GB2427173A

GB2427173A - Buoyant fluid for use in controlling the buoyancy of a structure

Info

Publication number: GB2427173A
Application number: GB0611868A
Authority: GB
Inventors: Patrick Joseph Collins
Original assignee: Ythan Environmental Services Ltd
Current assignee: Ythan Environmental Services Ltd
Priority date: 2005-06-15
Filing date: 2006-06-15
Publication date: 2006-12-20
Anticipated expiration: 2026-06-15
Also published as: GB2427173A8; GB0611868D0; GB2427173B; GB0512154D0; GB2427173B8

Abstract

A buoyant fluid comprises a liquid and a plurality of rigid containers, the rigid containers each having a sealed void containing a gas. A particular use for the fluid is in transporting heavy objects subsea. The gas in the rigid containers provides buoyancy but does not compress at different subsea pressures. Therefore effective buoyancy control subsea is much easier compared to known methods. For certain embodiments, a secured supply container 11 may be provided subsea to supply the liquid and rigid containers to a lifting device 1 having a container therefor and an attachment mechanism 6 secured to the object 8 being transported. The liquid is preferably a biodegradable substance such as vegetable oil.

Description

1 "Method and Apparatus" * ** * * * 2 **..

* 3 This invention relates to a fluid, method and *0*S 4 apparatus for providing buoyancy, particularly f or moving heavy objects underwater. S.. * 6

* 7 Buoyancy techniques are well known and frequently *:*. 8 applied for the movement or retrieval of structures 9 underwater. In general, they comprise of a container or bag that is attached to the structure 11 that needs to be moved together with a gas which is 12 used to fill or partially fill the container 13 exerting a buoyant force on the structure allowing 14 it to be lifted.

16 While this approach is effective in shallow water, 17 it becomes problematic in deeper water. This is 18 because gas being compressible will require to be 19 applied at a pressure exceeding that of hydrostatic pressure in order to provide buoyancy. Furthermore, 21 on moving towards the surface the gas will expand 22 rapidly increasing buoyancy and the rate which the 1 container, together with its tethered structure, 2 rises to the surface accelerates and becomes 3 uncontrollable.

An alternative approach involves the construction of 6 rigid buoyancy elements using syntactic materials 7 which are weighted. These are affixed to the 8 structure and the weights removed by, for instance, 9 a remote operating vehicle from the buoyancy elements. This approach has the disadvantage that 11 once released of their weights, the buoyancy ::. 12 elements exert a sudden upward force which can be . 13 difficult to control and could cause damage to S...

subsea equipment, such as ROVs, and personnel.

16 To tackle this problem, the weight of the structure 17 to be lifted is determined and complex calculations

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18 performed so that a suitable amount of buoyancy is 19 provided. However sometimes it is difficult to know the weight of the structure to be lifted and it has 21 been known for calculations to be incorrect, 22 resulting in the dangerous sudden upward movement of 23 the buoyancy elements and attached structure.

Moreover, such buoyancy elements must be returned to 26 the surface when structures of different weights 27 need to be lifted.

29 According to a first aspect of the present invention, there is provided a buoyant fluid 31 comprising a liquid and a plurality of rigid 1 containers, the rigid containers each having a 2 sealed void containing a gas.

4 Preferably the buoyant fluid has a specific gravity of less than 0. 78g/cm3, more preferably less than 6 O.70g/cm3, even more preferably less than O.65g/cm3, 7 especially less than below O.60g/cm3 and more 8 especially less than O.56g/cm3.

The rigid containers may be between 5 microns and 11 5mm in diameter, preferably between 10 microns and ::::. 12 500 micron in diameter and more preferably between 13 20 micron and 200 micron in diameter.

S S* S "Rigid" in this context means that the rigid 16 containers are incompressible at the pressures found 17 in underwater environments. I...

19 Preferably, the rigid containers are microspheres.

21 The buoyant fluid may comprise a hydrocarbon 22 (preferably low toxicity) such as an aliphatic oil, 23 poly alpha olef in, alkyl ester or vegetable oil that 24 is a triglyceride such as one having the structure: y c) H C () () IF(. ' oc R. H,, 0 26 Trig'yceride 1 where R1, R2, and R3 are hydrocarbon chains 2 typically with a chain length of between C12 and C22 3 to give a range of fatty acids and between zero to 4 three double bonds in the hydrocarbon chain length.

