Abstract:
A vessel includes a riser and/or tendon tensioning construction. A connector, such as an arm or deck structure, is suspended from cables movable relative to cable guides. The connector carries two or more risers and/or tendons extending from a subsea structure to above water level. The free ends of the cables are attached to weights for exerting a tensioning force on the risers, which are substantially decoupled from pitch, roll and heave motions of the vessel.
Abstract:
The invention relates to a storage structure having a fluid transfer boom for transfer of cryogenic liquids such as liquified natural gas (LNG) from a first storage structure to a vessel. The boom has two arms which are rotatably connected at their first ends via a swivel joint. In one embodiment a liquified natural gas duct is supported within the first and second arms which form a gas tight housing around the liquified natural gas duct. The transfer boom according to the present invention provides a redundant containment system wherein the LNG duct is supported by the structurally strong and self-supporting transfer boom which confines the natural gas in case of a leak in the inner LNG duct. In a further embodiment the transfer boom comprises seven swivel joints in total such that rotation in all directions is possible when the vessel is moored to the storage structure and has to cope with relative motions of roll, pitch, yaw, heave, sway and surge. The first arm may be suspended from the storage structure in a generally vertical direction, the second arm extending between the first end of the first arm and the vessel in a generally horizontal direction. Hereby a reliable, self-supporting construction can be achieved without the use of counterweights or tensioning cables for the vertical arm. Preferably the swivel joints are each of a substantially similar construction such that the costs of manufacture can be reduced. Another embodiment provides for the inner LNG duct being provided with leak containment means and with deformable wall parts for allowing thermal expansion.
Abstract:
A vessel includes a riser and/or tendon tensioning construction. A connector, such as an arm or deck structure, is suspended from pivot arms on the vessel. The connector carries two or more risers and/or tendons extending from a subsea structure to above water level. The free end of the pivot arms is attached to a weight for exerting a tensioning force on the riser which is substantially decoupled from pitch, roll and heave motions of the vessel.
Abstract:
The transfer system includes two generally vertically oriented duct sections which are placed at an angle with the vertical. These two sections are connected to a substantially horizontal third member, for instance a third duct section. Near the connection points of the vertically oriented duct sections and the horizontal member, a tensioning weight is provided such that a tensioning force in the horizontal duct section is created. Hereby bending/kinking and/or buckling due to currents or floating systems dynamics is reduced. A relatively long horizontal duct section can be used which is preferably made of hard pipe, having a reduced swing.
Abstract:
An offshore system is provided of the type that includes riser pipes (30) extending up from the seafloor (44) to a tall and narrow caisson (12) at the sea surface, with the caisson moored by mooring lines (34) extending to the seafloor and anchored thereat, which minimizes bending of the upper portion of the riser pipes when the caisson drifts in severe weather. Although the caisson has a Ballasted lower end and buoyant upper end to keep its axis (20) vertical, a device is provided for applying a horizontal force (54) to a location along the caisson that is vertically spaced from the upper ends of the mooring lines, to tilt the caisson so the axis of the caisson is parallel to portions (82) of the riser pipes lying immediately below the caisson. In one arrangement, a second set of mooring lines (60) is provided, that have upper ends coupled to second locations (64) along the caisson that are vertically spaced from the upper ends of the first mooring lines. Also, a motor driven device (70) is provided for pulling on selected ones of the lines to tilt the caisson. In another arrangement, largely horizontal force transmitting members (132) extend from a lower portion of the caisson to a location (136) along a single set of mooring lines (126). In still another system, thrusters (152, 154) are used to push at locations along the caisson to tilt it.
