GB2257405A

GB2257405A - Tether deployment system

Info

Publication number: GB2257405A
Application number: GB9211934A
Authority: GB
Inventors: Keith Shotbolt
Original assignee: Bechtel Ltd
Current assignee: Bechtel Ltd
Priority date: 1991-06-27
Filing date: 1992-06-04
Publication date: 1993-01-13
Also published as: NO922483L; NO922483D0; BR9202379A; GB9211934D0

Abstract

A tether deployment system for a buoyant mid-water support 2 attached to flexible risers 1 that link subsea production equipment 17 to a production platform 16 comprises a base 5 provided with rotatably mounted storage drums 4 located on the seabed 6. Tethers 3 are fixed at a first end to the storage drums 4 and at a second end to the mid-water support 2. The tether deployment system may include means 9, 10 to prevent the storage drums 4 rotating and motors to drive the storage drums 4 to wind and unwind the tethers 3. When the tether deployment system is installed in very deep water pressure resistant buoys can initially be attached to the tethers and substituted for the mid-water support after deployment. <IMAGE>

Description

TETHER DEPLOYMENT SYSTEM The present invention relates to a tether deployment system for a buoyant mid-water support and associated flexible risers.

Mid-water supports are used to support flexible risers that link production platforms to subsea production equipment. They are used to minimize the weight suspended from the production platform and help to stabilize the motion of the flexible risers caused by wave action.

"Offshore Engineer" magazine of July 1987 describes a method of installing a buoyant mid-water support. The mid-water support is attached to a base by tethers and the complete assembly is lowered to the desired location on the seabed. After the base has been located on the seabed, the mid-water support can be released to float upwards, where it is held in its working position by the tethers. A production platform can then be connected to the seabed wells via flexible risers laid over to the mid-water support. In relatively shallow waters, the tethers can be made of chain and stored in cans prior to release.

See also EP-A-0251488 and corresponding US-A479373.

As oil and gas fields are being found in ever deeper water, the tethers holding the mid-water support to the base need to become longer. The long chain tethers needed in deep water are excessively heavy and so ropes made of wire or synthetic fibre are preferred. It is desirable to store these rope tethers in a relatively neat manner prior to the release of the mid-water support.

Furthermore, in areas of the world where icebergs occur, such as off the East coast of Canada, there is a danger to floating production platforms from collision with icebergs propelled by wind or currents. Although it is possible to disconnect the production platform from the flexible risers to which it is attached, and so allow it to move out of the path of the iceberg, a danger of collision between the mid-water support and the underwater section of the iceberg remains. It would therefore be desirable to be able to lower the mid-water support and associated flexible risers towards the seabed until the threat of collision has passed.

The present invention provides a tether deployment system that allows the easy installation of a mid-water support.

According to the present invention a tether deployment system for a buoyant mid-water support for one or more flexible risers is provided comprising a base which is located on the seabed, a storage drum rotatably attached to the base for example by rotary bearings, and a tether fixed at a first end to a buoyant body and at a second end to the storage drum, so that the tether can be reeled around the storage drum to store the tether prior to the deployment of the buoyant body. The buoyant body may be the mid-water support itself, or a pressure resistant buoy which is subsequently replaced by the support, as explained below.

Preferably the storage drum is provided with locking means to prevent premature deployment of the tether which is a hydraulically operable latch attached to the base which is engageable with the storage drum, thereby to prevent the rotation of the storage drum. The tether can be deployed by disengaging the hydraulic latch from the drum, allowing the storage drum to rotate freely.

In the case where two or more storage drums, each attached to a tether, are used they can be advantageously connected by a shaft to ensure that the winding and unwinding of the tethers is synchronised.

In one embodiment the storage drum is provided with means to wind and unwind the tether to allow the operating depth of the mid-water support to be changed.

The tether deployment system is installed by lowering it to the desired location on the seabed where the base remains in a fixed position either due to its own weight or is fixed to the seabed by other means, for example by piles driven into the seabed through sleeves integral with the base.

The mid-water support can be deployed by releasing the mid-water support to float upwards under its own buoyancy to its working position, usually around lOOm below the surface of the sea, unreeling the tether from the storage drum as it rises. The flexible risers can then be laid over to the mid-water support.

In very deep water it is undesirable to subject the mid-water support to high external pressure by installing it with the base on the seabed. In this case, pressure resistant buoys are attached to the tether in place of the mid-water support. The tether deployment system can then be installed on the seabed as described above. The tether is then deployed upwards by the pressure resistant buoy and the mid-water support lowered to meet it. The tether can then be connected to the mid-water support, e.g. by a diver or remotely operated vehicle.

In one embodiment the present invention can be used to reduce the danger of collision between a floating obstacle, e.g. an iceberg, and the mid-water support and its associated flexible risers. If a floating obstacle is on a collision course with a production platform or the midwater support then the production platform can be disconnected from the flexible risers. The tether can then be rewound onto the storage drum, lowering the mid-water support to a depth where collision with the floating obstacle is avoided.

The invention will now be described by way of example with reference to the embodiments shown in the accompanying drawings in which: Figure 1 shows a production platform moored by flexible risers to the tether deployment system of the present invention; Figure 2 shows an elevation of the storage drum of one embodiment of the invention; Figure 3 shows an elevation of the storage drum of a further embodiment of the invention.

