Abstract:
A spar platform for use in the offshore drilling or production of fossil fuels includes a hull having a centerwell. An airtight and watertight barrier traverses the centerwell, forming a variable buoyancy compartment in the centerwell. In certain embodiments the centerwell is open at the bottom to the sea, while in certain other embodiments the centerwell is sealed at the bottom. At least one sleeve for accommodating a riser extends through the barrier and the variable buoyancy compartment to the bottom of the centerwell. The sleeve has an open upper end to provide a drain for water accumulating in the centerwell, and it forms an airtight and watertight seal at its juncture with the barrier. In some embodiments, two or more airtight and watertight barriers are provided across the centerwell, defining an airtight and watertight fixed buoyancy chamber between each adjacent pair of barriers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from provisional application Ser. No. 60/822,631, filed Aug. 16, 2006, the disclosure of which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present embodiments relate to offshore drilling and production platforms, particularly spar-type platforms. 
   2. Description of Related Art 
   Spar-type offshore drilling and production platforms typically include vertically elongated buoyant hulls. For example,  FIG. 1  illustrates an example spar platform  100  having an outer hull  102  with a hollow centerwell  104  that is open to the sea at its lower end, and open to the atmosphere at its upper end. The hull  102  supports a deck (not shown) on which drilling and production equipment (not shown) may be mounted, along with other structures. The hull  102  includes a plurality of buoyancy tanks  106  surrounding the centerwell  104 . The buoyancy tanks  106  define voids or compartments  108  that may be selectively filled with air or water to provide varying degrees of buoyancy to the platform  100 . The buoyancy tanks  106  extend down to a truss structure  110 , which, in turn, extends down to a ballasted keel  112 . The ballasted keel  112  at the bottom of the truss structure  110  lowers the center of gravity of the platform  100  and improves the stability of the platform  100 . One or more mooring lines (not shown) may be used to keep the platform  100  over its station. 
   Spar platforms are typically used in conjunction with one or more risers that extend under tension from the platform to a wellhead or an anchor on the seafloor. For example, the platform  100  of  FIG. 1  includes top-tensioned risers (TTRs)  116 . The TTRs  116  extend downward through the centerwell  104  from hydraulic-pneumatic tensioners (not shown) supported on a top-tensioned riser support frame  118 . The hull  102  supports the support frame  118  above the surface  120  of a body of water (e.g., the sea). In alternative spar platforms, the TTRs may be supported by a buoyancy can (not shown) floating in the open centerwell. Such a configuration is disclosed in U.S. Pat. No. 6,176,646, the disclosure of which is incorporated herein by reference in its entirety. Alternative spar platforms may include catenary risers and/or bottom tensioned risers (BTRs) that are used to import oil and/or gas from remote fields or to export oil and/or gas to the shore or to other platforms. These risers are generally located in the open centerwell, and the platform may include pull tubes or containment tubes for surrounding and containing the risers. In some cases, catenary risers may be located on the outside of the platform and mm along the length of the platform. Other utility pipes that are open at the bottom may also be located in the centerwell. 
   As understood from  FIG. 1 , the centerwell  104  is open to the sea at its bottom and flooded with sea water. Accordingly, the centerwell  104  does not contribute to the buoyancy of the platform  100 . 
   SUMMARY OF THE INVENTION 
   The preferred embodiments of the present spar platform have several features, no single one of which is solely responsible for their desirable attributes. Without limiting the scope of the present embodiments as expressed by the claims that follow, their more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of the Preferred Embodiments.” one will understand how the features of the present embodiments provide several advantages, including, without limitation, increased buoyancy, reduced size and weight, and simple and effective means to adjust the buoyancy of the platform as conditions change. 
   One aspect of the present spar platform includes the realization that in a typical spar platform, the centerwell is open to the sea and flooded. The centerwell thus provides no buoyancy contribution to the platform. Sealing off part or all of the centerwell would advantageously increase the buoyancy of the platform and enable the centerwell to provide adjustable buoyancy to the platform. Sealing off part or all of the centerwell would also advantageously help to reduce the diameter and size of the spar platform, thereby generating weight savings. The reduction in weight and volume would also enhance the ability of the spar platform to be built and transported in one piece using existing heavy lift vessels. 
