GB2283775A - Fluid storage stuctures - Google Patents

Abstract

An oil storage structure for location on the sea bed has a thick roof slab (2), a thick floor slab (4) and an array of thin intersecting walls (6) all formed by an integrated reinforced prestressed concrete structure. The walls (6) define an array of interconnecting cells (14) which can accommodate water and oil. Oil is stored by displacing water from the cells (14) into the ambient sea. The walls (16) which define the periphery of the structure are of arcuate shape to resist the hydrostatic pressure of the surrounding sea. The structure has three towers (8, 10 and 12) arranged to support a jack-up oil rig. The structure is also provided with guide holes 32 to act as a template for drilling into the sea bed below. <IMAGE>

Description

FLUID STORAGE STRUCTURES The present invention relates to fluid storage structures in particular, but not exclusively, for storing oil on the sea bed.

Rigs for producing oil from below the sea bed may be of the "jack-up" type. These are generally floated out to a site and are jacked up so that three legs engage the sea floor and a working platform is raised above sea level.

When such platforms are located some distance from land, a succession of oil tankers may be used to convey the oil from the platform to a land based terminal rather than using an oil pipe line.

Because of complications in shutting down the oil well between tankers, oil is allowed to continue its flow but is stored in an appropriately sized container so that no interruption in the flow of oil occurs. Such storage containers, to date, have for many different reasons not been satisfactory.

It is an object of the invention to provide an improved oil storage container.

According to the present invention there is provided a fluid storage structure comprising an array of intersecting I-beams defining an array of storage cells and a peripheral wall comprising a plurality of outwardly arcuate contiguous panels; the I-beams and panels being of an integral and of prestressed reinforced concrete.

According to the present invention there is further provided a fluid storage structure comprising a floor slab, a roof slab and an array of intersecting walls supporting the roof slab on the floor slab and defining an array of storage cells; the walls defining the periphery of the structure being outwardly arcuate to resist outer pressure, the walls, the roof slab and the floor slab comprising an integral of prestressed reinforced concrete structure.

An oil storage container embodying the present invention, will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of the container; Figure 2 is a cross-section of the container of Figure 1 to an enlarged scale on line II-II; Figure 3 is a cross-section of the container of Figure 1 to an enlarged scale on line III-III; Figure 4 is a cross-section of the container of Figure 1 to an enlarged scale on line IV-IV; and Figure 5 is a transverse section of the container of Figure 1 to an enlarged scale.

The oil storage container to be described, is in the form of a prestressed reinforced concrete structure which, in addition to storing oil, can be used as a base on which to locate a jack-up platform, and also can be used as a template on the sea bed through which drilling can take place.

As shown in Figure 1, the structure comprises a relatively thick T-shaped roof slab 2, a relatively thick T-shaped floor or base slab 4 and a series of relatively thin intersecting walls 6 supporting the roof 2 on the floor 4.

The walls 6 are arranged to divide the space between the roof 2 and floor 4 into an array of substantially similar cells 14 of generally square crosssection.

The thick roof and floor and the thin walls, all being of prestressed reinforced concrete, act as an integrated array of intersecting I-beams and so provide the structure with the considerable strength needed to withstand the pressures present on the sea bed.

Supported on the base 4 and extending upwardly through corresponding openings in the roof 2, are three hollow cylindrical towers 8, 10 and 12. The towers 8, 10 and 12 are located at the corners of an isosceles triangle and are also of reinforced concrete and are integral with floor, roof and walls. Each tower 8, 10 and 12 carries a corresponding upstanding locating pin 18, 20 and 22 to locate and support a corresponding leg of a jack-up oil rig (not shown).

Those cells 14A which extend around the perimeter of the roof and base have their sides closed off by relatively thin walls 16 of arcuate configuration. The walls 16 are bowed outwardly to resist the external pressure of the sea on the structure. The walls are of reinforced prestressed concrete and are integral with the rest of the structure.

Located on the underside of the floor 2 is an array of downwardly depending steel skirts 24. These skirts 24 are arranged to penetrate the sea bed when the structure is lowered thereon. The floor 4 may be provided with openings (not shown) so that sand or other ballast may be pumped into such cavities, if any, formed between the skirts 24 and the floor 4 to provide a satisfactory foundation on which the structure can rest when located in place.

Around the perimeter of the structure are provided a series of open topped buoyancy cells 30. These cells are also of reinforced prestressed concrete and are made integral with the structure. These cells 30 have two functions. The first is to provide additional buoyancy to allow the structure to be readily floated out of its construction drydock. The second is to hold ballast which will provide a lower centre of gravity for the structure to ensure the structure can be towed to the selected site in a controlled manner. The ballast also provides additional weight to drive home the skirts into the sea bed when lowered thereon. The ballast also provides additional lateral and vertical stability to the founded structure.

Each tower 8, 10 and 12 has a respective upper buffer chamber 8A, 10A and 12A. The lower part of each buffer chamber is provided with openings (not shown) to allow the sea to enter.

