Bistable valve and use thereof in the controlled submergence of a foundation structure TECHNICAL FIELD

The present invention relates to a bistable valve. The bistable valve may, for example, be used to control fluid flow between first and second regions or to vent excess fluid pressure in the event of an overpressure condition. The bistable valve could form part of a foundation structure and may be used to control the submergence of a foundation structure so that an object, such as a renewable energy turbine, mounted on the foundation structure can be located on a sea bed or river bed.

TECHNICAL BACKGROUND

It can sometimes be desirable to provide a valve which is capable of operating automatically so that the valve can move, without external intervention, from a closed position in which it prevents fluid flow to an open position in which it permits fluid flow, and vice-versa. For example, such valves might be used to control fluid flow in chemical processes or to vent excess pressure in the event of an overpressure condition. In foundation structures which are commonly used to locate objects such as renewable energy turbines on sea beds and river beds, fluid can be introduced into and removed from one or more tanks to control the buoyancy of the foundation structure. The foundation structure (typically a gravity base structure) can be lowered in the water, towards the sea bed or river bed, by flooding one or more tanks with water and raised from the sea bed or river bed by introducing gas, such as air, into the one or more tanks to expel water.

In order to ensure that the foundation structure remains stable, it is advantageous to be able to vary the buoyancy of at least one tank. Ideally, the tank should have a large buoyancy to enable controlled submergence of the foundation structure at relatively low speed and minimal buoyancy when the foundation structure is located on the sea bed or river bed to provide maximum submerged stability. SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a bistable valve comprising a bistable disc movable between a first position in which a first side of the bistable disc has a concave shape and a second position in which the first side of the bistable disc has a convex shape, the bistable disc including a peripheral sealing region on the first side which cooperates with a static part when the bistable disc is in the first position to create a seal between the bistable disc and the static part. According to a second aspect of the present invention, there is provided a foundation structure comprising a buoyancy tank including at least one flow control valve to enable fluid to be introduced into or removed from the buoyancy tank, wherein the at least one flow control valve is a bistable valve according to the first aspect of the present invention.

The static part, with which the sealing region on the first side of the bistable disc cooperates when the bistable disc is in the first position, typically forms part of the bistable valve and may be substantially annular. An annular fluid flow path may, thus, be defined between the periphery of the bistable disc and the annular static part when the bistable disc is in the second position in which the peripheral sealing region is spaced from the annular static part. The annular fluid flow path is closed by the bistable disc when the bistable disc is in the first position in which the peripheral sealing region cooperates with the annular static part. The bistable valve thus enables fluid flow to be controlled in a simple and effective manner based on the position of the bistable disc.

The bistable disc may include a sealing member which may extend around the peripheral sealing region and which may cooperate with the static part. The static part may include a sealing element with which the sealing member may cooperate when the bistable disc is in the first position. The sealing member and the sealing element may contact each other when the bistable disc is in the first position. The bistable disc may be supported on the first side for movement between the first and second positions. The bistable disc may be supported on the first side about a circumferentially extending support region which may be located radially inwardly of the peripheral sealing region. The bistable disc is, thus, static along the circumferentially extending support region and is able to flip between the first and second stable positions about the circumferentially extending support region. The movement of the bistable disc between the first and second stable positions will be dependent at least partly on the material properties of the bistable disc and the manner in which it is supported.

The bistable valve may include a rigid support body which supports the bistable disc about the circumferentially extending support region. The bistable valve may include a circumferentially extending flexible connecting member which may connect the rigid support body to the circumferentially extending support region on the first side of the bistable disc. The rigid support body and the static part may be attached to each other, for example by attachment members. The attachment may be semi-permanent. The rigid support body and the static part may alternatively be integrally formed as a single component. The rigid support body may define, in combination with the bistable disc and especially the first side of the bistable disc, a sealed fluid chamber. Fluid contained in the sealed fluid chamber may exert pressure on a central portion of the first side of the bistable disc. The fluid may be a gas and is typically an inert gas or air. The sealed fluid chamber may be pressurisable to a predetermined pressure. The sealed fluid chamber may include a fluid inlet to allow the introduction of fluid into the fluid chamber to pressurise the fluid chamber. The fluid inlet may be provided in the rigid support body. The fluid inlet typically includes a self-sealing fluid connector which permits the connection of a fluid pressure source to the fluid inlet and which seals the fluid inlet upon disconnection of the fluid pressure source from the fluid connector. The fluid chamber is, thus, automatically sealed upon disconnection of a fluid pressure source from the fluid inlet, thereby maintaining the predetermined pressure inside the fluid chamber.

