GB2330566A

GB2330566A - Oil and gas production vessel with bottom-mounted turret

Info

Publication number: GB2330566A
Application number: GB9722571A
Authority: GB
Inventors: Richard Stewart Whitttl Martin
Original assignee: LONDON MARINE CONSULTANTS Ltd
Current assignee: LONDON MARINE CONSULTANTS Ltd
Priority date: 1997-10-24
Filing date: 1997-10-24
Publication date: 1999-04-28
Also published as: GB9722571D0

Abstract

In a vessel for oil or gas production comprising a hull, a rotatable turret 2 extending upwardly through the hull, a conduit 6 for conveying oil or gas upwardly through the turret, and bearing means 3 taking substantially the whole weight of the turret, the bearing means comprises either a plurality of rotatable elements 20,23,26,27 or plain elements (60,61,62, Fig 9) located in the bottom half of the hull. As shown the bearing means is exposed to sea water. Alternatively sealing means (30,31 Figs 4-7) may be provided to prevent ingress of sea water into the bearing means.

Description

Oil or Gas Production Vessel with Bottom-Mounted Turret Description The present invention relates to a vessel for oil or gas production.

Ships and ship-like vessels are now commonly used for the extraction of offshore hydrocarbon resources and may be deployed in conjunction with other facilities, such as a platform, or platforms, which are fixed to the seabed, or may be deployed on their own, or together with other floating facilities.

The classes of vessel to which the present invention may be applied include:1. Floating storage and offloading vessel (ISO) or floating storage unit (FSU).

These vessels load onboard processed crude oil from another facility.

2. Floating storage production and offloading vessel PESO). These vessels receive the unprocessed wellstream oil or gas, process it and store it.

3. Floating production unit (FLU) and floating production and offloading vessel (FPO). This vessels are like the FPSOs but do not have storage capabilities.

These vessels, which may be purpose built or made by converting an oil tanker, typically have a rotatably mounted turret through which pipes carrying oil or gas pass. The turret often passes completely through the hull of the vessel. However, in some designs the turret is located in the bulge commonly found at the bow below the waterline in tankers. Due to their small size, turrets of this design have a very restricted capacity for carrying mooring loads, as compared to larger turrets passing through the main part of the hull. Furthermore, there is sufficient space for only a very few risers, i.e. pipes carrying oil or gas.

Since the turret is rotatably mounted, the vessel can "weathervane" about the turret in response to the wind, wave and currents. This avoids the pipes becoming twisted and entangled. In this system mooring lines, which may be formed of chains, wires, man-made fibre ropes, or any combination of these, lead from anchors or piles on the seabed, to the turret.

In designs where the turret passes through the main part of the hull, the turret is normally supported at the top deck of the vessel by bearings and depends therefrom through the hull. A bearing may be provided at the bottom of the turret but this is to provide lateral support rather than for supporting the weight of the turret.

The problem of transmitting the vertical loads due to the mooring line loads from the top deck to the bottom of the vessel has been addressed by installing a cylinder around the turret or by stiffening the ship's structure.

Another way of addressing this problem has in the past been to install, within the hull, roller bearings in which a plurality of rollers are mounted in a race. However, bearings of this type must be protected absolutely from the ingress of seawater and its components cannot be replaced whilst the bearing is in service. This means that a failure of the rollers, or of the surfaces on which they bear, results in the vessel having to be taken out of service for replacement of the complete bearing.

According to the present invention, there is provided vessel for oil or gas production comprising a hull, a rotatable turret extending upwardly through the hull, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means is arranged such that, when the vessel is at sea, it is exposed to seawater. The bearing means may indeed be submerged in seawater when the vessel is at sea.

From another perspective, the present invention provides a vessel for oil or gas production comprising a hull, a rotatable turret extending upwardly through the hull, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means comprises a plurality of exposed rotatable elements and is located in the bottom half of the hull.

