Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63C—LAUNCHING, HAULING-OUT, OR DRY-DOCKING OF VESSELS; LIFE-SAVING IN WATER; EQUIPMENT FOR DWELLING OR WORKING UNDER WATER; MEANS FOR SALVAGING OR SEARCHING FOR UNDERWATER OBJECTS
  • B63C7/00—Salvaging of disabled, stranded, or sunken vessels; Salvaging of vessel parts or furnishings, e.g. of safes; Salvaging of other underwater objects
  • B63C7/06—Salvaging of disabled, stranded, or sunken vessels; Salvaging of vessel parts or furnishings, e.g. of safes; Salvaging of other underwater objects in which lifting action is generated in or adjacent to vessels or objects
  • B63C7/10—Salvaging of disabled, stranded, or sunken vessels; Salvaging of vessel parts or furnishings, e.g. of safes; Salvaging of other underwater objects in which lifting action is generated in or adjacent to vessels or objects using inflatable floats external to vessels or objects

Definitions

• This invention relates to floatation devices and in particular to floatation devices for manipulation of devices in the subsea environment and raising items 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 include mining on the seabed, and constructing or decommissioning oil and gas platforms and ancillaries.
• the floatation device comprises 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.
• GB2435856 also discloses a method of raising an item from the seabed or lowering an item to the seabed using the device described above.
• the method comprises the steps of: attaching a floatation device as described above to the item and 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.
• extreme conditions gas cannot be created to flow into the gas chamber described in GB2435856 above.
• the conditions are sufficiently extreme the liquid nitrogen cannot evaporate into the gaseous phase.
• extreme conditions encompass depths in excess of 300m, i.e. pressures in excess of 32 Bar and/or temperatures close to or below O 0 C.
• a floatation device comprising: a buoyancy chamber; a cryogen reservoir; a heating pipe providing switchable fluid communication between the cryogen reservoir and the buoyancy chamber.
• a cryogen reservoir that may, in use, contain a fluid that transitions to a supercritical fluid enabling the floatation device to operate at extreme depths in the region of 2000m.
• the buoyancy chamber, cryogen reservoir and heating pipe may be partly or wholly contained within a housing.
• the housing may be provided with a reinforced portion adjacent the cryogen reservoir.
• the reinforcement ensures that the cryogen reservoir is sufficiently robust to operate at depths in the region of 2000m at which the pressure will be in the region of 200Bar.
• the device may further comprise a microprocessor and a plurality of sensors.
• the sensors are used to detect temperature and pressure at various key locations within the device. Yaw sensors are also provided to monitor the stability of the device. All of the sensors relay data to the microprocessor.
• the microprocessor may contain a unique identifier for the device.
• the housing may incorporate a blow-off portion which can be deployed if a pressure imbalance between the device and the surrounding sea exceeds a predetermined threshold.
• An imbalance could result in an uncontrolled rapid ascent to the surface which could result in the rapid expansion of the supercritical fluid which could transition to a gaseous phase and damage the housing.
• the buoyancy chamber may be subdivided into a plurality of compartments each of which may be provided with a diaphragm.
• the provision of a number of smaller diaphragms introduces an increased degree of redundancy into the system because, if one of the diaphragms fails, the remainder of the device may continue to function.
• the buoyancy chamber, or each compartment of the buoyancy chamber may be provided with at least one orifice to allow sea water to move into and out of the buoyancy chamber.
• the orifice(s) may be covered by a grill to prevent the ingress of marine life.
• the buoyancy chamber, or each compartment of the buoyancy chamber may be provided with at least one one-way valve to allow supercritical cryogenic fluid or gas to exit the device.
• the blow-off portion may be provided by the one-way valves which enable the egress of fluids if the pressure differential between the interior of the device and the surroundings has become too great.
• the heating pipe may be routed through the housing so that at least a portion of the heating pipe is adjacent an outer surface of the housing. This enables the heating of the cryogenic fluid by extracting heat from the surrounding sea water. This reduces or eliminates the need for a heat source within the device.
