Patent Publication Number: US-2016237766-A1

Title: Long Stroke Riser Tensioner System and Wellbay Structure for a Floating Unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/115,994, entitled “Long Stroke Riser Tensioner System and Wellbay Structure for a Floating Production and/or Drilling Unit” and filed Feb. 13, 2015, the contents of which application are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a floating unit or platform and, more specifically, to a semi-submersible floating platform comprising a low-profile wellbay structure and at least one long stroke riser assembly disposed within the wellbay structure, the at least one long-stroke riser assembly comprising at least one long-stroke riser and a respective top-mounted tensioner system for the at least one long-stroke riser for deep water and ultra-deep water field development. 
     BACKGROUND OF THE INVENTION 
     Offshore exploration has been moving into ultra-deepwater, especially for subsalt and lower tertiary reservoirs demanding large topside facilities with drilling capability. Floating system platforms, such as tension-leg platforms (TLPs) and spar platforms, encounter several disadvantages that may limit their use in ultra-deepwater. Due to technical and/or commercial challenges, TLPs are limited by water depth. Spar platforms may be constrained by topsides capacity, and they pose challenges in offshore installation in deep water (e.g., water having a depth between 1500 feet and 5000 feet, inclusive) and ultra-deep water (e.g., water having a depth greater than 5000 feet). 
     Although many floating systems could be used for dry tree solutions, there are substantial challenges to create a feasible and cost-effective solution for deep and ultra-deep water field development with dry trees. For a semi-submersible, an important challenge is the vertical or heave motion characteristics of the floater, which poses a significant impact on riser performance and demand a change of the hull and/or riser system design to compensate top-tensioned riser stroke motion. 
     Some long-stroke riser tensioners with stroke range up to 28 feet have been successfully used on production and drilling platforms for accommodating the vertical or heave motion characteristics of the floater. A 28-foot stroke range may be insufficient in some deep water and ultra-deep water applications. Increasing the stroke range further presents numerous design challenges, such as maintaining a sufficient air-gap in the vessel while not increasing its height too much to minimize the wind force on the vessel, providing stability to the long-stroke riser tensioners, etc. Furthermore, as the tensioner stroke range increases, the problems of cylinder rod loading (in compression with potential buckling), eccentric loading when one cylinder is out of service, trash getting into the cylinder head seals, and lateral side loading on the cylinder rods increase. 
     Floating system platforms, such as tension-leg platforms (TLPs) and spar platforms, have been used to support top-tensioned risers for dry tree solutions for several decades. For fabrication and operational reasons, a conventional floating vessel includes a wellbay and will normally have its top flanges flush with the lower deck elevation (normally at approximately 5 or more feet above the topside deck underside). The purpose is to avoid deck elevation step up coming in from surrounding areas. This means that the wellbay structure girders could protrude as much as 5 to 7 feet below the lower deck of the floater and could influence the air gap design for the entire topside. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a riser tensioner system including a first cassette, a second cassette, a tension joint, one or more centralizers, and a plurality of cylinders. The tension joint is slidably disposed through the first cassette and the second cassette. The one or more centralizers provide lateral support to the tension joint. Each of the plurality of cylinders comprises a first end, a second end, and an intermediate portion. The first end of each of the plurality of cylinders is secured to the first cassette, and the intermediate portion of each of the plurality of cylinders is secured to the second cassette. 
     In accordance with another aspect of the present invention, there is provided a wellbay structure for use in an offshore platform. The wellbay structure includes a plurality of transverse girders and a plurality of longitudinal girders. The plurality of transverse girders and the plurality of longitudinal girders form a grid comprising a plurality of slots. Each of the plurality of slots is configured for receiving and supporting a first cassette of each of a respective one of a plurality of riser tensioner assemblies. 
     In accordance with yet another aspect of the present invention, there is provided an offshore platform comprising a wellbay structure and at least one riser tensioner assembly. The wellbay structure comprises a plurality of transverse girders and a plurality of longitudinal girders. The at least one riser tensioner assembly comprises a first cassette, a second cassette, a tension joint, one or more centralizers, and a plurality of cylinders. The tension joint of the at least one riser tensioner assembly is slidably disposed through the first cassette and the second cassette. The one or more centralizers provide lateral support to the tension joint. Each of the plurality of cylinders of each of the plurality of riser tensioner assemblies comprises a first end, a second end, and an intermediate portion. The first end of each of the plurality of cylinders is secured to the first cassette, and the intermediate portion of each of the plurality of cylinders is secured to the second cassette. The plurality of transverse girders and the plurality of longitudinal girders of the wellbay structure form a grid comprising a plurality of slots. At least one of the plurality of slots is configured for receiving and supporting the first cassette. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustration, there are shown in the drawings certain embodiments of the present invention. In the drawings, like numerals indicate like elements throughout. It should be understood that the invention is not limited to the precise arrangements, dimensions, and instruments shown. In the drawings: 
         FIG. 1  illustrates a perspective view of a semi-submersible floating vessel comprising a deck structure comprising an upper level and a lower level, in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  illustrates an overall plan view of the lower level of the deck structure of the vessel of  FIG. 1 , the lower level having a wellbay area comprising a wellbay disposed therein, in accordance with an exemplary embodiment of the present invention; 
         FIG. 2A  illustrates a plan view of the wellbay area of  FIG. 2  disposed within the deck of the vessel of  FIG. 1 , in accordance with an exemplary embodiment of the present invention; 
