Abstract:
A spar-type offshore platform includes a buoyant upper hull structure supporting a deck and having lower end in which is received a buoyant lower mooring module. The upper hull structure is connected to the mooring module by connection lines. The upper hull structure is removed from the mooring module by disconnecting the connection lines from the upper hull structure while leaving the connection lines attached to the mooring module and while the mooring module remains moored to the seabed. The mooring module is lowered relative to the upper hull structure, allowing the latter to be moved away. The upper hull structure may be re-positioned over the mooring module, and the mooring module may be hauled upward into engagement with the lower end of the upper hull structure, so that the connection lines can be recovered and re-attached to the upper hull structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit, under 35 U.S.C. §119(e), of co-pending provisional application No. 60/617,346, filed Oct. 8, 2004, the disclosure of which is incorporated herein by reference. 

   FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   This application relates to offshore platforms for the exploration for, and production of, undersea petroleum deposits, and, in particular, to the various types of platforms generically known as spars, whether of the classic, truss, or cell spar variety. More specifically, the present invention relates to a spar-type platform, of the type having a buoyant upper hull structure and a lower buoyant section or module that supports the mooring and risers when the upper hull structure is detached, in which the lower section is constructed as a sub-sea mooring buoy (SSMB), wherein the upper hull section is detachably connected to the lower (SSMB) section. 
   The development of sub-sea petroleum and natural gas deposits in Arctic deep water regions presents special challenges for offshore platform designs. Specifically, platforms in these regions must be able to resist local and global loads from ice in addition to loads conveyed by wind, waves, and currents. In some cases, a platform must be moved to avoid contact with or collision with sea ice and icebergs. 
   One type of platform that has become widely used for the development of deep water deposits is the spar, especially spars that provide for the storage of petroleum or natural gas. The threat of ice would make it advantageous for the hull of the spar, containing storage or not, to be disconnectable or detachable from its mooring and riser system to avoid impact from the ice. Also, the staged development of a particular deposit may be facilitated by changing out topside facilities (by the detachment of the upper hull structure) as development progresses. 
   SUMMARY OF THE INVENTION 
   Broadly, the present invention is a spar-type platform comprising an upper hull structure that supports the topside facilities and equipment and that provides the buoyancy and ballasting functions and (optionally) a storage function (as in a typical spar), and a lower hull structure or module that forms part of the mooring system and that functions as a sub-sea mooring buoy (SSMB). The upper hull structure and the SSMB module are connected by a detachable connection mechanism, whereby the upper hull structure can be detached from the SSMB module and moved, either by towing or by an on-board propulsion system, to avoid or evade an environmental threat (e.g., floating ice or an iceberg), and then reattached to the SSMB module when the threat has passed. The SSMB module is sized for buoyantly supporting the mooring lines and the riser system that are left behind when the upper hull section is detached and removed. 
   In the attached condition, the two hull structures are connected by a plurality of connection lines (which can be chains, steel ropes, cables, or combinations thereof) that run from a plurality of chain jacks or fairleaders on the deck of the upper hull section, and through a fairleader/pulley mechanism mounted on the SSMB module. These connection lines can be part of the mooring system, or additional lines can be used to make the connection, or a combination of both can be used. (If additional lines are used in conjunction with the mooring lines, they are referred to as “tie lines.”) During the disconnection process, these lines are slackened at the deck level and removed from the fairleader/pulleys on the SSMB module. These lines are then carried away by the upper hull structure. 
   In a specific embodiment in which mooring lines are used to connect the two hull structures, the mooring lines are run through chain jacks and chain stoppers mounted on the upper hull structure in the traditional configuration, and they are run down the outside of the spar. Another set of chain stoppers is provided on the SSMB module to maintain tension in the mooring lines between the upper hull structure and the SSMB module. These lines are run through fairleaders mounted on the SSMB module. During the disconnection process, the mooring lines are lowered using guide lines and locked off at the fairleaders. They remain supported by the SSMB module during disconnection. The guide lines are dropped from the upper hull structure and allowed to hang from the SSMB fairleaders. 
   The weight of the mooring lines and risers, now unsupported by the buoyancy provided by the upper hull structure, causes the SSMB module to separate from the upper hull structure. The SSMB module, carrying the mooring lines and risers, moves downward, controlled by a chain in the centerwell, until the effective weight of the risers and mooring lines is decreased as they lay on the sea floor, until the weight of the mooring lines and risers is equal to the buoyancy of the SSMB module. 
   The upper side of the SSMB module carries a plurality of upwardly-extending guide posts and a guiding structure that is rigidly fixed to the top of the SSMB module. The guide posts and guiding structure provide the proper alignment between the upper hull structure and the SSMB. The guiding structure also includes a pneumatically- or hydraulically-controlled interface template for the connection of the risers between the upper hull structure and the SSMB module, and it provides structural strength to the connection by fitting inside the underside of the upper hull structure. 
