GB2458786A

GB2458786A - Wellhead cutting for abandonment

Info

Publication number: GB2458786A
Application number: GB0905334A
Authority: GB
Inventors: Jonathon Paul Edwards; Alexender Jeffrey Burns
Original assignee: Well Ops SEA Pty Ltd; WELL OPS UK Ltd
Current assignee: Helix Well Ops SEA Pty Ltd; WELL OPS UK Ltd
Priority date: 2008-04-05
Filing date: 2009-03-30
Publication date: 2009-10-07
Anticipated expiration: 2029-03-30
Also published as: EP2288471B1; WO2009122203A1; AU2009233524A8; AU2009233524B2; AU2009233524A1; EP2288471A1; AU2009233524B8; GB2458786B; GB0905334D0

Abstract

A high pressure abrasive cutting medium 103 is used to cut through a string of underwater concentric casings 111 to 114 such that their upper parts, and any associated wellhead, can be removed. A low density medium such as air is delivered at high pressure into the inner casing 111 via pipe 122 thereby to displace seawater 123 from the cutting zone. To prevent the escape of compressed air, apertures in the underwater structure may be plugged e.g. by socks 161 filled with a settable material and inserted in their unset state into cement ports 151. Various other mechanical and chemical plugging devices are also disclosed (figures 5 to 16).

Description

I

Method of Creating an Underwater Cutting Zone, and Related Plugging Devices and Methods The present invention relates to cutting environments, plugging devices and methods for plugging apertures in abandoned subsea weliheads for removal thereof from casing strings.

Many subsea oil and gas fields throughout the world are worked out and are now being abandoned. The wellheads of such wells represent a hazard to commercial fishers and it is a requirement in many countries that they are permanently removed and the surrounding seabed returned to its pre-oil-or-gas production state.

The most common method of removal is to blast the welihead away, using a quantity of high explosives. This method suffers from a major disadvantage amongst others, in that the explosive force that has to be used kills the marine life in a large area surrounding the explosion. Additionally, if the explosion fails to blast the wellhead away, it becomes increasingly unlikely that further successive explosive attempts will easily complete the severance and consequently far more explosives are used than is absolutely necessary to ensure success. Of course, this leads to even greater devastation of marine life over a much larger area.

Various proposals have been made for alternative and more environmentally-friendly methods for the severing of abandoned subsea oil and gas weliheads.

It has been realised that cutting the wellhead away would be more environmentally sound as well as safer for the operators carrying out the removal exercise. However, cutting through the steel wellhead casing at depth underwater raises particular problems. For example, a high pressure jet of abrasive medium has been proposed for such cutting. However, seawater and/or hydrocarbon fluids within the wellhead, and more particularly at the intended cutting area, impede the high velocity cutting medium and thereby reduce the cutting effect. This results in an increased cutting time, and thus increased wear on the cutting equipment, not least because of the additional internal wear on the cutting equipment (hoses and nozzle etc) caused by the abrasive medium flowing thereth rough.

For example, typically a string of concentric casings of the wellhead are located in the seabed. A cutting medium is ejected at high pressure from a nozzle at the end of a conduit. The objective is to cut through the casings such that their upper parts, and any associated wellhead, can be removed. However, in order to cut efficiently, the entrained grit must necessarily be very abrasive and this means that the conduit pipe work taking the grit to the cutting nozzle is unable to withstand the abrasive effects of the grit for long periods of time. The pipe work can be quite long -e.g. 300 metres. Accordingly, the pipes and fittings in these prior proposals wear out, in many cases before the cut is completed, and the pipes have to be renewed. This is not an easy procedure and is very time consuming. Another known problem is that when a jet of water passes through water, the surrounding water absorbs a high percentage of the total energy of the high-pressure jet. This means that less energy is available for the cutting action of the jet and it consequently takes longer to perform the required work. Of course, this longer time requirement leads to more abrasive wear in the conduits and fittings.

Wellheads and associated components can include external and/or internal apertures, such as through which liquid cement based medium pumped into the well casing and/or trapped gases may vent during installation or maintenance of the well bore.

It has been realised that it would be desirable to be able to carry out severing of the wellhead in an environment of reduced density medium, such as air. To this end, it would be beneficial to be able to exclude water or other liquid from the cutting zone, which may be achieved by introducing, such as by pumping, a lower density medium, such as air though not necessarily exclusively air, into the cutting zone. However, the apertures allow the air or other medium to escape which does not effectively then exclude the water from the cutting zone.

