The present invention relates to a buoyancy device particularly, but not exclusively, for attachment to large structures requiring to be lifted, lowered, positioned and transported via the ocean.

Conventionally, when an offshore drilling structure comes to the end of its working life, it is decommissioned. In the past, it was thought that decommissioning could entail sinking the drilling structure at the point where it once stood.

However, amongst other factors, environmental factors have recently increased the need for offshore drilling structures to be moved to shallower and calmer waters, or back on land so that the drilling structures can be dismantled safely.

Previously, moving the drilling structures has involved the use of flat back barges, onto which the drilling structures are hauled. However, these barges are expensive and costly in terms of man power requirements.

According to the present invention there is provided a buoyancy device comprising a plurality of buoyancy members substantially equi-spaced around the circumference of a coupling member, the buoyancy members being coupled to the coupling member, wherein at least two of the buoyancy members are inflatable members, the inflatable members being formed from a substantially flexible material, such that the inflatable member substantially collapses when deflated.

Preferably, the coupling member is coupled to a structure required to be moved in water, in use.

Typically the coupling member is a tubular member and the inflatable members may be coupled along the length of the tubular member.

Typically, the tubular member is substantially horizontal in use, when coupled to the structure required to be lifted, and after the inflatable members have been inflated.

Typically, the tubular member is coupled to the structure when the inflatable members are deflated. Preferably, the tubular member is coupled to the structure in an initially horizontal plane.

Alternatively, the tubular member is initially coupled to the structure such that longitudinal axis of the tubular member is approximately 45° to the horizontal plane.

Preferably, the inflatable member comprises an outer skin of substantially flexible material, the outer skin defining an inner space, the outer skin comprising a

body section, and an end section being sealably coupled to both ends of the body section. Preferably, the body and end sections comprise base edges by means of which the inflatable member is coupled to the tubular member. The base edges of the inflatable member may be spaced apart, and preferably, an inflation means inlet and a deflation means outlet are located between the spaced apart base edges .

Preferably, coupling devices are provided to couple the base edges of the inflatable members to the tubular member, and more preferably, a coupling device couples one side member of the body section of a first inflatable member in a back to back relationship with a side member of the body section of a second inflatable member. An inflatable member may have a cross-section which is substantially U-shaped, in use, when inflated.

This provides the invention with the advantage that the spaced apart base edges couple the inflatable member to the tubular member, and also provide access to the inner space from the tubular member to inflate or deflate the inflatable member. Thus, the outer skin of the inflatable member does not require to be pierced in order to provide access to the inner space.

Alternatively, the base edges of the inflatable member may be conjoined, and the inflatable members may be substantially wedge-shaped, in use, when inflated. The inflatable members may be movably coupled in a circumferential direction to the tubular member.

Preferably, the buoyancy device further comprises a pressure sensor to sense the pressure in the surrounding water, and may further comprise a displacement sensor to measure the displacement of the

buoyancy device, and may further comprise an acceleration sensor to measure the acceleration of the buoyancy device.

Preferably, there is provided a pressure sensor to sense the pressure within each inflatable member.

Typically, there is provided at least one valve to allow regulation of the pressure within an inflatable member. Preferably, there is at least one inflation valve to allow the pressure of air within each inflatable member to be increased and preferably, there is at least one deflation valve to allow the pressure of air within each inflatable member to be decreased.

Preferably, should one or more of the inflatable members deflate, the pressure within the remaining inflatable members may be increased to compensate for the deflated members. Preferably, the inflatable members are restrained from over-inflation by a restraining device.

Preferably, the buoyancy device further comprises a control system to allow variation of its buoyancy. More preferably, a number of inflatable members are provided with a control system to allow variation of the buoyancy of the inflatable members.

Preferably, the control system is connected to, and reads signals from, the surrounding water pressure sensor, the inflatable member pressure sensor, the displacement sensor and the acceleration sensor. More preferably, the control system varies the buoyancy of the inflatable member in response to the signals read.

Typically, when the pressure within the remaining

inflatable members is increased, the remaining inflatable members increase in size to occupy the space left by the deflated member(s).