Most typically such materials are derived from 6 nature as vegetable oils although synthetic 7 alternatives maybe made.

9 Thus for certain embodiments of the invention, the inherent environmental risk that some liquid therein 11 may leak is not a significant concern because ::::. 12 biodegradable oils may be used, such as vegetable 13 oil, which would not be a concern to wildlife in the S...

:. 14 unlikely event of a leak.

* S.. 15 16 The liquid may also comprise a viscosifying agent * 17 such as organophilic clay, dispersed silica, long S...

*:*. 18 chain polymeric materials, surfactants or mixtures 19 of the aforesaid agents.

21 Preferably the buoyant fluid exhibits viscoelastic 22 and or rheological properties.

24 At a low shear rate of 0.5rpm, the viscosity, as

measured on a Brookfield type viscometer, of the

26 buoyant fluid can optionally be between 10,000 and 27 100,000 centipoise, preferably between 20,000 and 28 100,000 centipose, more preferably between 40,000 29 and 80,000 centipose.

31 At a high shear rate of 30rpm, optionally the

32 viscosity as measured on a Brookfield type

1 viscometer, of the buoyant fluid can be between 500 2 and 10,000 centipose, preferably between 1,000 and 3 5,000 centipose, more preferably between 2,000 and 4 3,000 centipose.

6 Preferably, the buoyant fluid is an incompressible 7 fluid.

9 optionally the buoyant fluid may be used to displace water in subsea structures thereby generating a 11 buoyant force.

* * 12 * S 13 The buoyant fluid can be pumped into vessels, 14 structures, or bags rendering them buoyant or partially buoyant. This can be done prior to * 16 installation of subsea components, during 17 installation of subsea structures or as part of a ::: : 18 process of recovery of subsea structures.

According to a second aspect of the present 21 invention, there is provided a method of controlling 22 the buoyancy of a structure, the method comprising, 23 in any order: 24 (a) injecting or removing a buoyant fluid into or from a first container, said first container 26 connected to or integral with said structure; 27 (b) immersing the container in an immersion 28 fluid; 29 said buoyant fluid comprising a liquid and said buoyant fluid having a density which is less than 31 the density of the immersion fluid.

1 Preferably the buoyant fluid comprises a plurality 2 of rigid containers, the rigid containers each 3 having a sealed void containing a gas.

Thus preferably the buoyant fluid according to the 6 second aspect of the invention is the buoyant fluid 7 according to the first aspect of the invention.

9 According to a third aspect of the invention, there is provided a method of controlling the buoyancy of 11 a structure, the method comprising, in any order: 12 (a) injecting or removing a buoyant fluid into *I, I 13 or from a first container, said first container I...

14 connected to or integral with said structure; (b) immersing the container in an immersion 16 fluid; 17 said buoyant fluid comprising a liquid and a : 18 plurality of rigid containers, the rigid containers 19 each having a sealed void containing a gas; and said buoyant fluid having a density which 21 is less than the density of the immersion fluid.

23 Typically the immersion fluid is water, especially 24 sea water.

26 The buoyant fluid may also be added to or removed 27 from the first container before it is immersed in 28 the immersion fluid.

Preferably the buoyant fluid substantially comprises 31 liquid, as well as any rigid containers.

1 The gas in each rigid container may be air, 2 nitrogen, argon or another gas sufficient to achieve 3 a low bulk density.

Preferably, the buoyant fluid is an incompressible 6 fluid.

8 An advantage of embodiments of the present invention 9 is that the incompressible fluid does not undergo a volume change when the depth and therefore the 11 pressure of the first container is varied.

* 12 consequently, the first container of embodiments of a 13 the present invention will not accelerate as its * I as..

14 depth varies and so greater control of the structure is afforded.

17 According to a fourth aspect of the present : 18 invention, there is provided an apparatus to control * 19 the buoyancy of a structure, the apparatus comprising: 21 a first container having a first void suitable 22 for receiving a buoyant fluid, said first container 23 connectable to, or integral with, said structure; 24 an aperture in the first container, adapted to allow injection and removal of said buoyant fluid 26 into and out of the first container.