The bistable disc may adopt either the first stable position or the second stable position depending on the pressure differential across the bistable disc. The pressure differential across the bistable disc is typically dependent upon the fluid pressure inside the sealed fluid chamber. The pressure differential across the bistable disc is typically also dependent upon the fluid pressure acting on the second side of the bistable disc.

The bistable disc may adopt the first position, and is typically maintained in the first position, when the fluid pressure inside the sealed fluid chamber, acting on the central portion of the first side of the bistable disc, is greater than the fluid pressure acting on the second side of the bistable disc. As indicated above, when the bistable disc is in the first position, a seal is created between the peripheral sealing region on the first side of the bistable disc and the static part with which the peripheral sealing region cooperates. Fluid flow between the bistable disc and the static part is, thus, prevented.

The bistable disc may adopt the first position when the fluid pressure inside the sealed fluid chamber, acting on the central portion of the first side of the bistable disc, is substantially equal to the fluid pressure acting on the second side of the bistable disc.

The bistable disc may adopt the second position, and is typically maintained in the second position, when the fluid pressure acting on the second side of the bistable disc is greater than the fluid pressure inside the sealed fluid chamber, acting on the central portion of the first side of the bistable disc. As indicated above, when the bistable disc is in the second position, the peripheral sealing region on the first side of the bistable disc is spaced from the static part. Fluid can, thus, flow between the bistable disc and the static part.

Because the movement of the bistable disc is controlled based on the variation of the pressure differential between the first and second sides of the bistable disc, the operating characteristics of the bistable valve can be selected by pressurising the fluid chamber to a suitable predetermined pressure. The predetermined pressure can, for example, be selected having regard to the pressure on the second side of the bistable disc that needs to be attained to cause movement of the bistable disc from the first position to the second position to open the bistable valve.

When the bistable disc is in the second position, the bistable disc may be selectively movable from the second position to the first position. Movement of the bistable disc from the second position to the first position re-establishes the cooperation between the peripheral sealing region on the first side of the bistable disc and the static part, and hence re-establishes the seal between the two components.

Movement of the bistable disc from the second position to the first position may be effected by increasing the fluid pressure inside the fluid chamber, acting on the central portion of the first side of the bistable disc, or by decreasing the fluid pressure acting on the second side of the bistable disc, possibly so that the fluid pressure inside the fluid chamber, acting on the central portion of the first side of the bistable disc, is equal to or greater than the fluid pressure acting on the second side of the bistable disc.

Fluid may be introduced into the sealed fluid chamber, for example via the fluid inlet, to increase the fluid pressure inside the sealed fluid chamber so that the fluid pressure inside the sealed fluid chamber, acting on the central portion of the first side of the bistable disc, is equal to or greater than the fluid pressure acting on the second side of the bistable disc.

The bistable valve may include a reset device for moving the bistable disc from the second position to the first position. The reset device may comprise an actuator which may be mechanical, hydraulic or pneumatic. The actuator may be operable to move the bistable disc from the second position to the first position, irrespective of the fluid pressures acting on the first and second sides of the bistable disc. The actuator may comprise a pyrotechnic device or a pyromechanical device such as a protractor.

The reset device may comprise pressure control means for increasing the fluid pressure inside the fluid chamber, acting on the central portion of the first side of the bistable disc, possibly so that it is equal to or greater than the fluid pressure acting on the second side of the bistable disc, to thereby cause the bistable disc to move from the second position to the first position. The pressure control means may comprise heating means for heating the fluid in the sealed fluid chamber to thereby increase the fluid pressure inside the sealed fluid chamber.

Where the at least one flow control valve according to the second aspect of the present invention is a bistable valve according to the first aspect of the present invention, the bistable valve may be located at a lower end of the buoyancy tank and may selectively connect the interior of the buoyancy tank to the surrounding water to control the flow of water into and out of the buoyancy tank. The bistable valve may, thus, operate as a Kingston valve.