One or more of the inflatable members may be inflated by air. Alternatively, one or more of the inflatable members are inflated with an incompressible material having a density less than that of the surrounding water. Typically, the inflatable members are inflatable bags.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which;-

Fig. 1 is a perspective view of an uninflated lifting device attached in a first arrangement to a drilling structure; Fig. 2 is a perspective view of the lifting device of Fig. 1 after inflation; Fig. 3 is a side view of the lifting device of Fig. 1; Figs. 4 (a), (b) and (c) are transverse cross sections of the lifting device of Fig. 1; Fig. 5 is a longitudinal cross section of one of the inflatable bags of the lifting device of Fig. 1; Fig. 6 is a transverse cross section of the inflatable bag of Fig. 5; Fig. 7 is a perspective view of a number of uninflated lifting devices attached in a second arrangement to a drilling structure; Fig. 8 is a perspective view of the lifting devices of Fig. 7 after inflation; Fig. 9 is a perspective view of the drilling structure of Fig. 7 coupled to a tug boat;

Fig. 10 is a perspective view of the drilling structure of Fig. 7 being towed by a tug boat; Fig. 11 is a cross-sectional view of a side base edge coupling device for a second embodiment of a buoyancy device in accordance with the invention; Fig. 12 is a cross-sectional view of an end base edge coupling device for the buoyancy device of Fig. 11; Fig. 13 shows a diagrammatical side view of the buoyancy device of Fig. 11; Fig. 14 shows a schematic diagram of a buoyancy control system for controlling the buoyancy of the buoyancy device of Fig. 11; Fig. 15 shows a schematic diagram of a deflation system for the buoyancy device of Fig. 11; and Fig. 16 shows a cross-sectional side view of a venturi fluid flow device shown in the schematic diagram of Fig. 15.

Fig. 1 shows a first embodiment of a buoyancy device 1 attached in a horizontal position to an offshore drilling structure 3 requiring to be lifted up off the ocean floor and moved to a remote location. The buoyancy device of Fig. 1 is primarily a lifting device 1, such that when the legs 5 of the drilling structure 3 are cut and the lifting device 1 is inflated, the lifting device 1 raises the drilling structure 3 towards the water surface, as shown in Fig.2.

Figs. 3, 4(a), 4(b) and 4(c) show the lifting device 1 in more detail. A tubular member 7 is located at the centre of the lifting device 1. Attached around the circumference of the tubular member 7 are individual inflatable bags 9 which run the length of the tubular member 7. The inflatable bags 9 are restrained on their outside surface by webbing straps 11 which strive

to keep the inflatable bags 9 in the preferred wedge shape as shown in Figs. 4(a), 4(b) and 4(c), as the inflatable bags 9 may naturally attempt to obtain a more rounded and less efficient shape.

Fixed at either or both ends of the tubular member 7 are towing points (not shown) to which one end of a towing cable 30 can be coupled. The other end of the towing cable 30 is coupled to a tug 32, therefore allowing the drilling structure 3 to be towed, as can be seen in Figs 9 and 10.

The inflatable bags 9 each comprise a middle section 8 and an end section 10, which are sealably coupled to the middle section 8, with an outer skin of the inflatable bags 9 defining an inner inflatable space. The longitudinal distance D of each end section 10 is four meters.

At either or both ends of the lifting device 1, is a cone 13, in which is provided an air pressure sensor system (not shown) of a suitable type known from the prior art.

Alternatively, the air pressure sensor system is located on board the tug 32.

The air pressure sensor system is connected to each inflatable bag 9, and in the embodiment shown in Figs. 4(a), 4(b) and 4(c) there are nine inflatable bags 9.

Also located in the cone 13 is a manifold device (not shown) through which the individual bags 9 are inflated. The manifold is further connected to an air supply (not shown) on the tug boat 32 via an umbilical line (not shown) .

Initially, the inflatable bags 9 are inflated via the manifold to the required pressure. If the pressure in one of the inflatable bags 9 drops, then the air pressure sensor system will inform an operator of the system that the pressure has dropped. If required, the air pressure can be increased via the manifold into the required inflatable bag 9.

