or increasing pump pressure at the surface of the well. 
     There are occasions during drilling operations when a wellbore may penetrate a layer having a formation pressure substantially higher than the pressure maintained in the wellbore. When this occurs, the well is said to have “taken a kick.” The pressure increase associated with the kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high pressure kick tends to propagate from a point of entry in the wellbore uphole (from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. These “blowouts” often result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and in substantial injury or death of rig personnel. 
     Because of the risk of blowouts, blowout preventers are typically installed at the surface or on the sea floor in deep water drilling arrangements so that kicks may be adequately controlled and “circulated out” of the system. Blowout preventers may be activated to effectively seal in a wellbore until active measures can be taken to control the kick. There are several types of blowout preventers, the most common of which are annular blowout preventers and ram-type blowout preventers. 
     Annular blowout preventers typically comprise annular elastomer “packers” that may be activated (e.g., inflated) to encapsulate drill pipe and well tools and completely seal the wellbore. A second type of the blowout preventer is the ram-type blowout preventer. Ram-type preventers typically comprise a body and at least two oppositely disposed bonnets. The bonnets are generally secured to the body about their circumference with, for example, bolts. Alternatively, bonnets may be secured to the body with a binge and bolts so that the bonnet may be rotated to the side for maintenance access. 
     Interior of each bonnet contains a piston actuated ram. The functionality of the rams may include pipe rams, shear rams, or blind rams. Pipe rams (including variable bore rams) engage and seal around the drill pipe or well tool left in the wellbore, leaving the engaged objects intact. In contrast, shear rams engage and physically shear the drill pipe or well tools left in the wellbore. Similarly, blind rams engage each other and seal off the wellbore when no drill pipe or well tools are in the wellbore. The rams are typically located opposite of each other and, whether pipe rams, shear rams, or blind rams, the rams typically seal against one another proximate a center of the wellbore in order to seal the wellbore. 
     As such, many oil and gas bearing formations lie beneath large bodies of water. Producing wells extending into these formations are equipped with subsea wellheads and other underwater installations which rest at the ocean or sea floor. As such, it is customary to provide blowout protection and other related functions during subsea drilling operations. As such subsea blowout preventer installations may be equipped with numerous and varied types of valves, rams, and other operating controls that may be hydraulically, electro-mechanically, or electro-hydraulically operated to control wellbore fluids. 
     In shallow water, many subsea blowout preventer and flow control installations are controlled hydraulically. These all-hydraulic systems may include a bundle of hydraulic hoses and control lines extending between the surface and subsea facilities. Alternatively, individual hoses may supply hydraulic power from the surface to the subsea installation to monitor the status of the subsea equipment and perform control operations. Advantageously, these systems are simple, reliable, and inexpensive for relatively short hose lengths (i.e., water depths) although response time may be slow. However, in deep-water installations, the response time for a hydraulic system increases and its reliability decreases. 
     In response to the demands of deep-water subsea environments, electro-hydraulic systems were introduced to improve the performance of traditional hydraulic systems in deep water or over long distances. As such, an electro-hydraulic subsea control cable may employ a multiplex (MUX) hose in which several hydraulic control signals may be multiplexed (e.g., through digital time division) and transmitted. The multitude of signals may then be separated out at the end of the multiplex hose and used to manipulate valves in a control pod of a blowout preventer or another subsea component. While a multiplex umbilical line may be a hydraulic hose, it should be understood that an electrical line may also serve as a multiplexing conduit. 
     Blowout preventer stacks are typically custom fit during an initial assembly process, which requires the stack assemblies to be test fit together at the surface prior to installation subsea. Further, the current method of manufacture may produce assemblies requiring a custom fit which are not interchangeable. For example, many feed-thrus or interconnects, such as the hydraulic feed-thrus and/or electro-hydraulic cables, between a Lower Marine Riser Package (“LMRP”) and a Lower Stack, require adjustability upon assembly. 
     Accordingly, there exists a need for a stack assembly having complete interchangeability between an LMRP assembly and any Lower Stack assembly of the same design, without the need for any manual adjustments on the surface. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, embodiments disclosed herein relate to a method to interchangeably connect a plurality of Lower Marine Riser Packages with a lower BOP stack including engaging a Lower Marine Riser Package connector of the Lower Marine Riser Package, with a Lower Stack mandrel connector of a Lower Stack, thereby aligning the Lower Marine Riser Package and the Lower Stack axially about a vertical axis, engaging at least one ring alignment pin of the Lower Marine Riser Package with at least one alignment plate of the Lower Stack, thereby rotationally aligning the Lower Marine Riser Package and the Lower Stack within a specified angle about the vertical axis, and engaging feed-thru connections between the Lower Marine Riser Package and the Lower Stack. 
