GB2325485A

GB2325485A - Recoverable underwater storage tank

Info

Publication number: GB2325485A
Application number: GB9806259A
Authority: GB
Inventors: David Bone
Original assignee: RESOURCE MARGINAL SYSTEMS Ltd; RESOURCE TECHN DEV Ltd
Current assignee: RESOURCE MARGINAL SYSTEMS Ltd; RESOURCE TECHN DEV Ltd
Priority date: 1997-05-23
Filing date: 1998-03-25
Publication date: 1998-11-25
Anticipated expiration: 2018-03-25
Also published as: GB2325485B; GB9806259D0

Abstract

A recoverable underwater storage tank assembly comprises a main tank 10 for storing oil or other liquids and a plurality of ballast chambers 14 which can be de-ballasted during recovery of the assembly. The ballast chambers 14 extend below the main tank 10, so that the main tank is supported away from the sea bed S. The downwards forces on the assembly are applied to the sea bed S via foundation plates 18 engaged with the underside of each ballast chamber 14. The assembly further comprises means for disengaging the ballast chambers 14 from their foundation plates 18, so that the remainder of the assembly can easily be recovered, regardless of whether the plates 18 have become adhered to the sea bed S. As shown the disengaging means comprises compressed air supplied through lines 21 to a space formed between the chamber 14 and its foundation plate 18.

Description

Recoverable Underwater storage Tank This invention relates to a recoverable underwater tank for storing oil or other liquids.

Oil which is drawn from the sea bed by oil platforms is often stored in large underwater tanks arranged on the sea bed.

Such tanks are costly and thus it is desirable to be able to recover them once they have become redundant, so that they can be used elsewhere.

Hitherto, underwater oil storage tanks have been recovered by displacing the oil therein with air, thereby causing them to float to the surface.

A disadvantage of known underwater storage tanks is that they become adhered to or embedded in the sand and silt on the sea bed. Thus, during recovery a large amount of buoyancy has to be applied to the tank, before a sufficient force is generated to pull the base of the tank away from the sea bed. A disadvantage of this is that the very buoyant tank will rapidly rise, once it has released from the sea bed.

Hitherto, this problem has been overcome by attaching chains between the tank and the sea bed, which prevent the tank from rising to the surface, once it has been freed from the sea bed. Thus, a more controlled recovery of the tank can be achieved by reducing the buoyancy before the chains are released.

It will be appreciated that the above-mentioned procedure of tank recovery is time consuming and costly.

We have now devised a recoverable storage tank which alleviates the above-mentioned problems.

In accordance with this invention, there is provided an underwater storage tank assembly comprising a storage tank having a foundation member on its underside and release means for releasing the storage tank from said foundation member during recovery of the storage tank.

In use, the foundation member is disposed between the storage tank and the sea bed. In order to recover the tank, the release means is actuated, thereby freeing the tank from the foundation member, which may have become adhered to the sea bed.

Thus, no large force is required to free the tank from the sea bed and a controlled recovery of the tank can easily be effected.

Conventional recoverable oil storage tanks have hitherto been formed of a heavy gauge of metal with strengthening ribs, so that they can withstand the differences in hydrostatic pressure on the sea bed, between when they contain oil and when they contain air. Further strengthening is required to withstand the so-called hydrodynamic pressure differences caused by the waves and by the tide.

A disadvantage of strengthening the tanks in this way is that it makes them heavy and therefore more susceptible to becoming adhered to the sea bed.

Thus, preferably the storage tank comprises a main tank for storing oil or other liquids and a ballast chamber which can be de-ballasted, in order to recover the storage tank.

The main storage tank is not used for buoyancy and therefore can remain full of liquid until it reaches the surface of the water. Thus, the problems caused by the differences in hydrostatic pressure can be avoided and accordingly corresponding strengthening is not required.

In one embodiment, the density of the ballast material is greater than the density of water, the material being displaced by water during recovery of the storage tank.

Oil is less dense than water and thus the tank will rise to the surface, once the ballast material has been released.

In an alternative embodiment, the ballast material is water or a material denser than water, the ballast chamber being arranged to receive air during recovery. The ballast chamber can be strengthened to withstand the differences in hydrostatic pressure. However, this will not substantially increase the weight of the storage tank, since the ballast chamber is small compared with the main tank.

Preferably the foundation member is disposed on the underside of the ballast chamber of the assembly, the ballast chamber being arranged to support the main tank above the surface on which the foundation member rests. Thus, the underside of the main tank will not become adhered to the sea bed.

Preferably the main tank comprises a depending peripheral skirt, which is arranged to prevent erosion of the sea bed under the storage tank. The skirt also prevents sand and silt from being deposited under the main tank.

A disadvantage of providing a skirt on the underside of the main tank is that it effectively forms a seal against the sea bed, which could make it difficult to pull the storage tank away from the sea bed during recovery.

Another disadvantage of the skirt is that any changes in hydrodynamic pressure will not be applied to the water underlying the main tank. Thus, preferably passageways extend through the main tank from its underside, so as to allow the free flow of water around the main tank.

Preferably the foundation member comprises a plate engaged with the underside of the storage tank, the release means being arranged to disengage the plate from the storage tank during recovery.

Preferably the release means comprises a fastening which engages the foundation member with the underside of the storage tank, said fastening being arranged to release during recovery.

Preferably the release means is arranged to apply a force to said fastening, in order to disengage the foundation member from the storage tank.

Preferably the release means is actuated by compressed air.

An embodiment 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 sectional view through a recoverable sub -sea tank assembly for storing oil; FIGURE 2 is a sectional view along the line II-II of Figure 1; and FIGURE 3 is an enlarged sectional view of the underside of a ballast chamber of the assembly of Figure 1.