         FIG. 2B  illustrates a perspective view of the wellbay area of the vessel of  FIG. 1 , in accordance with an exemplary embodiment of the present invention; 
         FIG. 2C  illustrates a cross-sectional view of the wellbay area of the vessel of  FIG. 1 , in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  illustrates a cross-section of a portion of the vessel of  FIG. 1  showing an exemplary embodiment of a riser assembly from the wellbay to a wellhead at seafloor, the riser assembly comprising a tensioner system, in accordance with an exemplary embodiment of the present invention; 
         FIG. 4  illustrates an exemplary embodiment of the tensioner system of  FIG. 3 , the exemplary embodiment of the tensioner system comprising a centralizing pipe sleeve and a plurality of centralizers, in accordance with an exemplary embodiment of the present invention; 
         FIG. 4A  illustrates a cross-sectional view of the tensioner system of  FIG. 3  and specifically a cross sectional view of the centralizing pipe sleeve, in accordance with an exemplary embodiment of the present invention; 
         FIGS. 4B and 4C  illustrate views of an exemplary embodiment of a tension joint and a layout of exemplary embodiments of centralizers, respectively, the tension joint replacing the centralizing pipe sleeve illustrated in  FIG. 4  and the centralizers replacing the centralizers of  FIG. 4 , in accordance with an exemplary embodiment of the present invention; 
         FIGS. 4D through 4F  illustrate close-up views of components of the tensioner assembly of  FIG. 3 , in accordance with an exemplary embodiment of the present invention; 
         FIGS. 5A and 5B  illustrate perspective view of the wellbay disposed within the deck illustrated in  FIG. 2 , in accordance with an exemplary embodiment of the present invention; 
         FIG. 5C  illustrates a perspective view of a slot of the wellbay of  FIG. 2  and a cassette of a riser tensioner system configured to be disposed in the slot, in accordance with an exemplary embodiment of the present invention 
         FIG. 5D  illustrates a perspective view of the cassette of  FIG. 5C  disposed within the slot of  FIG. 5C , in accordance with an exemplary embodiment of the present invention; 
         FIGS. 6A and 6B  illustrate a first embodiment of a tensioner lateral stabilization system, in accordance with an exemplary embodiment of the present invention; and 
         FIG. 6C  illustrates a second embodiment of a tensioner lateral stabilization system, in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference to the drawings illustrating various views of exemplary embodiments of the present invention is now made. In the drawings and the description of the drawings herein, certain terminology is used for convenience only and is not to be taken as limiting the embodiments of the present invention. Furthermore, in the drawings and the description below, like numerals indicate like elements throughout. 
     Referring now to  FIG. 1 , there is illustrated a perspective view of a semi-submersible floating vessel, generally designated as  1000 , for extracting oil and gas from a subsea reservoir, in accordance with an exemplary embodiment of the present invention. The vessel  1000  comprises a hull  1010  comprising a deck  2000 , a bottom level structure  1020 , and a plurality of columns  1030 A,  1030 B,  1030 C, and  1030 D (not illustrated) connecting the deck  2000  to the bottom level structure  1020 . The deck  2000  comprises a deck structure  2005  and a wellbay  5000  comprising a structure  5020  in which riser tensioners  4000  (illustrated in  FIG. 2B ) are disposed. The bottom level structure  1020  comprises a plurality of hull pontoons  1022 A,  1022 B,  1022 C, and  1022 D and a riser keel guide support structure  1024 . The vessel  1000  is semi-submersible and is chosen to have a hull draft that is suitable for quayside construction requirements and deep water/ultra-deep water oil and gas extraction operations. 
     Illustrated in  FIG. 2  is a plan view of the deck  2000 , and illustrated in  FIG. 2B  is a perspective view of a wellbay  5000  of the deck  2000 , in accordance with an exemplary embodiment of the present invention. Referring to  FIGS. 2 and 2B , the deck structure  2005  of the vessel  1000  comprises a top  2001  and a bottom  2003 . The wellbay structure  5020  is a structural support grid having a top surface  5001  and a bottom surface  5003 . The wellbay structure  5020  (also referred to herein as the “structural support grid  5020 ”) is configured to support a plurality of production riser assemblies  3000 B-F, I, M, and P-T and a drilling riser  2015 . 
     The wellbay  5000  is located centrally in the vessel  1000  to allow the production riser assemblies  3000 B-F, I, M, and P-T to be centrally located. The deck structure  2005  in wellbay  5000  comprises two large beams  2030 A and  2030 B that traverse the length of the deck structure  2005  along with two rows of columns  2030 C-H, J-K that support a drilling unit  1100  (illustrated in  FIG. 2C ) via a drilling support structure  1110  (also illustrated in  FIG. 2C ). Disposed between the columns  2030 C-H, J-K (not shown) is structural bracing. The wellbay  5000  is disposed between the beams  2030 A and  2030 B, which provide segregation of the wellbay  5000  from the areas adjacent to the outsides of the beams  2030 A and  2030 B. In an exemplary embodiment, the bottom surface  5003  of the wellbay structure  5020  may be flush with the bottom  2003  of the deck structure  2005 . 
     To facilitate operation of the vessel  1000  in deep/ultra-deep water, in accordance with an exemplary embodiment of the present invention, the riser assemblies  3000 B-F, I, M, and P-T are long-stroke riser assemblies. The deck  2000  of the vessel  1000  further comprises banks  2020 A and  2020 B of high pressure accumulators for the respective riser assembly  3000 B-F, I, M, and P-T. The banks of high pressure accumulators  2020 A and  2020 B are located adjacent to but outside of the wellbay  5000  for safety and to provide for a safer and less congested working area within the wellbay  5000 . 
     Illustrated in  FIG. 2A , is a close-up plan view of the wellbay  5000  of the vessel  1000 , in accordance with an exemplary embodiment of the present invention. As illustrated, there are 12 riser assemblies  3000 B-F, I, M, and P-T disposed within the wellbay  5000  of the vessel  1000 . Attached to the top of each riser assembly  3000 B-F, I, M, and P-T is a respective valve block (also known as a “Christmas tree”)  2010 B-F, I, M, and P-T. By virtue of their connection to the tops of the riser assemblies  3000 B-F, I, M, and P-T, the valve blocks  2010 B-F, I, M, and P-T are top-mounted surface valve blocks, i.e., dry trees. 
