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 2A-2A of FIG. 1;
FIG. 2B is a cross-sectional view of the platform of FIG. 1, taken along lines 2B-2B 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, 2A, 2B, 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 2 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 22a between the bottom wall 38 of the compartment 33 and the horizontal bulkhead 56, and a lower centerwell portion 22b 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 22a 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 60a and a lower riser portion 60b. Each of the upper riser portions 60a is connected at its top end to production equipment (not shown) on the deck 12, while the bottom end of each upper riser portion 60a is connected to the top end of a corresponding lower riser portion 60b 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 60a are decoupled from the lower riser portions 60b, as described below. The section of each upper riser portion 60a that extends through the upper centerwell portion 22a may advantageously be enclosed in a protective upper riser sleeve 64.
The lower riser portions 60b are mounted in, and extend through, the detachable riser support buoy 58 that is seated below and coaxial with the centerwell 2 of the hull 14 at the keel 20. Preferably, each of the lower riser portions 60b 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 60b 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'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 60a from the lower riser portions 60b, 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.
Patent Application
20080029013
