GB2336384A

GB2336384A - Foundations for submarine structures

Info

Publication number: GB2336384A
Application number: GB9823810A
Authority: GB
Inventors: David Bone
Original assignee: RESOURCE MARGINAL SYSTEMS Ltd
Current assignee: RESOURCE MARGINAL SYSTEMS Ltd
Priority date: 1998-03-13
Filing date: 1998-11-02
Publication date: 1999-10-20
Anticipated expiration: 2018-11-02
Also published as: GB9805286D0; GB2336384B; GB9823810D0

Abstract

A submarine structure such as a storage tank comprises a main body portion 10 eg. for storing oil, which is entirely supported above the marine bed S by one or more foundation members 20 fastened to its underside, so that the main body 10 is not in direct contact with the marine bed S. The body 10 of the structure can be recovered from the marine bed S, once it has become redundant, by actuating release means, which free the body 10 from the or each foundation member 20. In this manner, a controlled recovery of the body 10 of the structure can be effected, since no large forces are required to free it from the marine bed S, in the event that the structure has become embedded in the marine bed S.

Description

2336384 Foundations for Submarine Structures This invention relates to

foundations for submarine structures, such as storage tanks and production platforms.

It is well known to provide submarine structures, such as storage tanks and production platforms, which rest on the marine bed whilst in use. Generally, it is desirable to recover such structures, once they are no longer used, so that they can be scrapped or used elsewhere. However, a disadvantage of this is that over time, the structures become adhered to or embedded in the sand and silt on the marine bed, thereby making them difficult to remove.

Hitherto, such structures have been detached from the marine bed by applying an upwards force, which overcomes the suction and other forces holding the structure on the marine bed. The most convenient way of applying an upwards force to the structure is by filing a cavity in the structure with air, thereby providing buoyancy.

However, a disadvantage of applying an upwards force to free the structure from the marine bed is that the forces holding the structure down are unpredictable, with the result that the structure can rise very rapidly under the applied force, once the structure has released itself from the marine bed.

It has been proposed to overcome this problem by attaching chains between the structure and the marine bed, which prevent the structure from rising, once it has been released from the marine bed. Thus, a more controlled recovery can be achieved by reducin the applied force before the chains are released.

It will be appreciated that the above-mentioned procedure is time consuming and costly, since the chains have to be anchored to the marine bed by divers.

We have now devised a submarine structure which alleviates the abovementloned problems.

In accordance with this invention, there is provided a submarine structure comprising a main body portion, a foundation member fastened to the underside of the main body, 2 and release means for releasing the body of the structure from said foundation member during recovery of the body, said foundation member being arranged to entirely support the main body of the structure on the marine bed.

In uset the foundation member entirely supports the body of the structure on the marine bed, so that the main body is not in direct contact with the marine bed. The body of the structure can be recovered by actuating the release means, thereby freeing the body from its foundation member, which may have become embedded in the marine bed. Accordingly, a controlled recovery of the body of the structure can be effected, since no large forces are required to free it from the marine bed.

Preferably the upper surface of the foundation portion of the structure substantially corresponds in shape to the underside of the body portion.

Preferably an upstanding lip extends around the periphery of the upper surface of the foundation member, said lip being arranged to constrain the body portion against lateral movement relative to the foundation member.

Preferably the underside of the foundation member generally lies in a plane which extends parallel to the general plane of the underside of the body portion of the structure, so that the structure can be mounted upright on a horizontal marine bed.

However, in an alternative embodiment the general plane underside of the foundation member may be inclined relative to the general plane of the body portion of the structure, so that the structure can be mounted upright on an inclined marine bed.

Preferably the foundation member comprises a depending peripheral skirt, which is arranged to prevent erosion of the marine bed under the foundation member.

The body portion of the structure is preferably fastened to the foundation portion by frangible fastening 35 means.

In one embodiment, the release means comprises means for applying fluid pressure between the body and foundation portion of the structure, which fluid pressure is sufficient to break said frangible fastening means.

3 In an alternative embodiment, the frangible fastening means comprises an actuator which can be activated, say by fluid pressure or by mechanical displacement to release the body portion of the structure from the foundation portion.

Also, in accordance with this invention, there is provided a method of installing a submarine structure on a marine bed., the method comprising entirely supporting a body of the structure on a foundation member resting on the marine bed, the body of the structure being attached to the foundation member by releasable fastening means.