     Referring now to  FIG. 3 , there is illustrated a cross-section of a portion of the vessel  1000 , showing an exemplary embodiment of each of the riser (production) assemblies  3000 B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. As illustrated in  FIG. 3 , each riser assembly  3000 B-F, I, M, and P-T comprises a respective top-tensioned riser  3010 B-F, I, M, and P-T and a respective top-mounted tensioner  4000 B-F, I, M, and P-T (also referred to herein as “tensioner system  4000 B-F, I, M, and P-T”). Each top-tensioned riser  3010 B-F, I, M, and P-T conveys oil and gas from the seafloor  300 . Each valve block  2010 B-F, I, M, and P-T at the top the respective riser assembly  3000 B-F, I, M, and P-T controls the conveyance of oil and gas through the respective top-tensioned riser  3010 B-F, I, M, and P-T. Each top-mounted tensioner  4000 B-F, I, M, and P-T applies tension to its respective top-tensioned riser  3010 B-F, I, M, and P-T. Each riser  3010 B-F, I, M, and P-T comprises a respective outer casing  3015 B-F, I, M, and P-T, an inner casing (not illustrated) for a dual-casing riser configuration only (if there is any), and tubing, which conveys the oil and gas from the seafloor  300 . 
     Each top-tensioned riser  3010 B-F, I, M, and P-T is supported at its top  3011 B-F, I, M, and P-T by the deck  2000  (via an interface with the wellbay structural support grid  5020 ) of the vessel  1000 . An intermediate section of each top-tensioned riser  3010 B-F, I, M, and P-T is embedded within a respective intermediate section (keel joint)  3012 B-F, I, M, and P-T interfacing with a respective riser keel guide  1025 B-F, I, M, and P-T to constrain the lateral displacement of the each top-tensioned riser  3010 B-F, I, M, and P-T relative to the vessel  1000 . At the bottom  3013 B-F, I, M, and P-T of each top-tensioned riser  3010 B-F, I, M, and P-T is a respective riser joint  3020 B-F, I, M, and P-T. The bottom  3013 B-F, I, M, and P-T of the top-tensioned riser  3010 B-F, I, M, and P-T is connected to a tapered stress joint  3030 B-F, I, M, and P-T, which is secured to a respective well head  3050 B-F, I, M, and P-T in the seafloor  300  by a respective subsea connector  3040 B-F, I, M, and P-T. 
     In order to maintain stress of the risers  3010 B-F, I, M, and P-T under allowable design values at various loading conditions, the top-mounted tensioners  4000 B-F, I, M, and P-T (also referred to as a “riser motion compensation systems” herein) are coupled to the respective tops  3011 B-F, I, M, and P-T of the respective riser assemblies  3000 B-F, I, M, and P-T. An important feature of each tensioner system  4000 B-F, I, M, and P-T is its stroke length, which determines how much stroke of the respective riser  3010 B-F, I, M, and P-T will be compensated by each tensioner system  4000 B-F, I, M, and P-T with relative soft stiffness. Consequently, the strength and fatigue performance of each top-tensioned riser  3101 B-F, I, M, and P-T will be governed by the configuration its respective tensioner system  4000 B-F, I, M, and P-T and mechanical properties thereof. 
     Each top-tensioned riser  3010 B-F, I, M, and P-T, specifically the keel joint  3012 B-F, I, M, and P-T thereof, is laterally restrained below the surface  351  of the water  350  at the respective keel guide  1025 B-F, I, M, and P-T by the keel guide support  1024  at the hull pontoon  1022  level of the vessel  1000 . The keel guides  1025 B-F, I, M, and P-T act as guides for the vertical movement of their respective risers  3010 B-F, I, M, and P-T. They also withstand lateral and bending forces imposed on the risers  3010 B-F, I, M, and P-T to protect the tensioner systems  4000 B-F, I, M, and P-T. 
     Illustrated in  FIG. 4  is a perspective view of each tensioner system  4000 B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. Illustrated in  FIG. 4D  is a close-up perspective view of the interface between each tensioner system  4000 B-F, I, M, and P-T and the wellbay  5000  structural support grid  5020  (illustrated in  FIG. 2B  and  FIG. 2C ), in accordance with an exemplary embodiment of the present invention. Illustrated in  FIG. 4E  is a close-up perspective view of the top of each tensioner system  4000 B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. 
     Referring now to  FIGS. 4, 4D, and 4E , each tensioner system  4000 B-F, I, M, and P-T comprises a plurality of cylinders  4010 A through  4010 F, a load ring  4020 , an upper cassette  4040 , and a lower cassette  4050 . The plurality of cylinders  4010 A-F are connected to the high pressure accumulators  2020 A and  2020 B (illustrated in  FIG. 2 ) and low-pressure accumulators adjacent to the tensioner systems  4000 B-F, I, M, and P-T. Each tensioner system  4000 B-F, I, M, and P-T is designed to transfer tension force from its respective vertical riser  3010 B-F, I, M, and P-T back into the wellbay  5000  structural support grid  5020 . Desirably, each tensioner system  4000 B-F, I, M, and P-T works independently from one other. Failure or malfunction of one tensioner system  4000 B-F, I, M, and P-T will not affect the other tensioner systems  4000 B-F, I, M, and P-T. 
     For each of the tensioner systems  4000 B-F, I, M, and P-T, a top portion  4011 A through  4011 F of each respective cylinder  4010 A through  4010 F is secured to the upper cassette (herein also referred to as a “support frame”)  4040 . A middle portion  4012 A through  4012 F of each respective cylinder  4010 A through  4010 F is secured to the lower cassette  4050 . A lower portion  4013 A through  4013 F of each respective cylinder  4010 A through  4010 F is over hung below the lower cassette  4050 . The support frame  4040  supports the cylinders  4010 A through  4010 F and transfers the riser force from its respective vertical riser  3010 B-F, I, M, and P-T back into the wellbay  5000 . The cylinders  4010 A-F are disposed in a lower portion  4003  of each tensioner system  4000 B-F, I, M, and P-T. 
