WO2000073132A1

WO2000073132A1 - Method of positioning a vessel and vessel provided with turret braking and locking means

Info

Publication number: WO2000073132A1
Application number: PCT/EP2000/004985
Authority: WO
Inventors: Jean-Loup Isnard; Sipke Schuurmans
Original assignee: Single Buoy Moorings Inc.
Priority date: 1999-05-27
Filing date: 2000-05-29
Publication date: 2000-12-07
Also published as: GB2365399B; GB0129108D0; NO20015747D0; GB2365399A; NO20015747L; AU5218600A

Abstract

The invention relates to a method of positioning a vessel (1) comprising a hull (6), a cylindrical turret well in said hull and a turret (3), which is rotatably supported in said turret well. The turret may comprise a flange (15) on which two circular raceways (16, 17) are provided. Flange is supported on bogey wheels (12, 13). Via a hydraulic jack (20) carrying a friction pad (22), the rotational movements of the hull around the turret may be restricted until the hull is substantially stationary with respect to the turret, whereafter the position of the turret relative to the hull of the vessel may be locked, for instance by lifting the flange free from the bogey wheels. In this way the heading behaviour, especially during heavy seas, can be improved without the need for auxilliary positioning devices such as thrusters (8).

Description

Method of positioning a vessel and vessel provided with turret braking and locking means.

The invention relates to a method of positioning a vessel comprising a hull, a cylindrical turret well in said hull and a turret which is rotatably supported in said turret well, the turret being connected to the sea bed via anchoring means.

The invention also relates to a vessel for carrying out the method of the present invention, in particular to a floating production, drilling, storage and offloading (FPDSO) vessel. From EP-A-0 399 719 a floating production storage and offloading vessel is known, which is anchored to the seabed via a geostationary turret. Near deck level, the turret comprises a flange which is supported on slide pads on the vessel. During periods of stable wind and current conditions, the turret is supported on the slide bearings, which provide a relatively high friction support. During changing wind and current conditions, rollers may be raised such that the turret is lifted from llic slide bearings, to be supported with low friction, and to allow for easy weathervaning. Supporting the turret on the slide bearings reduces wear of the more expensive wheel bearing system, whereas during support on the wheels, maintenance on the slide pads can be carried out. From GB 2 286 167 a turret support construction comprising bogey wheels is known, wherein the rotation of the bogey wheels can be blocked via a brake mechanism to reduce resonance oscillations of the turret.

It is an object of the present invention to provide a method and vessel with wheatervaning capabilities with improved heading behaviour. It is another object of the present invention to provide a method and a vessel, wherein the static and dynamic stability is improved and wherein the tensions in the anchor legs and the turret excursions, in particular under heavy sea conditions, are reduced.

Thereto the method according to the present invention is characterised in that at predetermined wind, sea and current conditions, or material handling conditions the rotational movements of the hull around the turret are restricted by breaking means until the hull is substantially stationary with respect to the turret, whereafter the position of the turret relative to the hull of the vessel is locked by locking means. The invention is based on the insight that the heading behaviour, in particular in case of failure of the thruster heading control system and/or during heavy seas, can be improved by restricting the rotational movement of the vessel around the turret, and by locking the turret in a fixed preferred position. Also during material handling upon transfer of material from the vessel to the turret, such as heavy weight components such as swivels, or a blow out preventor (BOP), the method of the prevent invention can be carried out for safe handling in lower sea state conditions.