Thus, in order to achieve effective exclusion of water etc from the cutting zone, one or more apertures in the welihead casing need to be plugged. With plugging, low density medium can be pumped into the casing in an amount greater than can escape in a given time, thereby providing a build up of the medium (such as pressurisation within a section of the casing) and exclusion of the water from the cutting zone.

With the aforementioned in mind, at least one form of the present invention provides a method of creating a cutting zone in an underwater structure, including; plugging at least one aperture in the underwater structure; introducing a low density medium into the structure to remove higher density medium from a required cutting zone; whereby the plugging restricts escape of the low density medium to enable a low density medium cutting zone to be created in the underwater structure.

Thus, advantageously, cutting may be carried out on the structure more efficiently and effectively in a low density medium environment, such as cutting with an abrasive jet in air by expelling seawater from the required zone.

Plugging may include insertion of a mechanical device, or part thereof, into the at least one aperture. Alternatively, or in addition, plugging may include covering the at least one aperture with a cover device, such as a plate or cap.

Plugging may also or alternatively include locating a plugging device over and/or in the at least one aperture. This may assist in correct positioning of a plugging device and thus enhanced success of plugging.

Plugging may further include expanding a portion of a plugging device within and/or against the at least one aperture. For example, opposed mechanical means (such as cones) may be brought towards each other to expand a resilient means (such as a rubber sleeve or collar or similar material).

This may be achieved mechanically, such as by a helical drive, or hydraulically or pneumatically operated. Alternatively, biasing means, such as a spring drive, is envisaged.

The cover may be magnetised and/or include one or more magnets attached to a metal structure. For example, the cover may include one or more rare earth magnets having a relatively strong magnetic attraction compared with other magnets to thereby resist removal from the structure resulting from pressure caused by the pumped low density medium.

Alternatively or in addition to attaching the plugging device to the subsea structure by one or more magnets, adhesive(s) may be used. For example, one or more magnets may be employed to initially hold the device to the structure until an adhesive bond is sufficiently formed. Advantageously, adhesive(s) may reduce the need for strong magnets, such as rare earth magnets, which are relatively expensive, or reduce the number of magnets required. This can provide cost savings. It is also envisaged that an adhesive may be used when it is not desirable or necessary to recover and reuse the plug after the structure has been severed. It will be appreciated that suitable adhesive(s) may be used negating the need for magnetic attachment entirely.

The cover may, include a seal to assist in sealing the cover against the surface of the structure, which may help reduce loss of pressure/low density medium.

The plugging step may includes introducing a settable medium into the aperture and allowing the settable medium to set.

The settable medium may comprise at least two components that are mixed in the plugging step to react with one another and thereby cause the settable medium to set.

The plugging step may include introducing a low-temperature medium to the vicinity of the aperture, thereby to cause a medium in the vicinity of the aperture to solidify.

A further form of the present invention provides a plugging device for an aperture in an underwater structure, including means to restrict escape of low density medium from a required cutting zone within the structure.

The plugging device may include a seal to enhance restriction of escape of the medium.

In addition or alternatively, the device may include a resilient portion which, in use, inserts into and/or over the aperture to create a seal in and/or over the aperture respectively. Preferably the resilient portion may include rubber or similar material. The resilient portion may be a dumb plug.

The resilient portion may be expanded by mechanical and/or fluid means to seal within the aperture. For example, opposed cones brought together within a sleeve or collar may utilised. An expandable container is also envisaged, such as a balloon type means, in which may be filled an expandable settable (such as a foam) or non-setting fluid (such as a gas or liquid) material.

Alternatively, a non-expandable, settable material may be used, such as an epoxy resin or cement/grout.

The plugging device may be operated manually, or if preferred by mechanical means, such as a helical screw means and/or hydraulically or pneumatically driven.

It is further envisaged that the plugging device may include a locator to assist in positioning the device correctly over/within the aperture.

According to a further form of the present invention there is provided a method of cutting an underwater component that contains water and/or hydrocarbons, comprising: creating a seal in the component; introducing at least one conduit into the component; delivering pressurized gas into the component to displace at least some of the water and thereby create a cutting zone; and delivering via the at least one conduit a cutting fluid at high pressure at the cutting zone to cut the component.

An air pipe, and a hose preferably with a nozzle, may be passed into the component. This may be through the seal created in the component.