The bistable valve may be located at an upper end of the buoyancy tank and may selectively connect the interior of the buoyancy tank to the surrounding water to control the flow of gas, typically air, out of the buoyancy tank. The bistable valve may, thus, operate as a vent valve.

According to a third aspect of the present invention, there is provided a method for controlling the submergence of a foundation structure including a buoyancy tank comprising a first bistable valve located at a lower end of the buoyancy tank for controlling the flow of water into and out of the buoyancy tank and a second bistable valve located at an upper end of the buoyancy tank for controlling the flow of gas out of the buoyancy tank, the method comprising:

submerging the foundation structure; and introducing water into the buoyancy tank through the first bistable valve during submergence of the foundation structure;

wherein the first bistable valve moves from a closed position to an open position when the foundation structure has been submerged to a predetermined depth to allow water to be introduced into the buoyancy tank and the second bistable valve moves from a closed position to an open position after the first bistable valve has moved to the open position.

The introduction of water into the buoyancy tank, which is possible due to opening of the first bistable valve and assisted by the subsequent opening of the second bistable valve, decreases the total water displacement of the submerged foundation structure and thereby significantly increases its submerged stability. Because the first and second bistable valves each move automatically from the closed position to the open position without external intervention, the use of an umbilical system when the foundation structure is at depth is obviated, thereby simplifying the submergence operation.

The foundation structure may include one or more ballast tanks and the step of submerging the foundation structure may comprise introducing ballast material, normally water, into one or more of the ballast tanks. The method may comprise initially introducing water into selected ballast tanks to partially submerge the foundation structure. The foundation structure is positively buoyant in this partially submerged state. The method may thereafter comprise introducing water into further ballast tanks to render the foundation structure negatively buoyant, thereby causing full submergence of the foundation structure.

The gas contained in the buoyancy tank is typically air. The gas pressure inside the buoyancy tank may be substantially equal to atmospheric pressure prior to submerging the foundation structure and prior to opening of the first bistable valve to allow water to be introduced into the buoyancy tank. The gas in the buoyancy tank is typically compressed as water flows into the buoyancy tank through the open first bistable valve and the pressure of the gas inside the buoyancy tank thus increases. The predetermined depth at which the first bistable valve moves from a closed position to an open position to allow water to flow into the buoyancy tank is typically on or in close proximity to the sea bed or river bed on which the foundation structure is to be located. The water displacement of the submerged foundation structure does not, therefore, decrease until the foundation structure is at, or close to, its intended installation location on the sea bed or river bed. This ensures that the rate of submergence of the foundation structure can be carefully controlled by controlling the amount of water that is introduced into the one or more ballast tanks.

The hydrostatic pressure at the predetermined depth may cause the first bistable valve to move from the closed position to the open position. The first bistable valve can, thus, be arranged to open at a desired predetermined depth by determining the hydrostatic pressure at the desired predetermined depth prior to submerging the foundation structure.

As indicated above, the gas pressure inside the buoyancy tank increases as water is introduced into the buoyancy tank following opening of the first bistable valve. The increase in gas pressure inside the buoyancy tank causes the second bistable valve to move from the closed position to the open position. This allows the compressed gas to be vented from the upper end of the buoyancy tank and thereby enables the buoyancy tank to fill completely with water. The decrease in the total water displacement of the foundation structure is, therefore, progressive and this maintains the stability of the foundation structure during the final stage of the submergence operation.

The method may comprise the step of moving the second bistable valve from the open position to the closed position once the foundation structure has been located on the sea bed or river bed. A reset device may be used for this purpose. Movement of the second bistable valve from the open position to the closed position allows air to be introduced into the buoyancy tank through the first bistable valve so that water can be expelled from the buoyancy tank through the open first bistable valve. The resulting increase in the buoyancy of the buoyancy tank may assist with subsequent retrieval of the foundation structure.