   Broadly, and in accordance with the above realizations, a spar platform in accordance with the present invention comprises a hull; a centerwell disposed within the hull and having a lower end open to the sea; a transverse barrier disposed within the centerwell so as to define a variable buoyancy compartment at the lower end of the centerwell, the variable buoyancy compartment being open to the sea; and a sleeve extending through the barrier and the compartment, wherein the sleeve forms an airtight and watertight seal at its junction with the barrier. 
   In one specific embodiment, the barrier is an airtight and watertight deck. In another specific embodiment, the barrier comprises at least first and second airtight and watertight decks defining an airtight and watertight fixed buoyancy chamber between them, with the sleeve extending through the fixed buoyancy chamber from the first deck, through the second deck, and through the compartment. The sleeve may advantageously be configured to accommodate a riser extending through it. The sleeve forms airtight and watertight seals at its respective junctures with the first and second decks. 
   As used herein, the terms “invention” and “present invention” are to be understood as encompassing the invention described herein in its various embodiments and aspects, as well as any equivalents that may suggest themselves to those skilled in the pertinent arts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the present spar platform will now be discussed in detail with an emphasis on highlighting the advantageous features. These embodiments depict the novel and non-obvious spar platform shown in the accompanying drawings, which are for illustrative purposes only. These drawings include the following figures, in which like numerals indicate like parts: 
       FIG. 1  is a cross-sectional side elevation view of a prior art spar platform; 
       FIG. 2  is a cross-sectional side elevation view of one embodiment of the present spar platform; 
       FIG. 3  is a cross-sectional top plan view of the spar platform of  FIG. 2 , taken through the line  3 - 3 ; and 
       FIG. 4  is a cross-sectional top plan view of the spar platform of  FIG. 2 , taken through the line  4 - 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  illustrates a cross-sectional side elevation view of one embodiment of the present spar platform  200 . Although the present embodiments are described herein with reference to a truss spar platform, those of ordinary skill in the art will appreciate that the present embodiments encompass any floating production and/or drilling platform or vessel having an open centerwell configuration. 
   As shown in  FIG. 2 , the spar platform  200  includes a hull  202  having a centerwell  204 . The centerwell  204  has an upper end that is open to the atmosphere, and a lower end that is open to the sea. A plurality of airtight and watertight barriers  206 ,  208 ,  210  extend substantially horizontally across the centerwell  204 . In a specific embodiment, one or more of the barriers  206 ,  208 ,  210  may be in the form of a non-airtight/watertight deck. For simplicity, in the description below the barriers  206 ,  208 ,  210  will be referred to as decks, even though in certain embodiments one or more of these barriers  206 ,  208 ,  210  may not be airtight or watertight. 
   The first and second decks  206 ,  208  define a first airtight and watertight fixed buoyancy chamber  216  between them. The second and third decks  208 ,  210  define a second airtight and watertight fixed buoyancy chamber  218  between them. One or more support or guide frames  214  may be provided across the centerwell  204  below the third deck  210 . In the illustrated embodiment, two support or guide frames  214  are provided, with the lowermost frame  214  being located near the lower end of the centerwell  204 , as shown in  FIGS. 2 and 4 . Those of ordinary skill in the art will appreciate that fewer or more support or guide frames  214  may be provided. The function of the support or guide frames  214  is discussed in detail below. 