     In another aspect, embodiments disclosed herein relate to a method to design interchangeability between assemblies in a blowout preventer stack including providing over-sized mounting holes to receive critical components, establishing at least a first reference point on a first component, and calculating the locations of multiple feed-thru connections in the riser stack from the first reference point, allowing at least a first half of the feed-thru connections to float, self-aligning the first half of the feed-thru connection with a corresponding second half of the feed-thru connection of a second component, and establishing rotational and vertical alignments between the first and second components. 
     In another aspect, embodiments disclosed herein relate to an interchangeable blowout preventer stack including a Lower Marine Riser Package comprising a Lower Marine Riser Package female connector, a Lower Stack comprising a Lower Stack mandrel connector configured to engage and axially align with the Lower Marine Riser Package female connector, at least one ring alignment pin disposed on the Lower Marine Riser Package, at least one alignment plate disposed on the Lower Stack and configured to receive the at least one ring alignment pin, wherein the Lower Marine Riser Package and the Lower Stack are rotationally aligned within a specified angle, at least one final alignment pin disposed on the Lower Marine Riser Package, at least one final alignment pin receiver disposed on the Lower Stack and configured to receive the at least one final alignment pin, and a plurality of feed-thru connections between the Lower Marine Riser Package and the Lower Stack. 
     In another aspect, embodiments disclosed herein relate to a method to interchangeably connect a plurality of Lower Marine Riser Packages with a plurality of lower BOP stacks including using a reference template in constructing the lower BOP stacks and the Lower Marine Riser Packages such that they have aligning interfacing points. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an assembly view of a Lower Marine Riser Package and a Lower Stack in accordance with embodiments of the present disclosure. 
         FIG. 2  is an assembly view of an LMRP connector and a mandrel connector in accordance with embodiments of the present disclosure. 
         FIG. 3  is an assembly view of a ring alignment pin and an alignment plate in accordance with embodiments of the present disclosure. 
         FIGS. 4A-4C  are assembly views of a final alignment pin and a final alignment pin receiver in accordance with embodiments of the present disclosure. 
         FIG. 5  is an assembly view of a choke and kill connection in accordance with embodiments of the present disclosure. 
         FIGS. 6A-6C  are assembly views of a MUX pod wedge and receiver combination in accordance with embodiments of the present disclosure. 
         FIGS. 7A and 7B  are detailed views of a MUX pod receiver in accordance with embodiments of the present disclosure. 
         FIGS. 8A and 8B  are section views of a choke and kill connection before and after hydraulic engagement in accordance with embodiments of the present disclosure. 
         FIG. 9  is a flowchart showing an assembly process of a BOP stack in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, embodiments disclosed herein relate to subsea stack assemblies. In particular, embodiments disclosed herein relate to methods to design and assemble interchangeable subsea stack assemblies. 
     Referring to  FIG. 1 , a conventional subsea BOP stack  50  is shown in accordance with embodiments of the present disclosure. BOP stack  50  includes two main assemblies: a Lower Marine Riser Package (“LMRP”)  100  and a Lower Stack  200 . LMRP  100  may include a flexible riser joint  102  to which a riser (not shown) running up to a floating surface rig is attached, an annular blowout preventer  104  configured to seal an inner bore of LMRP  100 , and multiple feed-thru connections. Lower Stack  200  may include a number of ram-type preventers (not shown) that are used to ensure pressure control of a well, as is well known in the art. The configuration of Lower Stack  200  may be optimized to provide maximum pressure integrity, safety, and flexibility in the event of a well control incident. 
     The ability to close in a well automatically plays an extremely important role in offshore drilling operations, both for safety and environmental reasons. BOP stack  50  first allows the rig to disengage quickly from the riser in the event that dynamic positioning is lost. The loss of the dynamic positioning of the rig may produce a condition in which the rig drifts off location with the riser, LMRP  100 , and Lower Stack  200  still attached. Secondly, BOP stack  50  provides a means for protecting the integrity of the well during and after disconnect, as well as providing a means to protect the environment by preventing the release of drilling fluid or hydrocarbons into the ocean. 