The first bistable valve may be a bistable valve according to the first aspect of the present invention. The second side of the bistable disc may, thus, be exposed to the water in which the foundation structure is submerged such that the fluid pressure acting on the second side of the bistable disc is equal to the hydrostatic pressure of the surrounding water. The bistable disc may move from the first position to the second position when the hydrostatic pressure acting on the second side of the bistable disc exceeds the fluid pressure acting on the first side of the bistable disc. The fluid chamber on the first side of the bistable disc may be pressurised to a pressure which is less than the hydrostatic pressure of the water at or below the predetermined depth. This ensures that the bistable disc moves from the first position to the second position at the predetermined depth to thereby open the first bistable valve at the predetermined depth.

The second bistable valve may be a bistable valve according to the first aspect of the present invention. The second side of the bistable disc may, thus, be exposed to the gas inside the buoyancy tank such that the fluid pressure acting on the second side of the bistable disc is equal to the pressure of the gas inside the buoyancy tank. The bistable disc may move from the first position to the second position when the gas pressure acting on the second side of the bistable disc exceeds the fluid pressure acting on the first side of the bistable disc. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic cross-sectional view of a bistable valve in a closed position;

Figure 2 is a diagrammatic cross-sectional view of the bistable valve of Figure 1 in an open position;

Figure 3 is a diagrammatic side elevation of a foundation structure including a buoyancy tank comprising first and second bistable valves; Figures 4 to 10 are diagrammatic side elevations of the foundation structure of Figure 3 at various stages during the submergence operation; and

Figure 11 is a diagrammatic view of a reset device used to control the operation of a bistable valve.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings. Referring initially to Figures 1 and 2, there is shown a bistable valve 10 for controlling the flow of fluid between first and second regions 12, 14 on opposite sides of a component or structure 11 having an aperture in which the bistable valve 10 is installed. The bistable valve 10 comprises a bistable disc 16 which can move between a first stable position shown in Figure 1 in which it cooperates with an annular static part 18 fixed to the component or structure 11 and a second stable position shown in Figure 2 in which it is spaced from the annular static part 18 to allow fluid to flow through an annular fluid flow path 20 between the first and second regions 12, 14.

The bistable disc 16 includes first and second sides 22, 24. When the bistable disc 16 is in the first stable position illustrated in Figure 1, the first side 22 of the bistable disc 16 has a concave shape. When the bistable disc 16 is in the second stable position illustrated in Figure 2, the first side 22 of the bistable disc 16 has a convex shape.

The bistable valve 10 includes a rigid support body 26 which supports the bistable disc 16 on the first side 22 for movement between the first and second positions. In the illustrated embodiment, the annular static part 18 and the rigid support body 26 are integrally formed as a single component.

The rigid support body 26 is connected to the bistable disc 16 by a circumferentially extending flexible connecting member 28. The flexible connecting member 28 supports the bistable disc 16 about a circumferentially extending support region 30 and facilitates movement of the bistable disc 16 between the first and second stable positions relative to the rigid support body 26. It will be appreciated that the bistable disc 16 is static along the circumferentially extending support region 30 where it is connected to the flexible connecting member 28 and that the bistable disc 16 is able to flip between the first and second stable positions about this static circumferentially extending support region 30.

The bistable disc 16 includes a peripheral sealing region 32 on its first side 22 and a circumferentially extending sealing member 34 extends around the peripheral sealing region 32. The circumferentially extending support region 30 is located radially inwardly of the peripheral sealing region 32. When the bistable disc 16 is in the first stable position illustrated in Figure 1, the circumferentially extending sealing member 34 cooperates with a circumferentially extending sealing element 36 extending around the annular static part 18 and a fluid seal is thus created between the bistable disc 16 and the annular static part 18.

The rigid static support body 26 and the flexible connecting member 28 define, in combination with the first side 22 of the bistable disc 16, a fluid chamber 38 into which air or any other suitable fluid can be introduced to pressurise the fluid chamber 38. The rigid support body 26 includes an air inlet 40 to enable air to be introduced into the fluid chamber 38. The air inlet 40 includes a self- sealing fluid connector (not shown) which allows air to be introduced into fluid chamber 38 through the air inlet 40 when an air pressure source is connected to the air inlet 40 but which seals the air inlet 40, and thereby seals the pressurised fluid chamber 38, upon disconnection of the air pressure source from the self- sealing fluid connector.