In Figure 1 a production platform 16 is connected to subsea production equipment 17 by flexible risers 1 which are attached to and supported by a mid water support 2. The mid-water support 2 is preferably located around lOOm below the surface of the sea and is itself moored to a base 5 by rope tethers 3. The base 5 keeps its position on the seabed through its own weight or due to piles driven into the seabed 6 through sleeves integral with the base 5.

The tethers 3 are respectively attached to storage drums 4, the storage drums 4 attached to the base 5 by rotary bearings 7. Advantageously an interconnecting shaft 12 is provided to allow the winching action of the storage drum 4 to be synchronised, thereby allowing the tethers 3 to be deployed evenly.

Figure 2 shows a side view of the tether deployment system after installation on the seabed 6 and prior to the deployment of the mid-water support 2. The mid-water support 2 is attached to a first end of a tether 3 by a pin 8 and a second end of the tether is attached to the storage drum 4 by a pin 9. The tether 3 is wound around a storage drum 4, to store the tether 3 neatly prior to deployment. The storage drum 4 is mounted on rotary bearings 7 on a support 17, and the support 17 is attached to base 5.

The storage drum 4 is further provided with a pin 11 which engages a hydraulic latch 10 located on the base 5.

The hydraulically operated latch 10 and pin 11 prevent the inherent buoyancy of the mid-water support 2 prematurely deploying the tether 3 by rotating the storage drum 4. The assembled tether deployment system is then lowered to the seabed 6 and installed in position there.

When the mid-water support 2 needs to be deployed, the hydraulic latch 10 can be activated, for instance by a diver, or remotely operated vehicle, allowing the storage drum 4 to rotate freely and the mid-water support 2 will float upwards, unwinding the tether 3.

Figure 3 shows an embodiment of the tether deployment system that allows the tether 3 to be rewound onto the storage drum 4.

The storage drum 4 is provided with a gear 13 which meshes with a smaller gear 15 driven by a hydraulic motor 14. The operation of the hydraulic motor 14 can rotate the storage drum 4, reeling the tether 3 in or out and lowering or raising the mid-water support 2. The hydraulic latch 10 can then be activated to engage pin 11 and prevent rotation of the storage drum 4 and hold the mid-water support 2 in position.

The hydraulic motor 14 can also be used to control the rate of deployment of the tether 3 and to adjust the working depth of the mid-water support 2 in use.

When there is a threat of collision between the production platform 16 or the mid-water support 2 and an iceberg [or other obstacle], the production platform 16 can be disconnected from the flexible risers 1 and moved out of the path of the iceberg. Then the hydraulic latch 10 on each storage drum 4 can be activated to disengage from pin 11, and the hydraulic motor 14 used to rotate the storage drum 4 and so rewind tether 3 onto the storage drum 4. The mid-water support 2 and its associated flexible risers 1 are thereby lowered to a depth where collision is avoided.

When the tether deployment system is to be installed in very deep water, the first end of the tether 3 can be attached to a pressure resistant buoy (not shown) instead of the mid-water support 2, so that the mid-water support 2 is not subjected to high external pressure when the tether deployment system is installed on the seabed 6.

The tethers 3 can then be deployed upwards by the pressure resistant buoys and the mid-water support 2 lowered to meet them. A diver or remotely operated vehicle can then transfer the tethers 3 to the mid-water support 2 and the flexible risers 1 installed.

Claims

1. A tether deployment system for a buoyant mid-water support (2) comprising: a base (5) for location on the seabed, a storage drum (4) for tether (3) rotatably attached to the base (5), and a rope tether (3), fixed at a first end to the mid-water support (2) and at a second end to the storage drum (4).

2. A tether deployment system according to claim 1 further comprising locking means to prevent the storage drum (4) rotating before the tether (3) is deployed.

3. A tether deployment system according to claim 1 or claim 2 wherein the locking means comprise a pin (11) located on the storage drum (4) which engages a latch (10) mounted on the base (5).

4. A tether deployment system according to any preceding claim comprising two or more storage drums (4) rotatably mounted on a base (5), each attached to a tether (3) wherein the storage drums are connected by a shaft (12) so that the winding and unwinding of the tethers (3) is synchronised.

5. A tether deployment system according to any preceding claim further comprising means to drive the storage drum (4) to wind the tether (3) onto the drum.

6. A tether deployment system according to claim 5 wherein the means to drive the storage drum 4 comprises a hydraulic motor mounted on the base (5).

7. A method of deploying a tether (3) attached to a buoyant body, the tether (3) being fixed at a first end to the buoyant body and at a second end to a storage drum (4), the storage drum rotatably attached to a base (5), comprising the steps of: (i) winding the tether around the storage drum, (ii) installing the storage drum with its base on the seabed, (iii) allowing the storage drum to rotate and the buoyant body to move upward deploying the tether (3), and (iv) where the buoyant body is not a mid-water support (2) for flexible risers, lowering a said mid-water support (2) to the deployed depth of the buoyant body and transferring the tether (3) to the mid water support.

8. A method according to claim 7 wherein the buoyant body is a pressure resistant buoy.

9. A method of avoiding collisions between a floating obstacle and a production platform (16) connected to flexible risers (1) that are attached to a mid-water support (2) using the tether deployment system of claims 4 or 5 comprising the steps of: (i) disconnecting the production platform (16) from the flexible risers (1) and (ii) activating the means to drive the storage drums (4) to rewind the tethers (3) so that the mid water support (2) and the flexible risers (1) attached to it are lowered to a depth where collision with the floating obstacle is avoided.