Abstract:
An offshore system is described, of the type that includes a turret (20) anchored to the sea floor and connected by at least an upper bearing assembly (34) to the vessel hull (14) so the hull can weathervane about the turret, wherein the upper bearing assembly is of moderate cost and high reliability. In one construction, the upper bearing assembly includes upper and lower slider bearing rings (40, 42) that lie facewise adjacent at an interface (44), with the upper bearing ring fixed to the turret and the lower bearing ring supported on the hull through quantities of elastomeric material (102). The elastomeric material permits slight tilt of the turret upper portion without opening a gap at one side of the interface. The lower bearing ring is divided into segments (112A, 112B), and large turret tilt allows pressured lubricant to escape from only one or a few segments that begins to lift up during turret tilt. In another construction, a circle of bearing devices (202) is provided, wherein each device includes a cylinder (224) and piston (226) and a source of pressured fluid that is applied to the device.
Abstract:
An offshore system is described that minimizes mooring loads attributable to oscillating wave drift. A tension member (24, FIG. 1), such as a chain, has a lower end (32) fixed to the seafloor and has an upper end portion (40) coupled to the vessel (12), so wave drift that occurs in a storm and urges vessel drift away from a quiescent position, is resisted by tension in the tension member. To reduce the load on the tension member, the vessel is allowed to move while paying out the tension member so it elongates. However, a brake mechanism (70) is applied to the tension member being payed out to slowly pay it out and absorb energy, to thereby damp vessel movement and minimize the maximum mooring load on the tension member. Preferably, at least three tension members (24, 26, 28) extend in different headings from the vessel to fixed locations on the seafloor, so a tension member that is resisting vessel drift extends at a large angle (C) to the vertical.
Abstract:
A tension leg platform system includes a relatively small platform (12, FIG. 3 ) and relatively low capacity tendons (16), despite providing sufficient tension to risers (34) that carry hydrocarbons from seafloor wells to the platform. With the platform floating at the sea surface and held in position by the tendons, seafloor wells can be connected through risers to a side of the platform, with the tension of each riser compensated by adding buoyancy to the corresponding side of the platform, as by using pressured air (at 100) to blow water out of a platform compartment (94).
Abstract:
An arrangement is described for mounting a turret (18, FIG. 3) on the outer ends of beams, or mounts, whose inner ends are supported by a vessel hull (14) that can weathervane about the turret. An upper bearing (42) is supported on a largely rigid upper mount (30), by a resiliently deflectable support structure (50) that includes a plurality of elastomeric shear pads (52). Each shear pad subtends an angle (B, FIG. 5) of no more than 20.degree. about a common point (26) along the rotatable axis of the turret. This allows a shear pad to use flat rubber sheets instead of spherically curved sheets, as well as avoiding excess stiffness. The support structure includes six posts (54) extending radially from the common point along the turret axis, each post having a gap where a shear pad is mounted, with the area around each shear pad being open to facilitate replacement of a shear pad. A limiter which extends in parallel with the shear pads to prevent them from being torn loose in the event of bearing seizing, can include a limiter member (122, FIG. 11) fixed with respect to one mount plate (70) of a shear pad and surrounding the opposite mount plate (72) of the shear pad.
Abstract:
A vessel with a rotatable turret thereon is moored in a manner that minimizes turret tilt while avoiding the need to maintain precisely concentric upper and lower turret bearings. A mooring structure (152, FIG. 7) is formed by a group of mooring lines (162, 164) such as chains, with the upper ends of the lines coupled to the vessel through a connecting apparatus (154) that comprises a group of arms (166) each connected to a corresponding one of the lines. Each arm is pivotally mounted (at 170) on the turret to hang therefrom, so the arm transmits primarily vertical forces to the turret and the turret bearing (185) has to support primarily vertical forces. Each arm carries a bearing pad (174) that presses horizontally against a vessel lower bearing ring (182) mounted directly on the vessel hull independently of the turret. Substantially the entire horizontal components H of mooring force are transmitted from the arm pads to the lower vessel bearing ring, so the horizontal force components are not transmitted through the turret. As a result, the turret does not tend to tilt, and the vessel lower bearing ring which transmits horizontal mooring force components does not have to be mounted precisely concentric with the upper bearing.