GB2298175A

GB2298175A - Turret-moored vessels.

Info

Publication number: GB2298175A
Application number: GB9503855A
Authority: GB
Inventors: Jean-Paul Denise
Original assignee: Offshore Design Engineering Ltd
Current assignee: Offshore Design Engineering Ltd
Priority date: 1995-02-25
Filing date: 1995-02-25
Publication date: 1996-08-28
Anticipated expiration: 2015-02-25
Also published as: NO960660L; GB2298175B; NO960660D0; GB9503855D0

Abstract

A turret-moored floating vessel 25 includes a turret 27 which extends vertically through the vessel and is so connected to it that, in use, the turret may be anchored while the floating vessel is free to rotate about it. Flexible conduits 35, 36 extend between the turret and the vessel. One end of each conduit is connected to a vertical cylindrical structure 32 on the turret so as to become wrapped around, or unwrapped from, the structure as the vessel rotates about the turret. The other end of the conduit is wrapped around a vertical or horizontal winch-driven rotatable structure 39, 45 on the vessel so as to compensate for the wrapping or unwrapping of the conduit on the structure 32 on the turret.

Description

"Improvements in or relating to turret-moored vessels" The invention relates to turret-moored floating vessels for use in the commercial operation of undersea oil and natural gas fields. Such vessels, which are often referred to as turret-moored FPSOs (Floating Production Storage and Off-loading Vessels), include a turret extending vertically through the vessel and so connected to it that, in use, the turret may be anchored while the floating vessel is free to rotate about it. Such free rotation is necessary for the safety of the vessel, so that it may "weathervane", i.e.

may yield to the action of waves, wind and current by rotating about the anchored turret.

Oil or gas from the undersea field is delivered to the turret through fixed piping from the sea bed and is then transferred to storage tanks in the vessel through conduits extending from the turret to the vessel. It is also usually required to transfer water and chemicals between the turret and the vessel and appropriate conduits need to be provided for this purpose also. In addition to these main fluid transfer requirements it is also necessary to transmit power and control signals to the turret and to the sub sea control systems.

In view of the rotation of the vessel relative to the turret, it is necessary for the conduit system between the turret and vessel to be able to accommodate such rotation.

At present two types of transfer system are available, known respectively as a toroidal multi-path fluid swivel, and a flexible hose "drag chain" system. Other systems have been suggested, but none has yet been fully developed and applied in practice.

Both ofthe main transfer systems at present available have recognised technical limitations. Thus, the toroidal swivel is limited by the maximum size of seal diameter, presently ofthe order of 2.0m, which limits the number and size of flow paths which can be accommodated. The "drag chain" system, on the other hand, is capable of accommodating only limited rotation (in practice only plus or minus 270 ). Also, it is very bulky, typically being of 32m diameter for a 10 inch flow path and consequently requiring a large diameter turret for support.

There is therefore a requirement for an alternative system which can provide flowpaths sufficient to allow production flow rates of, typically, up to and above 100,000 barrels per day, which has a rotation capacity of at least plus or minus 360 , and can be installed on a reasonably small diameter turret. The present invention therefore sets out to provide an improved turret-moored vessel whereby these requirements may be met.

According to the invention there is provided a turret-moored floating vessel of the kind including a turret extending vertically through the vessel and so connected to it that, in use, the turret may be anchored while the floating vessel is free to rotate about it, and at least one flexible conduit extending between the turret and the vessel, characterised in that one end of the conduit is connected to a vertical structure on the turret so as to become wrapped around, or unwrapped from, the structure as the vessel rotates about the turret, and the other end of the conduit is connected to a rotatable structure on the vessel around which the conduit may also be wrapped in a manner substantially to compensate for the wrapping or unwrapping of the conduit on the structure on the turret.

Since the structure on the vessel is rotatable, the length of conduit wrapped around the structure may be adjusted independently of the length of conduit wrapped around the structure on the turret. Such arrangement thus allows for adjustment of the length of the stretch of conduit between the turret and the vessel, and also adjustment of the tension in the conduit.

The structure on the turret is preferably fixedly mounted on the turret.

The structure on the turret may be of circular cross-section and may be generally cylindrical. It is preferably substantially co-axial with the axis of rotation of the vessel about the turret.

Alternatively, the structure on the turret may be elongate in overall cross-section.

In this case said one end of the conduit is preferably connected to the structure adjacent the centre thereof, as viewed in cross-section. In one such embodiment the structure comprises two elements of similar construction disposed side-by-side and parallel. Each such element may, for example, be of circular cross-section and generally cylindrical.

