Patent Publication Number: US-2013239868-A1

Title: Dry tree jacket semi-submersible platform

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present invention claims the benefits of the filing date of U.S. Provisional Patent Application Ser. No. 61/612,124, filed on Mar. 16, 2012, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of floating offshore structures. Specifically, the present invention relates to a host platform for oil and gas production and drilling facilities, especially for dry tree operations. More specifically, the present invention relates to a floating offshore platform with a combination of a jacket structure that supports the deck and a semi-submersible hull structure that provides the in-place buoyancy and floating stability. 
     BACKGROUND OF THE INVENTION 
     A column stabilizing platform that hosts a dry-tree well system, comparing to that of a wet-tree well system, requires good stability and motion characteristics. The ideal floating offshore platform shall (i) have adequate stability to support the production and drilling operation especially with riser load applied at high elevation in the dry tree system; (ii) provide steady support to the deck that hosts the topside drilling and production facilities; and (iii) have superior motion characteristics so that the relative offset between the floating platform and the dry wellheads (riser stroke) is minimized during operation. 
     Currently, most common floating platforms for dry-tree operation include Tension Leg Platform (“TLP”) and SPAR. These two platforms have good motion characteristics and adequate in-place stability and are suitable not only for a dry-tree well, but also a wet-tree well. Application of dry tree system to semi-submersible platforms has been proposed but has yet been applied in real field development. 
     Nevertheless, each aforesaid platform has its own constraints. A TLP may have superior motion characteristics due to the vertical restraint of the tendons, but the tendon system also reduces the cost efficiency of the TLP applying in very deep water. SPAR is a deep draft floating platform, but it is hard to assemble quayside due to the very deep draft. Compared to TLP and SPAR platforms, a semi-submersible platform, which has a floating hull consisting of columns and pontoons for supporting a deck located above it as shown in  FIGS. 1A-4B , has some unique advantages, such as, wide range of water depth, suitability for quayside or offshore assembly, and a relatively short to medium development schedule. 
     Notwithstanding the foregoing, the semi-submersible platform still has several deficiencies in hosting a dry tree well system. The center of gravity of such a platform is relatively high, and as a result, either a large column water plane area or a large column span is required to provide the hydrostatic restoring force for stabilizing the platform. However, the large column water plane area would result in a smaller heave natural period to be in the range of maximum wave energy period which is detrimental to the heave motion. As the deck is directly supported by the columns, the large column span inevitably leads to deck design problems, e.g. the deck is subject to the pry-squeeze load caused by the wave induced movement of the columns, which are structurally connected with the deck. The worst is that all of the aforesaid solutions for stability would increase the platform motion. Alternatively, ballast can be added at the lower compartment of the platform, but it would result in a large platform displacement and increase steel weight and the total cost for platform construction. 
     Furthermore, the heave motion of the semi-submersible platform not only causes large relative platform to dry tree offset which leads large riser stroke and large tensioners, but also induces larger design loads and causes increased fatigue damage to the mooring system and risers applied with the platform. 
     Therefore, a need exists for a platform with good stability and superior motion characteristics. 
     A further need exists for a platform with deck that can be sized without the constraint of column spacing and subjects to minimal pry-squeeze load. 
     A further need exists for a platform with optimized column water plane area for better motion responses. 
     A further need exists for a platform with acceptable tension and fatigue lives of riser and mooring lines. 
     A further need exists for a platform with good riser protection and guide frames for riser operation. 
     A further need exists for a platform with minimum ballast and thus reduced overall weight. 
     A further need exists for an efficient and cost-effective platform. 
     The present embodiments of the present invention meet these needs and improve on the technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustrating purposes only of selected embodiments and not all possible implementation and are not intended to limit the scope of the present disclosure. 
       The detailed description will be better understood in conjunction with the accompanying drawings as follows: 
         FIG. 1A  illustrates a prior art of a conventional semi-submersible platform 
         FIG. 1B  illustrates a prior art of another conventional semi-submersible platform. 
         FIG. 2  illustrates a perspective view of a jacket semi-submersible platform with four columns and a pontoon structure according to some embodiments of the present invention. 
         FIG. 3  illustrates a perspective view of a jacket semi-submersible platform with four columns and a pontoon structure according to some embodiments of the present invention. The column space is further extended. 
         FIG. 4  illustrates a perspective view of a jacket semi-submersible platform with three columns and a pontoon structure with three sided jack frame according to other embodiments of the present invention. 
         FIG. 5  illustrates a perspective view of a jacket semi-submersible platform with three columns and a pontoon structure with four sided jack frame according to other embodiments of the present invention. 
         FIG. 6  illustrates a perspective view of a jacket semi-submersible platform with four columns, a pontoon structure, and a mooring system according to some embodiments of the present invention. 
