Floatation device

A floatation device is disclosed that comprises a container 10 containing a liquefied gas, a gas chamber (60, FIG. 2) and a remotely operable device 29. The remotely operable device is switchable between a closed state in which fluid communication between the container and the gas chamber is prevented, and an open state in which fluid communication between the container and the gas chamber is enabled and vaporization of the liquefied gas charges the gas chamber with gas. The liquefied gas may be liquid nitrogen and the container may be heat insulated with an insulating vacuum cavity. A buoyancy unit (40, FIG. 2) which comprises a rigid enclosure (42, FIG. 2) defining an interior volume and a flexible diaphragm (55, FIG. 2) that partitions the interior volume into first and second chambers is also disclosed.

This invention relates to floatation devices and in particular to floatation devices for raising and lowering items to and from the seabed.

Sea-going vessels such as ships and submarines often carry valuable cargo, and are generally very valuable themselves. If such vessels are damaged whilst at sea and subsequently sink to the seabed, it is highly desirable to be able to recover the cargo, or even the vessel itself. Recovery of items such as these requires a method of raising the items to the surface from the seabed. Other instances that require items to be raised and lowered to and from the seabed is when mining on the seabed, and when constructing or decommissioning oil and gas platforms, and ancillaries.

One method of recovering items from the seabed involves the use of floatation devices secured to the item. These floatation devices typically comprise a compressed gas canister contained within an inflatable body. In use, a large number of these floatation devices are secured to the item to be raised, and gas is then remotely released from the gas canisters thereby inflating the inflatable body. The upward buoyancy force exerted by the sea on the inflated floatation devices acts to raise the item to the surface.

Conventional floatation devices are only effective up to a certain depth. At greater depths, the pressure exerted by the surrounding water on the inflatable body is too great for the inflatable body to inflate sufficiently to raise the item from the seabed.

There has now been devised an improved floatation device which overcomes or substantially mitigates the above-mentioned and/or other disadvantages associated with the prior art.

According to a first aspect of the invention, there is provided a floatation device comprising a container containing a liquefied gas, a gas chamber and a remotely operable device, the remotely operable device being switchable between a closed state in which fluid communication between the container and the gas chamber is prevented, and an open state in which fluid communication between the container and the gas chamber is enabled and vaporisation of the liquefied gas, in use, charges the gas chamber with gas.

The device according to the invention is advantageous principally because the liquid gas is able to vaporise and charge the gas chamber with gas even when the surrounding pressure is great. The device according to the invention is therefore effective at greater depths than conventional devices that use compressed gas canisters.

The liquefied gas may be any suitable substance. Most preferably, the liquefied gas is liquid nitrogen. Liquid nitrogen is both relatively cheap and readily available.

The device may be formed in any materials which have the strength to withstand the pressures that the device will encounter during use. Suitable materials for the device include metals, such as austenitic steel and stainless steel, plastics materials, carbon-fibre materials, and glass-fibre materials.

The container is preferably heat-insulated. This heat-insulation may be achieved by any conventional means that is suitable for incorporation in a floatation device.

For instance, the container may be formed in, lined, or surrounded by a material having good heat-insulating properties. Most preferably, however, the container includes an insulating vacuum cavity. In addition, the container may incorporate a cooling system such as those conventionally used with liquefied gases.

The floatation device may include means for heating the liquefied gas within the container. Alternatively, the floatation device may include means for aiding heat conduction from the surroundings to the liquefied gas within the container. For example, the floatation device may include one or more remotely operable devices that conduct heat across insulating material, or an insulating vacuum cavity, when activated. However, the provision of such devices is entirely optional.

The container preferably has an opening or fluid conduit that leads into the gas chamber. The container preferably includes a fluid conduit that connects the interiors of the container and the gas chamber. The fluid conduit may be adapted so as to allow liquid nitrogen flowing through the fluid conduit to be heated.