If one of the inflatable bags 9 develops a leak such that the required air pressure cannot be maintained, as shown in Fig. 4(b), then the air supply via the manifold can be halted at the manifold.

As shown in Fig. 4(c), the remaining inflatable bags 9 expand by such an amount as to compensate for the failed bag. It is possible that extra air could be introduced into the remaining inflatable bags 9 to aid the compensation of the deflated bag.

For the embodiment with nine inflatable bags 9, the bags 9 are attached to the tubular member 7 every 40°, where each individual inflatable bag 9 is capable of increasing in width by 10°. Therefore, if one inflatable bag 9, as shown in Fig. 4(b) and 4(c) were to become deflated and therefore inoperative then the remaining eight inflatable bags 9 would increase in width by 5°. As each inflatable bag 9 is capable of increasing in width by 10", it is possible that two inflatable bags 9 could be deflated and that the seven remaining inflatable bags 9 would compensate.

Fig. 5 shows a cross-section along the length of an individual inflatable bag 9. There are attachment points 15 spaced along the length of the base of the inflatable bag 9 for attachment to the tubular member 7. The webbing straps 11 can also be seen which are

spaced along the length of the outer surface of the inflatable bag 9.

Fig. 6 shows a cross-section across the breadth of the inflatable bag 9. The attachment point 15 is shown as a rivet 25 passing through a reinforced weld 27, at 1 meter intervals. The two sides 17, 19 of the inflatable bag 9 are formed from a medium weight fabric, and the outermost section 21 is formed from a heavy fabric for improved strength. The outermost section 21 is connected to the two sides 17, 19 by welds 23, which are offset to avoid chafing between adjoining inflatable bags 9. The radial distance A from the reinforced weld 27 to the innermost weld 23 is approximately 2 meters, and the radial distance B plus C from the innermost weld 23 to the outer circumference of the inflatable bag 9 is approximately 1.5 meters.

The overall length of the inflatable bags 9 is in the region of 27 metres, with attachment points 15 spaced at 1 metre intervals along the base of the inflatable bags 9. There are ten webbing straps 11 in all, which are 2 metres in length and are spaced at 2 metre intervals along the outermost section 21 of the inflatable bag 9. The radius of the inflatable bag structure is in the region of 3.5 metres, giving an inflated volume of the lifting device 1 in the region of 1000m ³ . This provides a total lift in the region of 1000 tonne per lifting device 1.

However, the attachment points 15 may be provided by rings (not shown) spaced along the length of the tubular member 7 which engage with correspondingly sized holes (not shown) located on the inflatable bags

The tubular member 7 may be constructed from a material having suitable strength and weight characteristics and may be constructed from steel. Alternatively, the tubular member 7 may be constructed from a suitably reinforced plastic material. If required, an inflatable bag (not shown) would be inserted and inflated within the tubular member 7 to aid the buoyancy of the tubular member 7.

The lifting device 1 may be connected to the drilling structure 3 by connection devices (not shown) located at each end of the tubular member 7. In addition, or alternatively, padeyes (not shown) may project out from the tubular member 7 for connection to the drilling structure 3.

Fig. 7 shows a second arrangement for lifting a drilling structure 3 off the ocean floor, and subsequently towing the drilling structure 3 to a remote location. A number of lifting devices 1, as previously described are attached to the drilling structure 3 at an angle approximately 45° to the horizontal plane of the ocean surface.

The lifting devices are inflated, and lift the drilling structure 3 off the ocean floor, such that the drilling structure 3 is lifting into a tilted towing position, as shown in Fig. 8, the angle of tilt being approximately 45° to the horizontal plane of the ocean surface. The tilted towing position provides a more stable towing position, and provides a greater depth clearance for the bottom of the drilling structure 3.

As shown in Fig. 9, a towing cable 30 is attached at one end to the drilling structure 3, and at the other end to a tug boat 32. Fig. 10 shows the tug boat 32

towing the drilling structure 3 towards land. A second embodiment of buoyancy device 2 is shown in Figs. 11-16. The buoyancy device 2 of the second embodiment is similar to the lifting device 1 of the first embodiment, in that there are nine inflatable bags 9A, 9B (not all shown) attached around the circumference of a tubular member 7. The inflatable bags 9A, 9B are again restrained on their outside surface by a similar webbing strap arrangement.