In this case said one end of the conduit may be connected at a location between the two elements.

The rotatable structure on the vessel may be rotated by a power-driven winch, for example a constant-tension winch. The structure on the vessel is preferably of circular cross-section and may be generally cylindrical.

The axis of the structure on the vessel may extend substantially vertically and may be parallel to the axis of the structure on the turret. Alternatively, its axis may be generally horizontal on the vessel and may extend generally at right angles to the axis of the structure on the turret.

In any ofthe above arrangements there may be provided a plurality of conduits connected between structures on the turret and the vessel. Said conduits may include hoses for the transfer of fluids, and cables for the transfer of electrical power. The conduits may be connected to the structure on the turret at vertically spaced locations, so that each conduit wraps around a different portion of the structure.

On the vessel there are preferably provided a plurality of separate structures, each receiving one or more of said conduits. Said plurality of structures are preferably co-axial, their common axis being either vertically or horizontally disposed.

Two or more of the structures on the vessel are preferably independently rotatable by separate power-driven winches.

Where the axis ofthe structure on the vessel is vertical, the connection between each conduit and the structure on the vessel is preferably at generally the same vertical level as the connection between the same conduit and the structure on the turret.

In the case where the axis of the structure on the vessel is horizontal, guide means such as sheaves or diabolo rollers are preferably provided to guide the conduits on to the structures.

In any of the above arrangements successive convolutions of each conduit may lie side-by-side on the structure on which it is wound. Preferably, however, each convolution overlies the preceding convolution. In order to effect this, each structure may be formed with a peripheral groove or channel which is of insufficient width to accommodate convolutions side-by-side, but which is of sufficient depth to accommodate a number of convolutions laid on one another.

The following is a more detailed description of embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figures 1-3 show diagrammatically the disposition of one form of transfer system in three different rotational positions of the vessel relative to the turret, Figures 4-6 show diagrammatically similar relative positions for an alternative form of transfer system, Figure 7 is a diagrammatic vertical section through part of a vessel incorporating a transfer system in accordance with the invention, and Figure 8 is a plan view of part of the vessel of Figure 7.

Figure 1 shows diagrammatically one transfer system in accordance with the invention, the system being shown in an initial equilibrium position.

There is mounted on the anchored turret (not shown) a fixed structure in the form of a tower 10 of circular cross-section. One end 11 of a flexible hose 12 is anchored to the tower 10, being coupled through an appropriate coupling to a fixed conduit within the tower.

The outer periphery of the tower 10 is formed with a peripheral groove 13 which is only slightly greater in width than the diameter of the conduit 12. Two convolutions of the conduit 12 are wrapped around the tower 10, within the groove 13, and overlie one another, as shown, due to the narrow width of the groove.

Mounted on the floating vessel (also not shown) which is capable of rotating around the anchored turret, is a further cylindrical structure in the form of a drum 14 which is also of circular cross-section and extends vertically on the vessel so that its axis is parallel to the axis of the tower 10 on the turret. The drum 14 is also formed with a peripheral groove 15, similar to the groove 13 which, in the equilibrium position shown in Figure 1, also receives two overlying convolutions of the conduit 12. The drum 14 is mounted on the vessel so as to be rotatable about its vertical axis, for example under the action of a constant-tension power winch. In order to accommodate this rotation, the end of the conduit 12 is coupled, by a suitable connector, to rigid piping 16 supported on the drum 14.The piping 16 is connected to rigid piping on the vessel by a rotating swivel connector, indicated diagrammatically at 17.

As the vessel rotates relative to the turret, more of the conduit 12 becomes wrapped on to, or unwrapped from, the tower 10 on the turret. At the same time the drum 14 on the vessel is rotated so as to take up or unwrap a sufficient length of conduit to compensate for the change in the length of conduit wrapped around the tower 10, and preferably to maintain substantially constant tension in the stretch of conduit extending between the two structures.

Figure 2 shows for example the position where the vessel has rotated relative to the turret anti-clockwise through one and a half revolutions (540 ). It will be seen that a further one and a half convolutions of the conduit 12 have become wrapped around the tower 10 on the turret, while the drum 14 on the vessel has been rotated clockwise relative to the vessel to compensate for this transfer of conduit to the turret.

Similarly, Figure 3 shows the position after the vessel has rotated through approximately 585" clockwise relative to the turret so that more than one and a half convolutions of the conduit 12 have been unwrapped from the tower 10 and have been taken up by the rotatable drum 14 on the vessel.