     In the exemplary embodiment illustrated in  FIG. 4 , the tensioner systems  4000 B-F, I, M, and P-T each comprise six cylinders  4010 A through  4010 F. It is to be understood that the tensioner system  4000  is not limited to having six cylinders  4010 A through  4010 F. For example, it is contemplated that the tensioner systems  4000 B-F, I, M, and P-T may have four cylinders, eight cylinders, etc. in other exemplary embodiments. The tensioner systems  4000 B-F, I, M, and P-T comprise more than one cylinder  4010 A-F to allow for the loss or removal for maintenance of one of the cylinders  4010 A-F without hindering performance of the tensioner systems  4000 B-F, I, M, and P-T. 
     Each cylinder  4010 A-F includes a respective extendible and retractable cylinder rod  4015 A through  4015 F. The cylinder rods  4015 A-F are designed with sufficient capacity to handle maximum buckling loads expected during in situ operation of the vessel  1000 . In an exemplary embodiment, each of the cylinder rods  4015 A-F has a stroke length of up to 50 feet. 
     Each tensioner system  4000 B-F, I, M, and P-T further comprises a tension joint  4090 . The tension joint  4090  interface with the upper cassette  4040  and the lower cassette  4050  to prevent the tension joint  4090  and the load ring  4020  from rotating relative to the upper cassette  4040  and the lower cassette  4050 , thereby assuring that the top ends of the cylinder rods  4015 A through  4015 F do not rotate. Rotation of the top ends of the cylinder rods  4015 A-F would severely damage the cylinders  4010 A-F and the cylinder rods  4015 A-F. A top portion  4091  of the tension joint  4090  is centered within the load ring  4020  by a positioning ring  4031  and secured to the load ring  4020 . A connector  4032  secures the top portion  4091  of the tension joint  4090  to its respective riser  3010 B-F, I, M, and P-T, which passes through the tension joint  4090  of its respective tensioner system  4000 B-F, I, M, and P-T. A bottom portion  4093  of the tension joint  4090  is secured to its respective riser  3010 B-F, I, M, and P-T by a connector  4033 . Each tensioner system  4000 B-F, I, M, and P-T comprises an upper portion  4001  comprising the load ring  4020 , the positioning ring  4031 , the connector  4032 , and a top portion  4091  of the tension joint  4090 . 
     In an exemplary embodiment, illustrated in  FIGS. 4 and 4A , the tension joint  4090  of each tensioner system  4000 B-F, I, M, and P-T is a centralizing pipe sleeve which surrounds a special portion of the riser outer casing  3015 B-F, I, M, and P-T in the respective tensioner system  4000 B-F, I, M, and P-T. The centralizing pipe sleeve  4090  comprises at least two guide rails  4095 A and  4095 B disposed on an outer surface of the centralizing pipe sleeve  4090 . Disposed on the upper cassette  4040  are a plurality of upper centralizers  4060 A through  4060 D, and disposed on the lower cassette  4050  are a plurality of lower centralizers  4070 A through  4070 D. The guide rail  4095 A is disposed against the centralizers  4060 C,  4070 C and  4060 D,  4070 D, and the guide rail  4095 B is disposed against the centralizers  4060 A,  4070 A and  4060 B,  4070 B to prevent the centralizing pipe sleeve  4090  and the load ring  4020  from rotating or moving laterally relative to the upper cassette  4040  and the lower cassette  4050 . In such embodiment, the centralizing pipe sleeve  4090  is connected to the outer casing  3015 B-F, I, M, and P-T at the bottom portion  4093  of the centralizing pipe sleeve  4090  via the connector  4033 . 
     In another exemplary embodiment, illustrated in  FIGS. 4B and 4C , the tension joint  4090  of each tensioner system  4000 B-F, I, M, and P-T is a special portion of the riser outer casing  3015 B-F, I, M, and P-T in the respective tensioner system  4000 B-F, I, M, and P-T. Such exemplary embodiment is illustrated in  FIG. 4B  and is generally designated as  4090 ′. The tension joint  4090 ′ has tension joint upsets (also referred to as “tension joint protrusions”)  4095 A′ through  4095 D′. The tension joint upsets  4095 A′- 4905 D′ interface with a slot guide plate  4042  on the upper cassette  4040  and a slot guide plate  4072  on the lower cassette  4050 , illustrated in  FIG. 4C , to prevent the tension joint  4090 ′ and the load ring  4020  from rotating relative to the upper cassette  4040  and the lower cassette  4050 , thereby assuring that the top ends of the cylinder rods  4015 A through  4015 F do not rotate. Specifically, the tension joint upsets  4095 A′- 4905 D′ interface with slots  4043 A through  4043 D in the slot guide plate  4042  and with slots  4073 A through  4073 D in the slot guide plate  4072  to prevent the tension joint  4090 ′ and the load ring  4020  from rotating relative to the upper cassette  4040  and the lower cassette  4050 . Rotation of the top ends of the cylinder rods  4015 A-F would severely damage the cylinders  4010 A-F and the cylinder rods  4015 A-F. A top portion  4091  of the tension joint  4090 ′ is centered within the load ring  4020  by a positioning ring  4031  and secured to the load ring  4020 . The centralizers  4060 A-D and  4070 A-D further help to prevent lateral deflection of the tension joint  4090 ′. The tension joint  4090 ′ is connected to the remainder of the outer casing  3015 B-F, I, M, and P-T via the connector  4033 . It is to be understood that the tension joint  4090 ′ may be used in any of the tensioner systems  4000 B-F, I, M, and P-T described herein in place of the centralizing sleeve  4090 . 
     With reference to  FIGS. 4, 4D, and 4E , the tensioner system  4000 B-F, I, M, and P-T further comprises a plurality of loading cups (also referred to as “upper impact buckets”)  4030 A through  4030 F connected to the tops of the respective cylinder rods  4015 A-F. The loading cups  4030 A-F are disposed within respective openings  4022 A through  4022 F of the load ring  4020 . The loading cups  4030 A-F transfer compression from the load ring  4020  to the cylinder rods  4015 A-F. Thus, each tensioner system  4000 B-F, I, M, and P-T transfers riser load from its respective riser  3010 B-F, I, M, and P-T to its support frame  4040 . 