• the device may further comprise a plurality of heat conductors configured to introduce heat into the cryogen reservoir. The heat conductors also enable heat from the surrounding sea water to be introduced into the device, in this case into the cryogen reservoir directly. The heat introduced to the cryogen reservoir by the heat conductors helps to start the transition of the cryogenic fluid into a supercritical fluid and the subsequent movement of this fluid through the heating pipe.
• the heating pipe may be switchable using a valve.
• a method of raising an item from the seabed comprising the steps of lowering a floatation device to the seabed; attaching the floatation device to the item to be raised; creating a supercritical fluid within a portion of the floatation device; and allowing the floatation device and the item to rise to the surface using the buoyancy of the supercritical fluid to raise the item to the surface.
• the supercritical fluid may be supercritical nitrogen, carbon dioxide, helium or hydrogen.
• Figures 1 A and 1 B are schematic cross-sectional views through a buoyancy chamber that forms part of a floatation device according to the present invention
• FIGS. 2A and 2B are schematic cross-sectional views through a cryogen reservoir that forms part of the device according to the present invention.
• Figures 3A and 3B are schematic side- and cross-section views of a heating system for use with the floatation device according to the present invention
• Figures 4A and 4B are schematic cross-sectional views of the device according to the present invention in the configuration adopted when as the device descends to the sea bed;
• Figures 5A and 5B are schematic cross-sectional views of the device shown in Figures 4A and 4B in the configuration adopted when the buoyancy chamber is ready to be filled with a low-density fluid;
• Figures 6A and 6B are schematic cross-sectional views of the device shown in Figures 4A and 4B in the configuration adopted when the buoyancy chamber is being filled with a low-density fluid;
• Figures 7A and 7B are schematic cross-sectional views of the device shown in Figures 4A and 4B in the configuration adopted as the device ascends;
• Figure 8 is a schematic cross-sectional view of a further example of a floatation device according to the present invention.
• Figure 9 is a schematic cross-sectional view of a yet further example of a floatation device according to the present invention.
• the floatation device 100 has a buoyancy chamber 1 10; a cryogen reservoir 210; a heating system 302 and a control system 400.
• the buoyancy chamber 1 10 is configured to hold a fluid that is less dense than water and thereby facilitate the raising of the device 100 and a load to be raised.
• the cryogen reservoir 210 is configured to hold cryogenic fluid which is typically in a liquid phase at atmospheric pressure.
• the heating system 302 is configured to heat the liquid cryogen so that it transitions to form a super-critical fluid which can then move into the buoyancy chamber 110.
• a supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. Close to the critical point, small changes in pressure or temperature result in large changes in density.
• the floatation device 100 may extend between 1 and 20m in length and a device, or a number of devices 100 cooperating together may be capable of lifting loads weighing between 1 tonne and 2000 tonnes.
• the housing 1 12 has an elongate central portion 1 14 of circular cross section disposed between two hemispherical end portions 1 15, 1 16.
• the housing 112 envelops the buoyancy chamber 1 10; the cryogen reservoir 210; some parts of the heating system 302 and the control system 400.
• the housing 1 12 provides protection and prevents unwanted interactions between the various elements which could occur were they not all contained in a single unit.
• the housing 1 12 may be a formed as a single piece or it may be partially defined by the outer surfaces of the buoyancy chamber 110 and the cryogen reservoir 210.
• the housing is formed by the hemispherical end portion 1 15 and elongate central portion 1 14 of the buoyancy chamber 1 10 and part of the cryogen reservoir 210.
• an annulus of housing material is provided to join the above mentioned together.
• the housing 1 12 may be fabricated from glass-fibre reinforced plastics material (GRP), aluminium or a carbon composite. If Aluminium is used, then the housing 112 may be formed in several pieces that are subsequently fixed together using air-tight joins.