   For reconnecting the two hull sections, the upper structure is maneuvered above the SSMB module. A haul-in line (chain or steel rope) is lowered and connected to the apex of the guiding structure using a remotely-operated vehicle (ROV). To assist in aligning the upper hull structure and the SSMB, guide post lines are lowered through guide post receptacles on the upper hull structure from the deck of the upper hull structure. The guide post lines are attached to the tops of the guide posts by an ROV. In combination with the haul-in line attached to the apex of the guiding structure, the guide post lines pull the guide posts into their respective receptacles, thereby aligning the upper hull structure and the SSMB. A winch on the upper hull structure draws the SSMB module into a docking bay in the bottom of the wellbay of the upper hull structure, with the guide posts engaging the receptacles to guide the SSMB module into place. The mooring lines are retrieved using an ROV, and they are winched back to the chain jacks. The tie-lines, if used, are reconnected around the fairleaders and tensioned to the deck of the upper hull structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a spar-type platform in accordance with a first embodiment of the present invention; 
       FIG. 2  is a top plan view of the spar-type platform of  FIG. 1 ; 
       FIG. 3  is a detailed view of the portion enclosed within the dashed outline  3  in  FIG. 2 ; 
       FIG. 4  is a cross-sectional view taken along line  4 — 4  of  FIG. 3 ; 
       FIG. 5  is a cross-sectional view taken along line  5 — 5  of  FIG. 4 ; 
       FIG. 6  is a cross-sectional view taken along line  6 — 6  of  FIG. 1 ; 
       FIG. 7  is a cross-sectional view taken along line  7 — 7  of  FIG. 6 ; 
       FIG. 8  is a cross-sectional view taken along line  8 — 8  of  FIG. 7 ; 
       FIG. 9  is a side elevational view of the spar-type platform of  FIG. 1 ; 
       FIG. 10  is a side elevational view of a modified form of the spar-type platform of  FIG. 1 ; 
       FIG. 11  is a side elevational view of the spar-type platform of  FIG. 1 , showing the upper hull structure connected to the SSMB module, the latter being moored to the seabed; 
       FIG. 12  is a side elevational view of the spar-type platform of  FIG. 1 , showing the disconnection of the upper hull structure from the SSMB module; 
       FIG. 13  is a side elevational view of the spar-type platform of  FIG. 1 , showing the upper hull structure being towed away from the SSMB module after disconnection; 
       FIG. 14  is a side elevational view of the spar-type platform of  FIG. 1 , showing the upper hull structure positioned above the SSMB module during retrieval of the SSMB for re-attachment of the SSMB module to the upper hull structure; 
       FIG. 15  is a side elevational view of the spar-type platform of  FIG. 1 , showing the step of drawing the SSMB module toward the upper hull structure for re-attachment of the SSMB module to the upper hull structure; and 
       FIG. 16  is a detailed cross-sectional view showing a guide post, guide post receptacle, and guide post line used to align the SSMB module and the upper hull structure. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, and particularly  FIGS. 1 ,  2 , and  9 , a spar-type platform  10 , in accordance with a preferred embodiment of the invention, includes an upper hull structure  12  and a buoyant lower section or module, configured as a sub-sea mooring buoy (SSMB)  14 , wherein the upper hull structure  12  and the SSMB module  14  are detachably connected to each other so that the upper hull structure  12  can be removed from the SSMB module  14  and relocated, either by towing or under its own power. The SSMB module  14  can be subsequently retrieved and reconnected to the upper hull structure  12 . The invention is described herein in the context of a cell spar, but it will be understood that it is easily adaptable for use with a so-called “classic” spar and with a truss spar. 
   The upper hull structure  10  comprises a plurality of interconnected elongate, hollow cells  16 , each of which may be divided by a series of vertically-spaced, transverse bulkheads (not shown) into a plurality of compartments (not shown), as is well-known in the art. The uppermost compartments are typically air-filled to provide buoyancy, while the lowermost compartments are typically filled with seawater to provide ballast, so as to keep the platform upright. The intermediate compartments may be used for the storage of petroleum. The tops of the cells  16  support a deck  18 , on which are installed the topside facilities and equipment (not shown) that are typical for such platforms. 
   Spaced around the periphery of the deck  18  are a plurality of upper line holding elements  20 , which may be chain jacks or fairleaders. Similarly spaced around the periphery of the SSMB module  14  are a plurality of lower line holding elements  22 , which may likewise be chain jacks or fairleaders. A plurality of mooring lines  24  are anchored in the seabed  26  by anchors  28 . Each of the mooring lines  24  is run through one of the lower chain jacks or fairleaders  22 , then up the side of the upper hull structure  12  and through a guide element  30 , and then through one of the upper fairleaders  20 . The mooring lines  24  are secured to the upper hull structure  12  by means of upper chain stoppers or cable locks  32 , and to the SSMB module  14  by means of lower chain stoppers or cable locks  34 . Thus, the extended mooring lines  24  serve as connection lines for connecting the upper hull structure  12  to the SSMB module  14 . 
