Abstract:
A floating offshore platform configuration is provided, which decouples pitch, roll, and heave motions from acting on tensioned risers and accommodates the angular displacement induced by floating offshore platform surge and/or sway excursion without inducing bending in the riser at its entrance to the floating offshore platform. The risers are guided by an inner structure that is tethered from the sea floor and centered inside an outer hull structure. Outer hull structure heave, pitch and roll motions are substantially isolated from acting on the inner structure through a connection mechanism, and each riser is allowed to individually expand or contract.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a Continuation-in-Part Application which claims priority from U.S. patent application Ser. No. 10/017,175 filed on Dec. 7, 2001, which claims priority from U.S. Provisional Application Serial No. 60/251,938, filed on Dec. 7, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an arrangement for an offshore platform for drilling or workover operations, production and/or storage operations and in particular to an arrangement for coupling subsea risers to a floating offshore structure which substantially isolates the structure&#39;s heave, pitch, and roll motions from the risers. The term floating offshore structure in this specification includes SPARS, FPSO&#39;s, floating offshore drilling platforms and the like. 
     2. Description of the Prior Art 
     The prior art has sought arrangements for coupling subsea risers to floating offshore structures. For example, U.S. Pat. No. 4,606,673 discloses a riser support can for a SPAR Buoy where a single buoyant riser support can supports several risers. The SPAR Buoy, a floating deepwater production and oil storage vessel, includes a riser system whereby risers are connected to a riser float chamber that moves along guides within a vertical passageway within the vessel. The riser support includes an adjustable support which repartitions the load on the risers to assume that each riser is uniformly tensioned. Hull heave motion is decoupled from the riser, but pitch and roll motions of the hull are transferred to the risers. As a result, the risers of this configuration are subjected to cyclical bending. Furthermore, as the adjustable riser supports do not provide any capability of axial vertical flexure relative to one another, this arrangement of riser support does not permit individual riser length fluctuations commonly occurring as a result of operating riser temperature and internal pressure changes. The risers supported as shown therein are subject to cyclical variation in tension as well. The vessel does not have a moonpool and is not designed for drilling or extensive workover operations. If drilling equipment is desired on the SPAR Buoy of the U.S. Pat. No. 4,606,673 structure, the draw works of such drilling equipment would be mounted on the surrounding hull and therefore would require heave compensation. 
     U.S. Pat. No. 4,966,495 discloses a floating drilling and production structure that includes two independently floating bodies. An outer production and drilling semi-submersible vessel completely surrounds an independently floating wellhead support buoy and supports the weight of the drilling platform, machinery, etc. and is ballasted and anchored in a manner similar to a conventional semi-submersible vessel. An inner constant tension buoy supports many risers. Pitching of the outer vessel is decoupled from the single buoy which supports the risers. The inner buoy (or riser can) is centered within the hull by an annular bumper. The risers are attached to the single riser can by lockdown screws. A significant disadvantage of the U.S. Pat. No. 4,966,495 arrangement is that the semi-submersible “outer” vessel has a large waterplane area thereby producing large forces in tension due to wave action and hull extension. Another disadvantage is that the drilling or workover equipment is mounted on the outer vessel which induces bending in the drill pipe when there is relative pitch between the outer body and the inner riser support buoy. Thus, the advantage of decoupling the pitch of outer hull from the inner riser support body can only be accomplished when the drilling rig is not in use. Another disadvantage of the arrangement of U.S. Pat. No. 4,966,495 is that individual riser elongation due to temperature and/or internal pressure variation is not allowed for. Furthermore any pitch of the inner riser support buoy results in fluctuation in riser tension because of the large waterplane area of the inner support buoy. 
     U.S. Pat. No. 4,913,238 discloses a TLP moored riser support module with a conventionally moored semi-submersible hull. A drilling draw works is located on the semi-submersible hull. A relatively small tension leg platform provides a heave-restrained deck for surface wellhead equipment. The hull is free to pitch, roll and heave independently of the risers. The riser support module, being installed within a semi-submersible is exposed to the environment and suffers loading induced thereby. As the semi-submersible hull heave, pitch and roll motions are decoupled from the motions of the riser support module, and the draw works are installed on the semi-submersible hull, the draw works require heave compensation and riser bending due to semi-submersible hull pitch and roll is inherent with this design. 