• GRP glass-fibre reinforced plastics material
• the housing 112 should be capable of withstanding a 2 to 3 bar pressure differential between the interior of the housing 112 and the surrounding environment. If the pressure differential between the inside and the outside of the device exceeds around 3 bar, changes will be triggered that will equalise the pressure before damage is caused. For example, valves may be opened to equalise the pressure differential between the interior and exterior of the device 100.
• the housing 1 12 is provided with a strengthened section 118 adjacent the cryogen reservoir 210.
• the strengthened section 1 18 enables the device 100 to withstand the pressures of operating in extreme conditions at depths in the region of 2000m and therefore pressures of 200bar.
• the strengthened section 1 18 may be formed from the same or a different material from the housing 1 12.
• the housing 1 12 is provided with at least one fixing 130 that facilitates the attachment of the device 100 to an object to be raised from the seabed.
• the fixing 130 illustrated includes two sets of parallel protrusions that can be attached to the item to be raised.
• the fixing 130 may be attached directly to the item to be raised or the attachment may be achieved via an intermediate piece, such as a length of steel cable passed through an eyelet provided in the fixing 130.
• the configuration of the fixings 130 dictates the orientation in which the device 100 will operate.
• two sets of fixings 130 on the exterior of the housing 112 are provided. If the fixings are attached to the item to be raised in a horizontally spaced apart configuration then the major axis of the device 100 will lie substantially in the horizontal plane when the device 100 is attached to an item to be raised. Conversely, if the fixings are attached to the item in a vertically spaced apart configuration then the device will operate in the vertical configuration illustrated in Figures 1A to 7A.
• the buoyancy chamber takes the form of an open-bottomed caisson 122 i.e. a bell-shaped enclosure that is at least partly open at its base.
• the base of the caisson is indicated by the dotted line 124.
• the caisson 122 is open to the surroundings, the material from which it is fabricated is not required to deal with large pressure differentials. Therefore, it could be formed from a partially flexible material.
• the caisson may not be bell-shaped, but may instead be spherical or another shape.
• the caisson 122 is provided with fixings 130 and a lifting eye 140 similar to those described above with reference to Figures 1A to 7A.
• a lip 145 may additionally be provided in order to increase the stability of the caisson 122.
• the lip 145 prevents supercritical fluid from flowing out of the caisson 122 if the caisson 122 is close to being fully charged and is not fully vertical.
• the housing 112 or cryogen reservoir 210 may be provided with a stand including a base that rests on any suitable support surface, including the deck of a ship or the seabed, and four inclined struts that extend from the upper surface of the base and are fixed to the external surface of the housing 112 or cryogen reservoir 210 at one end, thereby supporting the device 100 in a vertical orientation.
• the housing 1 12 or cryogen reservoir 210 is provided with means by which the device 100 can interface with a cradle.
• the interface means could be the portions of the heating coil that wrap around the exterior of the housing 112 or cryogen reservoir 210.
• a plurality of tabs may be provided to allow the housing 1 12 or cryogen reservoir 210 to rest within a cradle.
• the housing is provided with additional fixings which facilitate the attachment of a supplementary cryogen tank to the exterior of the housing 1 12.
• the supplementary tank could be used when a number of partial descents and ascents are planned.
• the cryogen reservoir 210 could then be recharged from the supplementary tank without requiring the device 100 to resurface.
• the buoyancy chamber 110 is shown in detail in Figures 1 A and 1 B.
• a rigid shell 150 formed of glass-fibre reinforced plastics material (GRP).
• GRP glass-fibre reinforced plastics material
• the interior volume of the shell 150 has a central cylindrical portion
• This interior volume of the shell 150 is partitioned by a diaphragm 160 into a first chamber 162 and a second chamber 164.
• the diaphragm 160 is fixed at its periphery to the interior surface of the shell 150 in a plane that bisects the interior volume of the shell 150 along its longitudinal axis.