28 According to a fifth aspect of the invention, there 29 is provided an apparatus to control the buoyancy of a structure, the apparatus comprising: 1 a first container having a first void suitable 2 for receiving a buoyant fluid, said first container 3 connectable to, or integral with, said structure; 4 an aperture in the first container, adapted to allow injection and removal of said buoyant fluid 6 into and out of the first container; 7 wherein said buoyant fluid comprises a liquid and a 8 plurality of rigid containers, the rigid containers 9 each having a sealed void containing a gas.

11 Preferably, said first void is defined within a 12 bladder. Preferably, a second void is defined 13 between the bladder and the first container.

14 Preferably, a first valve is provided to communicate with the first void. Preferably the first valve is S..

* 16 arranged at said aperture to allow injection or 17 removal of the buoyant fluid into and out of the S...

18 first container.

Preferably, a second valve is provided to 21 communicate with the second void. The bladder is 22 preferably flexible so that the volume of the first 23 and second voids can vary although the sum of their 24 volumes typically remains constant.

26 The apparatus may comprise a supply container which, 27 in use, contains a buoyant fluid. Preferably the 28 buoyant fluid is the buoyant fluid described herein 29 with respect to earlier aspects of the invention 31 In use, the supply container is typically connected 32 to the first container via a line (preferably 1 flexible), the line suitable to transfer buoyant 2 fluid between the first container and the supply 3 container.

Preferably, the supply container comprises a first 6 void, defined within a bladder and a second void 7 defined between the bladder and the container.

9 Preferably, the supply container comprises a first valve to communicate with its first void and 11 preferably also a second valve to communicate with 12 its second void. S.. * S S..

:. 14 Preferably, the bladder is flexible so that the * 15 volume of the first and second void can vary, S..

* 16 although the sum of their volumes is typically * 17 constant. ***.

19 Alternatively, the first container may receive the buoyant fluid from a surface vessel, such as a ship 21 or oil rig, or any other suitable source.

23 Where utilised, preferably the supply container 24 comprises a stabilising means, such as weights, or a line, in order to maintain a generally constant 26 depth during use regardless of the amount of 27 incompressible fluid within the supply container at 28 any one time.

A portion of the buoyant fluid may be added to the 31 first container onshore and the container then 32 immersed in water.

2 Preferably, the apparatus comprises a pump to 3 transfer the buoyant fluid between the supply 4 container (or other source) and the first container.

6 Preferably all the valves are proportional valves 7 rather than on/off valves, especially the valves in 8 communication with the first voids. Thus accurate 9 control of the proportion of buoyant fluid present in the first container at any one time is provided.

12 To move the buoyant fluid between the first e..s 13 container and other source, preferably the pressure 14 in the container or source which is to reduce its : 15 buoyant fluid content is increased. a. * 16

17 To move the buoyant fluid from the supply container SSS* *:*. 18 to the first container, water may be injected into 19 the second void of the supply container to compress the bladder and increase the pressure in the supply 21 container, thus forcing the buoyant fluid out of the 22 first void of the supply container and into the 23 first void of the first container. Once sufficient 24 buoyant fluid has been transferred from the supply container to the first container, the structure will 26 become buoyant. It can then be moved and positioned 27 as required.

29 To remove the buoyancy of the structure, the buoyant fluid may be removed from the first container. To 31 remove the buoyant fluid from the first container, 32 water may be pumped into the second void of the 1 first container to compress the bladder of the first 2 container thus causing the buoyant fluid to move via 3 the line into the supply container, thus reducing 4 the buoyancy of the first container.

6 The invention also allows a structure to be filled 7 with buoyant fluid, attached to the first container 8 and the buoyant fluid gradually removed from the 9 first container in order to allow a controlled launch of the structure to the seabed or subsea : .". 11 installation. S... *SSS * * I...

13 Thus embodiments of the present invention provide 14 more control because the buoyant fluid can be added or removed from the container in situ, that is when 16 it is immersed in the water or other immersion S...

17 fluid. * 18

19 According to a sixth aspect of the present invention, there is provided a method of controlling 21 the buoyancy of a structure, the method comprising: 22 (a) immersing a first container in an immersion 23 fluid, said first container connected to or integral 24 with said structure; then, (b) injecting or removing a buoyancy fluid into 26 or from the first container; 27 said buoyancy fluid having a density which is 28 less than the density of the immersion fluid.

