GB2137679A - Underwater buoyant tension leg - Google Patents

Abstract

The disclosure relates to an underwater buoyant structure in the form of a rigid platform of buoyant tubular members (15, 16) on which sub-sea equipment (22, 23) is mounted. The platform has a number of anchorages (17) (e.g. four) to which tethering members (18) are connected to anchor the platform to the sea bed at a level above the sea bed but below the surface to place the platform away from some or most of the exciting forces of the surface and from wind force and risk of collision with surface vessels. <IMAGE>

Description

SPECIFICATION Underwater buoyant tension leg This invention relates to a restrained underwater buoyant structure primarily for offshore facilities.

Two basic types of offshore oilfield production installations are in use at present. The first of these is surface orientated production and is conducted on platforms having live conductors and cellar deck christmas trees.

The second type is known as subsea production and involves christmas trees on the seabed which generally lead to a subsea mainfold system and thence via a riser assembly to a surface facility. The surface facility may be a floating facility or may be a jack-up or seabed orientated facility. However, whether satellite or template wellheads are used, or a combination of both, and whether the wells serviced are oil or gas producers or injection wells, and whether or not a subsea manifold is used, subsea well completions require control systems and marine riser assemblies. In many such subsea production systems, especially those which are aimed at early production incorporating offshore loading, crude oil storage facilities are valuable to allow buffer capacity to cover oil tanker changes at loading terminals.In most subsea production systems as well as in most ancilliary systems for reservoir injection purposes, the riser assembly which connects the surface equipment with the subsea equipment has to be so arranged that it can be disconnected in bad weather, or for maintenance and afterwards reconnected as reliably and easily as possible. From time to time it also becomes essential or desirable for divers to be deployed to work at the lower end of the riser, either to service or maintain the same or to facilitate disconnections or reconnections or replacements, and now this can be a costly and time consuming operation and can be considerably assisted if the underwater work can be undertaken at a depth where air breathing surface orientated diving is possible, i.e. at 30 metres of seawater, or less.The same depth consideration is applicable to all diving operations on subsea equipment of whatever type or nature.

More specifically the invention provides an underwater buoyant structure having a plurality of members whose sole function is to tether and restrain the structure to the seabed or to another structure or structures connected to or on the seabed and intended to support subsea equipment at a depth below the surface which is less than the depth of the seabed and sufficient to remove it from some or most of the exciting forces of the sea surface, from the forces of wind, and from sea surface collision risks. The structure may also be used to reduce the length of marine riser systems which may be connected from it to a surface orientated facility, and with the objective of reducing either the cost or complexity of such riser systems.

The structure may comprise an underwater buoyant structure wherein the structure comprises a buoyant platform having two or more spaced tethering members for anchoring the structure to the sea bed with the structure spaced above the sea bed but below sea level.

The platform may comprise a framework of closed air or foam filled rigid tubular members to render the platform and the equipment thereon buoyant and the platform may be of generally rectangular shape and have anchorages at the four corners thereof for four spaced anchor lines.

The platform of the structure preferably comprises a first pair of parallel spaced tubular members and a second pair of parallel tubular members extending transversely to the first members and connected thereto and further bridging members extending between the first and/or second pairs of tubular members to receive and support said sub-sea equipment.

In any of the above arrangements the tethering members may be arranged to extend generally vertically downwardly from the structure.

Two (or more) such structures may be tethered together either horizontally or vertically disposed in relation to each other.

This invention thus provides an underwater buoyant structure which is adapted to carry subsea oilfield equipment up from the seabed to a more convenient depth beneath the surface, but still substantially beneath the more damaging effects of wave action and other surface environmental forces and risks.

The underwater buoyant structure will typi caliy be constructed of welded steel or another suitable mateial such as fibre reinforced plastic or marine quality aluminium. It may be made boyant by virtue of entrapped air or by filling it with any substance to make it of lesser density than seawater. It will always be tethered by two or more members to the seabed and have a sufficiently low density to provide buoyancy to apply a proper tension to the tethering members so that it remains stable in position whilst supporting whatever equipment it is designed to carry.

