Foundation structure

1. A foundation structure for locating an object mounted on the foundation structure on a sea bed or a river bed, the foundation structure comprising:

a plurality of integral buoyancy devices, the plurality of integral buoyancy devices including first and second static ballast tanks, and a third movable ballast tank having a rigid control surface and pivotably mounted on the first static ballast tank and above the second static ballast tank, each of the plurality of integral buoyancy devices having variable buoyancy, the third movable integral ballast tank being movable relative to the first and second static ballast tanks to vary the position of the rigid control surface and control the position of a center of buoyancy relative to the position of a center of gravity, wherein the foundation structure is movable between

a fully surfaced condition, wherein the foundation structure floats on a surface of a body of water, and wherein the center of buoyancy of the foundation structure is positioned below the center of gravity of the foundation structure, and

a fully submerged condition, wherein the first and second static ballast tanks of the foundation structure are adjacent to one another and each of the first and second static ballast tanks are directly in contact with a bed of the body of water and wherein the center of buoyancy of the foundation structure is positioned above the center of gravity of the foundation structure.

2. A foundation structure according to claim 1 , wherein each of the plurality of integral movable buoyancy devices comprises a plurality of rigid surfaces enclosing a buoyant volume.

3. A foundation structure according to claim 1 , wherein the third movable ballast tank is movable between a retracted position and an extended position.

4. A foundation structure according to claim 3 , wherein the third movable ballast tank is substantially aligned with adjacent surfaces of the first and second static ballast tanks when in the retracted position.

5. A foundation structure according to claim 4 , wherein the third movable ballast tank extends outwardly away from the first and second static ballast tanks when in the extended position.

6. A foundation structure according to claim 1 , wherein the movement of the third movable ballast tank relative to the first and second static ballast tanks is controlled by varying the gross buoyancy of the foundation structure.

7. A foundation structure according to claim 1 , wherein the movement of the third movable ballast tank relative to the first and second static ballast tanks is controlled by varying the buoyancy of one or more of the first and second static ballast tanks.

8. A foundation structure according to claim 1 , wherein the buoyancy of the third movable ballast tank is individually controllable to vary its position relative to the first and second static ballast tanks.

9. A foundation structure according to claim 1 , wherein the foundation structure includes one or more restraints to limit the movement of the third movable ballast tank relative to the first and second static ballast tanks.

10. A foundation structure according to claim 1 , wherein the foundation structure includes one or more actuators to control the movement of the third movable ballast tank relative to the first and second static ballast tanks.

11. A method for controlling the deployment and retrieval of a foundation structure having an object mounted thereon for location on a sea bed or river bed, the foundation structure comprising a plurality of integral buoyancy devices, the plurality of integral buoyancy devices including first and second static ballast tanks, and a third movable ballast tank having a rigid control surface and pivotably mounted on the first static ballast tank and above the second static ballast tank, each of the plurality of integral buoyancy devices having variable buoyancy, the method comprising:

varying the position of the third movable ballast tank relative to the first and second static ballast tanks to vary the position of the rigid control surface and control the position of a center of buoyancy relative to the position of a center of gravity, the center of buoyancy of the foundation structure being positioned below the center of gravity of the foundation structure when the foundation structure is in a fully surfaced condition wherein the foundation structure floats on a surface of a body of water, and the center of buoyancy of the foundation structure being positioned above the center of gravity of the foundation structure when the foundation structure is in a fully submerged condition, and wherein the first and second static ballast tanks of the foundation structure are adjacent to one another and each of the first and second static ballast tanks are directly in contact with the sea bed or river bed.

12. A method according to claim 11 , comprising:

varying the gross buoyancy of the foundation structure to vary the position of the third movable ballast tank relative to the first and second static ballast tanks.

13. A method according to claim 11 , comprising:

varying the buoyancy of the third movable ballast tank to vary its position relative to the first and second static ballast tanks.

14. A method according to claim 11 , comprising:

varying the buoyancy of one or more of the first and second static ballast tanks to vary the position of the third movable ballast tank relative to the first and second static ballast tanks.

15. A method according to claim 11 , comprising:

varying the position of the third movable ballast tank relative to the first and second static ballast tanks to vary the position of the rigid control surface.

16. A method according to claim 11 , comprising:

moving the third movable ballast tank to an extended position during submergence and surfacing of the foundation structure.

17. A method according to claim 16 , wherein the extended position of the third movable ballast tank is defined by limiting the movement of the third movable ballast tank.

18. A method according to claim 11 , comprising:

moving the third movable ballast tank to a retracted position when the foundation structure is located on the sea bed or river bed.

19. A method according to claim 11 , wherein the third movable ballast tank comprises a buoyancy tank and the method comprises varying the buoyancy of the buoyancy tank by varying the quantity of fluid inside the buoyancy tank.

20. A foundation structure according to claim 1 , wherein a combined depth of the second static ballast tank and the third movable ballast tank is about equal to a depth of the first static ballast tank.

21. A foundation structure according to claim 1 , wherein the first static ballast tank is located at a stern of the foundation structure and the second static ballast tank and the third movable ballast tank are located at a bow of the foundation structure.

22. A foundation structure according to claim 1 , comprising:

a tidal turbine mounted on the foundation structure.

23. A foundation structure according to claim 1 , comprising:

a pair of the plurality of integral buoyancy devices, which are arranged side by side.

24. A method according to claim 11 , wherein a combined depth of the second static ballast tank and the third movable ballast tank is about equal to a depth of the first static ballast tank.

25. A method according to claim 11 , comprising:

locating the first static ballast tank at a stern of the foundation structure and the second static ballast tank and the third movable ballast tank at a bow of the foundation structure.

26. A method according to claim 11 , comprising:

mounting a tidal turbine on the foundation structure.

27. A method according to claim 11 , comprising:

arranging a pair of the plurality of integral buoyancy devices side by side.

28. A foundation structure according to claim 3 , wherein the third movable ballast tank extends outwardly away from the first and second static ballast tanks when in the extended position.

29. A foundation structure according to claim 4 , wherein the third movable ballast tank becomes negatively buoyant in the fully submerged condition and moves to the retracted position.

30. A method according to claim 18 , comprising:

introducing water into the third movable ballast tank so that the third ballast tank becomes negatively buoyant; and

moving the third movable ballast tank in the fully submerged condition to the retracted position.

31. A method according to claim 30 , comprising:

substantially aligning the third movable ballast tank with adjacent surfaces of the first and second static ballast tanks when in the retracted position in the fully submerged condition.