• the diaphragm 160 is enlarged relative to the corresponding cross-sectional area of the shell 150 so that the diaphragm 160 may be displaced so as to lie alongside an interior surface of the shell 150. In this way, displacement of the diaphragm 160 varies the relative sizes of the first and second chambers 162, 164.
• the diaphragm 160 may be formed from an elastic material so that it is under tension when it is lying alongside the interior surface of the shell. Alternatively, the diaphragm 160 may be formed of a flexible material and it may be sized to lie alongside an interior surface of the shell 150.
• the shell 150 further comprises a set of orifices 180 that allow the passage of sea water into, and out of, the first chamber 162.
• Six orifices 180 are illustrated in the Figure 1A. However, the number of orifices and the size of the orifices may be altered.
• the orifices 180 may be provided with a coarse meshed grill in order to prevent large marine life from entering the device 100.
• a set of one-way valves 170 is provided that can be opened in order to allow fluid contained within the buoyancy chamber 1 10 to exit the device 100.
• the one-way valves 170 have a pressure-release mechanism whereby fluid is allowed to exit the buoyancy chamber 1 10 when the pressure within the buoyancy chamber 110 exceeds the pressure of the surrounding sea water by a predetermined threshold value, such as 1 Bar.
• a predetermined threshold value such as 1 Bar.
• Six one-way valves 170 are illustrated in the Figure 1A and two are illustrated in Figure 1 B at the apex of the bell- shaped caisson 122. However, the number of valves and the size of the valves may be altered. Because the caisson 122 is open at its base, it must be operated in the orientation shown in Figure 1 B.
• the one-way valves 170 are therefore provided at the top of the caisson 122 in order to release fluid from within the caisson 122 without destabilising the device 100.
• the cryogen reservoir 210 is shown in detail in Figures 2A and 2B.
• the cryogen reservoir 210 has an inner wall 212 defining an enclosure 214 suitable for containing cryogenic fluid 216, and an outer wall 218 wholly encompassing the inner wall 212.
• the enclosure 214 in both of the illustrated examples has a hemispherical portion
• the hemispherical portion 222 is configured to conform to the shape of the housing 112. Therefore, if the end portion 1 16 of the housing 1 12 adjacent the cryogen reservoir 210 is not hemispherical, the shape of the enclosure 214 will differ from the example shown in order that the shape of the enclosure 214 will conform to the shape of the end portion of the housing 1 12.
• the inner wall 212 of the cryogen reservoir 210 may be formed of austenitic steel but might be of some other material, for example, 9% nickel steel or cryogenic aluminium.
• the outer wall 218 may be formed of GRP but might be of some other material, for example, carbon composite.
• the outer wall 218 is separated from the inner wall 212 by a hermetically sealed cavity 226. During manufacture, a vacuum is formed within this cavity between the inner and outer walls 212, 218, thereby providing heat-insulation for the enclosure 214.
• a filler pipe 230 and a vent pipe 232 extend through the inner and outer walls 212, 218 of the cryogen reservoir 210.
• the filler pipe 230 extends substantially along the length of the enclosure 214 to a position near to the lowest point of the hemispherical portion 222.
• the vent pipe 232 terminates at the interior surface of the outer wall.
• the inner wall 212 includes at least one aperture 234.
• a cooling coil 236 extends from the vicinity of the aperture 234, and is coiled around the outer surface of the inner wall 212.
• the cooling coil 236 includes a pressure-release valve at its upper end that allows fluid flow from the cooling coil 236 into the buoyancy chamber 1 10 when the pressures within the cooling coil 236 is approximately 100 millibars higher than the pressure within the buoyancy chamber 1 10.
• the cooling coil 236 is formed predominantly of metal. However, the portion 238 of the cooling coil 236 that extends between the cryogen reservoir 210 and the buoyancy chamber 1 10 is formed of a plastic material having a low thermal conductivity coefficient.
• the cryogen reservoir 210 is enclosed in a strengthened spherical enclosure 240.
