GB2152987A

GB2152987A - Offshore system

Info

Publication number: GB2152987A
Application number: GB08500847A
Authority: GB
Inventors: Roger Dyer
Original assignee: SEAFORTH MARITIME Ltd
Current assignee: SEAFORTH MARITIME Ltd
Priority date: 1984-01-13
Filing date: 1985-01-14
Publication date: 1985-08-14
Also published as: GB8500847D0; GB2152987B

Abstract

An offshore production system consists of a toroidal structure (10) held in a submersed condition by a cylindrical columnar structure (12). Production risers (14) within the structure (12) extend from the sea bed to the toroid (10). The risers feed into a manifold (17) on the toroid and the manifold feeds product to the surface through pipes (18). The system is so arranged that the risers can be removed and installed independently of one another through the central aperture (11) of the toroid. <IMAGE>

Description

SPECIFICATION Improved offshore system The invention relates to an improved offshore system.

The problems associated with modern production risers and their continual maintenance under the present stringent conditions that obtain in offshore oil and/or gas field development are discussed by Morgan G.W., Cowan R., Krolikowski L.

and Nikkel K.G., in Modern Production Risers, Petroleum Engineer International from October 1980, p. 86-100 to April 1982, p. 100-132, and in an article by Shotbolt K., The Influence of Production Riser Design on the Configuration and Operation of Semi-submersible Floating Production Systems, Journal of the Society for Underwater Technology, 9 No. 3, Autumn 1983, p. 14-21.

There is at present no concensus concerning a structure that will allow freedom of access to the many risers and ancillary pipes that need to be brought to the surface from either the sea bed direct or via some form of buoy assembly such as that used in the Compliant Roseau (Reed) Tower for deep water. Anon 'Reed' Tower for Deep Water, Offshore Engineer, November 1983, p. 14.

According to the present invention these difficulties are overcome by the provision of a system for use in offshore operations, comprising a submersible toroid, means on the toroid for connection to production risers extending to the sea bed, and means for providing a fluid passageway from said connection means to extend away from the toroid, the system being so arranged that the production risers can be installed or removed independently of one another through the central opening of the toroid.

Herein, a toroid is to be understood to be a surface generated by the rotation of a plane area of any shape about an axis lying in its plane and not intersecting it. Such a surface may be generated by a plane square figure or other plane rectangular figures.

By means of the invention, it is possible for a sub sea riser system to be used for producing oil or gas from sub sea walls, the riser system feeding the product to the toroid which, in use, is conveniently at position just beneath the sea surface that is accessible with conventional diving i.e. without the requirement for personnel to undergo long periods of saturation, that is to say generally about 25 to 60 metres below the water surface. The system may include an articulated column structure to locate the toroid in its submersed condition, but it may also equally be incorporated into a sub sea version of the Tension Leg Platform concept where the structure connecting the toroid to the sea bed foundation will be in the form of tensioned anchor legs.Similarly a sub sea version of the Guyed Tower may be used to locate the toroid where flexure of the structure and restraining guys eliminate the need for a universal joint.

The system according to the invention has production risers not only separated from any primary structural loads but also able to be installed and extracted from the structure in a method normally associated with drilling sub sea wells. This may be achieved by providing manifolds into which the risers can be individually connected. These manifolds are laid out in a toroidal plan to enable the risers to be placed through the toroid. The manifold arrangement is encased within the toroid, which has the dual purpose of keeping the equipment in a controlled environment and providing buoyancy to make the system self supporting.

With the above arrangement the manifold arrangement need not be functionally connected to the column/TLD/Guyed Tower structure when the risers are removed. Thus the troid itself can now be retrieved from the structure with the minimum number of disconnectable elements. Retrieval of the toroid can be made easier by the provision of ballast trimming tanks within the toroid.

The invention will be more fully understood from the following description given by way of example only with reference to the figures of the accompanying drawings in which: Figure 1 is a schematic showing a system according to the present invention; Figure 2 is a sectional elevation of one preferred embodiment of toroid chamber and structure below it to the sea bed, and Figure 3 is a partial perspective view of the toroid shown in Figure 2, illustrating how the risers may be installed.

In Figure 1 a chamber 10 in the form of a toroid has a central opening 11. Beneath the chamber 10 is a structure 12 to the sea bed 13 to which it is connected at 13,, 132. The structure 12 will generally be a lattice structure of right cylindrical form.

The chamber 10 is situated at a distance d below the water surface at W,, W2 which distance is about 25 to 60 metres and below the wave generation area. Production risers 14,, 142 (only two are shown) rise from a foundation structure 15 to which an oil delivery pipe or pipes (not shown) will bring oil and/or gas. The risers 14,, 142 connect at 16,, 162 to suitable valves on the surface of the toroid. A manifold shown generally at 17 allows some six or more pipes 18 to be brought to the water surface at W,, W2.

In practice the chamber will be approximately seven metres in its diameter d2 and its diameter d, will be such as to allow access to human personnel and may be filled with a gas, e.g. nitrogen, and have a neutral or small positive buoyancy.