The upward forces of the buoyancy on the anchor legs will be sufficient to overcome most or all of the tendency of the environment at the depth at which it is established to cause it to move in any direction. It may also be used as a convenient storage container for crude oil or for any other substance light enough in density, or of small enough quantity not to overcome the designed upward force of the reserve boyancy.

The following is a description of a specific embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a perspective view of an offshore oil field including a sub sea structure for supporting part of the equipment for extract ing oil from the sea bed; and Figure 2 is a more detailed view of the sub sea structure shown in Fig. 1.

Fig. 1 of the drawings shows an off-shore oil well site comprising a number of well heads indicated at 10 on the sea bed, a sub sea above sea bed platform structure indicated at 11 for supporting sub sea equipment; an anchored vessel 1 2 which forms the subject of our U.K. Patent Application No. (Reference GCB/BA5463) and a single point mooring buoy 1 3 for delivery of oil to a tanker indicated at 14.

The sub sea platform structure 11 is shown in greater detail in Fig. 2 of the drawings to which reference will now be made. The platform structure comprises a pair of parallel tubular members 1 5 which are sealed and are air or foam filled to render them buoyant and which are connected together adjacent their respective ends by a further pair of transversely extending tubular members 1 6 which are also sealed and air or foam filled.

Triangular webs 1 7 are secured between the adjacent pairs of ends of the platform structure and four anchor chains are secured to the four webs respectively. The anchor chains 18 extend generally vertically downwardly from the corners of the platform structure and are attached to fixed anchorages 1 9 on the sea bed to support the platform structure above the sea bed at a level about 30 metres below sea level. At that position, the sub sea structure is effected only marginally by currents and water movement but is readily accessible to divers for maintenance and other work to be carried out on it.

The platform further comprises longitudinally extending members 20 and transversely extending members 21 which, with the main members of the platform, support a plurality af marine risers indicated generally at 22 and other pipework and connectors indicated at 23 for the extraction and distribution of oil from the oil well. As can be seen in Fig. 1, the sub sea platform has riser pipelines indicated at 24 connected to the vessel 1 2 for control and operation of the oil well and a further pipeline 25 connected to the single point mooring buoy 1 3 for the supply of oil to the tanker.

Claims (8)

1. An underwater buoyant structure having a plurality of members whose sole function is to tether and restrain the structure to the seabed or to another structure or structures connected to or on the seabed and intended to support subsea equipment at a depth below the surface which is less than the depth of the seabed and sufficient to remove it from some or most of the exciting forces of ther sea surface, from the forces of wind, an from sea surface collision risks.

2. An underwater buoyant structure as claimed in claim 1 wherein the structure comprises a buoyant platform having two or more spaced tethering members for anchoring the structure to the sea bed with the structure spaced above the sea bed but below sea level.

3. An underwater buoyant structure as claimed in claim 2 wherein the platform comprises a framework of closed air or foam filled rigid tubular members to render the platform and the equipment thereon buoyant.

4. An underwater buoyant structure as claimed in claim 3 wherein the platform is of generally rectangular shape and has anchorages at the four corners thereof for four spaced anchor lines.

5. An underwater buoyant structure as claimed in claim 4 wherein the platform comprises a first pair of parallel spaced tubular members and a a second pair of parallel tubular members extending transversely to the first members and connected thereto and further bridging members extending between the first and/or second pairs of tubular members to receive and support said sub sea equipment.

6. An underwater buoyant structure as claimed in any of the preceding claims wherein a plurality of tethering members extend generally vertically downwardly from the structure.

7. An underwater buoyant structure as claimed in any of the preceding claims wherein a further buoyant structure (or structures) is connected to the structure by tethers and is disposed vertically or horizontally in relation to the first mentioned structures.

8. An underwater buoyant structure substantially as described with reference to and as illustrated in the accompanying drawings.