     The tensioner systems  4000 B-F, I, M, and P-T are designed to be capable of withstanding and operating through repeated cylinder bottom-out situations. To this end, each tensioner system  4000 B-F, I, M, and P-T further comprises a plurality of stoppers (lower impact buckets)  4045 A through  4045 F secured to a top surface  4041  of the support frame  4040  so that respective cylinder rods  4015 A-F pass there through. Each stopper  4045 A-F comprises a respective shock absorbing element  4046 A through  4046 F. In an exemplary embodiment, the shock absorbing elements  4046 A-F are formed from urethane (or other energy absorbing materials). 
     As each dry-tree unit  2010 B-F, I, M, and P-T moves vertically in survival hurricane conditions, its respective tensioner system  4000 B-F, I, M, and P-T, specifically the cylinders  4010 A-F thereof, may be stroked downwardly to the down-stroke limit position, a “bottom-out” condition. This loading condition could result in a high energy impact load as the load ring  4020  comes in contact with the support frame  4040 . The shock absorbing elements  4046 A-F are included to absorb most of this impact load. Thus, the loading cups (upper impact buckets)  4030 A-F are compressed against the stoppers (lower impact buckets) buckets  4045 A-F during bottoming out. The shock absorbing elements  4046 A-F absorb most of the impact load and transfer the respective riser  3010 B-F, I, M, and P-T load from the cylinders  4010 A-F to the upper cassette frame  4040  via the load ring  4020 . The shock absorbing elements  4046 A-F also allow for fabrication misalignment tolerances. 
     The tensioner systems  4000 B-F, I, M, and P-T are also designed to be capable of withstanding and operating through repeated cylinder top-up situations. To this end, each cylinder  4010 A-F comprises a respective shock absorber (or hydraulic cushion)  4016 A through  4016 F, as illustrated in  FIG. 4F . The shock absorbers (or hydraulic cushion)  4016 A-F are disposed within respective cylinders  4010 A-F between an inner wall of the respective cylinders  4010 A-F and an outer wall of respective cylinder rods  4015 A-F. Each shock absorber (or hydraulic cushion)  4016 A-F is located between a respective flange  4017 A-F secured to a bottom end of the respective cylinder rod  4015 A-F and a respective head  4014 A-F secured to a head end of the respective cylinder rod  4015 A-F. Each shock absorber  4016 A-F may be a series of one or more energy absorbing springs, cushion material, or hydraulic cushion. 
     As a dry-tree unit  2010 B-F, I, M, and P-T moves vertically in survival hurricane conditions, its respective tensioner system  4000 B-F, I, M, and P-T, specifically the cylinders  4010 A-F thereof, may be stroked upwardly to the up-stroke limit position, a “top-up” condition. This loading condition could result in a high energy impact load as the cylinder rods  4015 A-F, specifically their respective flanges  4017 A-F, come in contact with the cylinder head  4014  when the cylinder rod  4015  is extended to its maximum. The shock absorbers  4016 A-F are included to absorb most of this impact load. Thus, the shock absorbers  4016 A-F are compressed between the respective heads  4014 A-F and flanges  4017 A-F of the respective cylinders  4010 A-F. The shock absorbers  4016 A-F progressively decelerate the topping up motion in the last few inches of stroke and absorb/dissipate the impact energy as the tensioner systems  4000 B-F, I, M, and P-T top up during survival conditions. The shock absorbers  4016 A-F of each tensioner system  4000 B-F, I, M, and P-T reduce the respective riser  3010 B-F, I, M, and P-T impact load from the cylinders  4010 A-F at the “top-up” condition. They protect the structural integrity of the cylinder heads  4014 A-F from a sudden topping up impact load as a result of the cylinder pistons  4017 A-F abruptly hitting their respective cylinder heads  4014 A-F from inside during a top-up condition. 
     As illustrated, each tensioner system  4000 B-F, I, M, and P-T comprises four upper centralizers  4060  mounted on a top surface of the upper cassette  4040  and four lower centralizers  4070  mounted on a top surface of the lower cassette  4050 , although it is to be understood that the tensioner systems  4000 B-F, I, M, and P-T are not so limited. Other numbers of upper centralizers  4060  and lower centralizers  4070  are contemplated. The centralizers  4060  and  4070  provide additional constraint on lateral force and bending moment on the tensioner systems  4000 B-F, I, M, and P-T induced by the inertial force of the trees  2010 B-F, I, M, and P-T or blow-out preventers placed at the top of the risers  3000 B-F, I, M, and P-T. The centralizers  4060  transfer lateral forces and bending moments applied to the tensioner systems  4000 B-F, I, M, and P-T to the wellbay  5000  via the upper cassette  4040  of each tensioner system  4000 B-F, I, M, and P-T. The centralizers  4070  transfer lateral forces and bending moments applied to the tensioner systems  4000 B-F, I, M, and P-T to a drop-down structure  5010  (described below) of the wellbay  5000 . The centralizers  4060  and  4070  can be mounted directly to the top of the upper cassette  4040  and the lower cassette  4050 , respectively, or mounted on a plate that is secured to the upper cassette  4040  and lower cassette  4050 , respectively. In exemplary embodiments in which a centralizer mounting plate is used, the mounting plate is removable and installed with the tension joint  4090  of each tensioner system  4000 B-F, I, M, and P-T at the middle of the upper cassette  4040  and lower cassette  4050  open to pass the subsea connector  3040 B-F, I,M,P-T. 
     Each accumulator in the bank of high pressure accumulators  2020 A and  2020 B is connected to a high pressure side of the cylinders  4010 A-F. The high pressure (HP) accumulators  2020 A-B provide nitrogen gas pressure, for example, to each cylinder  4010 A-F and are designed to provide the specified stiffness for the risers  3010 B-F, I, M, and P-T. The HP accumulators  2020 A-B are remotely mounted just outside the immediate wellbay  5000  area, and gas from them is piped to the cylinders  4010 A-F of each of the tensioner systems  4000 B-F, I, M, and P-T. 
