Method and apparatus

Abstract

The present invention concerns a buoyant fluid comprising a liquid and a plurality of rigid containers, the rigid containers each having a sealed void containing a gas. A particular use is to transport 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 may be provided subsea to supply the liquid and rigid containers to a lifting device having a container therefor and an attachment mechanism secured to the object being transported. The liquid is preferably a biodegradable substance such as vegetable oil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figure, in which:

FIG. 1 is a diagrammatic view of an apparatus in accordance with one aspect of the present invention; and,

FIG. 2 is a diagram showing the viscosity against shear rate for a buoyant fluid in accordance with one aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

The buoyancy device 1 is attached, via cables or shackles 6, to the object 8 on sea bed 18, and via a hollow umbilical line 3, to the supply unit 11. Buoyant fluid can be transported between the buoyancy device 1 and supply unit 11 via the umbilical 3, as described further below.

The buoyancy device 1 comprises a rigid housing 4. Inside the housing 4 is a bag or bladder 5 manufactured from a strong impermeable material such as rubber, polypropylene or reinforced fabric or material. In use, the bag 5 contains a certain amount of buoyant fluid, described further below. A space 7 is defined between the bag 5 and the inside of the housing 4. The inside of the bag 5 is in fluid communication with the umbilical 3, via a proportional valve 9.

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

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

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

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

A pump (not shown) is attachable to the valves 2, 12 in order to pump sea water from the surroundings into the spaces 7, 17 between the bags 5, 15 and the housings 4, 14 respectively.

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

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

TABLE 1
    viscosity
rpm cps
0.3 100560
0.5 55330
0.6 46045
1   29530
1.5 21360
2   16610
2.5 13830
3   11800
4   9350
5   7820
6   6690
10  4580
12  4030
20  2825
30  2220

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

The microspheres are small glass spheres with a hollow centre containing air or another gas. Since they contain air, they are relatively very buoyant compared to any type of liquid. Since the air is trapped inside the glass microspheres, the microspheres and the buoyant fluid as a whole are incompressible. The wall thickness of the microspheres may be varied but must be sufficient to withstand the hydrostatic pressure experienced in the depth of water or other liquid in which the apparatus 20 will operate.

The microspheres significantly contribute to the buoyancy of the buoyant fluid within the bags 5, 15. The microspheres are held within the buoyant fluid as a direct consequence of the fluid's viscosity. Thus the individual microspheres will not have sufficient buoyancy to move to the top of the (viscous) buoyant fluid but rather, they will remain in the body of the fluid. This allows the microspheres to mix with the buoyant fluid properly, rather than gather at the surface of the buoyant fluid. This in turn provides a more even balance to the buoyancy of the buoyancy device 1.

Suitable microspheres may be obtained from 3M corporation based in St. Paul, Minn. USA.

For certain embodiments of the invention, the microspheres can act to viscosity the fluid and so the addition of a further viscosifying agents is not necessary. In one example, a buoyant fluid was prepared in the following manner: 60 g of vegetable oil were placed in a beaker to which was added 40 g of S38 glass microspheres from 3M corporation and the mixture was stirred gently to form a fluid viscous mixture with the appearance and consistency of thick cream. To this mixture was added between 0.5 to 1.0 milliliter of water whereupon, surprisingly, the fluid viscosified to form a fluid which at low shear rates exhibits very high viscosity whereas at higher shear rates the viscosity is reduced and the mixture will flow such fluids are described as being viscoelastic. At this point the density of the material was measured and determined to be 0.588 g/cm3.

The viscosity of a sample was measured, as defined in ISO 2555, using a Haake ViscoTester 7L at 21.2 C. Viscosity measurements are in milliPascal seconds. The results are shown in table 2 below.

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

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

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

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

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

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

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

Unlike certain known systems, the decrease in depth of the buoyancy device 1 does not result in an increased volume of air and therefore a further increased buoyancy (which would cause upward acceleration of the device and attached object to the surface.)

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

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

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

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

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

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

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

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

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

An advantage of certain embodiments of the invention is that since the mixtures are incompressible fluids, buoyancy elements can be constructed of lightweight simple containers which can then filled with the buoyant fluid.

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

Claims

1. A buoyant fluid comprising a liquid and a plurality of rigid containers, the rigid containers each having a sealed void containing a gas.

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

3. A fluid as claimed in claim 1, which substantially consists of liquid and said rigid containers.

4. A fluid as claimed in claim 1, having a specific gravity of less than 0.60 g/cm3.

5. A fluid as claimed in claim 1, wherein the rigid containers are between 20 micron and 200 micron in diameter.

6. A fluid as claimed in claim 1, wherein the buoyant fluid exhibits viscoelastic and/or rheological properties.

7. A fluid as claimed in claim 1, wherein the buoyant fluid comprises hydrocarbons such as vegetable oil.

8. A fluid as claimed in claim 1, wherein at a low shear rate of 0.5 rpm, the viscosity as measured on a Brookfield type viscometer, of the buoyant fluid is between 40,000 and 100,000 centipose and at a high shear rate of 30 rpm, the viscosity as measured on a Brookfield type viscometer of the buoyant fluid is between 2,000 and 3,000 centipose.

9. A method of controlling the buoyancy of a structure, the method comprising, in any order: (a) injecting or removing a buoyant fluid into or from a first container, said first container connected to or integral with said structure;(b) immersing the container in an immersion fluid;the buoyant fluid being the fluid as claimed in claim 1 and further having a density which is less than the density of the immersion fluid.

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

11. A method as claimed in claim 9, wherein a supply container is provided comprising a first void comprising said buoyant fluid, defined within a bladder and a second void defined between the bladder and an outer housing of the supply container, wherein the supply container is connected to the first container via a line, the line suitable to transfer buoyant fluid between the first container and the supply container.

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

13. An apparatus to control the buoyancy of a structure, the apparatus comprising: a first container having a first void suitable for receiving a buoyant fluid, said first container connectable to, or integral with, said structure;an aperture in the first container, adapted to allow injection and removal of said buoyant fluid into and out of the first container;wherein the buoyant fluid is the buoyant fluid as claimed in claim 1.

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

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

16. An apparatus as claimed in claim 14, wherein the bladder is flexible so that the volume of the first and second voids is adapted to vary although the sum of their volumes remains constant.

17. An apparatus as claimed in claim 13, further comprising a supply container which, in use, contains a buoyant fluid, the supply container comprising a first void, defined within a bladder and a second void defined between the bladder and an outer housing of the supply container.