GB2234002A

GB2234002A - Protective structure for sub-sea well heads or Xmas trees

Info

Publication number: GB2234002A
Application number: GB8913886A
Authority: GB
Inventors: Victor Cooper
Original assignee: Northern Ocean Services Ltd
Current assignee: Northern Ocean Services Ltd
Priority date: 1989-06-16
Filing date: 1989-06-16
Publication date: 1991-01-23
Also published as: GB8913886D0

Abstract

A protective structure for subsea use for protecting, for example, an underwater valve comprises a centrally located cell 1 for the valve to be protected and satellite cells 5 disposed around the cell 1 for ballast to give an otherwise light structure a stabilising mass. The central cell 1 is provided with a protective cap (29 Fig. 6D not shown) and walls 3 partially defining the satellite cells 5 are shaped to promote the riding over of the structure by a trawl board, for example. Locating piles 16 extend from the base of the structure. The structure may be made of a rigid corrosion resistant composite. <IMAGE>

Description

PROTECTIVE STRUCTURE The present invention relates to a protective structure. The structure is intended particularly, but not exclusively, for sub-sea use.

The growth of sub-sea technology applying for example to oil and gas exploitation has created an industry-wide need for a rapid-deployment, low cost, efficient, system that can provide suitable protection for underwater valves, Xmas-trees and other equipment at acceptable cost.

Suitable equipment should give a measure of protection from impacts generated by dropped objects or by side impacts such as from fishing trawl boards.

To date, the industry has relied on three alternative schemes.

In the first scheme, a "space frame" composed of a plurality of tubes connected together is provided for location over the area to be protected. However, this is unsatisfactory because it does not provide full protection, is difficult to deploy, usually needs piling, may corrode, is expensive and can create a trap for trawl boards, anchors and the like.

In the second scheme, a massive concrete structure is provided. This is unsatisfactory because it is very costly, may weigh about 800 to 1000 tonne, is very difficult to install, limits maintenance access, is almost irremovable and can still trap anchors and trawl boards.

In the third scheme, no additional protection at all is provided, reliance being placed on the external structure steelwork of the unit to be protected itself.

This lack of any protection at all is inherently unsatisfactory. It is usually adapted by industry purely to contain costs and places environment and production at risk.

According to the present invention, there is provided a protective structure comprising two cells, one of which is disposed for deployment to encompass the object to be protected and the other of which is adapted to receive ballast to weight down the structure in its position of deployment.

In a preferred embodiment of the invention, the cel for the object to be protected is centrally located and one or more cells for ballast are disposed as satellites around a centrally located cell. The or each satellite cell is defined between the or each wall of the central cells and additional walls which extend out from the central cell. These additional walls may act as bracing walls and may also be so shaped to enable an object, such as a trawl board, contacting the structure to ride over the structure. Thus, for example, the additional walls may be triangular in shape extending between a or the wall of the protective cell and the base of the associated satellite cell. Instead of being straight the outer peripheral edge of the walls may be curved, for example, part sinusoidal.In one form-the cells are parallelpipedic and in another the protective cell is cylindrical and the satellite cell is annular.

Although the structure may be made of any rigid material, composite materials to give corrosion resistance are preferred. For example, high strength polymers reinforced with arymid fibres may be employed.

For added protection a cap may be disposed on top of the protective cell to close the cell from above. The structure may be constructed on a modular concept, the cells being detachable from and attachable to each other in any desired order and arrangement.

Piles may be provided extending from the underside of the unit to assist in locating the unit on the sea bed.

In order that the invention may be more clearly understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic perspective view of one form of protective structure, Figure la shows a diagrammatic perspective view of another form of protective structure, Figure 2 is a plan view of the arrangement of Figure 1, Figure 3 is a side elevational view in section of the arrangement of Figure 1 and 2, Figures 4 and 5 shows cross-sectional views through joints betwen adjacent walls of the arrangement of Figures 1 to 3, and Figures 6a to 6d illustrate the steps taken in deploying the structure of Figure 1 as a sub-sea structure.

Referring to Figure 1, the structure comprises a centrally located protective cell 1 of octagonal horizontal cross-section. This cell 1 is supported on a base 2 which extends out from the cell 1. The structure can be given a controlled weight by using a concrete ringbeam. Bracing walls 3 of triangular shape arranged in pairs extend radially outwardly from alternate walls 4 of the protective cell. These walls 3 define with a portion of the base 2 and wall or walls 4 of the cell 1, satellite cells 5 disposed around the protective cell 1.

A remotely operable vehicle access port 6 is provided in one of the walls 4 and in the base 2 beneath that port a formation 7 is provided to accommodate a pipeline extending from a well head. The pipeline 8 and equipment 9 (in this case a satellite well) are not shown in Figure 1 but are shown in Figure 3. Locating piles 16 extend beneath the base 2. The dimensions and shape of the protective cell 1 should be such as to provide sufficient space for the equipment to be protected whilst allowing enough room for a remotely operable vehicle or a diver to move around the equipment for maintenance and servicing. The sloping nature of the external edges of the walls 3 not only provides for direct protection, but also permits bodies being dragged along the seabed such as trawl boards to ride over the structure or at least to be deflected.