Preferably, the fluid conduit has the form of a pipe with a small cross-section and large length relative to the corresponding dimensions of the container. Most preferably, the fluid conduit is a pipe that is coiled about the external surface of the container. Where the container includes an insulating vacuum cavity, the fluid conduit is preferably situated within this cavity, and is most preferably situated adjacent to an inner surface of an outer wall of the container.

The gas chamber may comprise a containing wall that substantially surrounds the gas within the chamber. Alternatively, the gas chamber may have an open lower end, with the gas being retained within the gas chamber by its buoyancy within the water. In any case, the gas chamber preferably includes at least one pressure release vent that vents the vaporised gas, as necessary, during use.

The gas chamber may be integrally formed with the container, but is most preferably formed as a separate component which is secured to the container during use.

The gas chamber may be rigid or flexible in form. Where the gas chamber is flexible, the flexible gas chamber is preferably formed in a conventional coated fabric material and is preferably fixed to the periphery of the opening or the fluid conduit of the container. The flexible gas chamber is preferably deflated before use and is inflated by the vaporised gas in use. However, the gas chamber is preferably at least partially rigid in form. In particular, according to a further aspect of the invention, there is provided a buoyancy unit comprising a rigid enclosure defining an interior volume and a flexible diaphragm that partitions the interior volume into first and second chambers, the first chamber being adapted to contain a gas and having an inlet for connection to a gas supply, and the second chamber having a fluid outlet, wherein the diaphragm is movable on charging of the first chamber with gas so as to urge fluid within the second chamber out of the buoyancy unit through the fluid outlet.

The buoyancy unit according to the invention is advantageous principally because it functions in an analogous manner to a gas chamber formed by an entirely flexible enclosure, but is less likely to be damaged in harsh deep sea environments because the flexible diaphragm is protected within the rigid enclosure. In addition, the buoyancy of the buoyancy unit may be easily, and accurately, controlled by altering the ratio of the volume of gas within the buoyancy unit to the volume of fluid within the buoyancy unit.

The floatation device according to the invention preferably comprises a gas chamber that forms part of such a buoyancy unit.

By “rigid” enclosure is meant that the enclosure maintains its shape during normal use. The first chamber preferably includes at least one pressure release vent that vents the gas, as necessary, during use, and the fluid outlet of the second chamber is preferably a simple aperture. The first chamber may also include a remotely operable vent for controlling the buoyancy of the buoyancy unit.

Preferably, the diaphragm is fixed at its periphery to the interior surface of the rigid enclosure along a line that is confined to a single plane, and the diaphragm is enlarged relative to the corresponding cross-sectional area of the rigid enclosure so that the diaphragm may be displaced so as to lie alongside an interior surface of the rigid enclosure. Most preferably, the diaphragm is fixed at its periphery to the interior surface of the rigid enclosure along a line that is confined to a single plane that bisects the interior volume.

The remotely operable device may be a single valve, or a plurality of valves, which may be located within the opening or fluid conduit of the container and have an open state that allows fluid communication between the container and the gas chamber. Alternatively, the remotely operable device may be a device that either physically prevents or allows inflation of the flexible gas chamber, or movement of the flexible diaphragm, by the vaporised gas.

Furthermore, according to the present invention there is provided buoyancy unit comprising a rigid enclosure defining an interior volume and a flexible diaphragm that partitions the interior volume into first and second chambers, the first chamber being adapted to contain a gas and having an inlet for connection to a gas supply, and the second chamber having a fluid outlet, wherein the diaphragm is movable on charging of the first chamber with gas so as to urge fluid within the second chamber out of the buoyancy unit through the fluid outlet.

The first chamber may include at least one pressure release vent that vents the gas, as necessary, during use. The fluid outlet of the second chamber may be a simple aperture. The first chamber may include a remotely operable vent for controlling the buoyancy of the buoyancy unit.

The diaphragm may be fixed at its periphery to the interior surface of the rigid enclosure along a line that is confined to a single plane, and the diaphragm may be enlarged relative to the corresponding cross-sectional area of the rigid enclosure so that the diaphragm may be displaced so as to lie alongside an interior surface of the rigid enclosure.