In Figure 2 a further construction is shown based on the schematic of Figure 1 but with the introduction of practical features for the use of the system in actual offshore oil and/or gas drilling operations. In Figure 2, like parts to those of Figure 1, are shown with the same reference numeral.

The submersed toroid 10 is of rectangular section and is provided with an internal manifold arrangement 17. The manifold 17 is provided with inlet valves 16 mounted on a riser top support structure 20. The inlet valves connect through choke valves 21 to a manifold header 22. The manifold connects to an outlet valve 22 to which is connected flexible outlet pipe 18 that extends away from the toroid, to a surface ship. A plurality of such manifold arrangements may be provided.

The toroid 10 is typically made of steel and comprises inner and outer pressure hulls 23, 24 connected by top and bottom watertight bulkheads 25, 26. The toroid is large enough to allow personnel to enter, and provided with a suitable access hatch 27.

The typical support structure 12 consists of a hollow cylindrical member mounted on a universal joint 28 on a foundation structure 29 on the sea bed. The lower end of the structure 12 is provided with a stab in plate 30 to receive the production risers 14. Stab in connectors 31 on the plate 30 are connected by flexible conduits 32 to flowlines 33 on the sea bed.

Guide frames 34 for the risers are provided at regular vertical intervals along the central opening of the cylindrical support structure 12.

The member 12 is provided with internal bulkheads 35 to provide flotation chambers at its upper end, to make it self supporting when the toroid 10 is removed therefrom.

Mounted on the top of the toroid is a frame 36 carrying guide funnels 37 which guide the risers into appropriate positions as they are lowered through the central opening of the toroid.

Figure 3 shows in detail how the risers 14 may be lowered and raised through the toroid 10, utilising a spyder guide 38. A central t.v. hurdling probe 39 of conventional design may be utilised.

The system of the invention offers a number of advantages as follows: 1. Conventional steel pipe risers can be used together with existing equipment such as valves, screw fittings, guide clamps and stab-in connectors.

2. The risers are protected from primary stresses that would be associated with a free standing tensioned system, be these due to the tensioning system, movement of the surface vessel, action on the risers; both direct and vortex induced and also similar forces from current flow; together with the temperature stress differential created by the product in the risers.

3. The risers are similarly protected from accidental damage as they are incorporated in the central core of the structure.

4. Risers can be installed or withdrawn singularly in a conventionally accepted operation for inspection, maintenance or replacement.

5. No direct continuous tensioning of the risers is required from the surface vessel hence allowing the payload of the surface vessel to be used for other purposes.

6. The manifold chamber can be a controlled environment at an acceptable working pressure for humans. Equipment used is therefore of a standard surface type and direct maintenance is possible.

7. As the manifold chamber can be a controlled environment temperature diferential problems associated with sub sea chokes can be alleviated.

8. With the risers in place or withdrawn, the manifold chamber need not have any functional connection with the rest of the structure, and hence it can be removed for major inspection and maintenance on the surface.

9. The manifold chamber in the sub sea position enables the minimum number of production lines to be brought through the sea surface. These lines can then operate at a controlled pressure and are then more easily removed for inspection, maintenance and replacement because of their shorter length and fewer number.

Claims

1. A system for use in offshore operations, comprising a submersible toroid, means on the toroid for connection to production risers extending to the sea bed, and means for providing a fluid passageway from said connection means to extend away from the toroid, the system being so arranged that the production risers can be installed or removed independently of one another through the central opening of the toroid.

2. A system according to claim 1 including locating means for attachment to the sea bed to locate the toroid in a submersed condition.

3. A system according to claim 2 wherein said locating means comprises a cylindrical structure have a central opening to receive the production risers.

4. A system according to claim 2 wherein the locating means comprises a tension leg structure.

5. A system according to claim 2 wherein locating means comprises a Guyed Tower structure.

6. A system according to claim 2 wherein said locating means comprises an articulated leg.

7. A system according to any preceding claim wherein said connection means comprises manifold means provided with a plurality of inlet valves for connection to the risers.

8. A system according to claim 7 wherein said manifold means is provided with at least one outlet valve for connection to said fluid passageway.

9. A system according to claim 7 or 8 wherein said fluid passageway comprises at least one flexible pipe for extending to a surface vessel.

10. A system according to any one of claims 7 to 9 wherein said manifold means is arranged within the toroid.

11. A system according to any preceding claim wherein the toroid is large enough to permit manual entry therein, and is provided with hatch means to permit such manual entry.

12. An offshore system according to any preceding claim installed with said toroid in a submersed condition and with a plurality of risers connected to said connection means and extending to the sea bed.

13. An offshore system according to claim 11 wherein said toroid is submersed to a depth of between 25 and 60 metres.

14. An offshore system substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

15. An offshore system substantially as herein described with reference to Figure 2 of the accom panying drawings.

16. An offshore system according to any preceding claim and so arranged that the toroid can be removed independently of on-site removal of the risers.