The structure may be made of any rigid material, but is preferably made of a composite material such as fibre reinforced plastics material. Suitable materials are hiah strenath polymers reinforced with arvmid

fibres or epoxy resin bonded Twaron Lf ibres. The materials are light, strong and impact and corrosion resistant.

The structure is advantageou'lsy constructed on a modular basis. This provides design flexibility from a relatively small number of components. Existing units may also be modified to provide for changing requirements and conditions. To facilitate implementation of this modular concept, adjacent wall members, are connected by appropriate jointers. An example of a straight jointer 10 for connecting walls 11 in line is shown in Figure 4 and an example of an angle jointer 12 for connecting walls 13 at an angle is shown in Figure 5. Each wall to be joined is formed with an extension 14 of substantially circular cross-section which fits into a cavity 15 of substantially circular cross-section.

Referring to Figures 6a to 6b, the structure may be deployed on the seabed by lowering from a dive support vehicle (DSV) using a lifting frame 20 mounted ona crane 21 and guide wires 22 extending down from the frame to the well head 23 to be protected (Figue 6a). The guide wires facilitate the accurate placing of the structure so that the protective cell is correctly located over the well head. Once deployed on the seabed, the structure is released from the lifting frame 20 and the guide wires 22 retrieved. The satellite cells may then be filled with appropriate material such as gravel and/or concrete.This gives great weight and mass to what is otherwise a relatively light structure and pushes the structure into the seabed below the mud level indicated at 25. The material may be pumped into the cell by means of the pumping arrangement shown in Figure 6c or introduced by any other appropriate method. The pumping arrangement comprises a barge 27 carrying the material, a pumping station and a pipeline 28 extending from the station to the structure. For simplicity, piles extending beneath the structure have not been included in Figures 6a to 6d but where they are included the structure may be pushed to press the piles into the mud.When the satellite cells 5 are full of material, a cap 29 comprising a keyway lock 30 can be lowered onto the structure to close off the protective cell 5 from the top using the same lifting frame, DSV and guide wires previously mentioned.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the invention For example, the structure may have a frusto-conical shape in which case the protective cell maybe cylindrical and the satellite cell annular perhaps divided into segments. Several structures may be connected together if appropriate. The outer peripheral edge of the ballast walls 3 may be curved, for example, part sinusoidal. This arrangement is illustrated in Figure la in which parts equivalent to those of the embodiment of Figure 1 bear the same reference numerals and in which the sinusoidal edge is referenced 3a.

Claims

1. A protective structure comprising two cells, one of which is disposed for deployment to encompass the object to be protected and the other of which is adapted to receive ballast to weight down the structure in its position of deployment.

2. A protective structure as claimed in claim 1, in which the cell for the object to be protected in centrally located and one or more cells for ballast are disposed as satellites around the centrally located cell.

3. A protective structure as claim in cliam 1 or 2, in which the or each cell for ballast is defined between the or each wall of the cell for the object to be protected and additional walls which extend out from the cell for the object.

4. A protective structure as claimed in claim 3, in which the additional walls are bracing walls.

5. A protective structure as claimed in claim 3 or 4, in which the additional walls are shaped to enable an object contacting the structure to rise over the structure.

6. A protective structure as claimed in claim 5, in which the additional walls are triangular in shape and extend between a, or the, wall of the cell for the object and the base of the associated cell for ballast.

7. A protective structure as claimed in any of claims 3 to 6, in which the outer peripheral edge of each additional wall is straight.

8. A protective structure as claimed in any of claims 3 to 6, in which the outer peripheral edge of each additional wall is curved.

9. A protective structure as claimed in claim 8, in which the outer peripheral edge of each additional wall is sinusoidal.

10. A protective structure as claimed in any preceding claim, in which the cells are parallelepipedic.

11. A protective structure as claimed in any of claims 1 to 9, in which the cell for the object is cylindrical and the or each cell for ballast is cylindrical.

12. A protective as claimed in any preceding claim which is made of a rigid material.

13. A protective structure as claimed in any preceding claim which is made of a corrosion resistant composite material.

14. A protective structure as claimed in claim 13, in which the composite material is made of polymers reinforced with arymid fibres.

15. A protective structure as claimed in any preceding claim, in which a top is disposed on top of the cell for the object.

16. A protective structure as claimed in any preceding claim, in which the cells are detachable from one another and may be rearranged as modules and reattached in a different arrangement.

17. A protective structure substantially as hereinbefore described with reference to Figures 1 and 2 to 6d of the accompanying drawings or to these figures with the modification of Figure la.