GB1573393A

GB1573393A - Under water structures

Info

Publication number: GB1573393A
Application number: GB4230777A
Authority: GB
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1978-05-23
Filing date: 1978-05-23
Publication date: 1980-08-20

Description

(54) IMPROVEMENTS IN OR RELATING TO UNDER-WATER STRUCTURES (71) We, HUMPHREYS & GLAS GOW LIMITED,a British Company, of 22 Carlisle Place, London, SW1P 1JA, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to underwater structures for use in the exploitation of mineral resources located on or below the floor of the sea or of a lake.

In this specification, particular reference will be made to under-water structures that are designed for the production of oil from offshore oil fields in any depth of water. It will be appreciated, however, that this invention is equally applicable to any under-water structure to which access by personnel is required either during its construction or its operation.

When recovering oil from relatively shallow-water locations, it is possible to position a heavy base unit on the sea bed and then to construct an effectively rigid tower thereon which extends to the surface. The oil, without any processing, is in this case led directly via the base unit, which could function additionally as a storage unit, along the length of the tower to the surface. If the structure is close in shore, rigid pipe-lines for the oil can connect the structure to the shore, or alternatively for loading tankers at sea one or more articulated booms can be provided at the top of the tower. A platform providing oil processing and personnel accommodation facilities can also be provided on an extension of the tower which rises above the normal surface of the water, with an allowance being made for maximum wave height and tidal fluctuations.

The development of this type of fixed production structure has, however, been inhibited when associated with water depths of 200 metres or more. The capital cost of such structures and the practical difficulties encountered in their assembly and operation increase to a point where other methods of offshore production have to be considered.

One particular method being used for deep-water operations involves the use of a tethered buoyant platform to which either a fixed pipeline to shore or an articulated loading buoy are connected. Generally in this case, the oil is not processed at floor level before it is led to the surface, and thus serious problems arise in connection with the provision of a high pressure riser and high pressure flexible joints.

Investigations have shown that a complete under-water production system operating in an approximately one atmosphere pressure environment is practicable and economically viable for deep-water applications, but in general this would involve the use of submersible shuttle vehicles for the assembly and maintenance of the under-water facilities. Although the technology of these submersible shuttle vehicles is well developed, they do place severe restrictions on the size and weight of equipment which can be transferred and on the personnel able to use such a system.

The present invention seeks to provide an under-water structure which is particularly suitable for use in deep-water locations and to which comparatively simple access is possible.

In accordance with the present invention there is provided an under-water structure for use in the exploitation of mineral resources located on or below the floor of the sea or of a lake, which structure comprises a base unit adapted to rest on the floor of the sea or of a lake and to which unit access by personnel is required, a hollow tower attached at one end to and extending away from the base unit and having sufficient length for the top of the tower substantially to reach the surface of the water when the base unit is resting on the said floor, and means for providing buoyancy to the tower so that the tower floats in a substantially vertical position when attached to the base unit with the base unit resting on the said floor, wherein the tower is attached to the base unit by an articulated joint, and wherein means are provided to allow access by personnel to the base unit from the interior of the tower.

Preferably the articulated joint is a universal joint, and the access means comprises at least one (e.g. six or eight) sealed, flexible tunnel disposed adjacent the articulated joint. It is also possible to provide access to the base unit via a sealed, flexible passageway passing along the axis of the joint.

The flexibility of the access tunnels can conveniently be provided by the incorporation thereinto of at least one bellows unit of the type normally used for expansion joints.

Metallic bellows units are available which are capable of withstanding an external pressure of 300 psi and are therefore satisfactory for operations down to a depth of about 680 feet of sea water. Each bellows unit is generally designed for displacement oscillations of at least about 5 , and so if larger oscillations are to be catered for two or even three such units can be arranged in series along the length of the tunnels. The fact that the water pressure is applied evenly to the external surfaces of each bellows unit means that the unit is able to resist "squirm" and exhibits enhanced strength.

As a safety feature, each bellows unit is preferably of a multiple-wall construction, so that in the event of failure of one of the walls, the remaining wall or walls will enable the unit to continue functioning satisfactorily. As a further safety feature, a pressurised fluid, e.g. a vegetable oil, can be supplied to the space between the outermost wall and its next inner wall, and the pressure of the fluid monitored. Firstly, any leakage of the fluid caused by some failure of the sealing of a bellows unit can be immediately detected, and secondly by pressurising the fluid the differential pressure across the outermost wall of the bellows unit can be reduced substantially and the working life of the unit prolonged. Alternatively, this space could be left empty and the pressure thereof monitored.