   Alternatively, the mooring lines  24  may extend only between the SSMB module  14  and the anchors  28 , with the connection between the upper fairleaders  20  and the lower fairleaders  22  being provided by tie lines, as discussed below with reference to  FIG. 10 . The mooring lines  24  (or tie lines, as the case may be) are run through protective shrouds  36  that are advantageously provided on the exterior of the upper hull structure  12  at the water line. 
   As best shown in  FIGS. 3–5 , a preferred arrangement for the mooring/tie lines  24  is to have them wound on winches  38  (only one of which is shown in  FIG. 4 ) mounted below the deck  18  of the upper hull structure  12 . Each line  24  then extends through a deck aperture  40  before being run through the pulley of an upper fairleader  20 , then down through a guide element  30  and an upper cable lock or chain stopper  32  before passing through a shroud  36  as it descends along the side of the upper hull structure  12 . As shown in  FIGS. 6 and 7 , at the SSMB module  14 , each of the lines  24  passes through a lower cable lock or chain stopper  34  before passing through the pulley of a lower fairleader or chain jack  22 . 
     FIG. 8  illustrates one type of locking mechanism that may be used for the upper cable lock/chain stoppers  20  and/or the lower cable lock/chain stoppers  22 . In this mechanism (which is described by way of example only), a pair of opposing pivoting clamp arms  42 , actuated by a hydraulic or pneumatic cylinder  44 , clamp down on the line  24  when the locking mechanism is actuated, and they release the line when the mechanism is de-actuated. 
     FIG. 10  illustrates a modification of the present invention, in which a plurality of tie lines  45  are used as connection lines to connect the upper hull structure  12  to the SSMB module  14 , instead of or in addition to the extended mooring lines  24  described above. Each of the tie lines  45  is fed from a tie line winch  47  on the deck  18 , and then it extends down along the side of the platform  10 , loops around a line guide  49 , such as a pulley, on the SSMB  14 , and then extends back up to an anchor point  51  on the deck  18 . With this modification, the upper hull structure  12  is disconnected from the SSMB module  14  by feeding slack to the tie lines  45  until they decouple from their respective line guides  49 , freeing the SSMB module  14  from the upper hull structure  12 . The tie lines  45  are carried away with the upper hull structure  12 . In re-connecting the upper hull structure  12  to the SSMB module  14 , an ROV (not shown) is used to couple the tie lines  45  to their respective line guides while the winches  47  take in the slack to tighten the tie lines  45  up on the line guides  49 . 
   The SSMB module  14  comprises a plurality of buoyancy cells or chambers  46  ( FIG. 6 ) arranged around a central passage through which a lower riser casing  48  (see  FIG. 1 ) passes. The upper end of the lower riser casing  48  is detachably connected to the lower end of an upper riser casing  50  (see  FIG. 2 ) that extends axially through the upper hull structure  12 . A plurality of risers  52  (see  FIG. 1 ), having lower ends that are connected by flexible jumpers (not shown) to seabed wellheads (not shown), pass through the riser casings  48 ,  50  for connection to appropriate structures on the deck  18 , as is well-known and conventional in the art. The risers  52  are separable into bottom and top portions that are detachably connected to each other at a riser interface template (not shown) that is included in a guiding structure  54  fixed to the top of the SSMB module  14 , as further described below. The specific configuration of the interface template will depend upon the specific configuration of the risers and control lines in a given platform, but the use of such templates in offshore platform applications is known, and the design of suitable templates for the purpose of this invention is considered to be within the ability of those of ordinary skill in the pertinent arts. 
   As shown in  FIGS. 12–15 , the guiding structure  54  is affixed to the top of the SSMB module  14 . The guiding structure  54  is a truss structure that nests into a sub-surface docking bay  55  in the bottom of the upper hull structure  12 . The guiding structure  54  has an upper apex to which is fixed a connection element  56  (which may be a hook, a loop, or equivalent structure) that is removably connected to a haul-in line (cable or chain)  66  by means of a hook  68  or the equivalent thereof. The haul-in line  66  is raised and lowered by a haul-in winch  70  on the deck  18  of the upper hull structure  12  (see  FIG. 14 ), and it is employed in the process of separating the upper hull structure  12  from the SSMB module  14 , and in the process of re-attaching the SSMB module  14  to the upper hull structure  12 , as described below. The top of the SSMB module  14  also includes the above-mentioned interface template (not shown) for the attachment of the lower riser casing  48  to the upper riser casing  50 , and for the attachment of the top and bottom portions of the risers  52 , as mentioned above. The interface template may also include means for detachably connecting control lines (not shown) that typically extend from the deck  18  to the welIheads. 