     A small low pressure accumulator is connected to the head of each cylinder  4010 A-F of each tensioner system  4000 B-F, I, M, and P-T to accept the gas/fluid in the respective cylinders  4010 A-F as it strokes. These low pressure accumulators are mounted local to each tensioner system  4000 B-F, I, M, and P-T. With the passive nature of the tensioner systems  4000 B-F, I, M, and P-T, simple local manual controls are provided for operation and monitoring. Additional monitoring to a remote control room is accomplished with pressure transmitters monitoring the operating pressure of each individual cylinder  4010 A-F of each tensioner system  4000 B-F, I, M, and P-T. 
     Each riser tensioner system  4000 B-F, I, M, and P-T is operated through a control panel that manually monitors, controls and regulates the gas pressure in the high pressure accumulators  2020 A-B and the low pressure accumulators individually for each of the cylinders  4010 A-F of the tensioner systems  4000 B-F, I, M, and P-T. Pressure transmitters connected to the high pressure accumulators  2020 A-B allow for pressure monitoring at a remote location in a control room. 
     As discussed above, each riser tensioner system  4000 B-F, I, M, and P-T is secured to the hull  1010  of the vessel  1000  at two interfaces: an upper interface and a lower interface. Illustrated in  FIG. 2C  is a cross-sectional view of a portion of the vessel  1000  and specifically the deck  2000  and wellbay  5000  thereof. The wellbay  5000  comprises a drop-down structure  5010  connected to the bottom surface  5003  of the wellbay  5000  structural support grid  5020 . The drop-down structure  5010  provides the lower interface with the tensioner assemblies  4000 B-F, I, M, and P-T in which the lower cassette  4050  of each tensioner assembly  4000 B-F, I, M, and P-T is secured to the drop-down structure  5010 . The drop-down structure  5010  absorbs the lateral forces and bending moments imparted on the tensioner assemblies  4000 B-F, I, M, and P-T via the lower interfaces. Specifically, these forces and bending moments are passed to the drop-down structure  5010  via the centralizers  4070  and the lower cassette  4050  of the tensioner assemblies  4000 B-F, I, M, and P-T. The upper interface is between the upper cassette  4040  of each tensioner system  4000 B-F, I, M, and P-T and a wellbay structural beam slot  5050  (described below) in the wellbay structural support grid  5020 . 
     The structural support grid  5020  of the wellbay  5000  supports the loads of the riser tensioner systems  4000 B-F, I, M, and P-T. An important design consideration is accommodating loads when the total stroke of the risers  3010 B-F, I, M, and P-T exceeds the down stroke limit of the riser tensioner systems  4000 B-F, I, M, and P-T, thereby causing the riser tensioner systems  4000  to bottom out. In such a scenario, the loading cups (upper impact buckets)  4030 A-F engage with the stoppers (lower impact buckets)  4045 A-F of the tensioner systems  4000 B-F, I, M, and P-T. 
     Complete retraction of the riser tensioner systems  4000 B-F, I, M, and P-T may create an enormous load on the deck  2000  (specifically the deck structure  2005  and the structural support grid  5020  of the wellbay  5000 ) of the vessel  1000 . One solution could be to increase the stroke of the riser tensioner systems  4000 B-F, I, M, and P-T further. Increasing the stroke of the riser tensioner systems  4000 B-F, I, M, and P-T, however, comes with extra cost, may pose challenges in designing the riser tensioner systems  4000 B-F, I, M, and P-T to have sufficient strength, and increases the elevation of the wellbay  5000  and associated deck structure  2005  to provide for the necessary air gap between the bottom  2003  of the deck structure  2005  and the surface  351  of the water  350 . 
     As illustrated in  FIG. 2C , two sides of the wellbay  5000  are connected to overhead support structure beams for a drilling support structure  1110 . Although structural bracing may be desirable, the other two sides of the wellbay  5000  are relatively open to the adjacent areas of the vessel  1000 . This “open ends” design facilitates handling of the riser tensioner systems  4000 B-F, I, M, and P-T and also natural ventilation across the production riser assemblies  3000 B-F, I, M, and P-T within the wellbay  5000 . 
     Another solution is to design the wellbay  5000  to accommodate the enormous load created by limited stroke length of the riser tensioner systems  4000 B-F, I, M, and P-T when they bottom out. Illustrated in  FIGS. 5A and 5B  are perspective views of a portion of the deck structure  2005  and particularly of the wellbay  5000  structural support grid  5020 , in accordance with an exemplary embodiment of the present invention. The wellbay  5000  comprises a grid of box girders  5030 A through  5030 H and box girders  5040 A through  5040 D. The box girders  5030 A-H and  5040 A-D form a plurality of slots  5050 A through  5050 U. In an exemplary embodiment, the box girders  5030 A through  5030 H and box girders  5040 A through  5040 D are 10 to 12 feet high to support heavy riser systems. 
     The wellbay  5000  structural support grid  5020  is connected to the deck structure  2005  so that a top  5001  of the structural support grid  5020  is recessed from the top  2001  of the deck structure  2005 , and the bottom  5003  of the structural support grid  5020  is flush with the bottom  2003  of the deck structure  2005 . This arrangement of the wellbay  5000  preserves the airspace between the deck structure  2005  and the surface  351  of the water  350  and provides sufficient space above the structural support grid  5020  for the cylinder rods  4015 A-F to extend by their maximum lengths, e.g., 50 feet. Additionally, recessing the wellbay  5000  structural support grid  5020  reduces drilling floor elevation without interrupting motion of the tensioner systems  4000 B-F, I, M, and P-T, thus minimizing the impact of drilling rig structure on construction, topside integration, global performance, stability and mooring line force. In an exemplary embodiment, the top  5001  of the structural support grid  5020  is recessed from the top  2001  of the deck structure  2005  by 23 to 25 feet. 
