Ships are increasingly employed as floating platforms in offshore oil fields, and perform a variety of roles for the production, drilling, well intervention, storage and offloading of oil. Such ships typically comprise a cylindrical turret, which is mounted in a bore which extends from the deck of the ship to the keel of the ship.

Such turrets are called"internal"turrets. The turret is rotatably mounted, and is also moored to the seabed. Thus, wind and waves will cause the ship to rotate ("weathervane") about the turret. Riser lines extend from an underwater installation for producing, storing or offloading oil up through the turret to piping on the ship. Conventional riser lines are flexible to allow them to accommodate relative movement caused by wind and waves. However, they may be damaged by excessive twisting about their own axis. The riser lines therefore pass through a multi-path swivel or other means for accommodating rotation so that the riser lines are not twisted, and so damaged, when the ship rotates. Such devices for accommodating rotation are complicated, and therefore expensive.

It is an object of the invention to seek to mitigate this disadvantage.

Accordingly, the invention provides mooring and fluid transfer apparatus for a ship, the apparatus comprising a turret, means for mooring the turret to the seabed, means for allowing the ship to rotate about the turret, and first and second riser lines for transferring fluid to the ship, the first riser line being connected to the second riser line, substantially all of which is mounted within the ship, and the first riser line being mounted such that it-does not rotate with the ship, and the second riser line being mounted such that it does rotate with the ship, the second riser line being flexible so that as the ship rotates, it twists to accommodate the rotation of the ship.

The invention uses first and second riser lines to transfer fluid to the ship, one of which, the second riser line, is flexible so that it is able to twist to accommodate the rotation of the ship, thereby avoiding the need for a multi-path swivel or other similarly expensive means for accommodating rotation of the ship.

The second riser line may comprise swivel means to allow the riser line to twist about its axis. The second riser line will usually be mounted so that its axis is not in line with the axis of rotation of the ship. The second riser line will therefore be subject to twisting about the axis of rotation of the ship, as well as to twisting about its axis. Conventional riser lines are flexible to allow them to accommodate relative movement caused by wind and waves, but may be damaged by excessive twisting about their own axis. Conventional riser lines will therefore be able to twist about the axis of rotation of the ship, but they will not be able to twist about their own axis without risking damage. Accordingly, a conventional riser line may be used for the second riser line, but a swivel means should preferably be used to prevent damage caused by twisting about its own axis.

The means for allowing the ship to rotate about the turret may comprise a bearing.

The bearing may be positioned on or near the deck of the ship. This will allow ready access to the bearing, and avoids the need for a seal to keep out seawater.

Alternatively, the bearing may be positioned on or near the keel of the ship.

Access may be more difficult, and a seal may be required to keep out seawater.

However, it may be possible to have a smaller turret, thereby reducing costs.

The apparatus may comprise a plurality of first and second riser lines. The first riser lines will not rotate with the ship, and so the first riser lines may originate from different underwater installations, if desired.

The apparatus may comprise means for radially spacing the second riser lines in order to prevent them from being damaged by rubbing against each other as they twist to accommodate the rotation of the ship.

The spacing means may comprise a core member which extends along at least a part of the turret and keeps the second riser lines spaced apart along its length, thereby reducing damage caused by rubbing.

The apparatus may comprise means for supporting the second riser lines, thereby relieving tension in the riser lines.

The support means may comprise a plate comprising a plurality of means for receiving a second riser line, the means being spaced apart from one another, thereby further reducing damage caused by rubbing.

One end of the second riser line may be movably mounted to accommodate shortening of the second riser line on twisting. The end of the riser line may be freely movable. Such an arrangement is suitable for low loads, but it may not be suitable for high loads. Accordingly, the end of the second riser line may alternatively be moved by means of an active drive system.

The plate may be movably mounted to accommodate shortening of the second riser line. The plate may be slidably mounted on the core member.

The connection between the first riser line and the second riser line may be in any suitable position provided that substantially all of the second riser line is mounted within the ship. The second riser line may extend substantially from the deck of the ship to the keel of the ship.

One end of the second riser line may be mounted on the turret.

