Turret mooring system arrangement

Abstract

A turret mooring system for a floating process and storage offloading, FPSO, vessel or a FLNG vessel includes a turret structure and a bearing support structure. The turret structure is to be placed within a moonpool in a hull of the vessel and rotatably held within the moonpool via the bearing support structure. The bearing support structure includes a radial support and an axial support. The radial support includes a plurality of radial wheels, and the axial support includes a plurality of axial bogies. The vessel has a topside process deck and the turret structure includes a collar deck. The collar deck is positioned vertically above an upper side of the axial support and below an elevation of the topside process deck. An FPSO or FLNG vessel is provided which includes the turret mooring system.

Claims

1. A turret mooring system (100; 400) for a floating process and storage offloading, FPSO, vessel or a FLNG vessel (150; 250), said system comprising: a turret structure (120); and a bearing support structure (110, 115); the turret structure to be placed within a moonpool (155) in a hull of the vessel and rotatably held within the moonpool via the bearing support structure, the bearing support structure comprising a radial support (115) and an axial support (110); the radial support comprising a plurality of radial wheels, the axial support comprising a plurality of axial bogies; the vessel having a topside process deck (160) and a hull with a bottom (180), wherein the hull has a height of at least 35 meter between the bottom and the topside process deck, wherein the turret structure comprises a collar deck (125) and the collar deck is positioned vertically above an upper side (L1) of the axial support and below an elevation of the topside process deck (160), wherein the bearing arrangement is positioned at a vertical distance (D) below the elevation of the topside process deck (160) such that there is at least one interdeck space of at least 6 meters between the topside process deck (160) and a top level (L1) of the bearing arrangement (110, 115), and wherein said interdeck space is large enough to locate process equipment in said interdeck space, and wherein the radial support is vertically positioned (L2) in the turret structure between the collar deck and a bottom part of the hull, and the axial support is vertically positioned (L1) in the turret structure between the collar deck and the radial support.

2. The turret mooring system according to claim 1, wherein the bearing support structure further comprises a plurality of lower stoppers (210) arranged in a bottom region (215) of the moonpool.

3. The turret mooring system according to claim 2, wherein the lower stoppers are arranged at vertical distance of about 5 meter from the bottom opening (180) of the moonpool (155).

4. The turret mooring system according to claim 1, wherein the collar deck is separated from the topside process deck by at least one interdeck distance (170).

5. The turret mooring system according to claim 1, wherein the collar deck is separated from the topside process deck by an intermediate element (165).

6. The turret mooring system according to claim 5, wherein the intermediate element comprises at least one of a riser termination deck (165) and a manifold structure.

7. The turret mooring system according to claim 6, wherein risers terminate at either the collar deck or the top side process deck.

8. The turret mooring system according to claim 1, wherein the collar deck (125) sits on the axial bogies of the axial support (110).

9. The turret mooring system according to claim 1, further comprising a lip seal (130) and/or a water damming labyrinth (135) between the radial support and the bottom (180) of the hull.

10. The turret mooring system according to claim 9, wherein the radial support (110) is positioned (L2) above a designed maximum draught level (L3), said maximum draught level (L3) accounting for the sea water dynamic motions, to prevent exposure of radial wheels mechanical components to sea water.

11. The turret mooring system according to claim 10, wherein the lip seal and/or water damming labyrinth is vertically positioned between the designed maximum draught level (L3) of the vessel and the level of the radial support (L2).

12. The turret mooring system according to claim 1, wherein said collar deck comprises a floor, sidewalls and a ceiling, and is accessible from another deck of the turret which lies directly above the collar deck.

13. The turret mooring system according to claim 1, wherein the turret structure is a mooring buoy.

14. A FPSO or FLNG vessel comprising a hull and a turret mooring system, the turret mooring system comprising a turret structure and a bearing support structure wherein the turret structure is rotatably suspended from the hull of the vessel, the turret mooring system in accordance with claim 1.

15. The turret mooring system according to claim 2, wherein the collar deck is separated from the topside process deck by at least one interdeck distance (170).

16. The turret mooring system according to claim 3, wherein the collar deck is separated from the topside process deck by at least one interdeck distance (170).

17. The turret mooring system according to claim 2, wherein the collar deck is separated from the topside process deck by an intermediate element (165).

18. The turret mooring system according to claim 3, wherein the collar deck is separated from the topside process deck by an intermediate element (165).

19. The turret mooring system according to claim 4, wherein the collar deck is separated from the topside process deck by an intermediate element (165).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not as a restriction of the inventive concept. The scope of the invention is only limited by the definitions presented in the appended claims.

(2) FIG. 1A shows a cross-sectional view of a vessel comprising a turret mooring system according to an embodiment of the invention;

(3) FIG. 1B shows a detail of the collar deck of FIG. 1A;

(4) FIG. 2 shows a cross-sectional view of a detail from FIG. 1A;

(5) FIGS. 3A and 3B show details of a bearing structure;

(6) FIG. 4 shows a cross-sectional view of a vessel comprising a turret mooring system according to an embodiment of the invention;

(7) FIG. 5A shows a cross-sectional view of a detail from FIG. 4, and

(8) FIG. 5B shows a detail of a bearing structure.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1A shows a cross-section of a vessel comprising a turret mooring system 100 according to an embodiment of the invention.