Preferably the foundation member is attached to the body of the structure by said releasable fastening means prior to submerging the structure.

Preferably a foundation member is chosen having its upper and lower surfaces arranged such that the structure will be supported in its upright position on the marine bed, regardless of the inclination of the marine bed.

Preferably a foundation member is chosen having its upper and lower surfaces separated by a distance which is sufficient to ensure that the underside of the structure will remain above the marine bed even if the foundation member sinks into the marine bed.

Preferably the foundation member is fastened to the marine bed, say by applying a cementitious material therebetween. Said cementitious material may also be used to set the level of the structure.

Embodiments of this invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIGURE 1 is a plan view of an embodiment of underwater storage tank in accordance with this invention; FIGURE 2 is a sectional view along the line II-II of Figure 1; FIGURE 3 is a sectional view along the line III-III of Figure 1; FIGURE 4 is a siTailar section to Figure 3 through a second embodiment of underwater storage tank in accordance with this invention; FIGURE 5 is a sixilar section to Figure 3 through a 4 third embodiment of underwater storage tank in accordance with this invention; FIGURE 6 is a similar section to Figure 3 through a fourth embodiment of underwater storage tank in accordance with 5 this invention; and FIGURE 7 is a similar section to Figure 3 through a fifth embodiment of underwater storage tank in accordance with this invention.

Referring to Figures 1 to 3 of the drawings, there is shown a recoverable underwater storage tank comprising a hollow metal body 10 for storing oil on the sea bed. The oil is stored in a large substantially cylindrical main tank 11 having a convex top wall 12, a flat bottom wall 15 and radiallyextending internal strengthening walls 13. A plurality of smaller upright cylindrical ballast chambers 14 are recessed into the side wall 16 of the main tank 11, at equal distances from each other. In the example shown, there are four ballast chambers 14 arranged at 900 around the main tank 11. 20 The base of each of the ballast chambers 14 extends below the level of the f lat bottom wall 15 of the main tank 11, thereby effectively providing feet on which the body 10 of the tank stands. The underside of the base of each ballast chamber 14 is attached by means of frangible fastenings (not shown) to a foundation member 20 of the tank, which is formed of metal and/or concrete. Each foundation member 20 comprises an annular side wall 23, which is attached at its upper end to a top wall 21 having a flat upper surface. An upstanding lip 22 extends around the periphery of the top wall 21, the inner surface of the lip 22 being inclined outwardly towards its upper end. A hollow circular-section void 24 is formed under the top wall 21 of each foundation member 20.

In use, the tank is positioned on the sea bed S, as shown in Figure 1 by filling the main tank 10 with sea water. The ballast chambers 14 can also contain water or a denser material, such as sand. In any event, it will be appreciated that the tank will sink to the sea bed S, where it will settle.

The sea water in the main tank 10 is then displaced by the oil to be stored. The weight of the tank assembly and the ballast in the chambers 14 keeps the tank assembly on the sea bed S. The weight of the body 10 of the tank and its contents are applied to the sea bed via the foundation members 20 and accordingly, the foundation members 20 eventually become embedded in the sea bed S. The height of the foundation members 20 is preferably sufficient to support the body 10 so that it is not in direct contact with the sea bed S.

During recovery of the tank, a large upwards force would have to be applied to the tank, in order to overcome the suction and other forces acting between the sea bed S and the foundation members 20. However, in accordance with this invention, the body 10 of the tank can be released from the foundation members 20 by releasing the fastening means extending therebetween.

The body 10 of the tank can then be recovered independently of the foundation members 20, which remain embedded in the sea bed S. The foundation members 20 of the tank are relatively inexpensive compared to the cost of the body 10 of the tank and new foundation members 20 can be fitted if the body 10 of the tank is to be reused.

Alternatively, the foundation members 20 can be recovered independently of the body 10 of the tank, say by attaching a lifting device to hooks provided on each foundation member 20.

Referring to Figure 4 of the drawings, the thickness D of the upper wall 21 of some or all of the foundation members and/or their overall height H can be selected according to the location or type of structure.

Referring to Figure 5 of the drawings, the void 24 under the foundation member 20 of the tank may be filled with a cementitious grout 50, in order to provide a level support under the foundation member 20. The grout 50 can be shaped to allow for unevenness in the sea bed S.