Preferably the sixth aspect of the present invention 31 is performed with the method, apparatus and buoyant 1 liquid according to earlier aspects of the 2 invention.

4 The buoyant fluid may consist of or comprise air, but preferably comprises liquid.

7 Any feature of any aspect of any invention or 8 embodiment described herein may be combined with any 9 feature of any aspect of any other invention or embodiment described herein mutatis mutandis. * S. * S S

S. S S 12 An embodiment of the present invention will now be 5..

13 described, by way of example only, with reference to 14 the accompanying figure, in which: Fig. 1 is a diagrammatic view of an apparatus * 16 in accordance with one aspect of the present 5..

: 17 invention; and, * 18 Fig. 2 is a diagram showing the viscosity 19 against shear rate for a buoyant fluid in accordance with one aspect of the present invention.

22 Fig. 1 shows an apparatus 20 comprising a buoyancy 23 device 1 and a supply unit 11. The apparatus 20 may 24 be used to move an object, such as an object 8, from one subsea location to another (or even to or from 26 the surface.) This can be useful for constructing 27 oil well assemblies, laying pipelines, recovering 28 submerged objects, or any other reason for moving 29 objects underwater.

31 The buoyancy device 1 is attached, via cables or 32 shackles 6, to the object 8 on sea bed 18, and via a 1 hollow umbilical line 3, to the supply unit 11.

2 Buoyant fluid can be transported between the 3 buoyancy device 1 and supply unit 11 via the 4 umbilical 3, as described further below.

6 The buoyancy device 1 comprises a rigid housing 4.

7 Inside the housing 4 is a bag or bladder 5 8 manufactured from a strong impermeable material such 9 as rubber, polypropylene or reinforced fabric or material. In use, the bag 5 contains a certain : .". 11 amount of buoyant fluid, described further below. A S.,.

12 space 7 is defined between the bag 5 and the inside S'S.

13 of the housing 4. The inside of the bag 5 is in 14 fluid communication with the umbilical 3, via a proportional valve 9. *...

. : 17 In alternative embodiments, the housing 4 may not be * 18 a rigid structure but may be a bag or bladder 19 manufactured from a strong impermeable material such as rubber, polypropylene or reinforced fabric or 21 material.

23 A further valve 2 is provided on the outside of the 24 housing 4 to allow water from outside the housing 1 to enter and exit the space 7 between the bag 5 and 26 the inside of the housing 4.

28 The supply unit 11 takes on a similar configuration: 29 a bag 15 is provided within a rigid housing 14 and the inside of the bag 15 is in fluid communication 31 with the umbilical 3 via a proportional valve 19. A 32 space 17 is defined between the bag 15 and the 1 inside of the housing 4. The supply unit 11 2 comprises a further valve 12 on the housing 14 to 3 allow water to enter and exit the space 17 between 4 the bag 15 and the inside of the housing 14.

6 The supply unit 11 also has weights 16 which cause 7 it to sink and rest on the seabed 18. Buoyant fluid 8 is stored in the bag 15, but regardless of the 9 amount of buoyant fluid, the supply unit 11 will remain on the seabed 18 during use. * * * * a I...

12 A pump (not shown) is attachable to the valves 2, 12 a..

13 in order to pump sea water from the surroundings 14 into the spaces 7, 17 between the bags 5, 15 and the housings 4, 14 respectively.

:1:: : 17 Inside the bags 5, 15 is the buoyant fluid 18 comprising oil, a viscosifying agent and 19 microspheres. The oil is preferably a low toxicity oil such as a vegetable oil. The viscosifying agent 21 may be organophilic clay for example. The addition 22 of the viscosifying agent gives the buoyant fluid 23 viscoelastic rheological properties. Since the fluid 24 is viscoelastic it can be pumped easily but when the fluid is at rest the increased viscosity keeps the 26 microspheres in place ensuring a consistent 27 material.