   A plurality of sleeves  224  extend in a substantially vertical (axial) direction through the centerwell  204 , from the uppermost deck  206  to the bottom of the centerwell  204 . In the illustrated embodiment, five sleeves  224  are shown, but it will be appreciated that fewer or more sleeves  224  could be provided. One of the sleeves  224 , preferably near the center of the centerwell  204 , may be a moon pool sleeve  224   a  (see  FIGS. 3 and 4 ), and it may be larger in diameter than the other sleeves  224  so as to provide a moon pool  225  that extends downwardly from the uppermost deck  206  to the lower end of the centerwell  204 . The sleeves  224 ,  224   a  are supported by the support or guide frames  214  as the sleeves extend through the centerwell  204  below the decks  206 ,  208 ,  210 . The sleeves  224  are advantageously dimensioned to receive and accommodate risers  227 , which may be top-tensioned risers (TTRs), bottom-tensioned risers (BTRs), or steel catenary risers (SCRs), either with or without riser casings (not shown). The TTRs may be supported by a top-tensioned riser support frame  229  with associated conventional riser tensioners (not shown), as is well-known in the art. Other containment tubes and/or pull tubes (not shown), such as those for catenary risers, umbilicals, moon pools and/or caissons, may also be provided in the centerwell  204 . 
   The hull  202  includes a plurality of buoyancy tanks or hard tanks  226  surrounding the centerwell  204 . The buoyancy tanks  226  may be selectively and controllable filled with air or water, by conventional means, to provide varying degrees of buoyancy to the platform  200 . The buoyancy tanks  226  extend down to a truss structure  230 , which extends down to a ballasted keel  232 . The ballasted keel  232  at the bottom of the truss structure  230  lowers the center of gravity of the platform  200  and improves the stability of the platform  200 . One or more mooring lines (not shown) may be used to keep the platform  200  over its station. Those of ordinary skill in the art will appreciate that certain embodiments of the present spar platform may not include a truss structure or a ballasted keel. 
   As described above, the decks  206 ,  208 ,  210  are airtight and watertight. Accordingly, the intersections of the sleeves  224 ,  224   a  with the decks  206 ,  208 ,  210  are similarly airtight and watertight. For example, the sleeves  224 ,  224   a  may be welded to the decks  206 ,  208 ,  210  in an airtight and watertight fashion. Those of ordinary skill in the art will appreciate that as used herein the term “sleeve” encompasses both continuous and segmented structures. Thus, each sleeve  224 ,  224   a  may comprise a single unitary length of material extending from the uppermost deck  206  to the lowermost support or guide frame  214 , or each sleeve  224 ,  224   a  may be constructed of a plurality of shorter segments that may be connected together and/or connected to the decks  206 ,  208 ,  210  and guide frames  214 . In embodiments where the sleeve(s)  224 ,  224   a  are constructed of a plurality of shorter segments, openings in the deck(s)  206 ,  208 ,  210  may be considered to be part of the sleeves. 
   In certain embodiments, the airtight and watertight buoyancy chambers  216 ,  218  are filled with air, thus adding buoyancy to the spar platform  200 . Because the sleeves  224 ,  224   a  passing through the fixed buoyancy chambers  216 ,  218  are likewise airtight and watertight, as are the junctures between the sleeves  224 ,  224   a  and the decks  206 ,  208 ,  210 , any water in the sleeves  224  will not seep into the fixed buoyancy chambers  216 ,  218  and interfere with their buoyancy contribution to the spar platform  200 . Furthermore, the sleeves  224  have open upper ends in the uppermost deck  206 , so that any water accumulating on the uppermost deck  206  is drained through the sleeves  224  and into the sea. 
   A variable buoyancy compartment  220 , defined below the lowermost deck  210 , has an open bottom coinciding with the open bottom of the centerwell  204 . Because this variable buoyancy compartment  220 , also referred to as a compressed air over water chamber, is open to the sea, seawater  222  may move in and out of the compartment  220  naturally. The amount of air and water in the variable buoyancy compartment  220  may be adjusted by adding air from a source of compressed air (not shown) or by bleeding air from the compartment  220  to the sea or to the atmosphere. The provision of compressed air and the bleeding of air may be performed by conventional mechanisms that are well-known in the art, and therefore need not be described in this specification. By controllably changing the ratio of air to water within the compartment  220 , the buoyancy contribution of the variable buoyancy compartment  220  to the platform  200  may be controllably adjusted. Because the sleeves  224  passing through the variable buoyancy compartment  220  are airtight and watertight, any air and/or water in the sleeves  224  will not seep into the variable buoyancy open bottom compartment  220  and interfere with its buoyancy contribution to the spar platform  200 . 