         FIG. 7  illustrates a perspective view of a jacket semi-submersible platform with three columns, a pontoon structure with four sided jack frame, and a mooring system according to other embodiments of the present invention. 
         FIG. 8  illustrates a perspective view of a jacket semi-submersible platform with three columns, a pontoon structure, and a mooring system according to other embodiments of the present invention. The column space is further extended. 
         FIG. 9  illustrates a perspective view of a jacket semi-submersible platform with four columns, a pontoon structure with a smaller size, and a mooring system according to some embodiments of the present invention. 
         FIG. 10  illustrates a perspective view of a jacket semi-submersible platform with four columns and a mooring system, but without a pontoon structure according to other embodiments of the present invention. 
     
    
    
     The present embodiments are detailed below with reference to the listed Figures. 
     SUMMARY OF THE INVENTION 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or its entire feature. 
     In one preferred embodiment, an offshore floating platform includes a deck, a jacket frame attached to the bottom of the deck for supporting the deck, and a hull structurally coupled to the jacket frame for stabilizing the offshore floating platform. The hull is not directly assembled with the deck for reducing the pry-squeeze load caused to the deck. 
     In some embodiments, the hull includes a column. 
     In some embodiments, the shape of the column is selected from the group consisting of square, rectangular, round, stepped, and multi-sided. 
     In some embodiments, the number of the column is at least three. 
     In some embodiments, the columns are spaced apart for stability of the platform. 
     In some embodiments, the hull includes a pontoon structure. 
     In some embodiments, the offshore floating platform further includes a topside facility on the deck. 
     In other embodiments, the offshore floating platform further includes a riser extending from an underground oil well to the topside facility for oil and gas production. 
     In other embodiments, the riser extends through the hull and the jacket frame. 
     In other embodiments, the type of riser is selected from the group consisting of a top-tensioned riser, a centenary riser, a flexible riser, and a rigid riser. 
     In other embodiments, the offshore floating platform further includes a mooring system coupled with the hull for maintaining the position of the platform. 
     In another embodiment, the mooring system comprises multiple mooring lines. 
     In another embodiment, the offshore floating platform further includes at least one guide frame coupled to the jacket frame. 
     In another embodiment, the offshore floating platform further includes at least one heave plate coupled to the jacket frame. 
     In still another embodiment, the heave plate is positioned below the hull. 
     In another preferred embodiment, a jacket semi-submersible platform includes a deck, a jacket frame attached to the bottom of the deck for supporting the deck, a guide frame structurally coupled to the jacket frame, and multiple columns structurally coupled to the jacket frame for stabilizing the platform. The columns are not directly assembled with the deck for reducing the pry-squeeze load caused to the deck. 
     In some embodiments, the jacket semi-submersible platform further includes a riser to be guided by the guide frame. 
     In still another preferred embodiment, a method for providing an offshore floating platform includes providing a deck, attaching a jacket frame to the bottom of the deck for supporting the deck, and coupling a hull to the jacket frame without contacting the deck. 
     In some embodiments, the method for providing an offshore floating platform further includes coupling a guide frame to the jacket frame for allowing pipes extending through. 
     In some embodiments, the method for providing an offshore floating platform further includes coupling a mooring system with the hull for maintaining the position of the platform. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Before explaining the present apparatus in detail, it is to be understood that the present invention is not limited to the particular embodiments and that it can be practiced or carried out in various ways. 
     The present invention relates generally to the field of floating offshore structures. Specifically, the present invention relates to a host platform for oil and gas production and drilling facilities, especially for dry tree operations. More specifically, the present invention relates to a floating offshore platform with a combination of a jacket structure that supports the deck and a semi-submersible hull structure that provides the in-place buoyancy and floating stability. 
       FIG. 2  illustrates a perspective view of a jacket semi-submersible platform. The jacket semi-submersible platform can include a deck  104 , a jacket frame  106 , and a hull  114 . The jacket frame  106  can be attached to the bottom of the deck  104  for supporting the deck  104 . The hull  114  can be structurally coupled with the jacket frame for providing buoyancy and a restoring force to stabilize the platform, but without directly being assembled with the deck  104  to reduce the pry-squeeze load caused to the deck  104 . The hull  114  can include columns  108  and selectively a pontoon structure  110 . 
     When the columns  108  are not rigidly assembled with the deck  104 , the structure of the deck  104  will not be squeezed or twisted due to the movement of the columns  108  that are caused by surrounding wave actions. Without such a rigid bounding structure, as shown in conventional semi-submersible platforms shown in  FIG. 1A-4B , the column span can be increased because the deck  104  is free of the pry-squeeze load caused by the columns  108 . Accordingly, if the column span can be increased, the column water plane area can be optimized to a smaller scale, i.e. a column  108  with a reduced height. Overall, the total weight of columns  108  and deck  104 , their construction cost, and development complexity of the platform can be reduced. 