Referring to Figure 6 of the drawings, the body 10 of the tank may be attached to the foundation members 20 by means of frangible pins 60. The body 10 of the tank is seated on elastomeric seals 61, which extend around the periphery of the upper surface of each foundation member 20. A small cavity 62 6 is formed between the underside of the body 10 of the tank and respective foundation members 20. A pipe (not shown) carrying fluid such as water or air connects to the cavity 62 between the body 10 of the tank and each of the foundation members 20.

The body 10 of the tank can be released from the foundation members 20 by applying fluid pressure along the pipe, in order to pressurise the cavity 61, thereby exerting an upwards force on the body 10 of the tank, which is sufficient to break the frangible pins 61 and thus the body 10 is released from the foundation members 20 of the tank. The body 10 can then be recovered independently of the foundation members 20, as hereinbefore described.

Referring to Figure 7 of the drawings, in an alternative embodiment, the body 10 of the tank is attached to the foundation member 20 by elongate rods 70, which extend from the upper surface of the body 10 of the tank. The rods 20 can be broken during recovery, either by exerting a mechanical force on the projecting upper end of each rod 70, or by applying fluid pressure to the interior of the rods 70 or to a cavity between the body and foundation members 10,20.

A tank or other structure in accordance with this invention is uncomplicated and inexpensive to construct. Furthermore, the structure can be recovered quickly and easily, since no large forces are required to release the structure from the sea bed.

7

Claims

1) A submarine structure comprising a main body portion, a foundation member fastened to the underside of the main body, and release means for releasing the body of the structure from said foundation member during recovery of the body, said foundation member being arranged to entirely support the main body of the structure on the marine bed.

2) A submarine structure as claimed in claim 1, in which the upper surface of the foundation portion of the structure substantially corresponds in shape to the underside of the body portion.

3) A submarine structure as claimed in claims 1 or 2, in which an upstanding lip extends around the periphery of the upper surface of the foundation member, said lip being arranged to constrain the body portion against lateral movement relative to the foundation member.

4) A submarine structure as claimed in any preceding claim, in which the underside of the foundation member generally lies in a plane which extends parallel to the general plane of the underside of the body portion of the structure.

5) A submarine structure as claimed in any of claims 1 to 3, in which the general plane underside of the foundation member is inclined relative to the general plane of the body portion of the structure.

6) A submarine structure as claimed in any preceding claim, in which the foundation nember comprises a depending peripheral skirt.

7) A submarine structure as claimed in any preceding claim, in which body portion of the structure is preferably fastened to the foundation portion by frangible fastening means.

8 8) A submarine structure as claimed in claim 7, in which the release means comprises means for applying fluid pressure between the body and foundation portion of the structure, which fluid pressure is sufficient to break said frangible fastening 5 means.

9) A submarine structure as claimed in claim 7, in which the frangible fastening means comprises an actuator which can be activated by fluid pressure or by mechanical displacement to release the body portion of the structure from the foundation portion.

10) A method of installing a submarine structure on a marine bed, the method being substantially as herein described with reference to Figures 1 to 3, Figure 4, Figure 5, Figure 6 or Figure 7 of the accompanying drawings.

10) A submarine structure substantially as herein described with reference to Figures 1 to 3, Figure 4, Figure 5, Figure 6 or Figure 7 of the accompanying drawings.

11) A method of installing a submarine structure on a marine bed, the method comprising entirely supporting a body of the structure on a foundation member resting on the marine bed, the body of the structure being attached to the foundation member by releasable fastening means.

12) A method as claimed in claim 11, in which the foundation member is attached to the body of the structure by said releasable fastening means prior to submerging the structure.

13) A method as claimed in claims 11 or 12, in which a foundation member is chosen having its upper and lower surfaces arranged such that the structure will be supported in its upright position on the marine bed, regardless of the inclination of the marine bed.

14) A method as claimed in any of claims 11 to 13, in which a foundation member is chosen having its upper and lower surfaces separated by a distance which is sufficient to ensure that the underside of the structure will remain above the marine bed even if the foundation member sinks into the marine bed.

9 15) A method as claimed in any of claims 11 to 14, in which the foundation member is fastened to the marine bed.

16) A method as claimed in claim 15, in which the foundation member is fastened to the marine bed by applying a 5 cementitious material therebetween.

17) A method as claimed in claim 164. in which said cementitious material is used to set the level of the structure.