(10) The turret mooring system 100 is arranged in an offshore vessel 150, which may be a Floating Storage and Offloading (FSO) vessel or a Floating Production Storage and Offloading (FPSO) vessel or a FLNG, or other type of vessel equipped for receiving an internal turret. The offshore vessel is typically moored at sea at a location in an offshore oil and/or gas field.

(11) The turret mooring system 100 comprises in a moonpool 155 of the offshore vessel 150 a bearing arrangement 110, 115, which is arranged to accommodate an internal turret 120. The turret 120 is typically connected at its lower end, i.e., the chain table, to mooring or anchoring lines that are attached to the seabed.

(12) When the turret 120 is positioned in the moonpool 155, the vessel 150 is thus in a moored position. In this position, the bearing arrangement 110, 115 allows rotation of the vessel 150 about a substantially vertical central axis, thus providing the vessel with a capability to weathervane under influence of waves, wind, etc.

(13) The bearing arrangement 110, 115 comprises an axial support 110 comprising a plurality of axial bogies positioned at a first vertical bearing level L1 and a radial support 115 comprising a plurality of radial wheels at a second vertical bearing level L2. The second bearing level L2 is positioned below the first bearing level L1.

(14) The axial bogies are designed to take an axial load transferred from the vessel 150 to the turret 120, and the radial wheels are configured to transfer a radial load from the vessel to the turret.

(15) In the embodiment as shown in FIG. 1A, the bearing arrangement 110, 115 is positioned at vertical distance D below the elevation level of a topside deck 160 of the vessel 150, such that there is at least an interdeck space 170 between the topside process deck 160 and the top level, i.e., the first level L1 of the bearing arrangement 110, 115. An interdeck space is defined as a vertical distance between two successive decks within the hull of the vessel 150, which is large enough to locate relevant equipment, such as process equipment.

(16) One interdeck space has a typical vertical distance of at least 5-6 meters.

(17) In this manner, the bearing arrangement 110, 115 is positioned relatively closer to the keel 180 of the hull and closer to a level of the chain table 140 of the turret 120 where the mooring lines 300 are connecting with the turret structure 120. In this manner the bending forces and moments on the turret structure are relatively reduced. The elevation (second level L2) of the radial wheels (with respect to the keel 180/chain table 140) has effect on the load on the axial bogies in the axial support 110.

(18) In an embodiment, the bearing arrangement 110, 115 is positioned close to a designed maximum draught line L3, which is based on the maximum draught of the vessel under full load.

(19) The level L3 of the maximum draught line is below the second level L2 of the radial support. The radial support 115 is positioned in the dry at a distance Z from the draught line L3 in order prevent the radial wheels to be exposed to the sea water.

(20) In an embodiment, the radial support 115 is positioned at some vertical distance Z above the designed maximum draught line L3.

(21) The skilled in the art will appreciate that the vertical distance Z will vary with inter alia operating conditions, the vessel type, and regional weather and/or swell conditions depending on the location of the vessel. The vertical distance may be between about 5 and about 10 meter. In FIG. 2 a simplified cross-sectional view of FIG. 1A is presented, indicating the vessel 150, the turret 100, the bearing arrangement 110, 115. Further the levels of the axial bogie 110, of the radial wheels 115 and the maximum draught line are shown indicated by lines referenced L1, L2 and L3, respectively.

(22) FIGS. 3A and 3B show details of the bearing structure. In FIG. 3A the axial bogie 110 is shown, positioned on the internal moonpool wall under the collar deck 125. In FIG. 3B an arrangement of radial wheels 115 on the internal moonpool wall is shown.

(23) Referring again to FIG. 1A, in a further embodiment, the bearing arrangement 110, 115 is provided with a lip seal 130 between the maximum draught line L3 and the level L2 of the radial support 115, to protect the radial support against sloshing of water in the gap 145 between the moonpool 155 and the turret structure 120.

(24) Additionally or alternatively, a water damming means can be provided for the same purpose between the maximum draught line L3 and the level L2 of the radial support 115. In an embodiment, the water damming means comprises a water labyrinth 135 that prevents sloshing water to reach the radial support.

(25) In an embodiment, the lip seal 130 and water labyrinth 135 are applied in combination, in which the water labyrinth 135 is positioned vertically between the draught line L3 and the level of the lip seal 130.

(26) Further, a collar deck 125 is provided that substantially sits on the axial support/axial bogies 110. The collar deck 125 enables transfer of axial loads from the axial bogies to the vessel via the bearing system. Typically the collar deck is a structural deck with relatively high stiffness that sits on the axial bogies.