29 The viscosity of a sample was measured, as defined in ISO 2555, using a Haake ViscoTester 7L at 23 C 31 using an L3 spindle, Viscosity measurements are in 1 centipoise. The results are shown in table 1 below 2 and in Fig. 2.

viscosity 6 rpm cps 7 0.3 100560 8 0.5 55330 9 0.6 46045 1 29530 11 1.5 21360 12 2 16610 I. 13 2.5 13830 14 3 11800 4 9350 16 5 7820 6 6690 & (S 1 10 4580 19 12 4030 2825 2220

22 Table 1.

24 Thus the table and graph show that the mixture has viscoelastic properties, that is, at low shear rates 26 the mixture is very viscous. As the shear rate 27 increases, the viscosity decreases. This is an 28 important benefit of certain embodiments of the 29 invention because the high viscosity at low shear rates allows microspheres to be generally evenly 31 distributed within the body of the liquid, rather 32 than rise to the top where they could cause an 1 imbalance in the liquid. The lower viscosity at 2 higher shear rates facilitates the pumping of the 3 fluid into the buoyancy device 1 and supply unit 11 4 during set up.

6 The microspheres are small glass spheres with a 7 hollow centre containing air or another gas. Since 8 they contain air, they are relatively very buoyant 9 compared to any type of liquid. Since the air is trapped inside the glass microspheres, the :,:::. 11 microspheres and the buoyant fluid as a whole are I...

12 incompressible. The wall thickness of the :. 13 microspheres may be varied but must be sufficient to * 14 withstand the hydrostatic pressure experienced in S..

the depth of water or other liquid in which the 16 apparatus 20 will operate.

IS * S S * S.

18 The microspheres significantly contribute to the 19 buoyancy of the buoyant fluid within the bags 5, 15.

The microspheres are held within the buoyant fluid 21 as a direct consequence of the fluid's viscosity.

22 Thus the individual microspheres will not have 23 sufficient buoyancy to move to the top of the 24 (viscous) buoyant fluid but rather, they will remain in the body of the fluid. This allows the 26 microspheres to mix with the buoyant fluid properly, 27 rather than gather at the surface of the buoyant 28 fluid. This in turn provides a more even balance to 29 the buoyancy of the buoyancy device 1.

31 Suitable microspheres may be obtained from 3M 32 corporation based in St. Paul Minnesota USA.

2 For certain embodiments of the invention, the 3 microspheres can act to viscosify the fluid and so 4 the addition of a further viscosifying agents is not necessary. In one example, a buoyant fluid was 6 prepared in the following manner: 60g of vegetable 7 oil were placed in a beaker to which was added 40g 8 of S38 glass microspheres from 3M corporation and 9 the mixture was stirred gently to form a fluid viscous mixture with the appearance and consistency 11 of thick cream. To this mixture was added between a...

12 0.5 to 1.0 millilitre of water whereupon, :. 13 surprisingly, the fluid viscosified to form a fluid 14 which at low shear rates exhibits very high a *s viscosity whereas at higher shear rates the 16 viscosity is reduced and the mixture will flow such a..

17 fluids are described as being viscoelastic. At this 18 point the density of the material was measured and 19 determined to be 0.588 g/cm3.

21 The viscosity of a sample was measured, as defined 22 in Iso 2555, using a Haake ViscoTester 7L at 21.2 C. 23 Viscosity measurements are in milliPascal seconds.

24 The results are shown in table 2 below.

1 Spindle Viscosity 2 rpm (mPas) 3 1 L3 81,760 4 1.5 L3 51,270 2 L3 42,580 6 2.5 L3 32,030 3 L3 28,340 8 4 L3 12,030 L3 8,960 * 10 6 L3 8,250 *.:.. 11 10 L3 5,500 12 20 L4 5,330 :. 13 30 L4 4,420 * *;: 14 50 L4 3,880 L4 3,630 S 100 L4 3,390

*. : 17 Table 2. * **

19 Thus the table shows that the mixture has viscoelastic properties, that is, at low shear rates 21 the mixture is very viscous while as the shear rate 22 increases, the viscosity decreases.

24 Although inclusion of the microspheres is preferred, certain embodiments of the invention do not require 26 microspheres. Instead a buoyant fluid with a 27 density less than water may be used. The relatively 28 reduced density will provide buoyancy. Many buoyant 29 fluids may be used, including for example diesel or methanol.