In the alternative arrangement shown diagrammatically in Figure 4 the fixed structure on the turret comprises two parallel cylindrical towers 18, 19, of circular crosssection, one end 20 of a conduit 21 being secured at the centre of the structure, between the two towers 18, 19. The opposite end of the conduit is again coupled to rigid piping 22 supported on a rotatable drum 24 on the vessel, the piping 22 being coupled by a swivel connector 23 to rigid piping on the vessel. Figure 4 shows the equilibrium position where none of the conduit is wrapped on the towers 18, 19 and slightly more than two convolutions are wrapped on the drum 24.

Figure 5 shows the position where the vessel has rotated through approximately 405" anti-clockwise about the turret. It will be seen that due to the increased overall peripheral dimension of the twin tower structure on the turret, the cable 21 has not yet perforrned a complete wrap of one convolution about the twin towers although one and three quarter convolutions have been unwrapped from the drum 24.

Figure 6 shows a corresponding position where the vessel has rotated clockwise through about 405" relative to the turret and, again, one and three quarter convolutions of the conduit have been unwrapped from the drum 24 to provide less than one additional convolution around the twin tower structure on the fixed turret.

The advantage of the arrangement of Figures 4-6 over the arrangement of Figures 1-3 is that it requires a shorter conduit length for the same rotational capacity.

It also provides space between the two towers 18, 19 for easy connection of the flexible conduit 21. The arrangement allows rotation of the vessel relative to the turret through plus or minus 405" to be achieved with less than a single wrap of the conduit on the towers 18, 19. It will be appreciated that the rotational capability could be increased by allowing more than one convolution to be wrapped around the towers.

Figures 7 and 8 show in greater detail one particular embodiment of a transfer system according to the invention applied to a turret-moored vessel.

Referring to Figure 7, the hull 25 of the vessel is formed amidships with a vertical aperture 26 of circular cross-section which receives the main body of a turret 27. The upper part of the turret 27 is rotatably mounted in a bearing structure 28 on the main deck 29 of the vessel, so that the vessel may rotate relative to the turret 27 about its central vertical axis.

The turret 27 is fixedly anchored to the sea bed by chains, indicated diagrammatically at 30, and conduits from the undersea oil or gas well lead to main conduits 31 extending upwardly through the turret.

There extends upwardly from the turret 27, above the deck of the vessel 25, a transfer structure 32 similar to that shown diagrammatically in Figures 4-6 and comprising two spaced parallel cylindrical towers 33, 34. Nineteen flexible conduits are connected at one end to the structure 32, ten of the conduits, indicated at 35, extending to starboard and the other nine conduits, indicated at 36, extending to port. The end of each conduit is connected through a suitable connector, as indicated for example at 38 in Figure 8, to a fixed conduit within one or other of the towers 33, 34 of the transfer structure. The connections are spaced one above another along the height of the structure, as may be seen from Figure 7, so that the conduits extend at different vertical levels, conduits passing alternately to port and to starboard.The conduits include flexible hoses and cable bundles to accommodate oil production, test, water injection, gas injection or gas lift, power and control requirements as follows: 2 x 10" Oil production 1 x 6" Oil test 1 x 10" Water injection Hoses 1 x 8" Water injection 2 x 8" Gas injection or lift 2 x 8" Spare flow paths 1 x 4" Vent to flare 1 x 4" Fire water to turret Cables 8 x 4" Bundles of cables for power, control and instrumentation, on turret and subsea.

The conduits 35 extending from turret structure 32 to the starboard side of the vessel are wrapped around respective drums or spools 39 arranged in two vertical arrays in a fixed tower structure 40 mounted on the deck of the vessel. The spools 39 in each array are rotatable about a common vertical axis by power-driven winches and may be individually driven or driven as a unit or in groups. The end of each conduit 35 is connected to its respective spool through a swivel connection 41 to a fixed conduit 42 which leads to the appropriate location in the vessel.

The conduits 36 extending to the port side of the vessel are guided horizontally and vertically by diabolo rollers 43 and 44 to respective drums or spools 45 mounted at the base of a fixed structure 46 for rotation about a horizontal axis extending in the fore and aft direction. Again, the end of each conduit 36 is connected to its spool 45 through a swivel connector 47 to a fixed conduit 48 leading to the appropriate location on the vessel The spools 45 are again rotatable by power-driven winches, either individually or in unison or in groups.