(27) FIG. 1B shows the collar deck 125 of FIG. 1A in greater detail. The collar deck is provided with a collar 127 on the outer surface 121 of the turret 120, wherein a lower side 128 of the collar 127 is arranged on the axial bearing supports 110. When the turret rotates around its axis of rotation R relative to the moonpool 155, the lower side of the collar is moved relative to the axial bearing supports 110. Above the lower side 128, the collar deck comprises a floor 122 having a top surface 123 on which equipment may be arranged and across which personnel may move, and having a bottom side 124 which may act as a ceiling for the deck 168 immediately below the collar deck 125. The floor preferably comprises an assembly of steel plates that are welded together and which extend between sidewalls 134 of the collar deck to cover a substantial part of the interior diameter of the turret. The floor thus improves the structural stability of the turret. The collar deck further comprises a ceiling 133.

(28) Riser deck 165 lies immediately above the collar deck 125, and comprises an opening for passage of a conduit 166 which is connected to or part of a riser line. Flow of hydrocarbons from the conduit 166 to processing equipment on or above the topside process deck can be controlled by means of valve 167.

(29) In order to allow personnel access to the collar deck 125 from the deck directly above or below the collar deck, stairs 131 and 132 are provided. The stairs 131 run from the collar deck 125 to the riser deck 165, and stairs 132 run through an opening in the floor 122 from the collar deck 125 to the deck 168. At the level of the top-side process deck 160 additional bearings 162 are provided. As will be apparent to the skilled person, the bearings 162, in contrast to the axial support bearings 110, are unsuitable for supporting a substantial portion of the weight of the turret or the collar of the turret, but merely provide support for the cover between the turret and the top side process deck of the vessel which covers the opening of the moonpool at the level of the top-side process deck.

(30) Lowering the collar deck has the effect that equipment is located lower in the turret which saves space on the manifold or has a lower manifold. Additionally, in case where the upper part of the turret should be winterized, or encapsulated and ventilated, lowering the collar deck in the moonpool will have the effect to reduce the volumes to be winterized/ventilated and accordingly the weight of winterization panels/air demand and HVAC equipment.

(31) In an embodiment, a riser deck 165 is provided above the collar deck 125. The riser deck 165 is arranged to hold manifold foundations (not shown in detail) such that riser terminations (not shown in detail) are at the level of the topside process deck 160.

(32) Preferably, the riser deck 165 is arranged at substantially the same level as the topside process deck 160, but the skilled in the art will appreciate that the riser deck 165 may be positioned also at an intermediate level between the level of the collar deck 125 and the topside process deck 160.

(33) On the topside process deck 160 above the moonpool 155/turret structure 120, the offshore vessel 150 is equipped with a supporting construction 350 which holds a swivel stack that couples to riser lines and/or other equipment that interacts with the turret structure 120 in the moonpool 155. Such swivel stack and equipment are well known in the art.

(34) FIG. 4 shows a cross-section of a vessel 250 comprising a turret mooring system 400 according to an embodiment of the invention.

(35) In FIG. 4 entities with the same reference number as shown in the preceding FIGS. 1, 2, 3A, 3B refer to corresponding or similar entities.

(36) FIG. 4 shows that a third type of bearing element, i.e., lower stopper 210 is added at a bottom region 215 of the moonpool 155. The bearing arrangement 110, 115, 210 comprises axial bogies 110, radial wheels 115 and lower stoppers 210 in this embodiment. Lower stoppers 210 are used as stoppers to limit the total load applied to the axial bogies and radial wheels in case of extreme events.

(37) In the embodiments of the invention relating to FIG. 4, instead of a riser deck 165 at substantially the level of the topside process deck 160, the collar deck 125 provides a support for manifold foundations. Manifolds 126 are mounted on the collar deck 125 while riser terminations 161 are provided at the level of the topside process deck 160. Since a braced structure like manifolds typically represents less weight than a turret cylinder of same height, the structural weight of the vessel can be reduced in this respect.

(38) The skilled in the art will appreciate that as shown in FIG. 4 the collar deck 125 lowered in the hull is not a suitable space to locate hydrocarbon production-related equipment, due to the relatively confined area and the required ventilation. However, the collar deck 125 may be adapted to locate equipment that is not a potential hydrocarbon leak source, such as various hydraulic power units, and/or a hook-up winch for mooring lines and/or risers, depending on the selected arrangement for pull-in.

(39) In FIG. 5A a simplified cross-sectional view of FIG. 4 is presented, indicating the vessel 250, the turret 100, the bearing arrangement 110, 115 and the lower stoppers 210. Further the levels of the axial bogie 110, of the radial wheels 115 and the maximum draught line are shown indicated by lines referenced L1, L2 and L3, respectively.

(40) FIG. 5B shows details of the lower stoppers 210, as positioned on the internal moonpool wall of the vessel 250.

(41) The advantages of the invention are the following: Locating the radial wheels as low as possible (above water level) has the effect of also lowering the loading applied to the axial bogies (hence allows for a reduction of the number of axial bearing elements), Reducing the total height of the lower turret from keel to collar deck and save structural weight thanks to: Reduction of Bogie Support Structure total height and weight, Reduction of turret cylinder total height and weight, Reduction of elevation of the overall turret center of gravity.

(42) The invention has been described with reference to some embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.