However, the inflatable bags 9A, 9B are attached to the tubular member 7 in an arrangement that provide them with a cross-sectional shape having parallel side members 12 which are sealed at their top by a curved roof portion (not shown). The two parallel side members 12 of an inflatable bag 9A or 9B are coupled to the tubular member 7 in a spaced apart relationship, and provides the inflatable bag 9A, 9B with a cross- section which is substantially U-shaped. The two parallel side members 12, and the roof portion form a body section 12. This provides the advantage that access to the inner space of the inflatable bag 9A or 9B, as defined by the outer skin of the inflatable bag 9A, 9B, can be gained through the side wall of the tubular member 7.

A coupling device 14 for coupling the parallel side members 12 to the tubular member 7 is shown in Fig. 11. The coupling device 14 comprises an inflatable member base edge securing rail 40 which is welded to the tubular member 7 by welding 46. The securing rail 40 may be formed from a suitable metallic material such as steel or aluminium, and its lower face 47 is curved to correspond to the curvature of the tubular member 7. The securing rail 40 has a T-shaped recess 48 running along its entire length. The base 50 of the parallel

side member 12 is formed by folding the edge 51 of the parallel side member 12 around a rope filler 42, and welding the flap 51 to the parallel side member 12. The flap 51 is welded to the parallel side member 12 by high frequency ultrasonic welding 52, and the rope filler is formed from a suitable material, which is typically plastic. An example of the inflatable bag 9A, 9B fabric is PVC coated woven polyester fabric.

The base 50 is thus formed to have a shoulder 53 which co-operates with one side of the T-shaped recess 48.

The base 50 of one of the inflatable bags 9A is inserted into one end of the coupling device 14, and pulled along the entire length of the T-shaped recess 48, so that the entire length of the base 50 is located within the T-shaped recess 48. Then, the base 50 of the other inflatable bag 9B is inserted into one end of the coupling device 14 and also run along the entire length of the T-shaped recess 48 so that the entire length of the base 50 of the inflatable bag 9B is located within the T-shaped recess 48. Thus, with the two parallel side members 12 of the inflatable bags 9A and 9B in a back-to-back relationship, the bases 50 are retained within the T-shaped recess 48 by their respective shoulders 53.

Alternatively, both bases 50 of the inflatable bags 9A and 9B may be run into the T-shaped recess 48 at the same time.

Each inflatable member 9A, 9B also has two parallel end members 55 which are sealably coupled to the roof portion and the two parallel side members 12, and form end sections 55 which seal the ends of the inflatable members 9A, 9B. The two parallel end members 55 have a

base 50, which is similar in construction to the base 50 of the parallel side members 12. The base 50 of one of the parallel end members 55 is shown in Fig. 12, and is secured to the tubular member 7 by a second coupling device 16. The second coupling device 16 has an L- shaped recess 57 into which the base 50 of the parallel end member 55 is secured. However, in order to locate the base 50 into the L-shaped recess 57, a portion 16A of the second coupling device 16 is removed from the coupling device 16. The base 50 can then be inserted into the L-shaped recess 57, and when properly located, the removable portion 16A is then re-attached by means of a retaining bolt 18.

In order to provide a further sealing capability to the inflatable bags 9A, 9B when the pressure within the inflatable bags 9A, 9B is increased the flap 51 will naturally fit around the curved upper face 54 of the first and second coupling devices 14, 16.