The diaphragm may be fixed at its periphery to the interior surface of the rigid enclosure along a line that is confined to a single plane that bisects the interior volume.

According to a further aspect of the invention, there is provided a method of raising an item from the seabed, or lowering an item to the seabed, which method comprises the steps of: (a) attaching a floatation device as described above to the item, and (b) switching the remotely operable device from its closed state to its open state, such that the gas chamber is charged with gas resulting from vaporisation of the liquefied gas.

Where an item is being raised from the seabed, the floatation device is attached to the item whilst the item is on the seabed. In this case, the floatation device may be lowered down to the seabed, or allowed to descend under the influence of gravity. The step of attaching the floatation device to the item on the seabed is preferably performed by a remotely operable means, such as a robot. The switching of the remotely operable device from its closed state to its open state is preferably achieved by a user at the surface of the sea transmitting signals, for example electromagnetic radiation signals, to the device.

A first embodiment of a floatation device according to the invention is shown inFIGS. 1 to 5. The first embodiment comprises a container10, which is shown inFIG. 1, and a lift unit40, which is shown inFIGS. 2 to 5.

Referring firstly toFIG. 1, the container10comprises an inner wall12defining a chamber16suitable for containing liquid nitrogen, an outer wall14wholly encompassing the inner wall12, and a stand18. The chamber16is generally cylindrical in shape but has end portions that are hemispherical in shape. The stand18comprises a base that rests upon the ground, and four inclined struts that extend from the upper surface of the base and are fixed to the external surface of the outer wall14at one end of the chamber16, thereby supporting the chamber16in a vertical orientation.

The inner wall12is formed of austenitic steel, and the outer wall is formed of glass-fibre reinforced plastics material (GRP). The outer wall14is separated from the inner wall12and a hermetically sealed cavity is formed between these walls12,14. During manufacture, a vacuum is formed within this cavity between the inner and outer walls12,14, thereby providing heat-insulation for the chamber16.

At the upper end of the chamber16(as viewed inFIG. 1), an inlet pipe22and a vent pipe24extend through the inner and outer walls12,14of the container10.

The inlet pipe22extends along the length of the chamber16to a position near to its lower end. The vent pipe24terminates at the interior surface of the inner wall12at the upper end of the chamber16.

A mount20is formed on the upper external surface of the outer wall14with the inlet and vent pipes22,24extending through the mount20, and branching horizontally away from one another, such that the pipes22,24project from the mount20in opposing horizontal (as viewed inFIG. 1) directions. The end portions of the inlet and vent pipes22,24that project from the mount20each include a valve23,25for controlling flow along the pipes22,24. The mount20, and the end portions of the inlet and vent pipes22,24that project from the mount20, are enclosed in use within a hermetically sealed, and pressure-resistant housing35that is releasably fastened to the upper external surface of the outer wall14.

At the lower end of the chamber16, the inner wall12includes a pair of circular apertures. A cooling pipe26extends from one of the circular apertures, and coils around the outer surface of the inner wall12towards the upper end of the container10. Near to the upper end of the container10, the cooling pipe26joins an outlet pipe30that extends upwardly (as viewed inFIG. 1) through the outer wall14and communicates with the lift unit40, as described below. The cooling pipe26includes a pressure-release valve27at its upper end that allows fluid flow from the cooling pipe26into the outlet pipe30, and hence the lift unit40, when the pressure within the cooling pipe26is approximately 100 millibars higher than the pressure within the lift unit40. The cooling pipe26and outlet pipe30are formed predominantly of metal. However, the portion of the outlet pipe30that extends across the cavity between the valve27at the outer surface of the inner wall12and the inner surface of the outer wall14is formed of a plastics material having a low thermal conductivity coefficient.