• the enclosure 240 must be thick-skinned and may be fabricated from a carbon composite.
• the enclosure 240 must be able to withstand pressure differentials of 1 Bar to 203 Bar so that it can operate at depths of 2000m.
• additional strengthening may be provided by ribbing either internal or external to the enclosure 240.
• the spherical enclosure 240 may be provided with watertight pressure-resilient inspection covers (not shown) in order to allow access within the spherical enclosure 240 for maintenance or other purposes.
• the heating system 302 is shown, primarily in Figures 3A and 3B although some features are shown in Figures 2A and 2B.
• the heating system 302 includes a heating pipe 310 that extends from the aperture 234 in the inner wall 212 of the enclosure 214; a heating chamber 336 and a pair of electrical heating elements 346.
• the heating pipe 310 is routed into the heating chamber 336.
• the path of the heating pipe 310 is tortuous and a portion that passes through the housing 1 12 and wraps around the outer surface of the housing 1 12.
• the portion of the heating pipe 310 that is outside the housing 112 is reinforced to withstand the pressures exerted as a result of operation of the device 100 under extreme conditions.
• a protective grill (not shown) is provided to protect the heating pipe 310 when it is outside the housing 112.
• the provision of a grill reduces the risk of damage to the heating pipe 310 occurring as a result of contact between the heating pipe 310 and the item to be lifted, or the sea bed or any other hazards.
• the heating system 302 also includes a valve 338 to control the flow of fluid through the heating pipe 310 and hence into the buoyancy chamber 130.
• the valve 338 is positioned adjacent to the inner surface of the outer wall 218.
• the heating system 302 also includes a heat shield 330 that is provided between the cryogen reservoir 210 and the buoyancy chamber 110.
• the heat shield 330 includes a first portion 332 that is adjacent the buoyancy chamber and a second portion 334 that is adjacent the cryogen reservoir 210.
• the two portions 332, 334 of the heat shield 330 define the annular heating chamber 336.
• the heating chamber 336 is thermally isolated from both the buoyancy chamber 110 and the cryogen reservoir 210.
• a plurality of remotely operable heat conductors 340 are fastened to the inner surface of the outer wall 218 around the hemispherical portion 222 of the enclosure 214.
• the heat conductors 340 each comprise a copper rod 342 which is pivotably mounted on the inner surface of the outer wall 218 of the cryogen reservoir 210.
• the copper rods 342 are pivotable between an active position in which the rods extend substantially perpendicularly to the surface of the outer wall 218 and a deactivated position in which the rods lie substantially parallel to the surface of the outer wall 124.
• the length of the rods 342 is selected to match closely the distance between the inner wall 212 and the outer wall 218 so that, when the rods are activated, they provide a path for heat conduction into the enclosure 214.
• the inner and outer walls of the enclosure are substantially parallel and therefore all of the rods 342 are substantially the same length.
• the number of rods 342 shown in the figures is illustrative only. There may be in the region of 30 provided. If the heat introduced into the cryogen reservoir 210 by the heat conductors 340 is insufficient, then this can be boosted by electrical heating from electrical heating elements 346.
• heating elements 346 Although a pair of electrical heating elements 346 is provided in the example illustrated, the number and size of heating elements may be varied according to the size of the device 100 and/or the intended field of use. For example, more heating elements may be required for a device deployed in the polar regions.
• Figures 4A and 4B each show a complete device 100 as described above including the buoyancy chamber 1 10, the cryogen reservoir 210, heating system 302 and control system 400.
• the width of the caisson 122 is substantially equal to the diameter of the spherical enclosure 240, the caisson 122 could be wider or less wide than the spherical enclosure 240.
• the spherical enclosure 240 is connected to the caisson using at least one connecting spar 242.
• the connecting spar 242 may extend partially or completely around the circumference of the spherical enclosure.
• a plurality of spars can be used. In this later case, each spar may be elongate, i.e. its length far exceeds its width.