It is also possible to mount a secondary bellows unit of smaller diameter within each of the main bellows units, so as to counter the possibility of the failure of the entire main bellows unit. Again a pressurised fluid can be supplied to the space between the main and the secondary bellows units.

The buoyancy means may conveniently take the form of a buoyancy chamber which is integral with and positioned at the end of the tower remote from the base unit. In order to provide oil handling and loading facilities as well as accommodation for personnel, a platform can be mounted on an extension of the tower which is designed to rise above the highest possible water level.

When exploiting an oil or gas field, the present structure can be provided with plant or machinery located in the base unit for processing the crude oil or gas before it reaches the tower. Advantageously, pressure reduction facilities can be provided here so as to avoid the necessity of high pressure pipework extending from the base unit to the surface. Storage facilities for oil or gas can also be provided within the base unit.

In view of the limitation that must at present be placed on the depth of working of the bellows units, the base unit can be provided with an elongate portion to carry the joint which, when the base unit is resting on the floor of the sea or of a lake, extends vertically away from the floor in order to allow the articulated joint to be located at a comparatively shallow depth of water.

The present invention also provides a mineral resource when exploited using an under-.water structure of the present invention.

One embodiment of the present invention will now be described, by way of example, with reference to and as illustrated in the accompanying drawings, in which: Figure 1 is an elevational view, partially shown in cross-section, of a complete under-water structure designed for oil production; Figure 2 is an enlarged sectional view of the articulated joint of the structure of Figure 1; Figure 3 is a further enlarged sectional view of one of the access tunnels shown in Figure 2; and Figure 4 is an alternative design of base unit of the structure shown in Figure 1 wherein oil storage facilities are absent from the base unit.

The base unit shown in Figure 1, which is generally indicated as 1, comprises a plurality of oil storage vessels 2 and a process plant 3 to which access is required. The base unit is of an approximately cylindrical shape with all of the utilities being supplied from a central position 4. The base unit 1 is arranged to rest firmly on the floor of the sea or of a lake by means of ballast 5.

Attached to and extending vertically away from the top of the base unit 1 is a hollow tower 6 which carries at its upper end a buoyancy chamber 7. The buoyancy chamber 7 is designed to allow the tower 6, when attached to the base unit 1 with the base unit resting on the floor of the sea or of a lake, to float in a substantially vertical position. Provision can also be made inside the lower end of the tower for the inclusion of ballast to assist the trim of the tower. The length of the tower 6 is such that when the base unit 1 rests on the floor of the sea or of a lake the buoyancy chamber 7 just reaches the water surface. The tower 6 is attached to the base unit 1 by means of a universal joint 8, so as to allow the tower to be displaced from its vertical position as a result of wind and wave action.

On an extension of the tower 6, a platform is carried above the maximum possible height of water and on which power generation and personnel accommodation facilities 9 and 10 are provided. Oil handling facilities including a flare stack 11, a tanker loading boom 12, and helicopter landing facilities are provided on the uppermost level 13 of the platform which can rotate.

The universal joint 8 at the lower end of the tower 6 is positioned on the centre-line of the tower, and symmetrically around the centre-line are arranged six access holes.

Identical access holes are provided in the top of the base unit 6 such that when the tower is vertical the centre-lines of the corresponding holes coincide. Each pair of corresponding holes is connected by a sealed, flexible tunnel as shown in Figure 2, whose flexibility is provided by two primary and two secondary, multiple-walled bellows units 14. Each set of bellows is manufactured from a nickel-based alloy such as INCONEL (Registered Trade Mark) 625 which is capable of withstanding high stresses and can resist sea-water corrosion. Displacement oscillations of up to about 10 or even 15 per bellows set can be tolerated.A pressurised, sealing fluid is provided between the walls of each multiplewalled bellows unit, and the pressure of the space 15 between the primary and the secondary bellows units as shown in Figure 3 is constantly monitored at 16.

The process and the utilities pipework, which is generally designated 17 in Figure 3, passes from the interior of the hollow tower 6 into the base unit 1 via the access tunnels.

Bellows units are also provided in this pipework as it passes through the tunnels so that displacement oscillations of the tower can be accommodated. Access manholes 18 and slam-shut valves are provided at either end of each tunnel and the access tunnels are segmented to enable a damage control system to operate to save human life in the event of a catastrophic failure.