A heating pipe28extends from the other circular aperture at the lower end of the chamber16, and coils around the chamber16along the inner surface of the outer wall14towards the upper end of the container10. Near to the upper end of the container10, the heating pipe28also joins the outlet pipe30that extends upwardly through the outer wall14. The heating pipe28includes a valve29at its lower end, adjacent to the inner surface of the outer wall14, that controls fluid flow into the heating pipe28, and hence into the outlet pipe30and the lift unit40.

The heating pipe28is formed predominantly of metal. However, the portion of the heating pipe28that extends across the cavity between the circular aperture of the inner wall12and the valve29at the inner surface of the outer wall14is formed of a plastics material having a low thermal conductivity coefficient.

Finally, a number of remotely operable heat conductors32are fastened to the inner surface of the outer wall14around the central cylindrical portion of the chamber16. The heat conductors32each comprise a copper rod which is disposed alongside the inner surface of the outer wall14when deactivated, and extends outwardly from the inner surface of the outer wall14into contact with the outer surface of the inner wall12when activated. Hence, when activated, the heat conductors32conduct heat from the outer wall14, through the copper rods, and into the chamber16through the inner wall12.

Turning now also toFIGS. 2 to 5, the lift unit40comprises a rigid shell42formed of glass-fibre reinforced plastics material (GRP). The interior volume of the shell42has a similar shape to the chamber16of the container10in that it comprises a central cylindrical portion and hemispherical end portions. This interior volume of the shell42is partitioned by a diaphragm55into a sea water chamber50and a gas chamber60. The diaphragm55is fixed at its periphery to the interior surface of the shell42in a plane that bisects the interior volume of the shell42along its longitudinal axis.

The diaphragm55is enlarged relative to the corresponding cross-sectional area of the shell42so that the diaphragm55may be displaced so as to lie alongside an interior surface of the shell42. In this way, displacement of the diaphragm55varies the relative sizes of the sea water chamber50and the gas chamber60.

At the lower end of the lift unit40(as viewed inFIGS. 2 to 5), a gas inlet pipe62extends through the wall of the shell42and into the gas chamber60. The gas inlet pipe62of the lift unit40is connected by a connecting pipe (not shown in the Figures) to the outlet pipe30of the container10. In addition, the lift unit40is firmly secured to the container10, in use, by high-strength connecting cables (not shown in the Figures), or other suitable means, that may be clamped, bolted or welded to the container10and the lift unit40.

The shell42also comprises a set of vents64having remotely operable valves that either allow or prevent the exit of gas from the gas chamber60, and a set of apertures52that allow the passage of sea water into, and out of, the sea water chamber50. The vents64also have a pressure-release mechanism whereby gas is allowed to exit the gas chamber60when the pressure within the gas chamber60exceeds the pressure of the surrounding sea water by a predetermined threshold value, such as 100 millibars. Finally, the external surface of the shell42is provided with suitable means44for attaching the lift unit to a load to be lifted.

In use, the chamber16of the container10is firstly charged with liquid nitrogen.

This process is usually carried out while the floatation device is aboard a ship or the like. Initially, the housing35is unfastened from the outer wall14, the inlet valve23and vent valve25are put into an open state that allows fluid flow along the inlet and vent pipes22,24, and the outlet valve29is put into a closed state that prevents fluid flow from the chamber16into the heating pipe28. A supply of liquid nitrogen is then connected to the inlet pipe22so that liquid nitrogen enters the chamber16at its lower end. Since the interior of the chamber16is at a relatively higher temperature than the liquid nitrogen, nitrogen gas will be produced which will pass through the vent pipe24and vent valve25, and into the surrounding atmosphere. This process is continued until the interior of the chamber16is at a low enough temperature for the chamber16to begin charging with liquid nitrogen. Once the chamber16has been fully charged with liquid nitrogen, the inlet valve23and vent valve25are closed, and the housing35is fastened to the outer wall14of the container10. When the inlet valve23and vent valve25are closed, the pressure-release valve27ensures that the pressure within the chamber16is maintained at an acceptable level, as discussed below.