1 Thus to operate the apparatus 20, the buoyancy 2 device 1 and supply unit 11 are lowered to the 3 vicinity of the object 8 to be moved. The buoyancy 4 device 1 is attached to the object 8 via the cables 6. A remotely operated vehicle (Roy) may be 6 utilised to attach the cables 6. The buoyancy 7 device 1 will be assumed to have sufficient buoyancy 8 at this stage to support itself, but if not its 9 buoyancy can be increased in the same way as that : 10 described below for raising the object 8.

12 To increase the buoyancy of the buoyancy device 1 :. 13 and attached object 8, the pump (not shown) is * 14 attached to the valve 12 of the supply unit 11 and is activated causing water to be gradually injected * 16 into the housing 14 of the supply unit 11 in the 17 space 17 between the bag 15 and the outside of the 18 housing 14 causing an increased pressure within the 19 supply unit 11. Valve 19 in the supply unit 11 and valve 9 in the buoyancy device 1 are opened to allow 21 the buoyant fluid, which is being forced out of the 22 bag 15 in the supply unit 14 by the increased 23 pressure, to travel through the umbilical 3 to the 24 bag 5 in the buoyancy device 1. The valve 2 in the buoyancy device 1 is also opened. Water in the 26 buoyancy device 1 in the space 7 between the bag 5 27 and the inside of the housing 4 can escape through 28 the opened valve 2.

The buoyancy of the buoyancy device 1 is thus 31 gradually increased by the gradual addition of 32 buoyant fluid until it is of a sufficient magnitude 1 to lift the object 8. The amount of lift or buoyancy 2 imparted is directly proportional to the volume of 3 buoyant fluid pumped into the buoyancy device 1.

Once the object 8 is raised from the seabed 18, the 6 pump attached to the valve 12 can be stopped and the 7 valves 9, 19 are closed to prevent further variation 8 of buoyancy of the buoyancy device 1. Valve 2 is 9 also closed.

:,:::. 11 Unlike certain known systems, the decrease in depth S...

12 of the buoyancy device 1 does not result in an :. 13 increased volume of air and therefore a further * : 14 increased buoyancy (which would cause upward *** acceleration of the device and attached object to * 16 the surface.) *.S. a. S * S * * S.

18 Also, the change in buoyancy of the buoyancy device 19 is gradual, rather than sudden as is the situation with a further known technique of removing weights 21 from a buoyancy device.

23 Thus embodiments of the invention are more 24 controllable and provide a safer means of raising immersed objects.

27 Referring back to the procedure for moving the 28 object 8, the ROV can then move the buoyancy device 29 1 and object to the appropriate place, relying on the buoyancy device 1 to provide the lift.

1 To remove the buoyancy from the buoyancy device 1, 2 the opposite procedure is followed. A pump is 3 attached to the valve 2 and pumps water into the 4 space 7 between the bag 5 and the inside of the housing 4. The valves 9, 19, as well as the valve 6 12 on the supply unit 11, are opened. The buoyant 7 fluid is thus forced by the increased pressure in 8 the buoyancy device through the umbilical 3. The 9 buoyant fluid proceeds to the bag 15 within the supply unit 11. Water in the supply unit 11 in the :,:::. 11 space 17 between the bag 15 and the inside of the S.., 12 housing 14 can escape through the opened valve 12.

14 The reduction in the amount of buoyant fluid within the buoyancy device 1 continues until it loses : 16 sufficient buoyancy and lowers the attached object 8 17 onto the seabed 18.

19 In alternative embodiments, there is no supply unit 11 and the buoyant fluid supplied to the buoyancy 21 device by a line extending to a surface vessel or

22 rig for example.

24 In an alternative use, the object could be removed from or placed onto another subsea object rather 26 than the seabed.

28 Thus the buoyant fluid can provide sufficient 29 buoyancy in a controlled manner to render a subsea element buoyant allowing it to be lifted by a remote 31 operating vehicle or submarine and manoeuvred into 32 the desired position or recovered to the surface 1 from a great depth. Once in place the buoyant fluid 2 can be removed allowing the subsea element to be 3 secured on the sea bed. This technique can also be 4 employed to lift items from the sea bed to the surface in a controlled manner.

7 Similarly, structures can be fabricated on shore 8 filled with buoyant fluid, towed out and placed on 9 the sea bed by pumping out the buoyant fluid such that the structure can be lowered into place. * .. * S S

*5** 11 SI..