To enable large pieces of equipment to be transferred from the inside of the tower 6 to the interior of the base unit 1, the access holes and tunnels are from 6 to 10 feet in diameter. Winches or lifts can be provided inside the tower 6 in order to transfer personnel and equipment down to the access tunnels.

A structure of the present invention can be satisfactorily designed for oil tanker loading with a platform payload of 3000 to 5000 tons and with a maximum tilt angle of 5 in 700 feet of water plus a wave height of 100 feet.

As a further safety feature, damping guys 19 can be provided to restrict excess movement and to damp oscillations of the tower down to an average of less than about 1210 from the vertical.

It is also possible to provide thrusters either on the platform or on the buoyancy chamber to counteract the displacement of the tower and platform by wind and wave action.

An alternative embodiment is shown in Figure 4, in which the base unit 1 is not provided with oil storage facilities. The height of the unit when resting on the floor of the sea or of a lake is, in this case, less than that of a corresponding base unit with storage facilities. If with such a reduction in height the bellows units would be forced to operate at a depth of water which was beyond their design capability, then it is necessary to provide an elongate portion of the top of the base unit, as shown in Figure 4, so that the articulated joint and its associated bellows units can be located at a comparatively shallow depth of water. Currentlyavailable metallic bellows units are available which are suitable for operations down to about 680 feet of sea water, although this figure can not be regarded as an absolute limit.

It will be seen that this invention is able to provide a practical and economic means of access from sea-level via an articulated tower to a production or habitable structure positioned on the floor of the sea or of a lake, substantially at a normal, one atmosphere, pressure condition. With the exemplified embodiment, normal ventilation, power generation, and venting of hazardous mixtures at the surface are possible, and furthermore the transfer of large items of equipment to sea-level operations become feasible, all utilising conventional technology with high safety and reliability. A "shirtsleeve" environment can be created under water which puts less demands on the personnel operating there than is possible with conventional underwater access systems.

WHAT WE CLAIM IS: 1. An under-water structure for use in the exploitation of mineral resources located on or below the floor of the sea or of a lake, which structure comprises a base unit adapted to rest on the floor of the sea or of a lake and to which unit access by personnel is required, a hollow tower attached at one end to and extending away from the base unit and having sufficient length for the top of the tower substantially to reach the surface of the water when the base unit is resting on the said floor, and means for providing buoyancy to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. lower end of the tower for the inclusion of ballast to assist the trim of the tower. The length of the tower 6 is such that when the base unit 1 rests on the floor of the sea or of a lake the buoyancy chamber 7 just reaches the water surface. The tower 6 is attached to the base unit 1 by means of a universal joint 8, so as to allow the tower to be displaced from its vertical position as a result of wind and wave action. On an extension of the tower 6, a platform is carried above the maximum possible height of water and on which power generation and personnel accommodation facilities 9 and 10 are provided. Oil handling facilities including a flare stack 11, a tanker loading boom 12, and helicopter landing facilities are provided on the uppermost level 13 of the platform which can rotate. The universal joint 8 at the lower end of the tower 6 is positioned on the centre-line of the tower, and symmetrically around the centre-line are arranged six access holes. Identical access holes are provided in the top of the base unit 6 such that when the tower is vertical the centre-lines of the corresponding holes coincide. Each pair of corresponding holes is connected by a sealed, flexible tunnel as shown in Figure 2, whose flexibility is provided by two primary and two secondary, multiple-walled bellows units 14. Each set of bellows is manufactured from a nickel-based alloy such as INCONEL (Registered Trade Mark) 625 which is capable of withstanding high stresses and can resist sea-water corrosion. Displacement oscillations of up to about 10 or even 15 per bellows set can be tolerated.A pressurised, sealing fluid is provided between the walls of each multiplewalled bellows unit, and the pressure of the space 15 between the primary and the secondary bellows units as shown in Figure 3 is constantly monitored at 16. The process and the utilities pipework, which is generally designated 17 in Figure 3, passes from the interior of the hollow tower 6 into the base unit 1 via the access tunnels. Bellows units are also provided in this pipework as it passes through the tunnels so that displacement oscillations of the tower can be accommodated. Access manholes 18 and slam-shut valves are provided at either end of each tunnel and the access tunnels are segmented to enable a damage control system to operate to save human life in the event of a catastrophic failure. To enable large pieces of equipment to be transferred from the inside of the tower 6 to the interior of the base unit 1, the access holes and tunnels are from 6 to 10 feet in diameter. Winches or lifts can be provided inside the tower 6 in order to transfer personnel and equipment down to the access tunnels. A structure of the present invention can be satisfactorily designed for oil tanker loading with a platform payload of 3000 to 5000 tons and with a maximum tilt angle of 5 in 700 feet of water plus a wave height of 100 feet. As a further safety feature, damping guys 19 can be provided to restrict excess movement and to damp oscillations of the tower down to an average of less than about 1210 from the vertical. It is also possible to provide thrusters either on the platform or on the buoyancy chamber to counteract the displacement of the tower and platform by wind and wave action. An alternative embodiment is shown in Figure 4, in which the base unit 1 is not provided with oil storage facilities. The height of the unit when resting on the floor of the sea or of a lake is, in this case, less than that of a corresponding base unit with storage facilities. If with such a reduction in height the bellows units would be forced to operate at a depth of water which was beyond their design capability, then it is necessary to provide an elongate portion of the top of the base unit, as shown in Figure 4, so that the articulated joint and its associated bellows units can be located at a comparatively shallow depth of water. Currentlyavailable metallic bellows units are available which are suitable for operations down to about 680 feet of sea water, although this figure can not be regarded as an absolute limit. It will be seen that this invention is able to provide a practical and economic means of access from sea-level via an articulated tower to a production or habitable structure positioned on the floor of the sea or of a lake, substantially at a normal, one atmosphere, pressure condition. With the exemplified embodiment, normal ventilation, power generation, and venting of hazardous mixtures at the surface are possible, and furthermore the transfer of large items of equipment to sea-level operations become feasible, all utilising conventional technology with high safety and reliability. A "shirtsleeve" environment can be created under water which puts less demands on the personnel operating there than is possible with conventional underwater access systems. WHAT WE CLAIM IS:

1. An under-water structure for use in the exploitation of mineral resources located on or below the floor of the sea or of a lake, which structure comprises a base unit adapted to rest on the floor of the sea or of a lake and to which unit access by personnel is required, a hollow tower attached at one end to and extending away from the base unit and having sufficient length for the top of the tower substantially to reach the surface of the water when the base unit is resting on the said floor, and means for providing buoyancy to

the tower so that the tower floats in a substantially vertical position when attached to the base unit with the base unit resting on the said floor, wherein the tower is attached to the base unit by an articulated joint, and wherein means are provided to allow access by personnel to the base unit from the interior of the tower.

2. A structure as claimed in Claim 1 wherein the articulated joint is a universal joint.

3. A structure as claimed in Claim 2 wherein the access means comprises a sealed flexible passageway passing along the axis of the joint.

4. A structure as claimed in Claim 2 wherein the access means comprises at least one sealed flexible tunnel disposed adjacent the articulated joint.

5. A structure as claimed in Claim 4 wherein the or each tunnel includes at least one bellows unit.

6. A structure as claimed in Claim 5 wherein the or each bellows unit is of a multiple-wall construction.

7. A structure as claimed in Claim 6 wherein a pressurised fluid is present between the outermost wall of the or each bellows unit and its next inner wall.

8. A structure as claimed in any of Claims 5 to 7 wherein the or each tunnel includes at least one set of bellows, the or each set comprising at least two bellows units diposed in parallel one within another.

9. A structure as claimed in Claim 8 wherein a pressurised fluid is present between the or any two of the bellows units of the or each set of bellows.

10. A structure as claimed in any one of the preceding claims wherein the buoyancy means comprises a buoyancy chamber integral with and positioned at the end of the tower remote from the base unit.

11. A structure as claimed in any one of the preceding claims wherein the base unit incorporates plant or machinery for processing mineral resources located on or below the floor on which the base unit will rest.

12. A structure as claimed in any one of the preceding claims wherein the base unit incorporates facilities for the storage of mineral resources located on or below the floor on which the base unit will rest.

13. A structure as claimed in any one of the preceding claims wherein the tower includes an extension at its end remote from the base unit, which extension carries a platform for the handling and/or processing of mineral resources located on or below the floor on which the base unit will rest, and wherein when the base unit is resting on the said floor the platform is disposed above the highest possible water level.

14. A structure as claimed in any one of the preceding claims wherein the base unit includes an elongate portion to carry the articulated joint which, when the base unit is resting on the said floor, extends vertically away from the floor in order to allow the articulated joint to be located at a comparatively shallow depth of water.

15. A structure as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 or Figure 4 of the accompanying drawings.

16. A mineral resource when exploited using an under-water structure as claimed in any one of the preceding claims.