Pressurised air contained in the sealed fluid chamber 38 exerts pressure on a generally circular central portion of the first side 22 of the bistable disc 16. Fluid pressure is also exerted on the second side 24 of the bistable disc 16. Because the second side 24 of the bistable disc 16 is exposed to fluid in the first region 12, the fluid pressure exerted on the second side 24 of the bistable disc 16 is equal to the local fluid pressure in the first region 12. The sealed fluid chamber 38 can be pressurised to any desired predetermined pressure. In order for the bistable valve 10 to operate in a desired manner, it is necessary to pressurise the fluid chamber 38 so that the bistable disc 16 moves from the first position to the second position, and vice-versa, based on the pressure differential across the bistable disc 16. The predetermined pressure is determined having regard to the mechanical and material properties of the bistable disc 16 and its support and having regard to the fluid pressure in the first region 12 that needs to be attained before fluid is allowed to flow from the first region 12 to the second region 14 through the annular fluid flow path 20.

According to one of a number of possible embodiments of the invention, when the air pressure inside the sealed fluid chamber 38 acting on the central portion of the first side 22 of the bistable disc 16 is greater than or equal to the pressure of the fluid in the first region 12 acting on the second side 24 of the bistable disc 16, the bistable disc 16 adopts the first stable position illustrated in Figure 1 and thereby prevents the fluid in the first region 12 from flowing through the annular fluid flow path 20 from the first region 12 to the second region 14. The bistable valve 10 is, therefore, closed when the bistable disc 16 is in this first position. If the pressure of the fluid in the first region 12 acting on the second side 24 of the bistable disc 16 increases to a level where it is greater than the air pressure inside the sealed fluid chamber 38 acting on the central portion of the first side 22 of the bistable disc 16, the bistable disc 16 flips from the first stable position illustrated in Figure 1 to the second stable position illustrated in Figure 2. This opens the bistable valve 10 and allows fluid to flow through the annular fluid flow path 20 from the first region 12 to the second region 14. The bistable disc 16 remains in the second position whilst the pressure differential across the bistable disc 16 is such that the fluid pressure acting on the second side 24 of the bistable disc 16 is greater than the air pressure inside the sealed fluid chamber 38 acting on the first side 22 of the bistable disc 16. The bistable valve 10 described above is particularly suitable for use as a flow control valve for a buoyancy tank of a foundation structure, as described below. Figure 3 illustrates a foundation structure in the form of a gravity base structure 44. The gravity base structure 44 is typically formed of steel and/or concrete and carries an object (not shown), for example a tidal turbine, that needs to be located on a sea bed or river bed 62 (see Figures 8 to 10). The foundation structure may include one or more of the features described in the applicant's UK patent application 0922075.7.

The gravity base structure 44 includes a plurality of integral buoyancy devices and more specifically a centrally located buoyancy tank 46, a pair of first ballast tanks 48 located on either side of the buoyancy tank 46 and a pair of second ballast tanks 50 located on either side of the first ballast tanks 48. Ballast material in the form of water can be introduced into the buoyancy tank 46 and the pairs of first and second ballast tanks 48, 50 to control the buoyancy of the gravity base structure 44. The buoyancy tank 46 acts as a ballast tank in the same way as the pairs of first and second ballast tanks 48, 50 but is conveniently referred to in this specification as a buoyancy tank to distinguish it from the pairs of first and second ballast tanks 48, 50.

The buoyancy tank 46 includes a first fluid flow control valve 52 at its lower end 56 and a second fluid flow control valve 54 at its upper end 58. Both the first and second fluid flow control valves 52, 54 are equivalent to the bistable valve described above with reference to Figures 1 and 2. The first and second fluid flow control valves 52, 54 will, therefore, be referred to hereinafter as first and second bistable valves 110, 210 and the features of those first and second bistable valves 110, 210 will be identified using reference numerals corresponding to those used in Figures 1 and 2 but prefixed by the numbers T and '2' respectively.