12 An advantage of certain embodiments of the invention 13 is that since the mixtures are incompressible 14 fluids, buoyancy elements can be constructed of * 15 lightweight simple containers which can then filled * 16 with the buoyant fluid. II.. S. 17

S S

S SI

18 Improvements and modifications may be made without 19 departing from the scope of the invention.

Claims

1 Claims 3 1. A buoyant fluid comprising a liquid and a 4 plurality of

rigid containers, the rigid containers each having a sealed void containing a gas.

7 2. A fluid as claimed in claim 1, which is an 8 incompressible fluid.

3. A fluid as claimed in claim 1 or claim 2, which :.:::. 11 substantially consists of liquid and said rigid S...

12 containers. :. 13

14 4. A fluid as claimed in any preceding claim, S..

having a specific gravity of less than 0.60g/cm3.

17 5. A fluid as claimed in any preceding claim, 18 wherein the rigid containers are between 20 micron 19 and 200 micron in diameter.

21 6. A fluid as claimed in any preceding claim, 22 wherein the buoyant fluid exhibits viscoelastic 23 and/or rheological properties.

7. A fluid as claimed in any preceding claim, 26 wherein the buoyant fluid comprises hydrocarbons 27 such as vegetable oil.

29 8. A fluid as claimed in any preceding claim, wherein at a low shear rate of 0.5rpm, the viscosity 31 as measured on a Brookfield type viscometer, of the 32 buoyant fluid is between 40,000 and 100,000 1 centipose and at a high shear rate of 30rpm, the

2 viscosity as measured on a Brookfield type

3 viscometer, of the buoyant fluid is between 2,000 4 and 3,000 centipose.

6 9. A method of controlling the buoyancy of a 7 structure, the method comprising, in any order: 8 (a) injecting or removing a buoyant fluid into 9 or from a first container, said first container connected to or integral with said structure; :.:::. 11 (b) immersing the container in an immersion a...

12 fluid; 13 the buoyant fluid being the fluid as claimed in * 14 any preceding claim and further having a density * 15 which is less than the density of the immersion * 16 fluid. as..

18 10. A method as claimed in claim 9, wherein the 19 immersion fluid is water, especially sea water.

21 11. A method as claimed in claim 9 or claim 10, 22 wherein a supply container is provided comprising a 23 first void comprising said buoyant fluid, defined 24 within a bladder and a second void defined between the bladder and an outer housing of the supply 26 container, wherein the supply container is connected 27 to the first container via a line, the line suitable 28 to transfer buoyant fluid between the first 29 container and the supply container.

31 12. A method as claimed in claim 11, wherein 32 movement of the buoyant fluid from the supply 1 container to the first container is effected by 2 injection of a fluid into the second void of the 3 supply container to compress the bladder and 4 increase the pressure in the supply container, causing the buoyant fluid to move from the first 6 void of the supply container into the first void of 7 the first container.

9 13. An apparatus to control the buoyancy of a structure, the apparatus comprising: ::::. 11 a first container having a first void suitable II..

12 for receiving a buoyant fluid, said first container 13 connectable to, or integral with, said structure; 14 an aperture in the first container, adapted to allow injection and removal of said buoyant fluid 16 into and out of the first container; II..

17 wherein the buoyant fluid is the buoyant fluid as 18 claimed in any one of claims 1 to 8.

14. An apparatus as claimed in claim 13, wherein 21 said first void is defined within a bladder and a 22 second void is defined between the bladder and an 23 outer housing of the first container.

15. An apparatus as claimed in claim 13 or claim 26 14, wherein a first valve is provided to communicate 27 with the first void and is arranged at said aperture 28 and a second valve is provided to communicate with 29 the second void; each to allow injection or removal of the buoyant fluid into and out of the first 31 container.

1 16. An apparatus as claimed in any one of claims 14 2 to 15, wherein the bladder is flexible so that the 3 volume of the first and second voids is adapted to 4 vary although the sum of their volumes remains constant.

7 17. An apparatus as claimed in any one of claims 13 8 to 16, further comprising a supply container which, 9 in use, contains a buoyant fluid, the supply container comprising a first void, defined within a 11 bladder and a second void defined between the 12 bladder and an outer housing of the supply 13 container.

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