The upper part of each chamber 8A, 10A and 12A connected by a pipe line (not shown) to the lower part of one or more cells 14. In this way, the cells 14 fill up with water through the buffer chambers 8A, 10A and 12A.

Apparatus (not shown) is provided at the top of each buffer chamber 8A, 10A and 12A to separate and recover any residual oil from the chamber. The buffer chambers thus provide a buffer between the cells 14 and the ambient sea with a view to keeping any contamination of the ambient sea with oil to a minimum.

The walls 6 may be perforated to allow water and oil to distribute freely throughout the cells.

Alternatively, selected walls may be imperforate to confine oil to particular groups of cells 14. Valves and pumps may be provided to transfer oil and/or water between different groups of cells as required.

As shown in Figure 4, a guide sleeve 32 is provided extending from the roof 2 to the floor 4 to provide a guide hole through the structure to guide a drill string into a selected location in the sea bed below. Several such guide sleeves may be provided. The structure thus acts as a template for drilling operations.

The concrete used in the structure may be lightweight concrete having a specific gravity less than 2.00 instead of the normal 2.4. This provides a light structure which can float more readily to facilitiate removal from a dry dock.

The structure is designed such that it can withstand high temperature differentials between the stored oil and ambient sea. Typically, the storage capacity of the container is between half a million and one million barrels of oil.

While the structure as described provides for an "open" storage system which stores varying proportions of oil and water it can be constructed as a closed system to store oil alone.

While the structure described is generally Tshaped in plan view, it will be appreciated that the cellular nature of the structure allows it to be made to conform to any general shape, for example, triangular, circular or rectangular, or "I" shape.

In operation, the structure is advantageously constructed in a dry dock of lightweight reinforced prestressed concrete as an integral structure. The dry dock is filled with water and buoyancy of the cells 14 and containers will allow the structure to float. The structure is then removed from the dry dock to a suitable deeper water location where the buoyancy cells 30 are charged with ballast. The structure is then towed by tugs to a desired location. The cells 14 are part-filled with sea water to cause the structure to sink in a controlled manner to the sea bed where the weight of the structure will cause the skirts to penetrate into the sea bed. Any cavities between the skirt and the base 4 are filled with grout to provide a stable foundation for the structure.

The three towers 8, 10 and 12 provide support bases for a jack-up oil rig. The rig is towed into position above the structure and the three legs of the rig are lowered onto the towers until they are engaged and located by the corresponding locating pins 18, 20 and 22. At this point, the legs can be secured to the structure and the working platform jacked-up to above sea level. The drilling for oil can now begin which can advantageously take place through the guide 32. As soon as oil begins to flow, it can be pumped into the cells 14 in the upper regions thereof for storage therein. The oil will displace water through the buffer chambers into the ambient sea.

Apparatus at the top of each tower acts to separate and remove any oil which has inadvertently reached the chamber so preventing contamination of the sea. Oil shuttle tankers can load oil at will from the storage structure via the jakcs up platform. With an appropriately sized storage capacity, the flow of oil does not need to be halted upon the departure of one tanker and before the arrival of the next.

When decommissioning the oil rig, the legs can be readily released from the locating pins to leave the lowheight structure on the sea bed thus minimising the hazard to shipping and fishing.

Claims (12)

1. A fluid storage structure comprising an array of intersecting I-beams defining an array of storage cells and a peripheral wall comprising a plurality of outwardly arcuate contiguous panels; the I-beams and panels being of integral and of reinforced prestressed concrete.

2. A fluid storage structure comprising a floor slab, a roof slab and an array of intersecting walls supporting the roof slab on the floor slab and defining an array of storage cells; the walls defining the periphery of the structure being outwardly arcuate to resist outer pressure, the walls, the roof slab and the floor slab comprising an integral reinforced concrete structure.

3. A structure according to Claim 1 or to Claim 2, wherein the walls are perforate to allow fluids to flow freely between the cells.

4. A structure according to Claim 1 or to Claim 2, wherein selected walls are perforate and selected walls are imperforate to divide the cells into separate groups of interconnecting cells.

5. A structure according to Claim 2, wherein the base slab supports a plurality of hollow towers extending through the roof slab, each tower being provided with locating means to locate and support the corresponding leg of an oil rig.

6. A structure according to Claim 5 wherein the towers are of reinforced concrete and integral with the floor and roof slabs.

7. A structure according to any one of Claims 2, 4 and 5 wherein the floor slab is provided with a downwardly depending skirt array to penetrate the sea bed.

8. A structure according to any preceding claim, including a plurality of open containers secured to the outer periphery of the structure to provide buoyancy cells to allow the structure to float or which can be filled with ballast to improve the structures floating stability and its stability on the sea bed.

9. A structure according to Claim 8, wherein said containers are of reinforced prestressed concrete and integral with the structure.

10. A structure according to any preceding claim, including guide channels through the structure to guide drill strings through the structure for drilling into the bed below.

11. A structure according to any preceding claim, wherein the reinforced prestressed concrete is of light weight concrete having a specific gravity of 2.0 or less.

12. An oil storage structure substantially as hereinbefore described with reference to the accompanying drawings.