GB2180809A

GB2180809A - Tethered buoyant system

Info

Publication number: GB2180809A
Application number: GB08622147A
Authority: GB
Inventors: William Norman Iball
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1985-09-24
Filing date: 1986-09-15
Publication date: 1987-04-08
Also published as: GB8523510D0; GB8622147D0; GB2180809B

Abstract

The system comprises a tethered buoyant platform (1), a tether base (2), tethers (4) and tether winches (5), the winches (5) being capable of tensioning the tethered buoyant platform at any desired position including and between the surface and the sea bed. The platform (1) can support a production system (8,9) suitable for use in deep water in an intermediate subsea position but can be raised to the surface for inspection and/or maintenance or lowered to the sea bed to avoid surface obstructions, such as icebergs. A flexible riser (10) is connected to a riser connector (9) on the platform and is looped over a saddle buoy (11) before being connected to a wellhead (not shown). Product is taken from platform (1) via a riser (20) connected to a swivel (8). <IMAGE>

Description

SPECIFICATION Tethered buoyant system This invention relates to a modified tethered buoyant platform.

Tethered buoyant platforms, sometimes called tension leg platforms, have been proposed for the production of oil and/or gas in offshore locations. Such platforms in use are moored vertically beneath their floating positions by means of tethers to which tension is applied to counteract part of the buoyancy and maintain the platform in position.

Such platforms have been proposed for use in connection with relatively large oilfields in relativeiy deep water (i.e. too deep for a free standing platform). These platforms are permanently tethered on station and the superstructure is permanently above the water level and supports a full range of production facilities.

In recent years a number of offshore oilfields have been located which are too small to be economically developed using fixed production platforms supporting a number of producing wells. However, these oilfields are believed to contain significant amounts of recoverable oil and there is therefore a need for a production system which is less expensive than fixed platforms.

The possible alternative is to rely on subsea wellhead equipment with production offtake to a tanker through a mooring buoy.

The servicing of subsea well heads presents a problem, however, particularly in deep waters or in areas subject to severe environmental conditions.

We have now devised a modified tethered buoyant system which can incorporate a production system suitable for use in deep water in which a tethered buoyant platform supporting production facilities is normally employed in an intermediate subsea position but can be raised to the surface for inspection and/or maintenance or lowered to the sea bed to avoid obstructions, such as icebergs.

Thus according to the present invention there is provided a tethered buoyant system (TBS) comprising a tethered buoyant platform, a tether base, tethers and tether winches, the winches being capable of tensioning the tethered buoyant platform at any desired position including and between the surface and the sea bed.

The tethers preferably co-operate with a tether base mounted on a piled or gravity base fixed to the sea bed.

Preferably the tethers run from winches mounted on the platform around pulleys mounted on the tether base and back to anchor points on the platform.

A surface marker buoy is preferably attached to the platform.

The tethered buoyant system is particularly suitable for use as a production platform for the offiake of production from a subsea well to a sea-going vessel.

In this case, the tethered buoyant system also comprises a riser manifold connected at one end to a connector for connection to a riser to a ship, and at the other to a flexible riser connected to a subsea wellhead.

Preferably the riser to the wellhead is supported by a saddle buoy restrained by an anchor.

Most preferably the saddle buoy is tethered to the platform in such manner that at the platform rises, the saddle buoy sinks, and vice versa.

This can be achieved by running a cable from the buoy to the anchor, from the anchor to the tether or gravity base of the platform, and from the tether or gravity base of the platform to the platform itself.

That section of the riser suspended betweeen the platform and the saddle buoy should have sufficient length to accommodate movement so that relative movement between the riser and the buoy at the point of contact is unnecessary, thereby eliminating wear on the riser from this case.

The connector for connection to a riser to a ship may incorporate a swivel, if desired.

The tethered buoyant platform component may be built onshore and floated to site, with its gravity or base section. Should a piled base be required this can be preinstalled.