     Designing Interchangeability Between the LMRP and the Lower Stack Assembly 
     BOP stack assemblies are typically custom fit during an initial assembly process before they are installed subsea. Feed-thrus from the Lower Marine Riser Package to the Lower Stack assembly are aligned and fixed so that each LMRP and Lower Stack assembly may unlatch and separate, and then re-mate after separation and continue operation from a drilling vessel. Feed-thrus may include, but are not limited to, choke and kill (“C/K”) lines, hydraulic BOP operating fluid stabs, and MUX pod wedge block and receiver combinations. Conventional methods of manufacture render each stack assembly unique, and by definition, not interchangeable. 
     As defined herein, an interchangeable subsea stack design may allow the LMRP and Lower Stack to be assembled separately without having to first mate them together to make any necessary adjustments. For example, if a production set includes two stacks, “Stack  1 ” and “Stack  2 ,” and both are comprised of two parts “LMRP 1 /LS 1 ” and “LMRP 2 /LS 2 ,” respectively, then LMRP 1  should also be able to mate with LS 2 , and LMRP 2  should be able to mate with LS 1  without any adjustment or intervention. 
     A significant obstacle that must be overcome to accomplish interchangeability is manufacturing the tolerances for the frame structures. All of the critical components (i.e., MUX pod system, choke and kill connections, hot stabs, and alignment system) that are installed on the frame must be aligned within a few thousandths of an inch for them to mate with their corresponding counterparts. Due to the size of the frames, features may only be fabricated within ¼ inch (i.e., ±⅛ inch) in certain embodiments, and standard milling machines may not be large enough to accept the frames. 
     Embodiments disclosed herein overcome this by using a combination of design techniques, which include the following. First, “over-sized” mounting holes on the frames which accept critical components during the assembly process may be used. Every critical component installed onto a stack frame fits into a corresponding opening or mounting hole. Typically, a majority of the stack frames received from various fabricators may have at least one feature or mounting hole that is out of position, which in turn may require added repair or rework of the equipment along with downtime. Standard manufacturing tolerances for a welded structure of this size may be as much as ±⅛ inch. Attempting to hold any tighter tolerance may only make the frames extremely costly and increase the number of repairs. Therefore, as defined herein, the over-sized mounting holes may be configured large enough to allow a certain margin of error when engaging connections. In certain embodiments, the over-sized mounting holes or openings may be over-sized by ½ inch radially or 1 inch diametrically. This provides the ability to position or locate fixed critical components accurately (e.g., within ±0.015 inches) on the frames, in the event that their corresponding mounting holes or openings are not manufactured exactly to print. 
     Next, in order for the fixed critical components to be positioned accurately (i.e., within ±0.015 inches), there should be two points of reference to locate them relative to: a vertical datum axis and a horizontal datum axis. Since the main connectors of a stack assembly, namely an LMRP connector and Lower Stack mandrel connector, are machined components having a known tolerance, their centerline may serve as the vertical datum axis. Further, front edges of the LMRP and Lower Stack frame may serve as the horizontal datum axis. The horizontal datum axis is used to locate the fixed components rotationally about the vertical axis, and ensures that when the LMRP and Lower Stack are mated together, their edges will be parallel to one another. 
     The next design technique involves a “floating” concept between corresponding components to aid in assembly. For the purpose of interchangeability, the term “float” may be defined as the ability of a component to move freely or float within a defined boundary, essentially allowing for some slight “play” between corresponding components. Three or more degrees of freedom may be incorporated into the critical components, such as a floating MUX pod receiver, and a floating choke and kill connector. As used herein, “floating” refers to both translational and rotational movements between mating components. Thus, both may be allowed to translate and rotate about a central axis by a an amount. In certain embodiments, both may allowed to translate off centerline in an XY plane (horizontal) and allowed to rotate approximately about the Z (vertical) axis. This will be described further in the description of the assembly process. One skilled in the art will understand that the amount that the components are allowed to float may vary without departing from the scope of the present embodiments. 
     Finally, another feature which makes interchangeability possible while keeping it cost effective is a precise measurement system. In embodiments disclosed herein, a laser measurement system may be used, as it is ideal for measuring large structures such as the ones used. However, one skilled in the art will understand alternative precise measurement systems available without departing from the scope of the present embodiments. The laser measurement system used with embodiments of the present disclosure may be capable of measuring a 200 foot circle within 0.005 inches, and a 20 foot circle within 0.001 inches. Additionally, the laser measurement system may be used to construct a “blue print” of each stack which may be followed during assembly. The blueprint may allow for a more accurate and reliable manufacturing process, as well as help with mass production of the assemblies. 
