GB2148202A

GB2148202A - Re-injection from a floating structure

Info

Publication number: GB2148202A
Application number: GB08320937A
Authority: GB
Inventors: Alexander George Copson
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-08-03
Filing date: 1983-08-03
Publication date: 1985-05-30
Also published as: GB8320937D0

Abstract

In the method of re-injecting fluid to an underwater reservoir, the fluid is supplied first to a take-off point 15 disposed outwardly remote from the vertically extended perimeter of the floating structure 1,2,3, and then to or towards the injection site by flexible hosing 16. In the event of gas escape at the hose connection a plume of aerated water is not formed directly beneath the floating structure. The structure has a pivoted boom 8 above or beneath the sea surface with the take-off point 15, having supply pipe 12, 13 from said floating structure. <IMAGE>

Description

SPECIFICATION Re-injection from a floating structure The present invention relates to the injection of fluid, especially gas, to an underwater reservoir.

When gas is forced into a sub-sea bore hole there is a risk during the operation that a supply line failure will cause a sudden release of gas into the surrounding water. Failure is most common in flexible jumper hoses used to connect a main gas supply line to a well-head template, particularly where a flexible hose joins a non-flexible line.

The operation, known as gas re-injection", is particularly hazardous when carried out from a floating vessel. A hose rupture near the seabed could create a large plume of gas bubbles rising rapidly to surface. The resultant aerated water might not provide sufficient displacement to support the vessel and the sudden loss of buoyancy could cause sinking or capsize. At present gas reinjection can only safely be performed from bottom supported vessels, with the resultant depth limitation to operation.

The present invention provides a method of reinjecting fluid from a floating structure to an underwater reservoir in which the fluid is supplied first to a take-off point disposed outwardly remote from the vertically extended perimeter of the floating structure, and then to or towards the injection site by flexible hosing In the event of gas escape at the hose connection a plume of aerated water is not formed directly beneath the floating structure. The method of the invention may be performed in a variety of ways; for example by using a floating structure having a take-off point for flexible supply hose disposed remote from the vertically extended perimeter of the floating structure, said take-off point having supply pipe from said floating structure. Preferably the take off point is mounted on a rigid support extending from and attached to the floating structure.

The invention is particularly suited for the reinjection of gas to an oil production well from a semi-submersible marine platform. Preferably the gas take-off point should be 'hard-plumbed' to a platform e.g. by steel or steel-reinforced pressure piping. Most preferably the take-off point would be 20 outrigged underwater from a platform on a suitable framework or boom. This would minimise both the risk of collision damage from surface vessels and environmental damage from wave action, wind etc.

Floating structures usable according to the invention will now be described and exemplified with reference to the accompanying drawings, in which: Figure 1 shows a partial elevation view of a floating structure according to the invention; and Figure 2 shows a partial elevation view of a second floating structure according to the invention.

In the drawings like numerals are used to signify like structures.

In the embodiment of Figure 1 a semi-submersible marine platform has a deck 1 supported over a base structure 2 by columnar support means 3 extending above mean water level 4. The semi-submersible marine platform is positioned above an oil well-head template (not shown) for the purpose of oil extraction. The platform has an underwater boom 8 in hinged attachment to the base 2, the boom 8 being supported by a hinge 9 and restraining cable 10.

Non-flexible gas supply line 12 is hard-plumbed from the deck 1 to the boom hinge 9 where line 12 is swivel connected to boom supply line 13. The hinge 9 and swivel connection provide freedom of movement for the boom 8 and boom supply line 13 in the vertical plane. The angle of the boom 8 relative to the upright columns 3 may be controlled by adjustment of the restraining cable 10. At the distal end of the boom 8 is a take-off point 15 for flexible supply hose 16.

In this embodiment the flexible hose 16 is shown supported by a buoy 17 in a "lazy-S" configuration in order to allow for motions of the platform.

