[0029] FIG. 1 shows a buoyancy device 1 which may be arranged in any suitable arrangement, such as that shown in FIG. 12 of WO 00/29285 and hence which may be attached in a substantially vertical orientation to legs of an offshore drilling structure requiring to be lifted from the ocean floor and moved to a remote location.

[0030] However, it should be noted that the buoyancy device 1 may be attached in any suitable arrangement to objects other than offshore drilling structures, such as large ships, super tankers, cruise liners, etc. which may be required to be lifted in order to allow them to clear the open-sea end of dry docks.

[0031] A clamping device, similar to the clamping device 207 of WO 00/29285 may be used to clamp around a suitable number of buoyancy devices 1 of the present invention to attach them to the structure to be moved.

[0032] The buoyancy device 1 comprise a rigid outer hull 3 which may be formed from suitable material such as glass reinforced plastic (GRP), carbon fibre or any other suitable material; it should be noted that GRP is preferred.

[0033] FIG. 3 shows the buoyancy device prior to inflation with either water or air; the outer bag 5 (shown in long dotted lines) is substantially cylindrical in nature and, when deflated, can be thought of having substantially the same shape as a windsock. The outer bag 5 is coupled to the outer shell 3 at both ends 5A, 5B by means of an adapter ring flange 7. It should be noted that the “wet” end 5B of the outer bag 5 is open to the outer environment, such that sea water can pass into the interior of the outer bag 5 via the opening or bore 14B in the annular flange 7B. An anti-extrusion sieve plate 9B is formed across the end 5B and is arranged such that it permits water to pass through itself, but prevents the passage of other unwanted solid material into the interior of the outer bag 5. A similar anti-extrusion sieve plate 9A is located across the other end 5A of the outer bag 5.

[0034] The “dry” end 3A of the outer shell 3 and the outermost end of the adapter flange 7A, are coupled to a shell closure plate 10 which provides rigidity to the dry end 3A. An aperture 12 is provided at the centre point of the shell closure plate 10 and an air line 11 is secured to the aperture 12 via a valve 13, such that pressurised air can be introduced into the bore 14 of the adapter flange 7A by opening the valve 13. The air line 11 is coupled to a compressor which provides the pressurised air. In this manner, air can be introduced into the interior of the outer bag 5.

[0035] An inner half bag 15 or diaphragm 15 is secured around the diameter of the middle of the outer bag 5 and a suitable fixing is shown in the form of a flexible joint in FIG. 2.

[0036] Both the outer bag 5 and the diaphragm 15 are preferably formed from a suitably strong and flexible material or fabric and a most preferred material is polyester coated with a plasticised PVC which may have a weight of in the region of 900 mg per square metre. A suitable example of such a material is sold under the Trade Mark DURASKIN(^RTM).

[0037] The joint 17 is preferably formed by using known and conventional welding techniques to weld mid-pieces 19, 21 to the outer bag 5 and the diaphragm 15. Indeed, the welding process makes the joint 17 or hinge 17 stronger than the outer bag 5 or diaphragm 15 alone. Accordingly, the joint 17 will act as a soft flexible hinge to permit the diaphragm 15 to move from the configuration shown in FIG. 1 to the configuration shown in FIG. 4 and all points in between, including the non-inflated configuration shown in FIG. 3.

[0038] Prior to introduction of the buoyancy device 1 into water, the outer bag 5 and diaphragm 15 will take up a configuration something akin to that shown in FIG. 3. However, when the buoyancy device 1 is introduced into water, the water will flow into the interior of the outer bag 5 and will move the outer bag 5 toward the inner surface of the outer shell 3. At the same time, the diaphragm 15 will move from right to left as shown in FIG. 3 until it takes up the configuration as shown in FIG. 1. The air that was previously in section A (as shown in FIG. 3) of the interior of the outer shell 3 is able to escape through a pair of non-return valves 23, 24 provided in respective adapter rings 7A, 7B. Therefore, the non-return valves 23, 24 communicate with the interior of the outer shell 3 to permit air to escape the interior. However, the non-return valves 23, 24 prevent air from passing through themselves in the direction from outside the outer shell 3 to inside Section A of the interior of the shell 3.

[0039] The anti-extrusion sieve plate 9A also has the important task of preventing the diaphragm 15 from over inflation, that is, passing into the bore 14 of the adapter ring 7A.

[0040] When the time has come to inflate the buoyancy device 1 to aid in lifting a structure to be moved in the water, the valve 13 is opened and pressurised air will enter into bore 14 and hence into the interior of the outer bag 5. Since the joint 17 also provides a seal between the outer bag 5 and the diaphragm 15, the pressurised air in Section B (see FIG. 3) will cause the diaphragm 15 to move from left to right as shown in FIG. 1 until it takes up the fully inflated configuration shown in FIG. 4. It is thought that the working pressure of the buoyancy device 1; that is the pressure of the air within the buoyancy device 1; will be in region of 2.5 atmospheres, although it is likely to be operable up to in the region of 6 atmospheres or higher.

[0041] The anti-extrusion sieve plate 9B also has the important task of preventing the diaphragm 15 from moving into the bore 14B of the flange 7B.

[0042] Accordingly, the main purpose of the outer bag 5 is to provide a reasonably flexible mounting point for the joint 17 and diaphragm 15. Those skilled in the art will appreciate that the advantage provided by such a flexible joint 17 is that there is a far reduced risk of localised stresses building up in the wall of the outer shell 3.

[0043] It should also be noted that FIGS. 1 to 7 are not to scale. Indeed, the outer shell is in the region of 16 m in length and approximately 2.3 m in diameter.

[0044] Modifications and improvements may be made to the foregoing without departing from the scope of the invention.