     U.S. Pat. No. 4,735,267 discloses a floatation buoy with ballast for supporting multiple risers. The buoy is arranged to be pulled within a moonpool of a buoyant hull. The buoy allows angular flexing of the risers. Individual riser length adjustment is accounted for by allowing the risers to take a catenary shape. The buoy is rigidly connected to the hull of the production vessel. Hanging the risers from the top of the buoy results in static instability, because as the buoy is pulled into the buoyant hull, it becomes unstable and tends to invert unless the buoy is ballasted to negative buoyancy. Because the floatation buoy is not tethered vertically to the sea floor, it is free to heave with the floating production platform, suffering the motions and loads induced thereby. 
     International patent publication WO 00/58598 shows a riser guide frame which is retractable in the vertical direction for one or more risers on a semi-submersible production vessel. The guide frame provides lateral support for individual riser support buoys. The arrangement of the WO 00/58598 publication provides for lowering the riser support buoys to a point below the splash zone with only the tops of the risers protruding through the splash zone. The riser frame is not tethered to the sea floor, does not have buoyancy, and is rigidly connected to the semi-submersible hull during operation, so the riser frame induces wear through its contact with the risers and their main buoyancy members due to semi-submersible heave. Bending is induced into the risers due to semi-submersible pitch, roll displacements and surge and sway excursions. 
     U.S. Pat. No. 3,601,075 discloses a system for riser support and guidance within a weathervaning hull. A guide decouples hull heave from riser tension by guiding the riser within a sleeve having rollers with horizontal axes. The system is pendular and allows angular deflection of the riser upon hull excursion through rotation on a spherical bearing or gimbals. The riser includes a buoyant element, but tensioning is accomplished by a hydraulic draw works. Mechanical means maintain the tower and draw works in a vertical position, and the guides act directly on the riser rather than on the riser buoy. The buoy allows bending to occur in the riser, because the buoy is not guided within a framework, so the riser bends when the riser is not vertical. 
     French patent publication 2,574,367 shows a variety of drilling production and storage platforms which include a central TLP moored-core buoyant structure surrounded by a hull capable of production and storage. The surrounding hull is free to heave up and down on the Tension Leg Pylon or free to heave and rotate on the Tension Leg Pylon or constrained by its own Tension Leg Moorings. Drilling rig and production equipment are disclosed as being placed on the TLP core. The French patent discloses a floating platform with tension leg means for station keeping. 
     U.S. Pat. No. 6,161,620 shows a riser can which accepts sliding on the surface of the can, rather than on a riser stem. 
     U.S. Patent Publication 6,176,646 B1 shows a riser arranged pendularly within the riser can. The riser can has an open bottom and an arrangement which allows riser flexing without over bending at the bottom of the riser can through supports which guide the riser, thereby limiting its minimum bend radius due to spar pitch, roll, surge and/or sway. 
     U.S. Pat. No. 4,702,321 shows a spar with individual flotation buoys attached for tensioning the top ends of each individual riser connected to the sea floor. The patent shows guides for handling the relative motion between the floating structure and each sea floor fixed riser. Stems above and/or below the buoys are described which cooperate with penetrations in the decks to control the relative position of the riser axis while suffering the relative motion of the floating structure. 
     Because the guides are connected directly to the platform hull, any hull pitch, roll, surge or sway motion is directly transferred to the risers through those guides. Furthermore, all heave motion of the hull is taken at the interface between the hull and the riser stems. 
     Prior art buoyancy cans for risers are also known that have flatbars welded to their sides which may be designed as sacrificial members to protect the integrity of the buoyancy cans due to their inherent obligation to withstand all relative motion at that interface. 
     IDENTIFICATION OF OBJECTS OF THE INVENTION 
     A primary object of the invention is to provide an improved arrangement for decoupling heave, pitch, and roll motions between a floating offshore platform and risers. The object is to provide an arrangement for supporting subsea risers which is applicable to semi-submersible, SPAR, TLP and FPSO platforms and can be installed within a moonpool or turret thereof. 
     Another object of the invention is to provide a riser support arrangement for a floating offshore platform that provides pendular support between the risers and a surrounding hull, to allow the risers to tilt in a pendular manner in response to lower frequency surge and sway excursion motions. 
     Another object of the invention is to provide individual riser buoyancy modules installed in a floating framework which is attached to the sea floor through either a drilling riser or a tendon with a drilling rig installed on the floating framework. 
     Another object of the invention is to provide an arrangement for centering a floating framework within a centerwell of the platform which includes link arms between the floating framework and the platform. 