• the spars may be rigid or flexible.
• a universal coupling could be used in place of the spar(s) in order to connect the caisson 122 to the spherical enclosure 240.
• the heating pipe 310 terminates in a cryogen feed vent 244 which protrudes from the spherical enclosure 240 and into the caisson 122.
• the cryogen feed vent 244 is provided with a one-way valve 245 to prevent the ingress of sea water 300 from the caisson 122 into the heating pipe 310.
• the control system 400 includes a microprocessor 402 mounted within the housing 1 12 of the device 100 and a remote control device (not shown) which is typically located on the ship from which the device 100 is launched and controlled.
• the microprocessor 402 communicates with the shipboard control device via a data link
• the data link 404 may be provided by a fibre optic link or a copper cable which is encased in a protective coating.
• the data link 404 may be detachable from the device 100.
• the microprocessor 402 mounted within the housing 112 is configured to collate data from sensors mounted within the device 100 and to relay this information to the remote control device.
• each device 100 has a unique identifier, which is stored on the microprocessor 402 and transmitted, together with position data, through the data link 404. This allows two or more devices 100 to be controlled to cooperate and thereby lift an item that is either too large or of such a shape that a single device 100 would be insufficient.
• the data may be sent directly to the remote control device or it may be shared between cooperating devices 100 directly.
• sensors mounted at different locations within and on the outer surface of the device 100.
• the illustrated positions of the sensors 410, 412, 414, 418, 420 within the device are merely exemplary. Sensors may be provided at further locations, or a number of the sensors described below may be provided at any one of the locations illustrated in the accompanying drawings.
• the sensors are configured to monitor the process conditions and to feed back information to the microprocessor 402.
• Some sensors, for example sensor 410 may be a temperature or pressure sensor.
• the provision of a number of pressure sensors allows pressure differentials between different parts of the device 100 to be calculated either by the microprocessor 402 or by the remote control device. In addition, the pressure external to the device 100 correlates strongly with the depth of the device 100.
• sensor 412 may be configured to contribute to resource management within the device.
• the sensor 412 may indicate how much cryogen remains in the reservoir 210.
• Sensors such as sensor 414 may be provided to ensure the integrity of sensitive parts of the device 100.
• Sensor 414 is mounted in the vicinity of the microprocessor 402 which is isolated from the device 100 by a heat shield 330. This sensor 414 will monitor the conditions to which the microprocessor 402 is subjected.
• the orientation of the device 100 may be monitored by a yaw sensor 418.
• the yaw sensor 418 gives an indication of the stability of the device 100 and identifies problems evolving when a number of devices 100 are working together to retrieve an object that is either irregular in shape or partially submerged in the sea bed. Information from the yaw sensor 418 can be used to identify if one part of the item is free whilst another remains trapped in the sea bed.
• heat conductors 340 are illustrated in all of the figures they are not essential to the operation of the device 100 and the cryogen reservoir 210 could achieve the required heat transfer by alternative means.
• Figure 9 shows a further alternative configuration with an open caisson 122.
• the caisson 122 in the example shown in Figure 9 differs from the caisson 122 shown in Figures 1 B to 7B in that the caisson 122 of Figure 9 is only partly open.
• the caisson 122 is also elongate in the horizontal, rather than the vertical plane.
• the cryogen reservoir 210 is located within the caisson 122. This configuration allows very convenient attachment of the device 100 to an item to be lifted using the fixings 130. In order to improve the stability of the device 100, two lifting eyes 140 are provided.
• cryogenic fluid 216 This can be liquid nitrogen, carbon dioxide, helium or hydrogen.
• Nitrogen may be used down to depths in the region of 1000m.
• Helium and hydrogen can be used at depths down to and in excess of 1000m, including 2000m.
• the process of priming or charging the device 100 is usually carried out while the floatation device is aboard a ship or the like.
• a supply of cryogenic fluid 216 is introduced into the device 100 through a pressure proof hatchway 350 and attached to the filler pipe 230.