From a further perspective, the present invention provides a vessel for oil or gas production comprising a hull, a rotatable turret extending upwardly through the hull, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means comprises a plain bearing, for example comprising a plurality of low friction pads, and is located in the bottom half of the hull.

The afore-mentioned rotatable elements may indeed be rollers. However, they are to be distinguished from the rollers of the prior art roller bearing in that they operate exposed to seawater rather than being enclosed and sealed.

Preferably, the bearing means includes a stationary part mounted at the bottom part of the hull. This configuration avoids the need for strengthening the structures of the hull over its full depth. However, this is not essential and in some applications it may be preferable to mount the main bearing higher up in the hull, for instance at the level of the vessel's neutral axis. The level of the neutral axis is the level at which longitudinal bending stresses in the vessel are zero. Depending on the loads involved, and in the case of tanker conversions the original strength of the vessel, it may be necessary to reinforce the bottom of the hull where the stationary part is mounted.

Preferably, the turret includes a substantially annular, radially extending flange arranged to contact the bearing means. This flange may provide the point of contact on the turret for the bearing means. Bearing elements may contact bottom, radial and top surfaces of the flange to receive loads in different direaions.

Preferably, the bearing means comprises a plurality of rotating elements, e.g. wheels or rollers, the rotatable elements being mounted to the hull and being arranged to contaa the turret. If wheels are used, they are preferably formed with integral axles.

Preferably, rotatable elements rotate within phosphor bronze bushings or the like.

Other materials which are resistant to corrosion when immersed in seawater may be used instead of phosphor bronze. The rotating elements themselves may be formed from stainless steel, or other corrosion resistant material. Alternatively, the rotating elements may be made of steel with stainless steel or plastics resin coatings on their wearing surfaces.

In order to ease maintenance, vessel may include a compartment surrounding the bearing means, and means for supplying compressed air to the compartment to reduce the water level therein.

Whilst a bearing as employed in the present invention may be tolerant of contact with seawater, sealing means may be provided for countering the ingress of seawater into the bearing means.

Preferably, the turret includes a chamber about its lower end.

Preferably, the turret has a waist in the region of the sealing means. This means that the length of the sealing means is less than would otherwise be the case. As a result the seal is more reliable and friction at the seal is reduced so that the vessel weathervanes more easily.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows a vessel according to the present invention in use; Figure 2 is a partial sectional view of a first embodiment of a vessel according to the present invention; Figure 3 is a detailed view of part of the main bearing of the embodiment of Figure 2; Figure 4 is a partial sectional view of a second embodiment of a vessel according to the present invention; Figure 5 is a sectional view of the upper seal of Figure 4; Figure 6 is a sectional view of the lower seal of Figure 4; Figure 7 is a partial sectional view of a third embodiment of a vessel according to the present invention; Figure 8 is a partial sectional view of a fourth embodiment of a vessel according to the present invention; and Figure 9 is a detailed view of part of an alternative main bearing structure.

Referring to Figure 1, a vessel 1 has a turret 2 extending vertically from its top deck to the bottom of its hull. The weight of the turret 2 is taken in the main by a main bearing 3. The top of the turret 2 is supported against sideways motion by a subsidiary bearing 4.

The vessel 1 is anchored to the seabed by a mooring system comprising a plurality of anchors or piles connected by twelve mooring lines or chains 5 (only two shown) to the underside of the turret 2. Wellstream flow is brought to the vessel 1 via a plurality of pipes 6 (only one shown) which pass up through the turret 2 to manifold and pigging facilities 7 mounted to the top of the turret 2. The manifold and pigging facilities 7 include fluid swivels or other means connected to piping 8 leading to processing or storage facilities.

The turret 2 is held in a fixed position by the mooring lines or chains 6 and the vessel 1 is able to weathervane about the turret 2 in response to winds, waves and currents.