     Another object of the invention is to provide a floating framework and platform arrangement where flotation elements of the framework are completely submerged so that no waterplane area exists in order to exert a constant buoyant force on the framework. 
     Another object of the invention is to provide a floating framework and a platform hull arrangement that provides individual riser buoyancy, a draw works decoupled from hull motion of the platform, and decoupling of hull motion from the risers so as to eliminate cyclical bending of the risers. 
     Another object of the invention is to provide a floating framework and a platform hull arrangement characterized by decoupling of the risers from hull pitch and constant riser tension regardless of hull motion, thereby avoiding cyclical tension of the risers. 
     Another object of the invention is to provide individual riser floating framework that is centered within a platform hull arrangement where the floating framework does not have significant variation in tension due to wave action and hull excursion. 
     Another object of the invention is to provide a floating framework within a platform hull where a drilling rig is mounted to the floating framework so that it does not require heave compensation and does not induce bending due to the elimination of relative pitch between the surrounding hull and riser support buoy. 
     Another object of the invention is to provide a floating framework within a platform hull where the floating framework with riser support buoyancy modules is completely submerged, with the result that tension load fluctuations are minimized. 
     Another object of the invention is to provide a floating framework within a platform hull where protection is provided to the riser support module, and the draw works of a drilling rig is mounted on that module to decouple heave, pitch and roll from the risers and the module supports each riser through individual buoyancy devices. 
     Another object of the invention is to provide a floating framework within a platform hull where vertical risers are supported from the framework with allowance for individual expansion and angularity as a bundle, where risers are decoupled from hull heave and pitch, where the draw works is mounted on the protective guide frame, and where a tendon is moveable to maintain a constant height of the framework above the sea floor. 
     Another object of the invention is to provide a floating frame within a platform hull where the frame is buoyant and tethered to the sea floor, has a draw works mounted on it, provides pendular coupling between frame and hull so as to avoid inducing bending of risers carried by the frame and positions the risers within a central vertical opening of the protective hull. 
     Another object of the invention is to provide a floating frame pendularly coupled to a platform hull with guided buoyancy dividers within the frame for tensioning of multiple risers, and with a drilling rig mounted on the floating frame. 
     Another object of the invention is to provide a floating frame within a platform hull with an arrangement which allows for independent variation in riser length, with process equipment mounted on the hull and with a buoyant frame tethered to the sea floor by a tendon, but with station keeping of the arrangement accomplished with conventional mooring of the hull. 
     Another object of the invention is to provide a floating frame within a platform hull with a riser can arrangement pendularly coupled to the hull. 
     SUMMARY OF THE INVENTION 
     A floating offshore arrangement substantially decouples pitch, roll and heave motions between an outer hull structure and buoyantly supported risers which are vertically oriented by a frame or support buoy positioned within the interior of the hull structure. The risers are arranged and designed to slide vertically with respect to the support buoy. The support buoy is coupled to the outer hull structure by a mechanism that allows it to remain in a nearly vertical orientation at a fixed distance above the sea floor while the outer hull is free to heave, roll and pitch. The support buoy is allowed to rotate in a pendular fashion in response to the angularity of risers produced by outer hull excursions in surge and sway. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the invention in a first embodiment of an offshore platform showing an inner structure pendularly centered inside an outer hull structure; 
     FIG. 1A shows a framework for an inner structure of FIG. 1 with a support buoy and drilling works mounted thereon and with a central shaft tethered to the sea floor and with riser guides mounted thereon; 