The second side 124 of the bistable disc 116 of the first bistable valve 110 is exposed to the water into which the gravity base structure 44 is submerged. The fluid pressure acting on the second side 124 of the bistable disc 116 is, therefore, equal to the local hydrostatic pressure of the water. The first bistable valve 110 acts as a Kingston valve to control the flow of water into the buoyancy tank 46 and to allow air to be introduced into the buoyancy tank 46 to expel water therefrom. The second side 224 of the bistable disc 216 of the second bistable valve 210 is exposed, at least initially, to air contained in the buoyancy tank 46. The fluid pressure acting on the second side 224 of the bistable disc 216 is, therefore, equal to the air pressure inside the buoyancy tank 46. The air inside the buoyancy tank 46 is at substantially atmospheric pressure prior to submergence of the gravity base structure 44, as will be described later in this specification. The second bistable valve 210 acts as a vent valve to allow air to be vented from the upper end 58 of the buoyancy tank 46 and to thereby enable the buoyancy tank 46 to be completely flooded with water. The ballast tanks 48, 50 each include vent valves 48a, 50a at their upper ends to enable air to be vented from the tanks and limber holes 48b, 50b at their lower ends to enable water to be introduced into the tanks and to allow air to be introduced into the tanks to expel water therefrom. The illustrated gravity base structure 44 also includes locating members 60 which enable it to be located on the sea bed or river bed 62 as shown in Figures 8 to 10.

Figure 4 shows the gravity base structure 44 in a fully surfaced condition in which it is normally towed to a position above an intended location site on the sea bed or river bed 62. In this fully surfaced condition, the buoyancy tank 46 and the pairs of first and second ballast tanks 48, 50 do not contain water and instead contain only air at substantially atmospheric pressure. The vent valves 48a, 50a are closed to ensure that air cannot escape from the upper ends of the ballast tanks 48, 50. This prevents water from entering into the ballast tanks 48, 50 through the limber holes 48b, 50b. The first and second bistable valves 110, 210 are both in the closed position to prevent the escape of air from, and the entry of water into, the buoyancy tank 46. The gravity base structure 44 has a large positive buoyancy in this state and this results in a low waterline 64 relative to the gravity base structure 44.

In order to commence the submergence operation, the vent valves 48a of the pair of first ballast tanks 48 are initially opened. Air contained in the ballast tanks 48 escapes through the vent valves 48a allowing water to flow into the ballast tanks 48 through the limber holes 48b. Once the ballast tanks 48 are completely flooded with water, the gravity base structure 44 sinks lower in the water resulting in a high waterline as shown in Figure 5. The vent valves 48a are then closed as shown in Figure 6. At this stage of deployment, the gravity base structure 44 is still positively buoyant and does not become fully submerged.

Either at this stage of deployment or earlier in the deployment operation, air is introduced into the fluid chamber 138 of the first bistable valve 110 to pressurise the fluid chamber 138 so that the bistable disc 116 will move from the first position to the second position to open the first bistable valve 110, and thereby allow water to flow into the buoyancy tank 46, at an external local hydrostatic pressure corresponding to the hydrostatic pressure of the water at a location in close proximity to the sea bed or river bed 62 or on the sea bed or river bed 62. Air is also introduced into the fluid chamber 238 of the second bistable valve 210 to pressurise the fluid chamber 238 so that the bistable disc 216 will move from the first position to the second position to open the second bistable valve 210, and thereby allow air to be vented from the buoyancy tank 46, when the air pressure inside the buoyancy tank 46 rises substantially above atmospheric pressure. As described above with reference to Figures 1 and 2, air pressure sources are connected to the air inlet of each fluid chamber 138, 238 to pressurise the fluid chambers 138, 238.