Especially in vessels such as FPDSO's, the turret diameter is relatively large (larger than 10 meter) for accommodating production risers and drill piping. For station keeping, the vertical center line of the turret is placed within half the distance between the bow of the vessel and the midpoint, i.e. relatively close to the midship position, and the turret is moored to the seabed via taut anchor legs, such as polyester cables of a length of for instance 1500 m. Such an anchoring system is capable or exerting a large restoring moment on the hull, compared to the moment exerted by the external forces, compared to systems which utilise a turret that is of a relatively small diameter, and that is located relatively close to the bow. Locking of the turret in such a case would result in winding up the anchoring system, with consequently larger anchor line angles at the fairleads. A suitable vessel for carrying out the method according to the present invention is a FPDSO vessel with active 270 ° turret rotation through a drag chain construction, that is for example described in Offshore Technology Conference paper OTC 881 1, as presented by applicant at the 1998 Offshore Technology Conference held in Houston, Texas, 4-7 May 1998. The turret braking and locking can be carried out by separate braking and locking means. In a preferred embodiment the turret is supported in the hull via bogey wheels which are mounted on one of the hull and the turret, the bogey wheels having contact surfaces running on a raceway mounted on the other of the hull and the turret, the braking means comprising a lifting device with which the turret is lifted such that the raceway and the contact surfaces of the bogey wheels are moved apart. The braking means may comprise hydraulic jacks, which are provided with friction pads. Alternatively, the braking means may for example comprise a motorised gear and pinion system. The turret may comprise at its upper end a flange on which the raceway is mounted, which flange rests on sets of bogey wheels on the vessel. By raising the hydraulic jacks, which are connected to the hull, the turret may be lifted from the bogey wheels, and an increasing braking force is exerted to counteract rotational movements of the vessel. The locking means may be mechanical locking means, such a means to block the movement of the drag chain drive mechanism, but are preferably formed by the hydraulic jacks. For turrets of sufficient weight, the frictional engagement of the jacks lifting the turret away from the supporting bogey wheels is large enough to counteract environmental torque and to thus effectively lock the turret within the hull. To increase the frictional force of the hydraulic jacks, the weight of the turret may be increased by adding additional weight elements, such as concrete or filling buoyancy parts in the turret with water, to ensure that the product of the coefficient of friction of the locking means and the turret weight is larger than the environmental moment created on the turret via the vessel.

The turret, which may for instance have a radius of 13,5 m, can be supported on a number of spaced apart sets bogey wheels, such as described in patent application no. WO 98/31535 and GB 2 320 231 , both filed in the name of the applicant. A hydraulic jack may be provided for each bogey wheel.

The turret can also be supported by a hydraulic bearing system, such as described in the not published US patent application SN 08/911924. In this case the braking, locking and bearing systems can be integrated in one system.

An embodiment of the method and the device according to the invention will, by way of example, be described in the following figures. In the figures:

Figure la, b show a FPDSO vessel according to the present invention, in side view, top view respectively Figure 2 shows a top view of a part of a turret supported on bogey wheels, and

Figure 3a, b show a view along the lines A-A and B-B in figure 2 respectively.

Figure 1 shows a floating production, drilling, storage and offloading vessel 1 comprising a drilling derrick 2 which is placed above a turret 3. The turret 3 is supported in a cylindrical well in the hull 6 of the vessel, relatively close to midship. The turret 3 is anchored with taut polyester lines 4, such as four groups of three lines each, to the seabed 5. The vessel 1 can weathervane around the turret 3, through which product risers and drill piping extends from a subsea wellhead to the vessel 1. Rotation of the vessel and pay-out of the flow lines may be assisted via a drag chain drive 7. Station keeping of the vessel 1 can be assisted by a dynamic positioning system comprising thrusters 8 at the stern or at the bow and the stern (not shown).

As is shown in figures 2 and 3, the turret 3 is supported on sets 10,1 1 of four bogey wheels 12, 13 each. The sets of bogey wheels 10,1 1 are mounted on the hull 6 of the vessel 1 , and support a flange 15 of the turret 3, on which two circular raceways 16, 17 are provided. The radius of the turret 3 may for instance be 1 3,5 in, whereas the number of bogey wheel sets 10,1 1 may be twelve. For each set of bogey wheels, a hydraulic breaking and locking mechanism is provided in the form of hydraulic jacks 20. The hydraulic jack 20 comprises a jack support structure 21, supported by the hull 6, a hydraulic cylinder 23 and a friction pad 22 on the end of a piston which is movable within the cylinder 23. The pad 22 is pushed against the flange 15 of the turret 3, such that the flange 15 is lifted off the bogey wheels 12, 13.