     FIG. 1B illustrates buoyancy cans mounted on a riser which is guided by upper and lower guides; 
     FIGS. 1C and 1D illustrate the orientation of the outer hull and the inner structure under a calm vertical condition (FIG. 1C) and under a surge and/or sway angularly displaced position (FIG.  1 D); 
     FIG. 2 is a cross section of FIG. 1 looking downward along lines  2 — 2  showing the details of a connection mechanism which decouples motions of the outer hull structure from acting on the inner structure; 
     FIG. 3 is a cross section of FIG. 1 looking downward along lines  3 — 3  showing the support buoy with riser openings and a central opening for a central column; 
     FIG. 4 is a cross section of FIG. 1 looking downward along lines  4 — 4  showing the arrangement of individual risers floats around a central column; 
     FIG. 5 illustrates the invention in another embodiment of an offshore platform showing an alternative connection mechanism; 
     FIG. 6 illustrates the invention in another embodiment of an offshore platform with riser support provided by individual cylindrical buoyancy cans; 
     FIG. 7 is a cross-section looking downward along lines  7 — 7  of FIG. 6 showing the arrangement of the cylindrical buoyancy cans; 
     FIG. 8 illustrates a slidable coupling arrangement between an individual buoyancy can and a support structure; 
     FIG. 9 is a cross section looking downward along lines  9 — 9  of FIG. 8 showing how the support structure serves as guidance for the buoyancy can; 
     FIG. 10 illustrates an alternative coupling arrangement for a square buoyancy can; 
     FIG. 11 is a cross section of FIG. 10 looking downward along lines  11 — 11  showing how the sliding shoes couple to the corners of the square buoyancy cans; 
     FIG. 12 illustrates another slidable coupling between an individual buoyancy can and a support structure; 
     FIG. 13 is a cross section of FIG. 12 looking downward along lines  13 — 13  showing a channel track/sliding shoe interface; 
     FIG. 14 is an alternative arrangement for square buoyancy cans using corner reinforcement devices; and 
     FIG. 15 is a cross section of FIG. 14 looking downward along lines  15 — 15  showing the arrangement of the comer reinforcement devices. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates the invention in an embodiment of a floating offshore platform  10 . In this embodiment, the floating offshore platform  10  includes an inner structure  15  and an outer hull structure  30 . The inner structure  15  includes a riser guide structure  35  and a support buoy structure  45  with a drilling draw works  120  mounted thereon. 
     The riser guide structure  35  is fixed with respect to the sea floor by means of at least one riser or tension member. A support buoy structure  45  is fixedly coupled to the riser guide structure  35 . The outer hull structure  30  is coupled to the support buoy structure  45  by a connection mechanism  55 . The heave, pitch and roll motions acting on the outer hull structure  30  are decoupled from inner structure  15  (riser guide structure  35  and support buoy structure  45 ) by means of the connection mechanism  55 . Transfer of the heave forces on the buoy structure  45  to the riser guide structure  35  is reduced, if not completely eliminated, by means of the tethered connection to the sea floor of the riser guide structure  35  and the fact that the buoy structure  45  is free to slide vertically with respect to the risers which are independently supported by one buoyancy module per riser. As a result, heave, pitch, and roll motions all are effectively decoupled from the risers  90  and their buoyancy modules  50 . 
     In other words, the hull  30  is free to move in pitch, roll and heave without such pitch, roll and heave movements being transferred to the inner structure  15 , because of the connection mechanism  55 , and because support buoy structure  45  (on which drilling rig equipment  120  is mounted) and riser guide structure  35  are fixedly connected to one another and tethered to the sea floor preferably by a central tether member  100  as shown in FIGS. 1 and 1A. (Other tethering arrangements may be provided as described below.) The inner structure  15  (including riser guide structure  35  and support buoy structure  45 ) is free to translate due to surge forces of the hull structure  30  being transferred to the inner structure  15 , via connection mechanism  55 . Such translation causes risers  90  to pivot from their tethered connection to the sea floor. When the risers pivot, the risers  90  coupled to buoyancy modules  50  are free to slide vertically within lateral guides  85  and  86  of riser guide structure  35 . 