   In certain embodiments the sleeves  224  are open at both ends. The sleeves  224  are thus at least partially filled with seawater that enters through the lower end of each sleeve  224 . As mentioned above, the sleeves  224  also advantageously act as drains for the uppermost deck  206 . Water or other liquids collecting on the deck  206  may drain through the open upper ends of the sleeves  224  and drain down through the sleeves  224  to the level of seawater contained in each sleeve  224 . The drainage advantageously prevents excessive accumulation of liquids on the deck  206 , which could increase the weight at the upper end of the platform  200  and possibly upset the balance of the platform  200 , or cause sloshing or other detrimental effects. 
   The embodiments described above advantageously provide watertight compartments  216 ,  218  in the centerwell  204  that increase the buoyancy of the spar platform  200 . Sealing off the lower part of the centerwell  204  by at least one watertight and airtight transverse barrier or deck also advantageously helps to reduce the diameter and size of the spar platform  200 , thereby generating weight savings. The reduction in weight and volume also enhances the ability for the spar platform  200  to be built and transported in one piece using existing heavy lift vessels. 
   The embodiments described above also advantageously provide the variable buoyancy or compressed air over water compartment  220 . The adjustable buoyancy of the variable buoyancy compartment  220  provides a simple and effective means for adjusting the buoyancy of the spar platform  200  as conditions aboard the platform  200  change. For example, as risers and/or topside equipment is added or removed over the life of the platform  200 , the buoyancy of the variable buoyancy compartment  220  may be adjusted to maintain the balance of the platform  200 . The compressed air buoyancy system is also advantageously simpler than a water ballast system using marine ballast pumps. 
   Although the illustrated embodiment includes three airtight and watertight decks  206 ,  208 ,  210  and two airtight and watertight compartments  216 ,  218  in the centerwell  204 , those of ordinary skill in the art will appreciate that the present embodiments encompass a centerwell having any number of airtight and watertight decks and compartments. Specifically, the advantages of the present spar platform, as described above, may be realized by employing only a single airtight and watertight transverse barrier or deck (e.g. the deck  206  shown in the drawings). In such an embodiment, the single barrier divides the centerwell into an upper portion that is open to the atmosphere, and a lower portion, open to the sea, that provides the variable buoyancy compartment  220 , and there are no buoyancy chambers defined between two or more decks. Similarly, if only two airtight and watertight barriers or decks are provided, there will be a single buoyancy chamber defined between them. In another embodiment, three or more such barriers or decks may be provided, with a buoyancy chamber defined between each adjacent pair of barriers or decks. 
   In an alternative embodiment of the present spar platform a lower end of the centerwell may be sealed by an airtight and watertight barrier. The airtight and watertight barrier may be substantially identical to the decks  206 ,  208 ,  210  described above and illustrated in  FIGS. 2 and 3 . In this embodiment seawater may not flow in and out of the centerwell naturally as in the embodiment of  FIGS. 2-4 . However, in certain embodiments having a closed lower end seawater may be added to and/or removed from the centerwell to adjust the buoyancy of the platform. The seawater may be added and/or removed using, for example, pumps (not shown). As in the embodiment of  FIG. 2 , airtight and watertight sleeves may extend through the centerwell, and in certain embodiments the sleeves may extend from the uppermost barrier or deck to the lowermost barrier or deck. 
   The above description presents the best mode contemplated for carrying out the present invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make and use this spar platform. The present invention is, however, susceptible to modifications and alternate constructions, in addition to those discussed above, that are fully equivalent. Consequently, the present invention is not limited to the particular embodiments disclosed herein. On the contrary, the present invention encompasses all modifications and alternate constructions coming within the spirit and scope of the invention, as generally expressed by the following claims, which particularly point out and distinctly claim the subject matter of the invention.