     The wellbay  5000  structural support grid  5020  comprises girder beams  5030 A-H which are connected at one end to the deck beam  2030 A and at their other end to the deck beam  2030 B. The deck structure  2005  further comprises a plurality of transverse beams  2040 A through  2040 H and  2050 A through  2050 H. Disposed on the beams  2040 A-G adjacent to the wellbay  5000  are tapered beams  2060 A through  2060 H, and disposed on the beams  2050 A-G are tapered beams  2070 A through  2070 H. The tapered beams  2060 A-H and  2070 A-G taper from the top  5001  of the structural support grid  5020  to the top  2002  of the deck beams  2040 A-G and  2050 A-H. 
     The arrangement of the wellbay  5000  within the deck  2000  allows the bottom  2003  of the deck structure  2005  to be flush with the bottom  5003  of the wellbay  5000  (and its disposed structural support grid  5020 ). This arrangement simplifies construction of the deck structure  2005  of the semi-submersible vessel  1000 . Furthermore, it enhances air gap for the semi-submersible vessel  1000  while reducing wave impact loading on the structure of the wellbay  5000  (via the structural support grid  5020 ). Additionally, it lowers the foundations of the riser tensioners  4000 B-F, I, M, and P-T and overall height of the wellbay  5000 , which reduces wind load on the drilling unit  1100  and improves stability and mooring performance of the vessel  1000 . 
     The slots  5050 A-U form the wellbay  5000  support structure grid  5020  (in a grid of 3×7 for this exemplary design). In the illustrated embodiment, twelve of the slots  5050 B-F, I, M, and P-T are used for production risers and one of the slots  5050 K is used for a drilling riser. The remaining grid openings  5050 A, G, H, J, L, N, O and U are used for access and other functions. Other embodiments in which the support structural grid  5020  comprises more or fewer slots is contemplated. 
     Each production riser unit  3000 B-F, I, M, and P-T and its respective tensioner system  4000 B-F, I, M, and P-T is supported by the structural support grid  5020  in a respective riser slot  5050 B-F, I, M, and P-T sized based on design requirements. In an exemplary embodiment, each slot  5050 B-F, I-M and P-T is a 16-foot by 16-foot square (but the slot size can be adjusted larger or smaller as needed), measured on the center lines of the beams  5030 B-G and  5040 A-D. In such embodiment, the wellbay  5000  is created by repeating the 16-foot spacing between the wellbay structural support grid beam  5030 B-G and  5040 A-D centerlines. 
     In a further exemplary embodiment, the center slot  5050 K is in many cases designed to be used for drilling purposes. Thus, the center slot  5050 K and adjacent slots  5050 D and  5050 R are larger than the other slots  5050 A-C,  5050 E-J,  5050 L-Q, and  5050 S-U. In such embodiment, the center slot  5050 K and the adjacent slots  5050 D and  5050 R may be between 20 feet by 20 feet to 25 feet by 25 feet, measured between the centerlines of adjacent beams  5030 D-E and  5040 A-D. The other slots  5050 B-C,  5050 E-F,  50501 -J,  5050 L-M,  5050 P-Q and  5050 S-T are sized to accommodate the upper cassettes  4040  of the tensioner systems  4000 . 
     Referring now to  FIGS. 5C and 5D , there is illustrated an exemplary embodiment of the slot  5050 B and the cassette  4040  of the tensioner system  4000 B, in accordance with an exemplary embodiment of the present invention. Although  FIGS. 5C and 5D  illustrate only the slot  5050 B and the cassette  4040  of the tensioner system  4000 B, it is to be understood that the other slots  5050 C-F, I, M, and P-T and cassettes  4040  of the tensioner systems  4000 C-F, I, M, and P-T may be constructed similarly. It is further to be understood that the slots  5050 D,  5050 K, and  5050 R and their corresponding cassettes  4040  may be larger or smaller, although they may be constructed similarly. 
     The slot  5050 B comprises a ledge  5051 B disposed therein. In an exemplary embodiment, the ledge  5051 B comprises horizontal members  5052 B. Corner brackets  5053 B may be used to support the ledge  5051 B. 
     The ledge  5051 B is recessed from the top surface  5001  of the wellbay  5000  structural support grid  5020  by a distance, d, that is equal to the height, h, of the upper cassette  4040 . When disposed within the slot  5050 B, the upper surface  4041  of the upper cassette  4040  is flush with the top surface  5001  of the wellbay  5000  structural support grid  5020 . Thus, the recessed ledge  5051 B provides for reduced overall tensioner assembly  4000 B stack-up height, generates a flush arrangement on the wellbay  5000  structure foundation, provides a simple installation and retrieval of the tensioner assembly  4000 B, evenly distributes riser  3010 B load on the cylinders  4010 A-F to the wellbay  5000  structure foundation, and eliminates fatigue issues at the tensioner assembly  4000 B frame  4040  and wellbay  5000  structural interface (via the interface to wellbay slot  5050 B). 
     The cassette  4040  is sized to interface with the slot  5050 B. The cassette  4040 B therefore transfers the load of the riser  3010 B evenly. Additionally, because the cassette  4040 B is seated on the ledge  5051 B of the wellbay beams  5030 B,  5030 C,  5040 C, and  5040 D, but not welded thereto, installation is simplified and the fatigue in the connection interface between the cassette  4040  and the wellbay beams  5030 B,  5030 C,  5040 C, and  5040 D is reduced. 
     The tensioner systems  4000 B-F, I, M, and P-T acts as a cantilevered structure when stroking out. As the cylinder rods  4015 A-F of each tensioner system  4000 B-F, I, M, and P-T move upward close to the up-stroke limit, they are potentially subject to lateral deflection due to inertia force induced on the tensioner systems  4000 B-F, I, M, and P-T by motion of the vessel  1000 . Specifically, the upper portion  4001  of each tensioner system  4000 B-F, I, M, and P-T is potentially subject to lateral deflection. The lateral deflection, especially in severe storm events, could impair performance of the tensioner systems  4000 B-F, I, M, and P-T. To provide lateral support of the upper portion  4001  of the tensioner systems  4000 B-F, I, M, and P-T, a tensioner lateral stabilization system may be introduced, as discussed below with respect to  FIGS. 6A through 6C . 