     The outer hull structure  30  includes an upper hull  42 , a middle hull  44 , and a lower hull  46 . The offshore platform  10  can take alternative forms. For example, the exterior shape of the outer hull structure may be a Tension Leg Platform (TLP), a SPAR (e.g., cylindrical shape), or FPSO or FSO ship shape. Its internal shape is preferably conical (as illustrated in FIG. 1) to allow the support structure  45  to pivot with respect to the tethering point on the sea floor (see FIG. 1D) when the hull  30  translates from its stable position due to sea surge and/or sway forces. One of the primary effects of the shape of the outer hull structure  30  is to protect the inner structure  15  from environmental effects such as wind and waves. In the embodiment of FIG. 1, the outer hull structure  30  circumferentially surrounds the inner structure  15  with the inner structure  15  being pendularly centered inside the outer hull structure  30 . Preferably, the lower hull  46  is at such a depth that the majority of sea forces hit the outer hull structure  30  while protecting the inner structure  15  from such radial sea forces. As shown in FIG. 1, the lower hull section  46  extends horizontally (outwardly) serving to suppress motions in response to vertical sea forces (heaving). The middle hull section  44  slants at an angle (that is, it is conically shaped) such that pitching motions of the outer hull  44  do not cause the hull  44  to contact the buoy structure  45  or riser support structure  35 . Preferably, the outer hull structure  30  is buoyant; and, if desired, the outer hull structure  30  can be ballasted and deballasted by any means known in the art. The hull as mentioned previously may be a SPAR, TLP or ship shaped hull. Its station keeping may be provided through catenary, inverted catenary, taut, semi-taut or other conventional means or through tension legs or dynamic positioning. In the event the hull  30  is allowed to weathervane, its interior conical structure would be provided by the geostationary portion of a traditional single point mooring turret allowing its connected FPSO hull to weathervane about the buoy and geostationary turret. Additionally, a production deck  66 , as known in the art of hydrocarbon production and storage, is mounted (if desired) to the upper hull section  42 . The outer hull structure  30  may be conventionally moored to the sea floor, e.g. by anchor legs  200  as shown in FIG.  1 . As mentioned above, dynamic positioning or tension legs could be used alternatively to anchor legs. Such mooring of the outer hull structure decreases or eliminates the need for complete reliance on the mooring connection of the riser structure  35 , illustrated below. The interior conical structure of moonpool  75  may be an opening in a TLP, SPAR or ship shaped hull. 
     The outer hull  30  is coupled to the inner structure  15  via the support buoy  45  by means of a passive, permanent connection mechanism  55 . Such connection mechanism is designed and arranged to allow the support buoy  45  to pivot about two horizontal axes with respect to hull  30  simultaneously. In other words, pendular coupling between support buoy  45  and hull member  42  is preferred. A preferred arrangement of the passive linkage and non-disconnectable connection mechanism  55  includes four legs or link arms  19  as seen in FIG.  2 . The connection mechanism  55 , which is permanently connected between outer hull  30  and inner structure  15 , substantially isolates motions of the hull section  42  in pitch and roll from the vertically oriented buoy  45  and simultaneously isolates heave motions of the hull section  42  from the support buoy  45 . This is accomplished by an arrangement of pinned flex arms  19  arranged as illustrated in FIG.  2 . The arms  19  centralize the buoy  45  within moonpool  75  inside the outer hull structure  30  and hull section  42 . Because the pinned link arms  19  can pivot at their connections  17 ,  13  to the buoy  45  and the hull section  42  and can flex as a result of spherical bushings  1 , heave, pitch and roll motions acting on the outer hull structure  30  are decoupled from acting on the buoy  45  and inner structure  15 . 
     The riser guide structure  35  guides individual risers  90  by means of individual floats  50  and riser guides  85  and  86 . As noted above, the riser structure  35  is vertically oriented and tethered to the sea floor, preferably by a central tether  100  which may be a riser or other type of tension member. The tether can be a riser, tendon, wire rope, chain, poly rope or combination thereof. The tether arrangement, whether it be by risers, tendons, chain or wire rope, etc., maintains the riser structure at a fixed distance above the sea floor and provides stability to the structure  15 . Risers  90  carry hydrocarbon fluids up to the riser structure  35  and through the support buoy structure  45  to the production deck  66 . While not shown in this embodiment, a drilling riser could also be used during drilling or workover operations through central shaft  60  or risers  90 . Production risers  90  are held in tension by means of individual riser floats  50  connected thereto. In the arrangement of FIG. 1, because individual riser floats  50  are used, independent expansion or contraction of the risers  90  can occur in each riser, because space exists above and below float  50 , with respect to buoy  70  bottom and guide platforms  85  or  86  as seen in FIG.  1 B. 
     Preferably, the individual riser floats  54 ,  52  are completely submerged beneath sea surface  5 , causing the upward buoyant force of the individual riser floats to remain approximately constant. The individual riser floats as shown in FIGS. 1 and 1B include an upper set of individual riser floats  52  and a lower set of individual riser floats  54 . Both sets of riser floats  52  and  54  are arranged around a central tubular shaft  60  as seen in FIG.  4 . 