The floatation device is then carefully lowered into the sea and allowed to descend down towards the load to be lifted (not shown in the Figures) on the seabed. Weights may be attached to the stand18of the container10to aid descent, if necessary. While the container10is descending towards the load, a certain amount of liquid nitrogen within the container10will vaporise to form nitrogen gas. Due to the construction of the container10, the liquid nitrogen within the cooling pipe26will tend to vaporise to form nitrogen gas rather than the liquid nitrogen within the chamber16. This vaporisation cools the cooling pipe26and therefore helps to keep the liquid nitrogen within the chamber16cool. The nitrogen gas formed in the cooling pipe26will escape through the pressure-release valve27into the outlet pipe30, and hence the lift unit40, when the pressure within the cooling pipe26is approximately 100 millibars higher than the pressure within the lift unit40.

On descent, the lift unit40has a configuration as shown inFIG. 2where the diaphragm55lies alongside an interior surface of the shell42such that the sea water chamber50almost completely fills the interior volume of the shell42. In order to prevent the lift unit40from becoming charged with nitrogen gas on descent, the vents64are maintained in an open state.

When the floatation device reaches the seabed and the load to be lifted, the load is secured to the attachment means44of the lift unit40by high strength steel cables, for example. The action of attaching the floatation device to the load is typically performed by a robot (not shown in the Figures) which is controlled by a user at the surface.

The floatation device is then actuated by transmitting signals to the outlet valve29and vents64so that the outlet valve29is set to its open state, whereby liquid nitrogen in the chamber16is allowed to flow into the heating pipe28, and the vents64are closed. Once the floatation device has been actuated, liquid nitrogen will flow upwards through the heating pipe28and will be heated until nitrogen gas is produced within the heating pipe28. This nitrogen gas will flow into the gas chamber60of the lift unit40, thereby inflating the gas chamber60so that the diaphragm55urges sea water out of the apertures52, as shown inFIG. 3.

Once the chamber60of the lift unit40is sufficiently charged with nitrogen gas, as shown inFIG. 4, the floatation device and load will begin to ascend due to the increased buoyancy force acting on the lift unit40. In order to control ascent of the floatation device and load, the vents64may be opened to allow nitrogen gas to exit the gas chamber60, and sea water to enter the sea water chamber50, thereby reducing the upwards buoyancy force, as shown inFIG. 5. As the floatation device and load ascend towards the surface, the pressure of the sea water will decrease. The pressure-release mechanism of the vents64will therefore allow nitrogen gas to exit the gas chamber60as the lift unit40ascends. Once the floatation device and load reach the surface, they are recovered and the floatation device is detached from the load. The floatation device may then be recharged with liquid nitrogen and reused.

The valves23,25,27,29, the heat conductors32and vents64are all electrically powered devices. The power supply may be a battery that is contained within a watertight and pressure-resistant compartment of the container10, such as the housing35, or the lift unit40. Alternatively, the power supply may be situated at the surface and connected by an umbilical cable (not shown in the Figures) to the floatation device. These devices23,25,27,29,32,64are also actuated by electrical signals. Where the floatation device includes a cable connection to the surface, the electrical signals may be passed down this cable. Otherwise, the electrical signals may be sent as electromagnetic waves from the surface, or may be initiated by ultrasonic signals sent from the surface to the floatation device.

A second embodiment of a floatation device according to the invention is shown inFIGS. 6 to 9. The second embodiment comprises a container100, shown inFIGS. 6 and 7, a lift bag120, shown inFIG. 8, and a support frame130, shown inFIG. 9.

Referring toFIGS. 6 and 7, the container100comprises a housing102, a chamber106, a heating pipe110and a vent pipe114. The housing102is generally cylindrical in form with a closed upper end that is slightly domed in shape and an open lower end.