The advantage of rotating the spools about a horizontal axis is that each rotatable structure can then be a conventional drum with a built-in winch and swivel, and it can be installed directly on to the main deck of the vessel.

As the vessel rotates clockwise or anti-clockwise about the turret 27, the conduits 35 and 36 wrap around the towers 33, 34 on the turret and are withdrawn from the spools 39 and 45 in similar fashion to that illustrated in Figures 4-6. If the vessel subsequently rotates in the opposite direction, the conduits are unwrapped from the towers 33, 34 and the spools 39 and 45 are rotated to take up the conduits and compensate for the lengths of conduit unwrapped from the towers. As previously mentioned, the power winches may be constant-tension winches so as to maintain substantially constant tension in the conduits 35 and 36.

Connection of the flexible hoses and cable bundles on the towers 33, 34 are by Grayloc type connectors to facilitate initial installation and later replacement of the conduits in the field where necessary. Connection of the conduits on the vessel-mounted spools is by more compact flange-type connectors.

The invention therefore provides an arrangement which provides large capacity operation of the vessel while allowing substantial rotation of the vessel about the turret under the action of waves, wind and current.

It will be appreciated that the initial orientation of the vessel with respect to the turret 27 is selected according to the prevailing conditions throughout the year at the site of operations. It is therefore unlikely that continuous rotation of the vessel in either direction beyond the limits of the system will occur. However, the vessel may be equipped with a system of orientable position thrusters which may be employed to rotate the vessel back to its original starting position, between changes of weather conditions resulting in its displacement from that orientation.

Claims

1. A turret-moored floating vessel ofthe kind including a turret extending vertically through the vessel and so connected to it that, in use, the turret may be anchored while the floating vessel is free to rotate about it, and at least one flexible conduit extending between the turret and the vessel, characterised in that one end of the conduit is connected to a vertical structure on the turret so as to become wrapped around, or unwrapped from, the structure as the vessel rotates about the turret, and the other end of the conduit is connected to a rotatable structure on the vessel around which the conduit may also be wrapped in a manner substantially to compensate for the wrapping or unwrapping of the conduit on the structure on the turret.

2. A vessel according to Claim 1, wherein the structure on the turret is fixedly mounted on the turret.

3. A vessel according to Claim 1 or Claim 2, wherein the structure on the turret is cylindrical and of circular cross-section and is substantially co-axial with the axis of rotation of the vessel about the turret.

4. A vessel according to Claim 1 or Claim 2, wherein the structure on the turret is elongate in overall cross-section.

5. A vessel according to Claim 4, wherein said one end of the conduit is connected to the structure adjacent the centre thereof, as viewed in cross-section.

6. A vessel according to Claim 5, wherein the structure comprises two elements of similar construction disposed side-by-side and parallel.

7. A vessel according to Claim 6, wherein each element is of circular cross-section and generally cylindrical.

8. A vessel according to Claim 7, wherein said one end of the conduit is connected at a location between the two elements.

9. A vessel according to any of the preceding claims, wherein the rotatable structure on the vessel is rotated by a power-driven constant- tension winch.

10. A vessel according to any ofthe preceding claims, wherein the structure on the vessel is generally cylindrical and of circular cross-section.

11. A vessel according to any of the preceding claims, wherein the axis of the structure on the vessel extends substantially vertically and is parallel to the axis of the structure on the turret.

12. A vessel according to any of the preceding Claims 1 to 10, wherein the axis of the structure on the vessel extends generally horizontally and extends generally at right angles to the axis of the structure on the turret.

13. A vessel according to any of the preceding claims, wherein there are provided a plurality of conduits connected between structures on the turret and the vessel.

14. A vessel according to Claim 13, wherein there are provided a plurality of separate co-axial structures on the vessel, each receiving one or more of said conduits.

15. A vessel according to Claim 11, wherein the connection between each conduit and the structure on the vessel is at generally the same vertical level as the connection between the same conduit and the structure on the turret.

16. A vessel according to Claim 12, wherein guide means are provided to guide the conduits on to the structures.

17. A vessel according to any of the preceding claims, wherein successive convolutions of each conduit lie side-by-side on the structure on which it is wound.

18. A vessel according to any of the preceding claims 1 to 16, wherein each convolution overlies the preceding convolution, each structure being formed with a peripheral groove or channel which is of insufficient width to accommodate convolutions side-by-side, but which is of sufficient depth to accommodate a number of convolutions laid on one another.

19. A turret-moored floating vessel substantially as hereinbefore described with reference to the accompanying drawings.