     In the arrangement of FIG. 1, there are two concentric rings of risers  90  around central tubular shaft  60  as seen in FIGS. 2,  3  and  4 . An inner ring A is placed around central tubular shaft  60 . Inner ring A risers are individually, connected to riser buoyant members or cans  54  while each riser of ring B is connected to an individual riser buoyant member or can  52  above guide  85 . See FIG.  1 B. The riser guides  85 ,  86  and the central tubular shaft  60  provide a framework for the individual cans  50  ( 54  and  52 ) in guiding the risers  90  up and through the support buoy  70 . Each riser is free to slide on interior guides  85 ,  86  and support buoy  70 . Flexible conductors (not shown) may be provided from the upper ends of the risers  90  to the production deck  66  mounted on the inner section  42  of hull  30 . Additionally, the riser guides  85 ,  86  are arranged to keep the risers  90  radially separated from each other, but they allow for any angularity or tilting of the risers  90  to occur as a bundle as illustrated in FIG.  1 D. Coupled to riser stems  90  of buoyancy modules  50  are stress joints  80 . The column  60 , if desired, can be arranged to allow a workover drill string or drilling bit to pass through its center to allow simultaneous drilling and production. Other equipment such as drill string risers (not shown) can be used for workover operations. 
     As described above, FIG. 1 shows a derrick  120  mounted on the support buoy  70 . Heave, pitch and roll motions acting on the outer structure  30  are decoupled from the support buoy  70 , because it is part of the inner structure  15 . As a result, workover operations can occur on this offshore platform  10 . Because there is no relative heave, pitch or roll motions between a drill string (not shown) of derrick  120  extending down through the column  60 , and support buoy  70 , drilling can be accomplished even when the outer hull structure  30  is heaving, pitching and rolling with minimal need for derrick heave compensation. 
     FIG. 3 is a cross section through support buoy  70  looking downward along lines  3 — 3  of FIG.  1 . When the offshore platform hull  30  moves in heave, it moves up and down with respect to support buoy  70 . The risers  90  and buoyant modules or cans  50  are independent in heave of the riser structure  35  including central tubular shaft  60 , and riser guides  86 ,  85 . The buoy  70  is free to move vertically relative to the risers  90  and the buoyancy modules  50  (e.g., buoyancy cans  52 ,  54 ). As illustrated in FIG. 3, riser openings  95  are provided in the support buoy  70  to accept risers. The tubular shaft  60  is welded to buoy structure  70  thereby creating opening  65 . If the support buoy  70  moves vertically, the risers  90  slide within openings  95 , and the tubular shaft  60  (which is connected to riser guides  86 ,  85 ) allow the risers  90  and buoyancy cans  50  to remain fixed relative to the sea floor with no loads exerted thereon. If desired, the riser openings  95  can be lined with material to facilitate the sliding. In a more complex embodiment, bearing surfaces can be attached to the sides of risers  90  or to the sides of riser openings  95  or both. A material with a low coefficient of friction can be used. Alternative embodiments of a slidable connection are shown in FIGS. 8-11. 
     FIG. 1A illustrates the connection between the support buoy  70  and the upper guide  85  and lower guide  86  with central tubular shaft  60 . The tubular shaft  60  is tethered to the sea floor by tether  100 . The support buoy  70  provides net upward force or tension to tether  100 . The couple created between the upward buoyancy provided by support buoy hull  70  and the downward tension in tether  100  makes the inner structure  15  statically stable. 
     FIG. 1B illustrates that each riser  90  includes a buoyancy module or cans, either  54  or  52  with the upper buoyancy module  52  guided by guide walls  7  within the support buoy  70  or guided on their stems at openings  95  and at  85  or at  85  and  86 . The buoyancy modules  54  and  52  provide upward buoyant force to maintain each riser in approximately constant tension. 
     FIG. 1C illustrates the offshore platform  10  under calm conditions where the tether  100  is substantially vertical with respect to its sea floor tethering point, and FIG. 1D illustrates the platform  10  where surge forces force the platform laterally from its vertical stable point. Risers  90  are free to expand or contract in length with respect to support member  70  due to internal pressure or temperature changes. FIG. 1D is an exaggerated illustration of the displacement of the offshore platform  10  when surge and/or sway forces cause the support buoy  70  to be displaced from calm vertical conditions of FIG.  1 C. As illustrated in FIG. 1D, the drilling rig draw works  120  remains aligned with support buoy  70 . No bending stresses are imparted to risers  90 , because tether  100  urges buoy  70  in a pendular fashion from the sea floor. 
     FIG. 5 is an illustration of the invention in an alternative embodiment. The floating offshore platform  10  is similar to that of FIG. 1 except that sliding bearings  130  are substituted for the connection mechanism  55  of FIG.  1 . In a similar manner to the link arms  19  and pins  13 ,  17  of FIG. 2, sliding bearings  130  are arranged and designed to allow the outer hull structure  30  to heave, pitch and roll with respect to the inner structure  15  in response to sea forces. Preferably, four sliding bearings  130  are provided, two for pitch motions and two for roll motions, all acting together for heave motions. The floating platform  10  is free to heave, pitch and roll with respect to the support buoy  70  without causing such motions in the support buoy. 
     FIGS. 6 and 7 illustrate another alternative embodiment of the invention. The offshore platform  10  works in the same manner as that of FIGS. 1 and 5 except that buoyancy cans of the risers are provided via individual cylindrical buoyancy cans  140 . As in FIGS. 1 and 5, the support buoy  70  tethered to the sea floor is free to slide relative to the individual buoyancy cans  140 . At space  150 , a connection mechanism can be provided to decouple heave and pitch and roll motions of the outer hull  30  from the inner structure  15 . Such connection mechanism can be link arms and pins as illustrated in FIG. 1 or sliding bearings as illustrated in FIG. 5, or any equivalent device known in the art. Preferably, the connection mechanism is designed to isolate the outer hull in heave, pitch and roll from the support buoy  70 . 
     FIG. 7 is a cross-section looking: downward along lines  7 — 7  of FIG.  6 . It shows the arrangement of the individual buoyancy cans  140 . In the embodiment of FIG. 7,  25  cans are provided—arranged 5 by 5 in a square. The central can is removed to provide tethering to the sea floor. As in FIGS. 1 and 5, the support buoy  70  is capable of sliding relative to the individual buoyancy cans  140 . Arrangements to facilitate this sliding include bearing surfaces of materials with low coefficients of friction and the like. Two alternative embodiments of a slidable connection are shown: a first in FIGS. 8 and 9; a second in FIGS. 10 and 11. 
     FIG. 8 shows an embodiment of a slidable coupling between an individual buoyancy can  160  and a support structure  180 . The support structure  180  can be a portion of the support buoy structure  70  (FIGS. 1,  2 ,  3 ,  5 ,  6 , and  7 ) and can serve as a guidance for the buoyancy can  160 . In this arrangement, an individual riser  90  is coupled to its individual buoyancy can  160 . The individual buoyancy can  160  contains a plurality of sliding shoes  170  located peripherally thereon. The sliding shoes  170  slidably couple with an inner wall  185  of the support structure  180 . This slidable coupling via the sliding shoes  170 —inner wall  185  interface can prevent contact between the individual buoyancy can  160  and the support structure  180 . The sliding shoes  170  can be coupled to the individual buoyancy can  160  through any means known to those skilled in the art. The sliding shoes  170  and the inner wall  185  are preferably made of a material with a low coefficient of friction—allowing the support structure  180  to move relative to the individual buoyancy can  160 . In an alternative embodiment, the sliding shoes  170  could be coupled to the support structure  180  with the sliding occurring between the sliding shoe  170  and the individual buoyancy can  160 . Alternatively, the sliding shoes  170  can be coupled directly to the riser  90  facilitating sliding such as that in FIG.  1 . As can be seen with the arrangement of FIG. 8, the sliding shoes  170  and individual buoyancy can  160  do not always have to be in contact, but can be arranged and designed to do so, if desired. 
     FIG. 9 is a cross section looking downward along lines  9 — 9  of FIG.  8 . The support structure  180  serves as guidance for the individual buoyancy can  160 . While the embodiments of FIGS. 8 and 9 show the support structure  180  as being substantially square, alternative shapes could be used including rectangular, circular, triangular, and the like. 
     FIG. 10 is an alternative slidable coupling arrangement for a square individual buoyancy can  160  in a square support structure  180 . The arrangement operates in a similar manner to that of FIGS. 8 and 9, except that the sliding shoes  170  have been moved to the comers of the square individual buoyancy can  160 . This arrangement allows for more buoyancy per unit length than that of FIG.  8 . 
     FIG. 11 is a cross section of FIG. 10 looking downward along lines  11 — 11 . This cross section shows how the sliding shoes  170  couple to the corners of square individual buoyancy can  160 , and how the sliding shoes  170  couple with the comers of the inner wall  185 . 