The chamber106is mounted within the housing102adjacent the closed upper end thereof so that there is a substantially empty volume at the lower end of the housing102. The chamber106is generally cylindrical in shape with upper and lower walls of domed shape, as shown inFIG. 7. The upper and lower walls of the chamber106each have a centrally positioned port, an upper port108and lower port109respectively, which are both in fluid communication with the interior of the chamber106. The external surface of the chamber106is covered by a layer of insulating material107. This layer of insulating material107may comprise a mesh bag, which surrounds the chamber106, and loose insulation material located within the volume between the mesh bag and the external surface of the chamber106. The upper and lower ports108,109extend outwardly from the chamber106to a position beyond the insulating layer107.

The vent pipe114extends from the upper port108of the chamber106, downwards along the length of the external surface of the chamber106, to a position beyond (i.e. below, as shown inFIG. 7) the lower wall of the chamber106. The end of the vent pipe114that is below the lower wall of the chamber106terminates in a pressure release valve115. In addition, immediately before the pressure release valve115, the vent pipe114includes a perpendicularly extending branch pipe that terminates in an outlet valve116.

The lower port109terminates in a T-junction from which extends an inlet pipe with an inlet valve111and an outlet pipe connected to a first port of a remotely operable three-way outlet valve112. The three-way outlet valve112also includes a second port connected to a relief nozzle113and a third port connected to the heating pipe110. The three-way outlet valve112is remotely switchable between a closed state, in which fluid communication between the three ports is prevented, a relief state, in which only the relief nozzle113is in fluid communication with the heating pipe110, and an active state, in which only the outlet pipe, and hence the lower port109, is in fluid communication with the heating pipe110.

The heating pipe110extends from the three-way outlet valve112, through the open lower end of the housing102, and upwards about the outer surface of the housing102in a helical fashion, as shown inFIG. 6. The heating pipe110terminates at the centre of the upper surface of the housing102, where it is connected to a remotely operable supply valve118.

FIG. 8shows the lift bag120that is connected to the container100by a port121which is connected to the supply valve118. The lift bag120is cylindrical in shape and is formed in a flexible material so that the lift bag120may be inflated and deflated, in use, between a folded state (not shown in the Figures) and an inflated state (as shown inFIG. 8). The lift bag120further includes pressure release vents124that release gas from within the lift bag120when the interior pressure becomes too great.

The supply valve118is switchable so as to either allow or prevent fluid communication between the heating pipe110and the interior of the lift bag120via port121. The lift bag120is orientated horizontally and a pair of lift straps123overlies the curved upper surface of the lift bag120and hang vertically down either side of the lift bag120as shown inFIG. 8. The lift straps122have connection rings123at each end thereof which allow connection of the straps122, and hence the lift bag120, to the support frame130shown inFIG. 9.

The container100is mounted, in use, within the support frame130shown inFIG. 9. The support frame130comprises a pair of similarly orientated upper and lower rings132that are connected together by four struts134that extend perpendicularly between the upper and lower rings132. Each strut134includes an outwardly extending connection ring135at each end thereof. The support frame130is formed in a high strength material, typically a metal, such as steel.

The four connection rings135located at the periphery of the upper ring132are connected, in use, to the connection rings123of the straps122of the lift bag120. The four connection rings135located at the periphery of the lower ring132are connected, in use, to the load that is to be lifted.

In order to use the second embodiment of the floatation device, the chamber106must firstly be charged with liquid nitrogen104. Again, this process is usually carried out while the floatation device is aboard a ship or the like. Firstly, the user ensures that inlet valve111and outlet valve116are in an open state, and three-way valve112and supply valve118are in a closed state. A supply of liquid nitrogen104is then connected to inlet valve111so that liquid nitrogen104enters the interior of the chamber106through the lower port109. Since the interior of the chamber106is at a relatively higher temperature than the liquid nitrogen104, nitrogen gas will be produced which will pass through the upper port108, through the vent pipe114, through the outlet valve116, and into the interior of the housing102. This process is continued until the interior of the chamber106is at a low enough temperature for the chamber106to become fully charged with liquid nitrogen104, at which point the user closes inlet valve111and outlet valve116. The three-way valve112is then set to its relief state whereby gas103within the housing102is allowed to enter the heating pipe110but the chamber106remains sealed.