For a better understanding of the invention, two embodiments will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a floating production storage offloading ship (FPSO) having a known mooring and fluid transfer apparatus; Figure 2 shows the turret of the apparatus of Figure 1; Figure 3 shows the multi-path swivel of the turret of Figure 1; Figure 4 shows a known"drag chain"system which may be used instead of the multi-path swivel of Figure 3; Figure 5 shows a mooring and fluid transfer apparatus according to the invention; Figure 6 shows a cross-sectional view along line A-A of the turret of the apparatus of Figure 5; Figure 7 shows an alternative mounting arrangement for the termination plate of Figure 5; Figure 8 shows a cross-sectional view taken along line B-B of the termination plate of the apparatus of Figure 7; and Figure 9 shows in schematic form a second embodiment of a mooring and fluid transfer apparatus according to the invention.

Figures 1,2 and 3 illustrate a floating production storage offloading ship (FPSO) 1 having a known mooring and fluid transfer apparatus 2. The apparatus 2 comprises a turret 3, which is mounted in a cylindrical bore which extends from the deck of the ship 1 to the keel of the ship 1. Such turrets are called"internal" turrets. The turret 3 is rotatably mounted, and is also moored to the seabed via mooring chains 4. Thus, wind and waves will cause the ship 1 to rotate ("weathervane") about the turret 3, which is prevented from rotating by the mooring chains 4.

Riser lines 5 extend from an underwater installation for producing, storing or offloading oil up through the turret 3 into a multi-path swivel 6 at the deck level of the ship 1. The riser lines 5 are flexible to allow for relative movement caused by wind and waves. The multi-path swivel 6 connects the risers 5 to piping on the ship 1, and accommodates rotation of the ship 1 so that the risers 5 are not twisted, and so damaged, when the ship rotates. The multi-path swivel 6 includes large number of separate paths 7 to cater for the differing needs of various oil wells, as well as for injecting water or gas back into the oil field, and exporting processed oil or gas.

Figure 4 shows a known"drag chain"system 8 which may be used instead of the multi-path swivel 6 of Figure 3. In the"drag chain"system 8, the riser lines 5 are laid out on the deck of the turret 3 so as to allow the risers 5 to unwind without twisting as the ship weathervanes.

A disadvantage of both known systems is that they are complicated, and therefore expensive.

Figures 5 and 6 show a mooring and fluid transfer apparatus for a ship comprising a turret 9, means 10 for mooring the turret to the seabed, means 11 for allowing the ship to rotate about the turret 9, and first and second riser lines 12,13 for transferring fluid to the ship, the first riser line 12 being connected to the second riser line 13, substantially all of which is mounted within the ship, and the first riser line 12 being mounted such that it does not rotate with the ship, and the second riser line 13 being mounted such that it does rotate with the ship, the second riser line 13 being flexible so that as the ship rotates, the second riser line 13 twists to accommodate the rotation of the ship.

The ship has a cylindrical bore 14, which extends from the deck 15 of the ship to the keel 16 of the ship. The bore 14 accommodates a cylindrical turret 9. The turret 9 extends from the deck 15 to the keel 16 of the ship. At its lower end, the turret 9 includes a conventional mooring spider 10, which is provided with a number of chains 17 for mooring the turret 9 to the seabed. At its upper end, the turret 9 includes an annular flange 18, which rests on a first bearing arrangement 19 mounted on the deck 15 of the ship. The bearing 19 provides both axial and radial support, and allows the ship to rotate relative to the turret 9, the turret being moored to the seabed so that it cannot rotate. A, second bearing arrangement 20 is also provided at the lower end of the bore 14 to provide further radial support.

A plurality of internal (second) riser lines 13 are accommodated inside the turret 9 and extend from the deck 15 to the keel 16 of the ship. The internal riser lines 13 are mounted in a circular array around the inner circumference of the turret 9 (see Figure 6). Each internal riser line 13 is connected to piping (not shown) on the ship at its upper end, and to a flanged connector (not shown) attached to the turret 9 at its lower end. Each connector connects an internal riser line 13 to an external (first) riser line 12 which extends to an underwater installation for producing, storing or offloading oil. The mooring spider 10 is moored to the seabed, and so cannot rotate. Thus, the external riser lines 12 will not rotate with the ship. However, the internal riser lines 13 are connected to the ship at their deck end via the piping, and so the internal riser lines 13 will rotate with the ship.