Figs. 13 and 14 show the control system for providing a controlled variable buoyancy to the second buoyancy device 2. Located within the tubular member 7 is an air reservoir 61 which can either be self-contained or can also be connected to a surface air reservoir (not shown) via an umbilical air supply (not shown) by conventional means which are well known in the prior art. Also mounted within the tubular member 7 are pressure 62, displacement 63 and acceleration 64 transducers which together form a transducer array. The pressure transducer 62 typically comprises a diaphragm (not shown) which has a strain gauge (not shown) attached thereto, one side of the diaphragm having a sealed known pressure acting on that side of the diaphragm, and the other side of the diaphragm being open to the ambient pressure of the outside

water. An example of a suitable pressure transducer is a DIGIQUARTZ( ^IM ) pressure transducer 62. An example of a suitable displacement transducer is a SIMRAD(™) acoustic tracking system. An example of a suitable acceleration transducer 64 is well known in the art as an accelerometer. Also located within the tubular member 7 is an acoustic transponder 65 which allows a computer control system 67 mounted on a surface ship to communicate with the control system located within the tubular member 7. An example of a suitable acoustic transponder 65 is an acoustic telemetry system such as a SIMRAD HPR 4000 (™) system. Alternatively, the computer control system 67 can communicate with the control system mounted within the tubular member 7 by means of a hardwire electrical cable (not shown) being connected between the tubular member 7 and the surface ship, whereby the signals to be communicated are multiplexed across the electrical cable, by conventional means well known in the art.

Alternatively, a computer control system 67 may be mounted within the tubular member 7.

The computer control system 67 allows the movement plan of the buoyancy device 2 to be pre-programmed, such that signals from the transducer array 62, 63, 64 are transmitted to the computer control system 67 which monitors the movement of the buoyancy device 2 and can send signals back to the control system to vary the buoyancy of the buoyancy device 2 as necessary.

Power is supplied to the buoyancy device 2 via a power unit 68 which is either located within the tubular member 7 in the form of a battery unit, or is located on a surface ship, and in the latter case the power is supplied from the power unit 68 to the tubular member 7

via an umbilical electrical cable (not shown) .

Each of the inflatable bags 9 has an airflow inlet (not shown) and an airflow outlet (not shown) mounted within the side wall of the tubular member 7 at a location that allows access to the inflatable bags 9A, 9B between the spaced apart parallel side members 12 and the parallel end members 55. Air is supplied into each inflatable bag 9A, 9B by two discrete mechanisms from the air reservoir 61. The first mechanism is an automatic regulation 77 of the inflatable bag 9 through a pressure relief valve mechanism (not shown) which regulates the flow of air supplied from the air reservoir 61, since the air reservoir 61 will be at a relatively high pressure with respect to the inflatable bag 9A, 9B. Also, by using this automatic regulation mechanism 77, a constant flow through of air into the inflatable bag 9A, 9B is maintained in order to compensate for air leakage from the inflatable bags 9A, 9B due to imperfections in the control system and the inflatable bag 9A, 9B structure. Secondly, there is an applied regulation mechanism 78 which operates by means of a control valve system (not shown) which regulates the pressure in each individual bag 9 in accordance with the calculated movement plan held within the computer control system 67. The pressure relief valve mechanism, and the control valve system, which together form an inflatable bag 9A, 9B inflation system 74, are connected in parallel between the air reservoir and the inflatable bag 9 air inlet by appropriate air supply conduits (not shown).

The air flow outlet of the inflatable bag 9 is connected to a second pressure relief valve mechanism (not shown) to provide an automatic venting mechanism 79 of the inflatable bag 9A, 9B if, in particular when

the buoyancy device 2, attached to the structure to be lifted, is raised through the water. This is required because as the buoyancy device 2 is raised, the surrounding ambient water pressure will reduce, but the air pressure within the inflatable bags 9A, 9B will remain the same. Therefore, this automatic venting mechanism 77 allows the buoyancy device 2 to be raised slowly without damage to the inflatable bags 9A, 9B. However, if the automatic venting mechanism 79 through the second pressure relief valve mechanism is not sufficient, then a second control valve system connected to the air outlet of the inflatable bag 9A, 9B provides an applied venting mechanism 80 to vent a greater amount of air. The second pressure relief valve mechanism and the second control valve system together form an inflatable bag 9A, 9B deflation system 75.

Each inflatable bag 9A, 9B is provided with an individual control system such that the distribution of air flow input and output from the inflatable bags 9A, 9B is controlled individually such that each inflatable bag 9A, 9B is a discreet subsystem of the overall buoyancy device 2.