The floatation device comprising the container100, which is fully charged with liquid nitrogen104, the lift bag120, which is in a deflated state, and the support frame130, which encases the container100, is then lowered into the sea. The floatation device is lowered into the sea in a vertical orientation with the open lower end of the housing102being submerged first. In this way a pocket of gas103is formed within the housing102. The user then causes the floatation device to descend, by attaching weights or otherwise, towards a load on the seabed.

As the floatation device descends, the pressure of the surrounding sea water will increase, as explained above. At the same time, nitrogen gas will be produced within the chamber106due to the ingress of heat from the surrounding sea water, thereby forming a pocket of nitrogen gas105at the upper end of the chamber106. The pressure of this nitrogen gas105will be allowed to increase only as high as a threshold pressure which is slightly higher, eg 100 millibars higher, than the pressure of the surrounding sea water by the pressure release valve115. Therefore, as nitrogen gas105is produced within the chamber106and the pressure of this gas105increases, nitrogen gas105will be vented through vent pipe114, and pressure release valve115, into the pocket of gas103within the housing102. The pressure of the pocket of gas103and the gas within heating coil110will therefore be maintained equal to that of the surrounding sea water, and the nitrogen gas105within the chamber106will be maintained at a pressure approximately 100 millibars higher than the pressure of the surrounding sea water.

Once the floatation device has reached the load on the seabed, the load is secured to the four connection rings135located at the periphery of the lower ring132of the support frame130by high strength steel cables, for example. The action of attaching the floatation device to the load is typically performed by a robot (not shown in the Figures) which is controlled by a user at the surface.

The floatation device is then actuated by transmitting signals to the remotely operable three-way valve112and supply valve118so that the three-way valve is set to its active state, whereby liquid nitrogen104in the chamber106is allowed to flow into the heating pipe110, and the supply valve118is opened, whereby fluid in the heating pipe110is allowed to enter the interior of the lift bag120.

Once the floatation device has been actuated, liquid nitrogen104will flow upwards through the heating pipe110and will be heated until nitrogen gas is produced within the heating pipe110. This nitrogen gas will flow through the supply valve118and port121into the interior of the lift bag120, thereby inflating the lift bag120.

Once the lift bag120is sufficiently charged with nitrogen gas, the floatation device and load will begin to ascend due to the increased buoyancy force acting on the lift bag120. As the floatation device and load ascend towards the surface, the pressure of the sea water will decrease. This will cause nitrogen gas to exit the lift bag120through the vents124during ascent. Once the floatation device and load reach the surface, they are recovered and the floatation device is detached from the load. The floatation device may then be recharged with liquid nitrogen and reused.

FIGS. 10 to 13show a third embodiment of a floatation device according to the invention which is generally designated210. The device210has the general shape of an inverted cone having a domed upper end and a smaller domed lower end (as shown inFIG. 10). The diameter of the device210therefore increases steadily from the lower end to the upper end thereof. The lower end of the device210includes a securing ring211that is used to secure the device210to the load226that is to be raised.

The device210comprises a heat-insulated lower chamber212and an upper chamber214that extends vertically upwards from the upper wall of the lower chamber212. The device210is formed in stainless steel with the lower chamber212being heat-insulated by conventional means. The upper chamber214is significantly greater in height than the lower chamber212and is not heat insulated. The volume ratio between the upper chamber and the lower chamber is approximately equal to the volume ratio between nitrogen gas at a temperature of 0 C and liquid nitrogen at a temperature in the region of −196 C.

The upper wall of the lower chamber212is angled slightly upwards from its periphery to an opening at its apex. This opening is sealed by a remote operation valve218which either allows or prevents the passage of gas through the opening. A funnel216extends upwardly from the valve218and opening and opens into the upper chamber214. The lower chamber212is therefore in fluid communication with the upper chamber214when the valve218is in an open position, and sealed when the valve218is in a closed position.