Whilst only one supply line 12 and flexible hose 16 have been illustrated it is envisaged that in practice a plurality of them would be used in conjunction with a single boom 8. Flexible hose 16 is preferably fabricated from individual hose lengths having non-return valve interconnections. It is preferred that flexible hose 16 be connected at the seabed to a terminal not lying directly beneath the platform and hard-plumbed into the template. This obviates the need for any flexible hose to run underneath the semi-submersible marine platform.

It is also preferred that the boom 8 be of length similar to the working draft of the platform; this allows connection of pipes and/or maintainance to be carried out by pivoting the boom to an upright position whereby the take-off point 15 is at or near the mean water level 4. A plurality of restraining cables 10 may be provided and each may be connected to a winch (e.g. mounted on the deck 1) to control pivoting of the boom 8.

It is clear that in the event of failure of flexible supply hose 16 near the joint with the non-flexible supply line 13 at the take-off point 15, any escaping gas will only cause the formation of a plume of aerated water near the surface and remote from the platform.

In a second embodiment, illustrated in Figure 2, the take-off point 15 is mounted at the distal end of a boom 8 mounted above mean water level 4 at deck height. The boom mounting may be either fixed or pivoted; a pivoted boom may be swung over the deck 1 to allow hose attachment and maintainance operations to be carried out more conveniently. It is envisaged that an above water boom arrangement would only be suitable for use in relatively calm waters since the boom and part of the hose are exposed to wind and wave action and are liable to damage in severe weather.

Whilst the invention has been illustrated by description of the use of semi-submersible marine platforms equipped with truss-type outrigged booms, other floating structures (e.g. barges) could be used and other means by which the flexible hose take-off point may be kept remote from the floating structure are possible for example, the safety advantage of the invention could equally well be obtained by rigidly outrigging a framework outwardly from and along a side of the floating structure. This framework would then carry hardplumbed supply lines to a remote take-off point.

In as much as the take-off point 15 has been described in use for passing gas from the floating structure to the flexible supply hose, it is understood that the same apparatus could be used for passing gas from flexible hose to the floating structure and the same safety advantage would be obtained.

Claims

1. A method of re-injecting fluid from a floating structure to an underwater reservoir in which the fluid is supplied first to a take-off point disposed outwardly remote from the vertically extended perimeter of the floating structure, and then to or towards the injection site by flexible hosing.

2. A method according to claim 1 wherein the floating structure is a semi-submersible marine platform.

3. A method according to claim 1 or 2 wherein the re-injection fluid is gas.

4. A floating structure having a take-off point for flexible supply hose disposed remote from the vertically extended perimeter of the floating structure, said take-off point having supply pipe from said floating structure.

5. A floating structure having a take-off point for flexible supply hose disposed remote from the vertically extended perimeter of the floating structure, said take-off point mounted on rigid support structure extending outwardly from and attached to said floating structure.

6. A floating structure according to claim 4 or 5 wherein the floating structure is a semi-submers ible marine platform.

7. A floating structure according to any of claims 4 to 6 wherein the take-off point is supplied from said floating structure by means of non-flexi ble pipe.

8. A floating structure according to any of claims 4 to 7 wherein the take-off point is disposed at the distal end of a truss-type boom attached to the floating structure.

9. A floating structure according to claim 8 wherein the boom is below mean water level.

10. A floating strcture according to claim 8 or 9 wherein the boom is pivotable about its attach ment to the floating structure.

11. A floating structure according to any of claims 4 to 7 wherein the take-off point is disposed on a framework rigidly outrigged outwardly from and along a side of the floating structure.

12. A semi-submersible marine platform having a take off point for a flexible supply hose to carry gas for re-injection to an underwater reservoir, the take-off point being disposed remote from the vertically extended periphery of the platform by means of a boom in hinged attachment to the platform and the take-off point itself being hardplumbed to the platform.

13. A floating structure substantially as hereinbefore described with reference to the accompanying drawings.