Complete locking of a turret having a braking diameter of 26,7m (diameter of the raceway 16) and a submerged turret weight (including mooring and production lines) of 76888 kN (extra weight added to the turret by water or concrete), the turret being supported on twelve jacks 20, equally spaced over 360 degrees, was achieved at a coefficient of friction of the pads 22 of 0.3, and a lifting capacity of 6407 kN per jack. The breaking force per jack being 6407*0.3 = 1922 kN. For 12 jacks a locking force of 23066 kN was achieved, which is larger than the maximum torque exerted on the turret by the environment (via the vessel).

From model testing of a locked turret and an unlocked turret, based on the same conditions (same environment, same vessel loading conditions, etc), including a 100- year sea state and no additional positioning thrusters active, the locking of the turret resulted in substantially lower tension in the anchor lines, a reduced maximum yaw angle, a reduced excursion of the vessel and reduced maximum roll angle, compared to the freely rotating turret.

Claims

1. Method of positioning a vessel comprising a hull, a cylindrical turret well in said hull and a turret which is rotatably supported in said turret well, the turret being connected to the sea bed via anchoring means, characterised in that al predetermined environmental conditions or material handling conditions the rotational movements of the hull around the turret are restricted by breaking means until the hull is substantially stationary with respect to the turret, whereafter the position of the turret relative to the hull of the vessel is locked by locking means.

2. Method according to claim 1 , wherein a vertical center line of the turret is located at a distance from the midpoint of the vessel which is not larger than half the distance between the bow of the vessel and the midpoint, preferably not larger than one third of the distance between the bow of the vessel and the midpoint, the turret diameter being larger than 10 m.

3. Method according to claim 1 or 2, wherein the anchoring means comprise substantially taut anchor legs extending between the turret and the seabed.

4. Method according to any of the preceding claims, characterised in that the turret is rotatably connected to the hull via bogey wheels which are mounted on one of the hull and the turret, the bogey wheels having contact surfaces running on a raceway mounted on the other of the hull and the turret, the breaking means comprising a lifting device with which the turret is lifted such that the raceway and the contact surfaces of the bogey wheels are moved apart.

5. Method according to claim 4, wherein the lifting device is provided with a friction material which is brought into engagement with the hull or with the turret, for providing an increasing breaking force.

6. Method according to claim 4 or 5, wherein the turret is supported by a number of bogey wheel sets which are distributed along the circumference of the turret, a lifting device being provided for each bogey wheel set.

7. Method according to claim 4, 5 or 6, wherein a weight is added to the turret whereby the breaking force that is exerted by the lifting device is increased.

8. Vessel comprising a hull, a cylindrical turret well in said hull and a turret which is rotatably supported in said turret well, the turret being conncctable to the sea bed via anchoring means, characterised in that the vessel comprises breaking means which at predetermined environmental conditions can be actuated to limit the rotational movements of the hull around the turret until the hull is substantially stationary with respect to the turret, and locking means for fixing the position of the turret relative to the hull of the vessel.

9. Vessel according to claim 8, the diameter of the turret being larger than 10 m, the anchor lines being substantially taut and a vertical center line of the turret being located at a distance from the midpoint of the vessel which is not larger than half the distance between the bow of the vessel and the midpoint, preferably not larger than one third of the distance between the bow of the vessel and the midpoint.

10. Vessel according to claim 9, wherein the turret is rotatably connected to the hull via bogey wheels which are mounted on one of the hull and the turret, the bogey wheels having contact surfaces running on a raceway mounted on the other of the hull and the turret, the braking means comprising a lifting device with which the turret is lifted such that the raceway and the contact surfaces of the bogey wheels are moved apart.

1 1 . Vessel according to claim 10, wherein the lifting device is provided with a friction material which is brought into engagement with the hull oi with the turret, for providing an increasing breaking force.

12. Vessel according to claim 1 1 , wherein the locking means are formed by the lifting device.

13. Vessel according to claim 10, 1 1 or 12, wherein the turret is supported by a number of bogey wheel sets which are distributed along the circumference of the turret, a respective lifting device being provided for each bogey wheel set.

14. Vessel according to any of claim 10 to 13, wherein a weight is added to the turret for increasing breaking force that is exerted by the lifting device.