The base section is then lowered to the sea-bed using the tethers (a gravity base would be ballasted using a flexible hose and for a piled base the tether base would be latched to the pile base).

The saddle buoy to provide a lazy or steep 'S' catenary for the flexible riser is then floated to site.

The prelaid flowline/transfer line/control and flexible lines are picked up and connected at the buoy to the riser lines. The buoy is then lowered and the riser lines connected to the TBS riser manifold.

The TBS platform is then lowered to its operating position below sea level and the saddle buoy automatically rises to componsate for this.

To service the saddle buoy or replace riser hoses the TBS platform is pulled to the seabed and the buoy rises to surface. To connect the new risers at the manifold or service the manifold the TBS riser platform is allowed to rise to surface with the buoy pulled deeper.

Should the system be in iceberg waters it is possible to retract the TBS platform to the sea-bed on the approach of an iceberg. It is then necessary to pull the buoy also to the sea-bed.

The winches on the TBS platform are controlled and powered from surface by an umbilical, to a service vessel.

For wells, it is envisaged that these could be modified so that the tree and control pod are lowered and connected with the flowline/umbilical already attached. The wellhead section of the flowline is then connected at surface to the prelaid flowline, which is picked up and the connected lines lowered to the sea-bed.

It is envisaged that the ease of installation and maintenance will more than compensate for the extra length of lines required by the above system.

The invention is illustrated with reference to the accompanying drawing.

A TBS platform 1 is shown moored in its normal operating position in solid outline and in other positions in broken outline. The platform is tethered to a tether base 2 mounted on a piled base 3. Tethers 4 run from winches 5 on the platform 1 down to pulleys 6 mounted on the tether base and up to anchor point 7 on the platform 1.

The platform 1 is fitted with a swivel connector 8 and a riser connector 9.

A flexible riser 10 is connected to the riser connector 9 and is looped over a saddle buoy 11 before being connected to a subsea wellhead (not shown).

The riser 10 and buoy 11 are also shown in their normal positions by solid outlines and other positions by broken outlines.

The buoy 11 moored to a piled anchor 12 by means of a cable 13 which runs from the buoy over a pulley 14 fitted to the anchor 12, over a pulley 15 mounted on the tether base 2 and over a pulley 16 to a winch 17 mounted on the platform 1.

A surface marker buoy 18 is attached to the platform 1 by means of a cable 19.

Product is taken from the platform 1 by a flexible riser 20 connected to the swivel 8, and loaded onto a tanker 21.

Claims

1. A tethered buoyant system (TBS) com prising a tethered buoyant platform, a tether base, tethers and tether winches, and winches being capable of tensioning the tethered buoyant platform at any desired position inciuding and between the surface and the sea bed.

2. A tethered buoyant system according to claim 1 wherein the tethers cooperate with a tether base mounted on a piled or gravity base fixed to the sea bed.

2. A tethered buoyant system according to claim 2 wherein the tethers run from winches mounted on the platform around pulleys mounted on the tether base and back to an chor points on the platform.

4. A tethered buoyant system according to any of the preceding claims also comprising a riser manifold connected at one end to a con nector for connection to a riser to a ship, and at the other to a flexible riser connected to a subsea wellhead.

3. A tethered buoyant system according to claim 4 wherein the riser to the wellhead is supported by a saddle buoy restrained by an anchor.

6. A tethered buoyant system according to claim 5 wherein the saddle buoy is tethered to the platform in such manner that as the platform rises, the saddle buoy sinks, and vice versa.

7. A tethered buoyant system according to claim 6 wherein a cable is run from the buoy to the anchor, from the anchor to the tether or gravity base of the platform, and from the tether or gravity base of the platform to the platform itself.

8. A tethered buoyant system according to any of claims 4 to 7 wherein the connector for connection to the riser to a ship incorporates a swivel.

9. A tethered buoyant system as hereinbefore described with reference to the accompanying drawing.