• the cryogenic fluid 216 passes along the filler pipe 230 and enters the enclosure 214 close to its base. Since the interior of the enclosure 214 is at a comparatively elevated temperature relative to the cryogenic fluid, some of the fluid will vaporise on contact with the interior of the filler pipe 230 and the enclosure 214 and the gas resulting from the evaporation of the cryogenic fluid will leave the enclosure 214 via the cryogen vent pipe 232.
• the device 100 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.
• sea water 300 enters the first chamber 162 through the orifices 180 as shown in Figure 4A.
• the sea water 300 enters the caisson 122 through the openings at the base of the caisson 122 as indicated by the arrows on Figure 4B.
• the sea water 300 displaces the air from the buoyancy chamber 110 and causes the diaphragm 160 to decrease the size of the second chamber 164 relative to the size of the first chamber 162.
• the device 100 may be positively buoyant as a result of the presence of air pockets in the heating chamber 336 or as a result of the density of cryogenic fluid 216.
• cryogenic fluid 216 For example, liquid nitrogen has a density in the region of 80% that of water. While the device 100 is descending towards the load, a small amount of the cryogenic fluid 216 will vaporise to form gas. Due to the construction of the device 100, the fluid in the cooling coil 236 will tend to vaporise rather than the cryogenic fluid 216 within the enclosure 214.
• This vaporisation cools the cooling coil 236 and therefore helps to maintain the cryogenic fluid within the enclosure 214 cool.
• the gas formed in the cooling coil 236 will move through the cooling coil 236 and into the second chamber 164 of the buoyancy chamber 110.
• the one-way valves 170 may be maintained in an open state. Alternatively, the one-way valves 170 may be held closed in order to prevent cryogen from boiling off and moving into the sea. This is especially applicable when the device 100 is retained at the sea bed for some time before being activated to lift a load.
• the load is secured to the fixings 130 in an appropriate manner.
• 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 100 is then actuated by transmitting signals to close the oneway valves 170 and to open the valve 338 bringing the heating pipe 310 into fluid communication with the cryogenic fluid 216 contained with in the enclosure 214.
• the cryogenic fluid 216 becomes supercritical as it passes through the heating pipe 310.
• the supercritical fluid moves into the buoyancy unit 110 and forces the diaphragm 160 to move increasing the size of the second chamber 164.
• the first chamber 162 reduces in size and the pressure of the sea water therein exceeds the pressure of the surrounding sea water and therefore sea water 300 exits through the orifices 180 as shown in Figure 6A.
• the sea water 300 exits through the open area at the base 124 of the caisson 122.
• the floatation device 100 and load will begin to ascend.
• the one-way valve 170 may be opened to allow supercritical fluid to exit the buoyancy chamber 110, and sea water 300 to enter the first chamber 162, thereby reducing the upwards buoyancy force.
• the supercritical fluid 216 may transition into a gaseous phase and will exit through the one-way valves 170 or through the open area at the base 124 of the caisson 122 as the device 100 ascends.
• the device 100 can be used to provide uplift to an item reducing its effective weight and thereby making it easier to manipulate under water. For example, if the device is attached to an item weighing 50 tonnes, and sufficient cryogen is transitioned to a supercritical state to reduce the effective weight of the object just 5 tonnes, then the item will be easier to manipulate.
• the load When the uppermost part of the device 100 reaches the surface of the water, the load will remain submerged to a greater or lesser extent, depending on the configuration of the device 100. From this position at or close to the surface of the water the floatation device 100 and load are recovered and the floatation device 100 is detached from the load. The floatation device 100 may then be recharged with cryogenic fluid 216 and reused.
• the housing 1 12 may be substantially spherical or it may take the shape of a regular, or irregular, polygon.
• the lozenge shape and the spherical shape are advantageous because they are formed solely from curved shapes and therefore avoid the stresses associated with sharp corners and edges.

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