Conventional riser lines are used for both the external 12 and internal 13 riser lines. As the internal riser lines 13 are mounted in a circular array, the axis of each internal riser line 13 will not be in line with the axis of rotation of the ship.

Each internal riser lines 13 will therefore be subject to twisting about the axis of rotation of the ship as well as to twisting about its own axis. Conventional riser lines are flexible to allow them to accommodate relative movement caused by wind and waves, but may be damaged by excessive twisting about their own axis. Accordingly, the conventional riser lines will be able to twist about the axis of rotation of the ship, but a single path swivel (not shown) is attached to the fixed end of each internal riser line 13 in order to accommodate the twisting of the internal riser lines 13 about their own axis, so preventing damage to the internal riser lines 13.

The riser lines 13 may also be damaged by rubbing against each other as they twist to accommodate the rotation of the ship. This is prevented by providing the turret 9 with a circular termination plate 23 and a central core 24 which extends from the top to the bottom of the turret 9. The termination plate 23 is provided with a plurality of flanged connectors 25 positioned about the circumference of the termination plate 23 so that each connector 25 is equidistant from adjacent apertures. The termination plate 23 also has a central aperture 26 which receives the core 24. An internal riser line 13 is fixed to each aperture 25, and the internal riser lines 13 are therefore held in a circular array, spaced apart from one another, at the level of the termination plate 23. The central core 24 passes through the centre of the circular array of internal riser lines 12, and so retains the internal riser lines 13 in their circular array throughout the whole length of the core 24.

The termination plate 23 is slidably and freely mounted on the central core 24.

Thus, the termination plate 23 is free to move to accommodate any shortening of the internal riser lines 13 caused by twisting of the riser lines.

The mounting arrangement of Figures 5 and 6 where the termination plate 23 may slide freely up and down the central core 24 is acceptable at low loads.

Indeed, if the load is sufficiently low, or sufficient overlength is provided in the internal riser lines 13, it may even be acceptable to have a fixed termination plate 23. However, the mounting arrangement of Figures 5 and 6 may not be acceptable at high loads as it may not be able to control the high levels of tension caused by the shortening of the internal riser lines 13.

Figures 7 and 8 show an alternative mounting arrangement for the termination plate of Figures 5 and 6, which is suitable for use at high loads. The termination plate 23 is not freely mounted on the central core 24, but is instead moved by four screw jacks 27 each driven by a motor 28. The screw jacks 27 allow vertical movement. The screw jacks 27 may be replaced by hydraulic jacks, but screw jacks are preferred as there will be no creep, and the screw jacks may be manually controlled in the event of failure of the motors. In addition, with screw jacks, it is easier to accommodate the non-linear relationship between rotation and vertical movement required to control the load in the risers.

Figure 9 shows a schematic view of a second embodiment of the mooring apparatus according to the invention.

The main difference between the embodiment of Figure 9 and the embodiment of Figure 5 is that the bearing arrangement 29 which supports the turret 30 is mounted near the keel 16 of the ship. This means that the turret 30 can be much smaller than the turret 9 shown in the embodiment of Figure 5. The embodiment of Figure 9 also uses a seal (not shown) to keep seawater away from the bearing arrangement 29. The internal riser lines 13 extend from the deck 15 of the ship to the keel 16 of the ship as in the embodiment of Figure 5. Each internal riser line 13 is connected to a pipe termination 31 at its upper end, and to the turret 30 at its lower end. The turret 30 is moored to the seabed as in the embodiment of Figure 5, and so cannot rotate. The pipe termination 31 rotates with the ship, and so the internal riser lines 13 will also rotate with the ship and twist to accommodate that rotation as in the embodiment of Figure 5. As in Figure 5, a single path swivel 32 is attached to the end of each internal riser line 13 in order to accommodate twisting of the internal riser lines 13 about their own axis.

In other respects, the embodiment of Figure 9 is similar to that of Figure 5.

The embodiment of Figure 5 provides better access to the bearing, and does not necessitate the use of a seal to keep out seawater. However, the embodiment of Figure 9 uses a smaller turret, thereby reducing costs.