     Referring now to  FIGS. 6A and 6B , there is illustrated an exemplary embodiment of a plurality of the tensioner lateral stabilization systems, generally designated as  6000 B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. Each lateral stabilization system  6000 B-F, I, M, and P-T is connected to the wellbay  5000  structure adjacent to its respective tensioner assembly  4000 B-F, I, M, and P-T and is also connected to its respective tensioner assembly  4000 B-F, I, M, and P-T. 
     Each tensioner lateral stabilization system (hereinafter also referred to as “stabilizer”)  6000 B-F, I, M, and P-T comprises a pair of vertical supports  6050  and  6060 . Each stabilizer  6000 B-F, I, M, and P-T further comprises a pair of respective horizontal support members  6020  and  6030  connected together by one or more cross members  6025  and a stabilization ring  6040 . A first end  6021  of the support member  6020  and a first end  6031  of the support member  6030  is connected to the stabilization ring  6040 . A second end  6022  of the support member  6020  is connected to the vertical support  6050 , and a second end  6032  of the support member  6030  is connected to the vertical support  6060 . 
     The stabilization ring  6040  is disposed around its respective riser  3010 B-F, I, M, and P-T. The pair of vertical supports  6050  and  6060  are secured to the wellbay  5000  structural support grid  5020  adjacent to the respective tensioner assembly  4000 B-F, I, M, and P-T. The upper end of the vertical supports  6050  and  6060  are fixed to the drilling support structure  1110  overhead or a structural member of the wellbay  5000 . At the end  6022  of the support member  6020  is a trolley  6024  that engages with the vertical support  6050 , and at the end  6032  of the support member  6030  is a trolley  6034  that engages with the vertical support  6060 . In an exemplary embodiment, the vertical supports  6050  and  6060  are rails on which the trolleys  6024  and  6034  ride. 
     The attachment of the trolleys  6024  and  6034  to the vertical supports  6050  and  6060  in each stabilizer  6000 B-F, I, M, and P-T prevents rotation of the stabilizers  6000 B-F, I, M, and P-T  6000 B-F, I, M, and P-T, while allowing the trolleys  6024  and  6034  to move up and down as the tensioner assemblies  4000 B-F, I, M, and P-T move up and down. Thus, the stabilizers  6000 B-F, I, M, and P-T provide lateral support to the risers  3010 B-F, I, M, and P-T throughout the full range of motion of the cylinders  4015 A-F. In an exemplary embodiment, the trolleys  6024  and  6034  employ roller bearings or sliding bearings for movement along the respective vertical supports  6050  and  6060 . 
     The vertical supports  6050  and  6060  are attached to the wellbay  5000  structure directly or through stays, depending on application requirements. The vertical supports  6050  and  6060  and the horizontal support members  6020  and  6030  may be formed from aluminum or steel, depending on application requirements. 
     Referring now to  FIG. 6C , there is illustrated an exemplary alternative embodiment of the stabilizers  6000 B-F, I, M, and P-T, which exemplary alternative embodiment is generally designated as  6000 B′-F′, I′, M′, and P′-T′ (hereinafter designated as  6000 ′ for brevity), in accordance with an exemplary embodiment of the present invention. Each stabilizer  6000 ′ comprises, respectively, a pair of respective horizontal support members  6020 ′ and  6030 ′. The support members  6020 ′ and  6030 ′ are connected together by one or more respective cross members  6025 ′ and  6035 ′. 
     A first end  6021 ′ of each support member  6020 ′ and a first end  6031 ′ of each support member  6030 ′ are secured to a respective platform  2090 B-F, I, M, and P-T. Such attachment prevents rotation of the stabilizers  6000 ′. A second end  6022 ′ of the support member  6020 ′ is connected to the vertical support  6050 , and a second end  6032 ′ of the support member  6030 ′ is connected to the vertical support  6060 . At the end  6022 ′ is a pair of trolleys  6024 ′ and  6026 ′ that engage with the vertical support  6050 , and at the end  6032 ′ is a pair of trolleys  6034 ′ and  6036 ′ that engage with the vertical support  6060 . 
     The trolleys  6024 ′,  6026 ′,  6034 ′, and  6036 ′ move up and down as the respective tensioner assembly  4000 B-F, I, M, and P-T moves up and down. Thus, they provide lateral support to the riser  3010 B-F, I, M, and P-T throughout the full range of motion of the cylinders  4015 A-F. In an exemplary embodiment, the trolleys  6024 ′,  6026 ′,  6034 ′, and  6036 ′ employ roller bearings or sliding bearings for movement along the respective vertical supports  6050  and  6060 . 
     In another exemplary embodiment, the trolleys of either of the stabilizers  6000  or  6000 ′ are designed to have a gap between them and their respective vertical supports  6050  and  6060 . In such embodiment, the stabilizers  6000  or  6000 ′ arrest horizontal bending of the tensioner assemblies  4000 B-F, I, M, and P-T only when large unwanted deflections occur. 
     The tensioner assemblies  4000 B-F, I, M, and P-T and the wellbay  5000  provide a feasible and cost-effective solution for deep and ultra-deep water field development using top-tensioned risers  3010 B-F, I, M, and P-T on a floating system, such as the semi-submersible vessel  1000 , a spar platform, a TLP, and other floaters. This design provides not only a more efficient concept for the production units  3000 B-F, I, M, and P-T and the drilling unit  1100 , but also especially enable a floater with larger motion characteristics applicable to deep/ultra-deep water field development. For example, the vessel  1000 , a dry tree semi-submersible, comprising the wellbay  5000  and long stroke riser tensioner systems  3000 B-F, I, M, and P-T is able to overcome the challenges due to large platform motion at severe field conditions, thus enable the dry tree semi-submersible for deep/ultra-deep water field development. In addition, the tensioner assemblies  4000 B-F, I, M, and P-T are able to compensate long stroke of risers due to significant thermal expansion during deep water and ultra-deep water extraction, which is extremely valuable for high temperature field development. 
     These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it is to be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is to be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention.