Abstract:
A spar-type platform includes a hull defining a centerwell extending downward to a keel. The hull includes a reduced diameter neck portion for diverting ice flow. Adjustable ballast tanks allow the hull to be moved between a ballasted down position defining an upper water line, and a ballasted up position defined by a lower water line. A riser a support buoy is disposed in the keel. Risers extend through the centerwell, each having an upper portion extending upward from the support buoy and a lower portion supported in the support buoy. A disconnect system detachably connects the support buoy to the hull and the upper portion of each riser to the lower portion thereof, whereby the hull and the upper portion of each riser are selectively detachable from the buoy and the lower portion of each riser for movement to avoid a collision with a floating object.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   Not applicable 
   FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to floating offshore production vessels for oil and gas, and in particular, to a deepwater spar vessel for ice flow conditions. 
   The arctic regions of the world are known to contain appreciable hydrocarbon reserves (petroleum and natural gas), and exploitation of these reserves is likely to occur in the near future. Some of these hydrocarbon reserves are in deep water, and currently there is not a proven floating system for the production of petroleum and natural gas from deep water in areas where ice flow conditions are common. 
   Icebergs and ice flow conditions existing in the arctic regions create a major hurdle to deepwater drilling operations. Ice flow from sheets of ice is caused by environmental forces, such as water currents and wind acting on the ice. A drilling platform may be severely damaged if left to take the full impact of the crushing force of ice flow conditions or left to suffer a collision with an iceberg. 
   Drilling platforms not suited for ice flow conditions must be removed to safer waters until the ice is sufficiently melted. Many work hours as well as production hours are lost during removal of a drilling platform as a result of severe ice flow conditions or an approaching iceberg. 
   Previous systems exist that melt or break ice flow as the ice flow approaches the drilling platform. Other systems suggested are structures that are physically capable of withstanding the crushing forces of ice flow. Still other systems use structures that merely redirect ice flow. These systems are typically costly and/or impractical. Further, these systems do not provide an efficient means for removal of the drilling platform in the face of an imminent iceberg collision. 
   Of the several generic types of offshore platforms for the exploitation of undersea hydrocarbon reserves, the spar-type platform is most promising for arctic conditions, since it has a smaller water plane area than other designs, and thus has a smaller hull section exposed to ice flows. Nevertheless, spar-type platforms can still suffer damage by ice flows, and destruction by icebergs, and are thus not suitable, in their present state of the art, for areas where these phenomena are prevalent. 
   A need therefore exists for a drilling platform system that can be quickly and efficiently moved temporarily to avoid an imminent iceberg collision, and that can still be quickly and easily restored to its original operation position after the possible danger has passed. It would also be advantageous to provide such a platform with the ability to withstand ice flow conditions. 
   SUMMARY OF THE INVENTION 
   Broadly, the present invention is a spar-type platform that comprises an elongate buoyant hull supporting a deck and extending vertically from the deck to a keel, the hull having an axial centerwell extending through its length and a reduced-diameter cylindrical neck section below a lower or “ice-flow” water line; a riser a support buoy disposed in the bottom of the centerwell at the keel of the hull; one or more risers extending through the centerwell, each of the risers having an upper portion extending from the deck to the top of the support buoy and a lower portion supported in the support buoy; and a disconnect system detachably connecting the riser support buoy to the hull and the upper portion of each riser to the lower portion thereof, whereby the hull and the upper portion of each riser are selectively detachable from the buoy and the lower portion of each riser for movement to avoid a collision with a floating object, such as an iceberg, and whereby the hull and the upper portion of each riser are re-connectable to the buoy and the lower portion of each riser after the danger of a collision has passed. 
   More specifically, the hull comprises an upper cylindrical section attached to the deck and connected to the reduced-diameter neck section by an upper tapered section. An upper or “ice-free” water line is defined around the upper cylindrical hull section, while the lower or “ice-flow” water line is defined around the upper tapered hull section. A plurality of adjustable or “soft” ballast tanks surround the centerwell, into which seawater can be selectively and adjustably introduced or evacuated with forced air to provide adjustable ballast for the hull. In normal (ice-free) conditions, the hull is ballasted down to the upper or “ice-free” water line, in which the reduced-diameter neck section is totally submerged. When ice flow conditions are encountered, the ballast is reduced so that the hull rises slightly to the lower or “ice-flow” water line, thereby bringing the reduced-diameter neck section closer to the surface so as to reduce the hull area exposed to ice flows. 
   Each riser in the riser assembly includes an upper riser portion that extends through the centerwell and that is detachably coupled, at the riser support buoy, to a lower riser portion that extends through the riser support buoy to the seabed. In a preferred embodiment, the disconnect system comprises a remotely operable riser coupler that releasably couples the upper portion of each riser to the lower portion thereof, a latch mechanism that is remotely-operable to releasably secure the buoy to the keel of the hull; and a buoy lowering mechanism, comprising a plurality of buoy chains or cables, each of which is detachably connected to the buoy and wound on a deck-mounted winch that is selectively operable to lower the buoy when the riser coupler(s) and the latch mechanism are released, and to raise the buoy back up into the keel when it is desired to re-connect the buoy to the hull. 
   In a preferred embodiment of the present invention, a plurality mooring lines enter the hull below the reduced-diameter neck section, and upon entering the hull are directed to a substantially vertical orientation by bending shoes mounted in the hull. The mooring lines extend upwardly through the hull to chain stoppers, located above the neck section, that take up the vertical forces on the mooring lines. At the top of the hull, the mooring lines pass over a series of sheaves that redirect the lines to tensioning windlasses. 
   In use, when it is desired to move the platform out of the path of an iceberg the riser coupler(s) and the latch mechanism are respectively actuated so as to disconnect the upper portions of each the riser from the lower portion thereof, and so as release the buoy from the keel. The winches are operated to lower the buoy out of the keel, and the buoy chains or cables are then detached from the buoy and recovered on the winches. This completes the separation of the hull from the buoy, the latter remaining fixed in place by the connection between the lower portion of each riser and the seabed. Finally, the mooring lines are cut just below the chain stoppers, allowing the hull and the deck of the platform to be moved (either by towing or by self-propulsion) out of harm&#39;s way. When the iceberg has passed, the hull and deck are moved over the buoy; the mooring lines are recovered and reattached to the hull, the buoy chains or cables are attached to the buoy; and, using the winches, the buoy is hauled upwardly into centerwell at the keel of the hull. Finally, the latching mechanism is actuated to secure the buoy to the hull, and the upper and lower portions of each riser are coupled together with a riser coupler. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view of a spar-type platform in accordance with the present invention; 
       FIG. 2A  is a cross-sectional view of the platform of  FIG. 1 , taken along line  2 A- 2 A of  FIG. 1 ; 
       FIG. 2B  is a cross-sectional view of the platform of  FIG. 1 , taken along lines  2 B- 2 B of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view taken along lines  3 - 3  of  FIG. 2A ; 
       FIG. 4  is a bottom plan view of the platform of  FIG. 1 , taken along line  4 - 4  of  FIG. 2B ; 
       FIG. 5  is a side elevational view of a spar-type platform in accordance with the present invention, showing the riser support buoy of the present invention being lowered from the hull of the platform; 
       FIG. 6  is a side-elevational view, partially in cross-section, of the spar-type platform, showing the riser support buoy being lowered from the hull; 
       FIG. 7  is a side elevational view of a spar-type platform in accordance with the present invention, showing the riser support buoy of the present invention after separation from the hull of the platform; and 
       FIG. 8  is a side-elevational view of the spar-type platform showing the riser support buoy after separation from the hull. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIGS. 1 ,  2 A,  2 B,  3 , and  4 , a spar-type platform  10 , in accordance with the present invention, is shown. The platform  10  includes a deck  12  and a hull  14 . The hull  14  includes one or more hard tanks  16 , one or more skirt tanks  18  and a ballasted keel or keel tank  20 . As is typical with spar-type platforms the platform  10  is provided with a mechanism (not shown) for selectively filling and evacuating the skirt tank or tanks  18  with seawater ballast, for purposes to be described below. The hull  14  defines an axial centerwell  22  to be described more fully below, that extends to the keel  20 . The hull  14  has an upper portion  24  secured to the deck  12 , and a lower portion  26  extending upward from the keel  20 . Between the upper hull portion  24  and the lower hull portion  26  is a reduced-diameter neck portion  28  that is joined to the upper hull portion  24  by a tapered (e.g., frusto-conical) upper transition portion  30 , and to the lower hull portion  26  by a tapered (e.g., frusto-conical) lower transition portion  32 . The purpose of the neck portion  28  will be explained below. 
   Contained within the upper hull portion  24  and secured to the underside of the deck  12  is an enclosed internal compartment  33  having a top portion defined by vertical upper side walls  34  attached between the deck  12  and the outer edges of a horizontal, inwardly-extending shelf  36 , and a narrower bottom portion defined by vertical lower side walls  37  attached between the inner edges of the shelf  36  and a bottom wall  38 . A plurality of mooring lines  40  (which may be cables or chains), securing the platform  10  to the sea bed, enter the lower portion  26  of the hull  14  below the neck portion  28 , each of the mooring lines  40  passing through a hawser pipe  42  that extends to the exterior of the hull  14  with a water-tight fit. Each hawser pipe  42  engages one of a plurality of bending shoes  46  secured to the inner wall of the hull  14  near the lower end of the neck portion  28 , thereby directing the mooring lines  40  into a substantially vertical orientation. Each hawser pipe  42  has an upper end that is secured in the bottom wall  38  of the internal compartment. Each of the mooring lines  40 , after emerging from its corresponding hawser pipe  42 , then passes through a corresponding one of a plurality of chain stoppers  48 , secured to the upper surface of the bottom wall  38  of the compartment  33 , which take up the vertical load of the mooring lines  40  and inhibit slippage in the mooring lines  40 . 
   From the chain stoppers  48 , each of the mooring lines  40  passes over a vertical sheave  50  attached to an inner edge of the shelf  36 , and then over a horizontal sheave  52  ( FIG. 3 ). The sheaves  50 ,  52  respectively direct the mooring lines  40  first from a vertical to a horizontal orientation, and then turn the mooring lines about 90° in the horizontal plane. As shown in  FIG. 3 , a windlass  54  is mounted in each corner of the shelf  36 , and the mooring lines from the adjacent sheaves  50 ,  52  are wound on each windlass  54 . In the specific example illustrated in the drawings, there are thirty-six mooring lines  40 , with nine mooring lines  40  wound on each windlass  54 . The windlasses  54  are operated so as to pay out the appropriate length of mooring line, and to apply the appropriate amount of tension to each line  40  to secure the platform  10 . By enclosing the chain stoppers  48 , the sheaves  50 ,  52 , and the windlasses  54  in the compartment  33 , these devices are shielded from harsh environmental conditions, such as wind and ice. 
   The centerwell  22  includes a horizontal bulkhead  56  that divides the centerwell into an upper centerwell portion  22   a  between the bottom wall  38  of the compartment  33  and the horizontal bulkhead  56 , and a lower centerwell portion  22   b  between the horizontal bulkhead  56  and the top wall of a detachable riser support buoy  58  (described more fully below) installed in the bottom of the centerwell  22  at the keel  20  of the hull  14 . The upper centerwell portion  22   a  defines an enclosure that provides some of the buoyancy lost due to the loss of hard tank capacity resulting from the smaller cross-sectional area of the neck portion  28  of the hull  14 . 
   Extending through the centerwell  22  is a riser assembly comprising one or more risers, each of which comprises an upper riser portion  60   a  and a lower riser portion  60   b . Each of the upper riser portions  60   a  is connected at its top end to production equipment (not shown) on the deck  12 , while the bottom end of each upper riser portion  60   a  is connected to the top end of a corresponding lower riser portion  60   b  by a remotely-operable releasable riser coupler  62 , of a type that is well-known and conventionally used in sub-sea petroleum and natural gas production systems. The couplers  62  may advantageously include self-sealing valves (not shown) to prevent or inhibit loss of fluid when the upper riser portions  60   a  are decoupled from the lower riser portions  60   b , as described below. The section of each upper riser portion  60   a  that extends through the upper centerwell portion  22   a  may advantageously be enclosed in a protective upper riser sleeve  64 . 
   The lower riser portions  60   b  are mounted in, and extend through, the detachable riser support buoy  58  that is seated below and coaxial with the centerwell  22  of the hull  14  at the keel  20 . Preferably, each of the lower riser portions  60   b  passes through a lower riser sleeve  66  that extends axially through the riser support buoy  58 . Each of the lower riser sleeves  66  terminates in a bend limiter  68  extending downwardly from the bottom of the support buoy  58 . Each of the lower riser portions  60   b  then extends from one of the bend limiters  68  to a wellhead (not shown) in the seabed, as is well-known in the art. 
   The riser support buoy  58  is secured to the hull  14  by a remotely-operated latching mechanism comprising a plurality of latches  70  ( FIGS. 2B and 4 ) mounted on the bottom of the keel  20 , each having a latching element  72  that is engageable with the bottom of the riser support buoy  58 . The latching mechanism is operable selectively to disengage the latching elements  72  from the support buoy, whereby the hull  14  of the platform  10  can be separated from the buoy  58 , as described more fully below. Suitable latching mechanisms are well-known in the art, and have been used, for example, for releasably securing a buoy in a bow turret of a floating production, storage, and offloading (FPSO) vessel. 
   As shown in  FIGS. 2A and 23B , the buoy  58  is supported in the centerwell  22  by a plurality of buoy-lowering lines  74  (which may be cables or chains), each of which extends down the centerwell  22  from a winch  76  secured to the deck  12 , passing through corresponding apertures in the bottom wall  38  of the enclosure  33 , and in the centerwell horizontal bulkhead  56 . The lower end of each of the cables or chains  74  terminates in a remotely-operable coupling socket  78  that releasably receives a mating ball  80  fixed to the top of the buoy  58  (see  FIG. 8 ). The remotely-operable ball-and-socket coupling mechanism  78 ,  80  may be of any conventional design that is known in the art. Alternatively, the ball-and-socket coupling mechanism  78 ,  80  may be operated by a remotely-operated vehicle (ROV) (not shown). When the buoy  58  is secured and supported in its hull-attached or raised position within the centerwell  22  by the latches  70  and the lowering chains or cables  74 , respectively, a first plurality of buoy stop elements  82 , mounted around the periphery of the top of the buoy  58 , seat against a corresponding second plurality of buoy stop elements  84  fixed to the top of the keel tank  20 , as shown in  FIG. 2B . 
   As described above, the platform  10  of the present invention is operable in at least two ways to minimize the risk of damage due to flow ice and icebergs. First, as shown in  FIG. 1 , the platform  10  has a first or “ballasted down” position, in which the neck portion  28  and the tapered upper transition portion  30  of the hull  14  are totally submerged below an upper or “ice-free” water line  90  that is defined on the upper hull portion  24  at a predetermined distance below the deck  12 . The “ballasted down” position is used for conditions in which large waves may be encountered, but ice flow conditions do not exist. By evacuating some of the ballast from the skirt tank(s)  18 , the platform  10  is movable to a second or “ballasted up” position during ice flow conditions. The controllable introduction and evacuation of ballast into and out of the skirt tank(s)  18  to create the ballasted up and ballasted down positions are performed by means well-known in the art, typically a system of conduits (not shown) and air pumps (not shown) that respectively admit seawater into the tank(s)  18  and blow the water out of them. In the ballasted up position, the upper part of the tapered upper transition portion  30  of the hull  14  is raised, so as to present a lower or “ice flow” water line  92 , represented by a broken horizontal line in  FIG. 1  extending across the upper transition portion  30 , above which at least the upper part of the upper transition portion  30  of the hull  14  extends. In the ballasted up position, the upper transition portion  30  of the hull  14  is thus at the lower water line  92 , and the reduced-diameter neck portion  28  is just below the lower water line  92 . The hull  14 , in this “ballasted up” position, thus presents the reduced cross-sectional areas of the upper transition portion  30  and the reduced-diameter neck portion  28  to the near-surface of the water, thereby reducing the surface area of the hull  14  that is exposed to flow ice impact. 
   When impact with an iceberg appears imminent, the hull  14  may be separated from the riser support buoy and moved out of harm&#39;s way by the process described below and illustrated in  FIGS. 5-8 . 
   As shown in  FIGS. 5 and 6 , with reference also to  FIGS. 2B and 4 , the latches  70  securing the riser support buoy  58  to the hull are released, as are the riser couplers  62 . These operations decouple the upper riser portions  60   a  from the lower riser portions  60   b , while also detaching the buoy  58  from the hull  14 . The riser buoy  58  is thereby freed to be lowered, relative to the hull  14 , by means of the buoy-lowering cables or chains  74  and the winches  76 , to a hull separation position, as shown in  FIG. 6 . 
   As shown in  FIGS. 7 and 8 , after the buoy  58  is lowered to the hull separation position and has achieved a stable equilibrium position, the coupling sockets  78  are actuated so as to release the coupling balls  80 , thereby completing the separation of the hull  14  from the buoy  58 . The equilibrium position is a position where the buoyancy of the support buoy  58  maintains it at a certain depth that would be below any approaching iceberg and at which the buoy is not exposed to excessive wave action or water currents. A weighted object, such as a chain supported by a light-weight polyester line (not shown) may be attached to the support buoy  58  to help establish an equilibrium position. 
   If the hull and deck of the platform  10  are to be moved, the mooring lines  40  must then be severed, preferably at or just below the chain stoppers  48 , and preferably after being slacked down a bit. The hull and deck may then be moved away, either by towing or by an onboard propulsion system (not shown). After the iceberg has passed or is otherwise deemed harmless, the hull and deck of the platform may be moved back over the buoy  58  for re-connection thereto by performing the above-described steps in reverse order after the mooring lines  40  have been re-connected. This reconnection may be performed, for example, by recovering the mooring lines  40  from the seafloor by attaching a retrieval line (not shown) to each of the mooring lines  40  using an ROV (not shown). Once the mooring lines are recovered to the surface, additional lengths of mooring line would be added, and the lines  40  would then be pulled through the hawser pipes  42  and secured by the chain stoppers  48 . 
   Although the present invention has been described herein in the context of several exemplary embodiments, it will be understood that a number of variations and modifications may suggest themselves to those skilled in the pertinent arts. Such variations and modifications should be considered within the spirit and scope of the present invention, as defined in the claims that follow.