The lower part of the side wall of the upper chamber214includes a number of openings220which are regularly spaced around the circumference of the upper chamber214. These openings220are located below the upper end of the funnel216and above the periphery of the upper wall of the lower chamber212.

The lower chamber212also includes a replaceable cap (not shown in the Figures) that allows the lower chamber212to be unsealed, filled with liquid nitrogen222and then sealed again ready for use.

In use, the lower chamber212is firstly charged with a quantity of liquid nitrogen222and then sealed with the cap. The valve218is set to the closed position.

The device210is then held within the sea, in a horizontal orientation for example, so that the upper chamber214charges with sea water224. Once the upper chamber214is sufficiently charged with sea water224, the device210is released and allowed to descend towards an item226on the seabed228, as shown inFIG. 11.

As the device210descends, the pressure of the sea water224will increase.

Sea water224will therefore enter the device through the openings220as the device210descends, as shown by the curved arrows inFIG. 11. Once the device210reaches the seabed228, the pressure of the sea water224will be great. If the seabed is at a depth of 300 m, for example, the pressure of the sea water224will be approximately 3 MPa, or 30 atmospheres.

Turning now toFIG. 12, once the device210has reached the seabed228, the securing ring211of the device is attached to the load226by a high strength steel cable235. The action of attaching the device210to the load226is performed by a robot (not shown in the Figures) which is controlled by a user at the surface.

Once the device210is secured to the load226, the liquid nitrogen222is allowed to heat up and become nitrogen gas240. The remote operation valve218is then switched to the open position and the nitrogen gas240is allowed to exit the lower chamber212and collect in the upper part of the upper chamber214. If the device210is lying on the seabed228, the gas240will still collect in the upper part of the upper chamber214due to the shape of the device210. When a sufficient volume of gas240has collected in the upper chamber214, the device210will orientate itself into an upright position, as shown inFIG. 12. The nitrogen gas240collecting at the upper end of the upper chamber214forces sea water224to exit the upper chamber214through the openings220, as shown by the arrows inFIG. 12.

When the upper chamber214is sufficiently charged with nitrogen gas240, the device210and load226will begin to ascend due to the increased buoyancy force acting on the device210, as shown inFIG. 13.

As the device210and load226ascend towards the surface, the pressure of the sea water224will decrease. This will cause nitrogen gas240to exit the upper chamber214through the openings220during ascent, as shown by the curved arrows inFIG. 13. Once the device210and load226reach the surface, they are recovered and the device210is detached from the load226. The device210may then be recharged with liquid nitrogen222and reused.

A fourth embodiment of a floatation device according to the invention is shown inFIGS. 14 and 15, and is generally designated310. The device310comprises a hollow body312that has the general shape of an inverted cone having an upper end with rounded edges and a smaller domed lower end (as shown inFIG. 14).

The body312is heat-insulated and is charged with liquid nitrogen320in use.

The lower end of the device310includes a securing ring314that is used to secure the device310to the load that is to be raised (not shown inFIGS. 14 and 15).

The upper end of the body312has a large opening that has a folded balloon316mounted therein. The folded balloon316is fixed to the periphery of the opening and held in position by a closure318which occludes the opening and seals the body312. One side of the closure318is hingedly mounted to the body312at the periphery of the opening, and the opposite side is secured to the body312at its opposite side by a remote operation fastening device (not shown in the Figures). The balloon316is preferably formed in a conventional coated fabric.

The fourth embodiment of the device310is used in a similar manner to the third embodiment210save that rather than the remote operation valve218being opened to allow the upper chamber214to charge with nitrogen gas222, the remote operation fastening device is released to allow the balloon316to charge with nitrogen gas322. In addition, the balloon316includes pressure release vents (not shown inFIGS. 14 and 15) which vent the gas322as the device310ascends towards the surface.