     Furthermore, since the deck  104  and the hull  114  are not directly connected, the designer can have the freedom to size the deck according to deck space requirement and size the hull for superior motion characteristics and adequate stability. 
     The deck can be completely supported by the jacket frame  106  in the middle, while the columns  108  are spaced apart to provide adequate stability of the platform. In such an arrangement, the deck size and design are not constrained by column spacing, and the hull configuration can be independently tuned for adequate stability and minimizing heaving motion. 
     In some embodiments, the jacket semi-submersible platform can install a topside facility  102  on the deck  104  and a riser  112  extending from an underground oil well (not shown in the figures), via the hull  114  and the jack frame  106 , to the topside facility  102  for oil and gas production. The hull  114  or the pontoon structure  110  can have a central opening structure  116  allowing the riser  112  or other piping to pass through. The riser  112 , which can be, but not limited to, a top-tensioned riser, a centenary riser, a flexible riser, or a rigid riser, can be well protected by the jacket frame  106  from boat impact. The good stability of the jacket semi-submersible platform according to the present invention can make the strokes (relative motion between the hull  114  and the riser  112 ) and the tension of the riser  112  to be within acceptable tolerance limits for facilitating the reliability of drilling and riser operation. 
       FIGS. 3˜5  illustrate perspective views of several jacket semi-submersible platforms according to other embodiments of the present invention. As shown in these figures, the number and size of the column  108  can be varied. However, the number of the column  108  is three (3) or more to achieve dynamical stability. The dimension of the column  108  is preferably optimized for motion purpose, i.e. reduced height, in consideration of the total column span. The shape of the column  108  also can be of any geometric shape, such as square, rectangular, round, stepped, and multi-sided. Accordingly, the number, size, and shape of the pontoon structure  110  can be optimized to match up the design of the column  108 . 
     Also as shown in  FIGS. 3˜5 , the size and points of support of the jack frame  106  can be varied with the arrangement of the columns  108 . The jacket frame  106  also can be sized for deck floatover installation. 
     Also as shown in  FIGS. 3˜5 , the column spacing can be further extended in a situation where small deck space is required while large column spacing is necessary to achieve stability requirement. 
       FIGS. 6˜8  illustrate perspective views of jacket semi-submersible platforms further having a mooring system  604  according to some embodiments of the present invention. The mooring system  604  for maintaining the position of the platform can include multiple mooring lines  602 , which are coupled with the hull  114  and anchored on the seafloor. The good motion characteristics of the jacket semi-submersible platform according to the present invention can reduce the fatigue of the mooring system  604 . 
     In some embodiments, one or multiple guide frames  600  can be coupled with the jacket frame  106  for guiding the riser  112  to reduce the effective riser load acting height. 
     In some embodiments, one or multiple heave plates (not shown in the Figures) can be coupled with the jacket frame  106  or the hull  114  for increasing damping to reduce the heave motion of jacket semi-submersible platform. The jacket frame  106  can be selectively extended below the hull  114  for connecting to the heave plates in a deeper position or additionally for guiding the riser  112  with a prerequisite that the buoyancy of the platform can support the total weight. 
       FIG. 9  illustrates a perspective view of a jacket semi-submersible platform with a pontoon structure  110  with a smaller size according to some embodiments of the present invention. 
       FIG. 10  illustrates a perspective view of a jacket semi-submersible platform without a pontoon structure  110  according to other embodiments of the present invention. The jacket semi-submersible platform can include a deck  104 , a jacket frame  106 , a guide frame  600 , and columns  108 . The jacket frame can be attached to the bottom of the deck  104  for supporting the deck  104 . The columns  108  can be structurally coupled with the jacket frame for providing buoyancy and a restoring force to stabilize the platform, but without directly being assembled with the deck  104  to reduce a pry-squeeze load caused to the deck  104 . One or multiple guide frames  600  can be coupled with the jacket frame  106  for guiding the riser  112  to reduce the effective riser load acting height. 
     A method for providing an offshore floating platform can include providing a deck, attaching a jacket frame to the bottom of the deck for supporting the deck, and coupling a hull to the jacket frame without contacting the deck. 
     In some embodiments, the method for providing the offshore floating platform can include coupling a guide frame to the jacket frame for allowing pipes extending through. 
     In some embodiments, the method for providing the offshore floating platform can include coupling a mooring system with the hull for maintaining the position of the platform. 
     In operation, the jacket semi-submersible platform according to the present invention can provide good stability and help decrease heave motion of the platform. Furthermore, the separation of the deck  104  supported by the jacket frame  106  from the hull  114  or the column  108  can help reduce the pry-squeeze load adding to the deck  104 . As a result, the column water plane area, the weight of the deck  104  and the hull  114 , tension of the riser  112  and mooring lines  602  can be optimized. Overall, the total construction cost and complexity of the platform can be decreased. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. It will be readily apparent to one skilled in the art that other various modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.