In order to continue the submergence operation and fully submerge the gravity base structure 44 in the water, the vent valves 50a of the pair of second ballast tanks 50 are opened. Air contained in the ballast tanks 50 escapes through the vent valves 50a allowing water to flow into the ballast tanks 50 through the limber holes 50b. Once the ballast tanks 50 are completely flooded with water, the buoyancy of the gravity base structure 44 becomes slightly negative which causes the gravity base structure 44 to fully submerge in the water and slowly sink towards the sea bed or river bed 62. This stage of the deployment operation is illustrated in Figure 7. The vertically downwards acceleration of the gravity base structure 44 is limited by the upwardly acting hydrodynamic drag forces and this helps to maintain the stability of the gravity base structure 44 during the submergence operation. The hydrostatic pressure of the water surrounding the gravity base structure 44 increases as the gravity base structure 44 sinks further into the water, towards the sea bed or river bed 62. As the gravity base structure 44 reaches the sea bed or river bed 62, the local hydrostatic pressure acting on the second side 124 of the bistable disc 116 of the first bistable valve 110 increases to a level at which it is greater than the pressure of the air inside the sealed fluid chamber 138 of the first bistable valve 110. This causes the bistable disc 116 of the first bistable valve 110 to move from the first position illustrated in Figure 7 to the second position illustrated in Figure 8, thus moving the first bistable valve 110 from the closed position to the open position. This allows water 42 to flow into the buoyancy tank 46 through the lower end 56. The total water displacement of the gravity base structure 44 gradually decreases as water 42 flows into the buoyancy tank 46 thereby increasing the submerged stability of the gravity base structure 44. As water 42 continues to flow into the buoyancy tank 46 through the open first bistable valve 110, the air in the buoyancy tank 46 is compressed and its pressure thus increases. Once the pressure of the compressed air inside the buoyancy tank 46 has increased to a level at which it is greater than the pressure of the air inside the sealed fluid chamber 238 of the second bistable valve 210, the bistable disc 216 of the second bistable valve 210 moves from the first position illustrated in Figure 9 to the second position illustrated in Figure 10, thus moving the second bistable valve 210 from the closed position to the open position. This enables air to be vented from the upper end 58 of the buoyancy tank 46 and allows the buoyancy tank 46 to become completely flooded with water. The total water displacement of the gravity base structure 44 thus decreases further, to a minimum, when the gravity base structure 44 is located on the sea bed or river bed 62. This maximises the submerged stability of the gravity base structure 44.

In order to retrieve the gravity base structure 44 from the sea bed or river bed 62, the bistable disc 216 of the second bistable valve 210 may need to be moved from the second position illustrated in Figure 10 to the first position illustrated in Figure 9, to thereby close the second bistable valve 210. Once the second bistable valve 210 has been closed, air can be introduced into the buoyancy tank 46 via the open first bistable valve 110. Air can also be introduced into the pair of first ballast tanks 48, whose vent valves 48a were previously closed during the submergence operation, via the limber holes 48b. As air is introduced into the buoyancy tank 46 and the ballast tanks 48, typically using a suitable umbilical system, water flows out of the buoyancy tank 46 through the open first bistable valve 110 and out of the ballast tanks 48 through the limber holes 48b, typically until the buoyancy tank 46 and the ballast tanks 48 are completely devoid of water. The gravity base structure 44 becomes sufficiently buoyant following removal of the water from the buoyancy tank 46 and the ballast tanks 48 that it rises to the surface of the water, thereby enabling the object located on the gravity base structure 44 to be retrieved. A number of possibilities are available to move the bistable disc 216 of the second bistable valve 210 from the second position to the first position to close the second bistable valve 210 and thereby enable air to be introduced into the buoyancy tank 46 to permit surfacing of the gravity base structure 44. For example, an umbilical system can be employed to increase the pressure inside the fluid chamber 238 to a level at which it is greater than or equal to the local hydrostatic pressure of the water inside the buoyancy tank 46. A reset device, such as an actuator, can alternatively be employed.

It is possible that the reset device could comprise pressure control means which operates automatically, to increase the air pressure inside the fluid chamber 38 of the second bistable valve 210 and thereby move the bistable disc 216 of the second bistable valve 210 from the second position to the first position at any time after the gravity base structure 44 has been located on the sea bed or river bed 62. Figure 11 provides a diagrammatic illustration of one embodiment of such a reset device 66 connected to the bistable valve 210. The reset device 66 includes a bellows arrangement 68 which is connected to the air inlet 240 of the fluid chamber 238 via a flow restriction 70. A rigid container 72 constrains the movement of the bellows arrangement 68 and includes apertures 74 which enable the container 72 to flood with water when submerged so that the bellows arrangement 68 is subjected to the local hydrostatic pressure of the water. The illustrated reset device 66 includes self-sealing fluid connectors 76 which enable a common fluid pressure source 78 to be used to introduce air or another gas into the fluid chamber 238 of the bistable valve 210 and into the bellows arrangement 68, via fluid lines 80 positioned either side of the flow restriction 70, so that they are pressurised to the same pressure. Prior to submergence of the gravity base structure 44, the air pressure source 78 is connected to the self- sealing fluid connectors 76 and the fluid chamber 238 and the bellows arrangement 68 are pressurised to the same pressure. The pressure is determined having regard to the air pressure that needs to be attained inside the buoyancy tank 46 to move the bistable disc 216 of the second bistable valve 210 from the first position to the second position, as explained earlier in this specification. Once the air pressure source 78 has been disconnected, the self-sealing fluid connectors 76 maintain the pressurisation inside the fluid chamber 238 and the bellows arrangement 68. Air does not flow between the fluid chamber 238 and the bellows arrangement 68 through the flow restriction 70 because the air pressures inside the fluid chamber 238 and the bellows arrangement 68 are equal.