Referring to the drawings, there is shown a recoverable tank assembly formed of metal for storing oil on the sea bed.

The oil is stored in a large substantially cylindrical main tank 10 having a convex top wall 10, a flat bottom wall 12 and radially-extending internal strengthening walls 13.

A plurality of smaller upright cylindrical ballast chambers 14 are recessed into the cylindrical side wall 15 of the main tank 10, at equal distances from each other. In the example shown, there are four ballast chambers 14 arranged at 900 intervals around the main tank 10. However, this number can be varied according to the capacity of the main tank 10.

Each of the ballast chambers 14 comprises a depending peripheral skirt 16, which is attached at its upper end to an annular leg 17 which extends around the flat circular base of the ballast chamber 14, as shown in Figure 3. The wall thickness of the annular leg 17 preferably reduces towards its lower end.

A flat metal foundation plate 18 is attached to the underside of each ballast chamber 14 by means of plastics nails 20. A compressed air line 21 extends from inside each chamber 14 through an outlet 22 into a void (not shown) formed between the chamber 14 and its foundation plate 18.

The ballast chambers 14 extend below the flat bottom wall 12 of the main tank 10, such that the ballast chambers 14 serve to support the main tank away from the sea bed S. A peripheral depending skirt 19 is provided on the underside of the main tank 10.

A plurality of passageways 23 extend through the main tank 10, between the top and bottom walls 11,12 and a settling tank 24 may be provided at the centre of the assembly.

In use, the tank assembly is positioned on the sea bed ,as shown in the drawings 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, the tank assembly will sink to the sea bed S, where it will settle.

The sea water ballast in the main tank 10 is then displaced by the oil to be stored and the settling tank 24 serves to remove any oil from the water, as it is discharged.

The weight of the tank assembly and the ballast in the chambers 14 keeps the tank assembly on the sea bed when in use.

The downwards forces on the assembly are applied to the sea bed S via the foundation plates 20 and accordingly the plates become adhered to the sea bed S by suction forces etc.

The bottom wall 12 of the main tank 10 is supported above the foundation plates 18 and will thus not become adhered to the sea bed S.

The skirt 19 on the underside of the main tank 10 seals against the sea bed S and on one hand prevents the flow of water from washing away the sea bed S and undermining the assembly, and on the other hand prevents sand and silt from building up underneath the main tank 10.

The water pressure surrounding the assembly varies according to the tide and the waves and this change in pressure is communicated with the water sealed under the main tank 10 by the passageways 23.

During recovery, a large upwards force would have to be applied to the assembly, in order to overcome the suction forces between the plates 18 on the underside of the assembly and the sea bed S. However, in accordance with this invention, the assembly can be released from the plates 18, by applying compressed air to the air lines 21, thereby causing a build up of pressure between the foundation plates 18 and the base of their respective ballast chambers 14.

At a predetermined pressure, the force exerted on the plates 18 is sufficient to break or disengage the nails holding them in place, thereby freeing the remainder of the assembly from the sea bed S.

The remainder of the tank assembly can then be brought to the surface by discharging the ballast in the ballast chambers 14 and/or by filling the chambers with air. The skirt and legs 16,17 on the underside of the ballast tanks 14 are easily pulled out of the sea bed as the tank rises and this is helped by the tapering profile of the legs 17.

The foundation plates 18 are left behind on the sea bed S. However, it will be appreciated that it is relatively inexpensive to fit new plates before reusing the assembly.

Alternatively, it may be possible to recover the plates, say by attaching a lifting device to hooks provided on the plates.

An assembly in accordance with this invention is uncomplicated and inexpensive to construct. Furthermore, the assembly can be recovered quickly and easily, since no large forces are required to pull the assembly away from the sea bed.

Claims

1) An underwater storage tank assembly comprising a storage tank having a foundation member on its underside and release means for releasing the storage tank from said foundation member during recovery of the storage tank.

2) An underwater storage tank assembly as claimed in claim 1, in which the storage tank comprises a main tank for storing oil or other liquids and a ballast chamber which can be deballasted, in order to recover the storage tank.

3) An underwater storage tank assembly as claimed in claim 2, in which the ballast chamber is arranged to contain a ballast material having a density greater than the density of water.

4) An underwater storage tank assembly as claimed in claim 2, in which the ballast chamber is arranged to contain water as a ballast material.

5) An underwater storage tank assembly as claimed in claims 3 or 4, in which the ballast chamber is arranged to receive air during recovery.

6) An underwater storage tank assembly as claimed in any of claims 2 to 5, in which the foundation member is disposed on the underside of the ballast chamber of the assembly, the ballast chamber being arranged to support the main tank above the surface on which the foundation member rests.

7) An underwater storage tank assembly as claimed in claim 6, in which the main tank comprises a depending peripheral skirt.

8) An underwater storage tank assembly as claimed in claim 7, in which a passageway extends through the assembly from the underside of the main tank.

9) An underwater storage tank assembly as claimed in any preceding claim, in which the foundation member comprises a plate engaged with the underside of the storage tank, the release means being arranged to disengage the plate from the storage tank during recovery.

10) An underwater storage tank assembly as claimed in any preceding claim, in which the release means comprises a fastening which engages the foundation member with the underside of the storage tank, said fastening being arranged to release during recovery.

11) An underwater storage tank assembly as claimed in claim 10, in which the release means is arranged to apply a force to said fastening, in order to disengage the foundation member from the storage tank.

12) An underwater storage tank assembly as claimed in claim 11, in which the release means is actuated by compressed air.

13) An underwater storage tank as claimed in any preceding claim, in which the storage tank comprises an outlet through which ballast water can be discharged, the outlet being connected to an outlet of the assembly via separation or filtering means.

14) An underwater storage tank assembly substantially as herein described with reference to the accompanying drawings.