The control system for the inflatable bag 9A, 9B is a closed loop feedback system, in which the pressure 62, displacement 63 and acceleration 64 transducers continually measure the pressure being applied to, and the speed and acceleration of the buoyancy device 2. Also me irec is the pressure within each inflatable bag 9A, .3 by means of a presure sensor (not shown) located within each bag 9A, 9B. These measured quantities are then compared to a pre-determined movement plan held within the computer control system 67 and corrections to the actual movement path of the

buoyancy device 2 can then be made by controlled operation of the first and second control valve systems .

Fig. 14 schematically shows the automatic 77 and applied 78 regulation mechanisms, and the automatic 79 and applied 80 venting mechanisms controlling the air flow into the air flow inlet and being vented from the air flow outlet.

In addition to the abovementioned buoyancy control mechanism, additional buoyancy control measures can be used. For example, a "bursting disc" may be incorporated into the outer skin of the inflatable bag 9A, 9B, the bursting disc comprising a metallic disc which will burst when the differential pressure across the metallic disc face reaches a pre-determined level. An example of a bursting disc is a SWAGELOCK( ^IM ) bursting disc. Also, by attaching a balance chain, which is well known in the art, the balanced equilibrium of the buoyancy device 2 will be reached at a certain ascent height. Also, a venturi suction system for rapid inflatable bag 9A, 9B, venting could also be utilised in the buoyancy device 2 and such a system is shown in Figs. 15 and 16.

In Figs. 15 and 16, a second air flow outlet 85 is provided from the inflatable bag 9A, 9B which leads by a conduit (not shown) to a tapping 86 in the throat restriction 87 of a venturi device 88. The venturi inlet 89 is connected by a conduit 90 to a relatively high pressure air reservoir 91, which may be for instance the air reservoir 61 mounted within the tubular member 7 or mounted at the sea surface. The venturi outlet 92 is connected by another conduit 93 to a relatively low pressure reservoir 94 which may be the

ambient pressure of the surrounding water. Therefore, if rapid inflatable bag 9A, 9B venting is required, air is pumped from the relatively high pressure reservoir 91 through the venturi device 88 and into the relatively low pressure reservoir 94, thereby creating a vacuum in the inflatable bag 9A, 9B. The use of the venturi device 88 is initiated according to the control system instructions.

In order to reduce the number of components in the buoyancy device 2, distinct types of inflatable bags 9 may be provided. The buoyancy device 2 may be provided with a combination of the following distinct types of inflatable bags 9. "Dumb" inflatable bags are provided with the abovementioned automatic regulation mechanism 77 and the abovementioned applied regulation mechanism 78, but are only provided with the abovementioned automatic venting mechanism 79, which obviates the requirement for a relatively expensive applied venting mechanism 80. Secondly, "intelligent" inflatable bags 9A, 9B have the abovementioned automatic 77 and applied 78 regulation mechanisms and the automatic 79 and applied 80 venting mechanisms. This provides the advantage that a number of "dumb" inflatable bags can be provided in combination with a number of "intelligent" inflatable bags 9A, 9B, thereby obviating the expense of a number of applied venting mechanisms 80. Further, a number of contingency redundant inflatable bags can be provided which have the characteristics of the "intelligent" inflatable bags but are normally redundant, these inflatable bags only operating in the event of compromise to the other inflatable bags 9A, 9B.

Further, a number of the inflatable bags 9 may be replaced with bags (not shown) which are filled with an

incompressible buoyancy material. Examples of such materials are alumina silicate microspheres (a bi- product of the coal fired power generation industry) which contain C0 ₂ gas, bitumen, oil based fluids, fresh water, and other incompressible substances whether fluid or solid which have a density lower than salt sea water which surrounds the buoyancy device. The advantage of providing some of these bags would arise particularly in deploying payloads in deep water. The buoyancy device 2 comprising a number of these bags would be attached to the payload required to be lowered in deep water, such that the payload and buoyancy device 2 combined have a slightly negative buoyancy with respect to the surrounding sea water. Therefore, for a large payload such as a well head Christmas tree, a relatively small crane can be used to deploy the payload.

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