Referring to Figure 2, the turret 2 comprises a tube 9 extending from above the top deck 10 to the bottom 11 of the hull. An annular chamber 12 is formed around the bottom end of the tube 9. The mooring lines 5 extend from the chamber 12 and can be paid out from or withdrawn into the chamber 12 as necessary. Immediately above the chamber 12, there is an annular flange 13, projecting radially from the tube 9.

The area 14 of the bottom 11 of the hull that lies around the chamber 12 is built up and reinforced. A ring of hooked fingers 15 projects from the top of the reinforced area 14. Each of the fingers 15 overhangs the flange 13. The reinforced area 14 and the fingers 15 carry wheels which are in contact with the flange 13 to form the main bearing 3.

No seals are provided and, consequently, seawater not only rises within the tube 9 but also finds its way between the chamber 12 and the reinforced area 14 into the compartment of the vessel that surrounds the turret 2. As a result the main bearing 4 is submerged.

The subsidiary bearing 4 comprises PTFE-coated pads 29 arranged around the top of the tube 9.

A segment of the main bearing will now be described in detail with reference to Figure 3 by way of example. The finger 15 comprises a vertical portion 16 rising from the top of the reinforced area 14 and a horizontal portion 17 which projects from the top of the vertical portion towards the tube 9 (see Figure 2). A first pair of trunnion blocks 18, 19 depend from the free end of the horizontal portion 17. The first pair of trunnion blocks 18, 19 supports a first wheel 20 which has an integral axle. The axis of rotation of the first wheel 20 extends radially with respect to the tube 9 (Figure 2).

A second pair of trunnion blocks 21, 22 are located mid-way up the tube-side face of the vertical portion 16. The second pair of trunnion blocks 21, 22 supports a second wheel 25 which has an integral axle. The axis of rotation of the second wheel 23 is parallel to the axis of the tube 9.

A third pair of trunnion blocks 24, 25 is located on the upper surface of the reinforced area 14 underneath the first pair of trunnion blocks 18, 19. The third pair of trunnion blocks 24, 25 support third and fourth wheels 26, 27 which are integrated with a common axle.

The axles of the first, second, third and fourth wheels 20, 23, 26, 27 rotate within phosphor bronze bushings, located in the trunnion blocks 18, 19, 21, 22, 24, 26.

Referring to Figure 4, a second embodiment is identical to that shown in Figure 2 except that upper and lower seals 30, 31 are provided to counter the ingress of water and debris into the vessel. The upper seal 30 is located around the top of the outer wall of the chamber 12. The lower seal 31 is fixed around the bottom end of the outer wall of the chamber 12. The upper seal 30 is primarily for countering the ingress of seawater and the lower seal 31 is primarily for countering the ingress of debris.

Referring to Figure 5, the upper seal 30 comprises two channels 32, 33 running circumferentially around the outer wall 34 of the chamber 12. Each channel 32, 33 has received therein a respective elastomeric sealing ring 35, 36. The sealing rings 35, 36 have radial flanges 37, 38. These flanges 37, 38 extend radially outwards and down, and press against a stainless steel insert 40 in the side wall 39 of the reinforced area 14.

An aperture 41 extends through the wall 34 of the chamber 12 between the channels 32, 33 so that the performance of the lower 36 of the two sealing rings can be monitored.

Referring to Figure 6, the meeting of the outer wall 34 of the chamber 12 and its lower end wall 42 is rounded. A groove 43 is formed around the outer wall 34 of the chamber 12 at the point where it begins to make the transition to the lower end wall of the chamber. A resilient annular member 44 is received in the groove 43 and bears against the surrounding hull portion 45.

Referring to Figure 7, a third embodiment of the present invention is the same as the above-described second embodiment save that the chamber 12 has a waist 50 towards its upper end. The upper seal 30 is located at the waist 50 so that the circumference of the sealing rings is reduced.