The gravity base structure 44 is submerged causing the first and second bistable valves 110, 210 to operate in the manner described above. The reset device 66 is also submerged with the gravity base structure 44 causing the container 72 to flood with water. The bellows arrangement 68 is thus subjected to the local hydrostatic pressure of the water and the air pressure inside the bellows arrangement 68 increases to match the local hydrostatic pressure. The air pressure inside the bellows arrangement 68 is, therefore, greater than the air pressure inside the fluid chamber 238. Because the bellows arrangement 68 and the fluid chamber 238 are connected, the air pressures will eventually equalise. However, this pressure equalisation takes place over an extended period of time due to the presence of the flow restriction 70. The flow restriction 70 is configured so that the air pressure in the fluid chamber 238 remains close to its initial value until the second bistable valve 210 has been open for a sufficient amount of time to enable the buoyancy tank 46 to become completely filled with water, as described above with reference to Figure 10. Once the air pressure inside the fluid chamber 238 has increased so that the air pressures inside the bellows arrangement 68 and the fluid chamber 238 have equalised, the bistable disc 216 moves from the second position illustrated in Figure 10 to the first position illustrated in Figure 9. This enables air to be introduced into the buoyancy tank 46 via the open first bistable valve 110 to expel water from the buoyancy tank 46 through the open first bistable valve 110.

Although embodiments of the present invention have been described in the preceding paragraphs, it should be understood that various modifications may be made to those embodiments without departing from the scope of the present invention, as claimed.

In particular, it will be apparent to those skilled in the art that any suitable pressure differential across the bistable disc 16 may be used to control the movement of the bistable disc 16 having regard to the mechanical and material properties of the bistable disc 16 and its support. For example, the bistable disc 16 could be arranged to move from the first position to the second position when the fluid pressure inside the sealed fluid chamber 38 acting on the central portion of the first side 22 of the bistable disc 16 is greater than the fluid pressure in the first region 12 acting on the second side 24 of the bistable disc 16. Alternatively, the bistable disc 16 could be arranged to move from the first position to the second position when the pressure inside the sealed fluid chamber 38 acting on the central portion of the first side 22 of the bistable disc 16 is substantially equal to the fluid pressure in the first region 12 acting on the second side 24 of the bistable disc 16.

The first and second fluid flow control valves 52, 54 may be respectively first and second bistable valves having a construction which is different to that described with reference to Figures 1 and 2. That said, the first bistable valve still moves from the closed position to the open position when the hydrostatic pressure increases above a predetermined level at a predetermined depth at which opening of the first bistable valve is desired. Likewise, the second bistable valve still moves from the closed position to the open position as a result of compression of the air inside the buoyancy tank 46, typically to a level substantially above atmospheric pressure.

The sealed fluid chambers 38, 138, 238 may be pressurised using a gas other than air. Similarly, any suitable gas can be introduced into the buoyancy tank 46 and the pairs of ballast tanks 48, 50 to control the overall buoyancy of the gravity base structure 44. The gravity base structure 44 may include any suitable number and/or arrangement of buoyancy tanks 46 and ballast tanks 48, 50 other than illustrated in Figures 3 to 10.

Although the illustrated foundation structure is a gravity base structure 44 that is held in place on the sea bed or river bed 62 solely by frictional forces, the foundation structure could be anchored to the sea bed or river bed 62 to hold it in place.