     Assembly of the LMRP and Lower Stack 
     Embodiments disclosed herein relate to a method to assemble an LMRP and a Lower Stack assembly subsea without requiring surface adjustments. The components include corresponding features which are “self-aligning,” and which engage each other in a specified manner and sequence until the LMRP and Lower Stack are fully mated and functional. 
     Referring to  FIGS. 1-6  and  9 , a make-up sequence between a LMRP assembly  100  and a Lower Stack  200  is described in accordance with embodiments of the present disclosure. LMRP assembly  100  and Lower Stack  200  may be axially aligned about vertical datum axis  5  and may be longitudinally aligned with horizontal datum axis  10 . Initially, a female LMRP connector  110  of LMRP assembly  100  makes contact with a corresponding mandrel male connector  210  of Lower Stack  200  as shown in  FIG. 2 . The engagement between LMRP connector  110  and mandrel connector  210  aligns LMRP  100  and Lower Stack  200  axially with each other. 
     As shown in  FIG. 2 , there may be a specified distance A of vertical travel remaining before LMRP connector  10  and mandrel connector  210  is fully engaged, as well as a specified distance B of vertical travel remaining before the next component is engaged. In certain embodiments, distance A may be between about 26 and 27 inches, while distance B may be between about 11.5 and 12.5 inches. Those skilled in the art will understand however, that distance A and distance B may vary without departing from the scope of embodiments of the present disclosure. 
     In some embodiments, as the make-up sequence continues between LMRP assembly  100  and Lower Stack  200 , an alignment ring pin  120  of LMRP assembly  100  may engage an alignment plate  220  of Lower Stack  200  as shown in  FIG. 3 . The engagement between alignment ring pin  120  and alignment plate  220  may pre-align LMRP assembly  100  and Lower Stack  200  rotationally within about a ½ degree (about Z axis). This pre-alignment may allow the next components, which may include a final alignment pin  130  and a final alignment pin receiver  230 , to be put into proper position and engage one another further along in the make-up sequence. At this stage, final alignment pin  130  and final alignment pin receiver  230  may have a distance C remaining before engagement. In certain embodiments, distance C may be between about 9.5 and 10.5 inches. Additionally, LMRP connector  110  may have further engaged with mandrel connector  210 . The distance A remaining before the LMRP assembly  100  and Lower Stack  200  are fully engaged may be reduced, and may now be between about 14 and 15 inches. 
     Referring now to  FIG. 4A-C , in some embodiments, the make-up sequence may continue as final alignment pin  130  mates with final alignment pin receiver  230 . Upon this engagement, LMRP assembly  100  and Lower Stack  200  may be rotationally aligned with each other, and only a small distance D of vertical travel may remain before the last component, which is a floating choke and kill connection  140 ,  240  engages. In certain embodiments, distance D may be between about 1 and 2 inches. Further, the distance A remaining before LMRP assembly  100  and Lower Stack  200  are fully engaged may be reduced to between about 4 and 5 inches. 
       FIG. 5  shows an initial engagement of a critical component between LMRP assembly  100  and Lower Stack  200 , which is floating choke and kill connection  140 ,  240 . At this point, the vertical distance A remaining before the fully mated condition between LMRP assembly  100  and Lower Stack is reduced to between about 2.5 and 3.5 inches. The initial engagement between a male connector body  140  and a female C/K bucket  240  pre-aligns the component within about 1/16 inch, however, a final alignment between the two is carried out once they are hydraulically engaged, which will be described later. As stated, the connection is a “floating” connection, thus connector body  140  is able to move freely in the XY plane up to about ¾ inch off an axial centerline, and it also may rotate freely about the X and Y axis approximately 1 degree of the vertical Z axis. 
     At this stage, LMRP connector  110  may “bottom out” on mandrel connector  210  leaving a gap between LMRP  100  and Lower Stack  200  of approximately 2 inches. LMRP connector  110  may then be hydraulically engaged and locked to mandrel connector  210  with a BOP hydraulic system, as will be understood by those skilled in the art. LMRP  100  and Lower Stack  200  are considered to be fully engaged at this stage; however Lower Stack  200  is not fully functional until critical components including MUX pod wedge and receiver  150 ,  250 , and choke and kill connections  140 ,  240  are hydraulically engaged. 