Referring to Figure 8, a fourth embodiment of the present invention is the same as the above-described first embodiment save that an air-tight compartment 55 is formed around the main bearing 4. The air-tight compartment 55 is defined by an inverted, dish-shaped wall 56 mounted to the reinforced area 14. An annular air-tight seal 57 is provided between the wall 56 and the turret 2. An air-lock 58 is provided to allow access to the air-tight compartment 55. An inlet 59 is provided in the wall 56 so that compressed air can be pumped into the compartment 55 by a compressor (not shown).

If the normally submerged main bearing 4 requires inspection or maintenance work, compressed air is pumped into the air-tight compartment 55 via the inlet 59. The increasing air pressure in the compartment 55 forces the level of the seawater down exposing the main bearing 4. Once, the main bearing 4 is exposed, a worker can enter the compartment 55 via the airlock 58.

When the inspection or maintenance work has been completed, the worker exits via the airlock 58 and the air, that had been pumped into the compartment 55, is allowed to escape via the inlet 59. As a result the water level in the compartment 55 rises and recovers the main bearing 3.

Referring to Figure 9, the main bearing structure as described above may be replaced with a plain bearing. In this case, a first pad is mounted underneath the free end of the horizontal portion 17 of a finger 15. A second pad 61 is mounted on the radially in with the directed face of the vertical portion 16 and a third pad is mounted on the upper surface of the reinforced area 14 underneath the first pad 60. The bearing surfaces of the first, second and third pads are provided with PTFE coating 63, 64, 65.

It will be appreciated that many modifications can be made to the above-described embodiments. For instance, although the present invention has been described with reference to a vessel receiving oil from a well, the incoming fluid stream may be from another vessel or facility. The turret may be provided with rails which are contacted by the wheels or rollers of the main bearing.

Claims

Claims 1. A vessel for oil or gas production comprising a hull, a rotatable turret extending upwardly through the hull aft of the bow, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means is arranged such that, when the vessel is at sea, it is exposed to seawater.
2. A vessel for oil or gas extraction comprising a hull, a rotatable turret extending upwardly through the hull aft of the bow, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means comprises a plurality of exposed rotatable elements and is located in the bottom half of the hull.
3. A vessel for oil or gas extraction comprising a hull, a rotatable turret extending upwardly through the hull aft of the bow, a conduit for conveying oil or gas upwardly through the turret, and bearing means taking substantially the whole weight of the turret, wherein the bearing means comprises a plain bearing and is located in the bottom half of the hull.
4. A vessel according to claim 1 and claim 2.
5. A vessel according to claim 1 and 3.
6. A vessel according to any preceding claim, wherein the bearing means includes a stationary part mounted at the bottom part of the hull.
7. A vessel according to claim 6, wherein the bottom part of the hull is reinforced where said stationary part is mounted.
8. A vessel according to any preceding claim, wherein the turret includes a substantially annular, radially extending flange arranged to contact the bearing means.
9. A vessel according to claim 8, wherein the bearing means comprises a bearing element contacting a bottom surface of the flange.
10. A vessel according to claim 9, wherein the bearing means comprises a further bearing element contacting a radial surface of the flange.
11. A vessel according to claim 9 or 10, wherein the bearing means comprises a further bearing element contacting an upper surface of the flange.
12. A vessel according to claim 1, 2 or 4, wherein the bearing means comprises a plurality of rotatable elements, the rotatable elements being mounted to the hull and being arranged to contact the turret.
13. A vessel according to claim 1, 2, 4 or 12, wherein the rotatable elements rotate within phosphor bronze bushings or trunnion blocks.
14. A vessel according to claim 1, including a compartment surrounding the bearing means, and means for supplying compressed air to the compartment to reduce the water level therein thereby making the bearing means available for maintenance.
15. A vessel according to claim 2 or 3, including sealing means for countering the ingress of seawater into the bearing means when the vessel is at sea.
16. A vessel according to any preceding claim, wherein the turret includes an chamber about its lower end.
17. A vessel according to claim 15, wherein the turret has a waist in the region of the sealing means.
18. A vessel substantially as hereinbefore described with reference to the accompanying drawings.