     FIG. 12 is another embodiment of a slidable coupling between an individual buoyancy can  160  and a support structure  180 . In this embodiment, the individual buoyancy can  160  has a plurality of channel tracks  190  provided on its periphery. The support structure  180  has a plurality of sliding shoes  170  coupled to the inner wall  185 . The channel tracks  190  and sliding shoes  170  are adapted to couple with one another creating a slidable connection between the support structure  180  and the individual buoyancy can  160 . Additionally, the channel tracks  190 /sliding shoes  170  interface can create a guide for the individual buoyancy can in the support structure  180 . The channel tracks  190 /sliding shoes  170  interface can be done in a variety of ways, which should be become apparent to those skilled in the art. The embodiment of FIG. 8 provides sliding shoes  170  that are complimentary to the channel tracks  190 . The channel track  190  at arrow  200  is cutout to show the channel track  190 /sliding shoes  170  interface. While this embodiment provides the sliding shoes  170  on the inner wall  185  and the channel tracks  190  on the individual buoyancy can  160 , in an alternative embodiment, the channel tracks  190  could be on the inner wall  185  and the sliding shoes  170  could be on the individual buoyancy can  160 . Additionally, in an alternative embodiment, the support structure  180  can be a different shape such as a circle, triangle, or the like. 
     FIG. 13 is a cross section of FIG. 12 looking downward along lines  13 — 13 . This figure illustrates how the channel tracks  190  and sliding shoes  170  can interface with one another. 
     FIG. 14 is an alternative embodiment of FIGS. 12 and 13 showing a square buoyancy can  160  slidably coupled inside a square support structure  180 . FIG. 14 works in a similar manner to that of FIGS. 12 and 13, except that comer reinforcements  210  are used instead of channel tracks  190 . As can be seen in this embodiment, sliding shoes  170  are provided on the square individual buoyancy can  160 . The support structure  180  contains corner reinforcements  210 , which are adapted to compliment the sliding shoes  170 . 
     FIG. 15 is a cross section of FIG. 14 looking downward along lines  15 — 15 . This cross section shows the corner arrangement of the corner reinforcements  210  and sliding shoes  170 . 
     It should be understood that the invention is not limited to the exact details of construction, operation, or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. For example, while the offshore platform  10  is designed to decouple heave, pitch, and roll motions of a hull from acting on risers  90 , the offshore platform  10  does not necessarily completely isolate such motions from the risers  90 . The offshore platform  10  can be used to reduce such motions from acting on the risers  90 . 
     Also, the outer hull structure  30  can be made of any material and by any means known to those skilled in the art. While in a preferred embodiment, the lower hull  44  extends horizontally outward, such may not be the case in other embodiments. Additionally, the outer hull structure  30 , if buoyant, can be ballasted by any means known to those skilled in the art. The hull  30  can provide storage or produced fluids. Furthermore, while in a preferred embodiment the outer hull structure  30  is tethered to the sea floor using mooring lines, in other embodiments, the hull can be kept in position using dynamic positioning and the like. 
     While the production deck  66  is preferably mounted to the outer hull structure  30 , other embodiments could mount the production deck  66  to either the support buoy structure  45  or the riser structure  35 , both part of the inner structure  15 . Still in other embodiments, a production deck  66  might not be needed. 
     The connection mechanism  55 , referenced in the preferred embodiments, can be one of many choices for decoupling heave, pitch and roll motions from acting on the inner structure  15 . These choices include, but are not limited to joints, bearing surfaces, spherical bushings, link arms, and the like. Additionally, while a single device is illustrated for each of the embodiments of FIGS. 1 and 5, a plurality of devices working together to decouple heave, pitch and roll motions could be used. For example, one device could be used to decouple the pitch motions and another could be used to decouple the roll motions. 
     With regard to a buoyant device, which provides tension to the risers  90 , two embodiments are shown: the individual riser floats  50  (see floats  54  of FIG. 4, which each provide a single buoyancy module for its riser) and individual buoyancy cans  140  (FIGS. 6 and 7) and  160  (FIGS. 8-15) (which provide a singled buoyancy can for each riser). However, other arrangements and devices that provide buoyant uplift, as known in the art, can be used. Additionally, the support structures used to support these buoyant devices, as known in the art, can be provided. Two embodiments are given with reference to FIGS. 1,  5 , and  6 . However, such structures can include, but are not limited to the embodiments of the structures disclosed with reference to FIGS. 8-15. While the preferred embodiment describes the buoyant device as a plurality of individual riser floats  50  and a plurality of individual buoyancy cans  140 , other embodiments may contain a single riser float or single buoyancy can  140 . Accordingly, the invention is therefore limited only by the scope of the claims.