     The critical components of the assembly include the MUX pod wedge and receiver combination, and the choke and kill (C/K) feed-thrus. Proper engagement of these critical components is necessary to allow them to provide the proper functionality and allow communication between LMRP assembly  100  and Lower Stack  200 , as they are used to control or manipulate various valves in the BOP assembly during operation. Further, proper engagement between the critical components is important so as to prevent damage to the critical components during engagement, which could lead to costly repairs and downtime. 
       FIGS. 6A-C  show a MUX pod wedge  150  and floating receiver  250  in both retracted ( FIG. 6A ) and extended ( FIG. 6B ) positions in accordance with embodiments of the present disclosure. A hydraulic cylinder  156  pushes wedge  150  downward along guide rails  152 . As wedge  150  travels downward, extensions  154  mounted on a bottom face contact alignment pins  252  mounted on receiver  250  causing the floating receiver  250  to align itself with wedge  150 . In certain embodiments, floating receiver  250  rests on a support plate  254  with no fasteners, allowing it to float. As shown in  FIG. 6C , receiver  250  may move freely in any direction on the XY plane, up to about ¾ inch off centerline, which allows for angular misalignment between wedge  150  and receiver  250 . 
     Referring to  FIG. 7A , a detailed section view of floating receiver  250  is shown in accordance with embodiments of the present disclosure. Receiver  250  “floats” on a set of springs  254  that are fastened to a spring frame  256 . Spring frame  256  is held in place between a support block  258  and a support plate  260  which are fastened together, and is free to move in any direction in the XY plane up to ¾ inch off centerline as previously mentioned. Further, receiver  250  may have about ¾ inch downward vertical travel (−Z direction) and may rotate about the X or Y axis to compensate for any angular misalignment between itself and wedge  150 .  FIG. 7B  shows a perspective view of floating receiver  250  in accordance with embodiments of the present disclosure. In addition, there may be a receiver plate  262  attached to the bottom of receiver  250 , which is configured to accept an additional guide pin (not shown) which is fixed to the center of wedge  150 . When wedge  150  is lowered into place, the guide pin makes contact with an opening in receiver plate  262 , thus causing receiver  250  to align itself with wedge  150 . 
     Referring to  FIG. 8 , section views of choke and kill (C/K) connector  140 ,  240  are shown in retracted ( FIG. 8A ) and extended ( FIG. 8B ) positions in accordance with embodiments of the present disclosure. Initially, female C/K connector  140  is aligned in C/K bucket  242 , after which piston  142  of female C/K connector  140  is extended and aligns over male C/K connector  240  (shown in  FIG. 8B ). Female C/K connector  140  is mounted to the LMRP frame  100  with a spring loaded spherical thrust bearing system  250 . Female connector  140  may be free to move in any direction in the XY plane up to about ¾ inch off centerline. The spherical bearing  250  may also allow connector  140  to rotate about the X and Y axis approximately 1 degree off the vertical Z axis as shown by rotation path  145 . 
     As previously mentioned, after fully engaging MUX pod wedge and receiver  150 ,  250 , and C/K connector  140 ,  240 , LMRP  100  and Lower Stack  200  are in communication with each other and may be considered fully functional. In the event that they should be separated, the critical components may first be disengaged and prepared for separation, followed by separation of LMRP  100  and Lower Stack  200 . Further, if the need arises, either LMRP  100  or Lower Stack  200  may be removed and replaced with another interchangeable LMRP  100  or Lower Stack  200 , of which the assembly will follow the procedure as outlined above. 
     Advantageously, embodiments of the present disclosure may allow the LMRP and the Lower Stack to be assembled separately without having to first be mated together for adjustments. The elimination of a unique and individual design for each assembly may allow a mass production of the assemblies because of their interchangeability. The ability to mass produce such assemblies may further lead to increased productivity of the assemblies and/or efficiency of manufacturing the assemblies. The increased efficiency of mass producing the interchangeable LMRP and Lower Stack assemblies may lead to decreased production costs. Further, interchangeable LMRP and Lower Stack assemblies may provide fewer occurrences of misfit features, which leads to costly rig downtime and multiple trips to and from the surface when installing the assemblies, 
     While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure as described herein. Accordingly, the scope of the disclosure should be limited only by the attached claims.