   A plurality of guide posts  58  are spaced around the periphery of the SSMB module  14  and extend upwardly therefrom. A plurality of guide post receptacles  60  are located around the periphery of the upper hull structure  12 , near the lower end thereof, so as to receive the guide posts  58  when the SSMB module  14  is connected to the upper hull structure  12 . As shown in  FIG. 16 , each of the guide post receptacles  60  is formed as a tubular element with an axial passage  72  therethrough. A plurality of guide post lines  74  (one of which is shown in detail in  FIG. 16 ) are raised and lowered through the guide post receptacle passages  72  by guide post line winches  76  and guide post line pulleys  78  mounted on the deck  18  of the upper hull structure  12 , and the end of each of the guide post lines  74  is detachably fastened to the top of a respective guide post  58 . 
   The process of disconnecting and removing the upper hull structure  12  from the SSMB module  14  is illustrated in  FIGS. 11–13 .  FIG. 11  shows the spar platform  10  with the upper hull structure  12  connected to the SSMB module  14 . At the beginning of the disconnection process, as illustrated in  FIG. 12 , the mooring lines  24  are lowered using guide lines  62  and locked off at the lower chain jacks or fairleaders  22 . The mooring lines  24  remain supported by the SSMB module  14  during disconnection. The guide lines  62  are dropped from the upper hull structure and allowed to hang from the SSMB chain jacks or fairleaders  22 . 
   The weight of the mooring lines  24  and the bottom portions of the risers  52 , now unsupported by the buoyancy provided by the upper hull structure  12 , causes the SSMB module  14  to sink, controlled by the haul-in line  66  and the guide post lines  74 , and thus to separate from the upper hull structure  12 . The SSMB module  14  continues to sink as the effective weight of the riser bottom portions and the mooring lines  24  decreases as they settle on the sea floor, until the weight of the mooring lines and risers is equal to the buoyancy of the SSMB module  14 . 
   As shown in  FIG. 13 , the haul-in line  66  is disconnected from the guiding structure  54  on the SSMB module  14  (the guide post lines  74  having been disconnected from their respective guide posts  58 ), and the upper hull structure  12  is towed away by a vessel  64 . Alternatively, the upper hull structure  12  may have its own propulsion system (not shown), so that it cam move away from the SSMB module  14  under its own power. 
   When it is desired to re-connect the upper hull structure  12  to the SSMB module  14 , the upper hull structure  12  is positioned over the SSMB module, as shown in  FIG. 14 , and the haul-in line  66  with the hook  68  on its end is lowered by the haul-in winch  70 . The hook  68  is engaged with the connection element  56  by means such as an ROV (not shown). The ROV also re-attaches the guide post lines  74  to their respective guide posts  58 . The SSMB module  14  is hauled upwardly toward the bottom of the upper hull structure  12 , as shown in  FIG. 15 , by the haul-in line  66 . As the SSMB module  14  rises, the guide posts  58  on the SSMB module  14  are aligned with their respective receptacles  60  by means of the guide post lines  74 , whereby each of the guide posts  58  registers with and enters the appropriate corresponding guide post receptacle  60  on the upper hull structure  12 . By this process, the guiding structure  54 , with its interface template, is properly seated in the docking bay  55  of the upper hull structure  12  for the re-attachment of the upper and lower portions of the risers  52  and for the re-attachment of any control lines that need to be re-connected. The ROV may then recover the guide lines  62  for re-attaching the mooring lines  24  to the upper hull structure  12  in the manner discussed above. 
   While a preferred embodiment of the invention has been described herein, it has been set forth by way of example only, and is meant to encompass a wide range of equivalent structures. It will be appreciated that a number of variations and modifications will suggest themselves to those skilled in the pertinent arts, and that many of the components and mechanisms specifically described in this specification will find equivalents in the technical arts that are applicable to the present invention. Thus, for example, as mentioned above, the present invention will be readily adaptable to the various types of spar-type platforms known in the art, and the modifications necessary or advantageous to accommodate the invention to various types of spars will be easily understood by those skilled in the pertinent arts. Also, as will be appreciated by those skilled in the pertinent arts, the term “line” as used in this specification, is meant to encompass a cable, a chain, a steel rope, or any functional equivalent thereof. Likewise, the line holding mechanisms described herein may encompass any suitable mechanism available in the art that may accomplish the functions ascribed to these mechanisms. These and other modifications and variations should be considered within the spirit and scope of the present invention, as defined in the claims that follow.