The cutting fluid may be delivered as a fluid jet through said nozzle, which jet may be directed at an area of said component from which the water has been displaced i.e. in the cutting zone, thereby to cut said area by said jet.

Preferably, a method as above further includes the step of passing a pressure relief conduit (e.g. a hose or pipe) into said component, preferably through said seal, to allow relief of pressure within said component. Other pressure relief means may be provided, such as a pressure relief uvalveP, which may be a plugging device configured to release or open at or above a required pressure. Preferably, such a pressure relief means is provided at or adjacent the underwater component to be cut.

Preferably, said component comprises a casing connected to a subsea wellhead. Preferably, said casing is one of a string of casings disposed one inside another.

Preferably, a pair of said casings defines therebetween an annular gap, wherein there is at least one aperture through one or both of the casings (which may or may not align), and sealing includes sealing the or each said aperture.

Preferably, the or each said aperture is sealed by means of cement.

Preferably, said cement is lighter than water. Preferably, said cement is a rapid setting cement.

Preferably, said nozzle directs said jet upwardly or downwardly, with respect to the horizontal. Preferably, said nozzle directs said jet at an angle in the range 50 to 15° with respect to horizontal.

In a yet further aspect, the invention provides a system for cutting an underwater component that contains water and/or hydrocarbons, the system comprising: at least one seal for the component; a gas delivery conduit to, in use, provide pressurized gas into the component for displacing water and/or hydrocarbons present in the component an abrasive fluid delivery conduit to, in use, deliver a cutting fluid therethrough into the component; and means for directing a jet of said cutting fluid at an area of said component from which the water and/or hydrocarbons has been displaced, thereby to cut said area by said jet.

Preferably, such apparatus is adapted to perform a method according to any of the preceding aspects of the invention.

The term TMsealing" used within the specification means total or partial sealing as required. That is sealing sufficient to create/maintain displacement of the water and/or hydrocarbons from the component.

Any of the methods, systems and apparatus according to these further forms of the invention may be combined with any of the methods, systems and apparatus according to the preceding forms of the invention.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 illustrates a string of concentric casings that are located in the seabed, together with an underwater cutting apparatus.

Figure 2 is a view similar to Figure 1, without the cutting apparatus but with a welihead attached.

Figure 3 is a view similar to Figure 2, showing the blocking of cement circulation ports.

Figure 4 is a view similar to Figure 3, showing an alternative blocking method.

Figure 5 shows an embodiment of the present invention mounted in place over an aperture in a structure.

Figure 6 shows an alternative embodiment of the present invention.

Figures 7a and 7b show a further embodiment of the present invention in alternative actuation positions.

Figures 8a and 8b show alternative actuation positions of an embodiment of the present invention.

Figure 9 shows a pad containing a hydrophilic hydro-expanding urethane.

Figure 10 shows a hydraulic ram with rubber ends.

Figure 11 shows a tool with four independent chambers for applying chemicals in various ratios.

Figure 12 is a view similar to Figure 11, but showing a tool with two independent chambers.

Figure 13 is a view similar to Figure 12, with the addition of a flexible expanding membrane.

Figure 14 shows an expandable rubber plug containing a settable mixture.

Figure 15 illustrates a rubber block that is expandable by a hydraulic actuator.

Figure 16 illustrates use of liquid nitrogen to freeze water in a localised area.

In the figures, like references denote like or corresponding parts.

Figure 1 illustrates a string of concentric casings 111 to 114 that are located in the seabed 102. An abrasive cutting medium 103 is ejected at high pressure from a nozzle 101 at the end of a pipe 104, which is a high-pressure hose. The objective is to cut through the casings 111 to 114 such that their upper parts, and any associated wellhead, can be removed.

In Figure 1, the culling jet 103 is inclined downwardly at a small angle to the horizontal -about 100 in this case. In known apparatus, the cutting jet 103 is substantially horizontal. Also a problem with known cutting apparatus of this type is that, as the abrasive water jet 103 passes through water that fills the casings 111 to 114, it loses energy and naturally breaks up into the surrounding water. As the abrasive water jet 103 works at increasing depths, this phenomenon becomes more noticeable until a stage is reached, at comparatively shallow depths, where the prevailing hyperbaric pressure effectively prevents efficient cutting.

In order to combat this problem, compressed air is delivered at high pressure into the inner casing 111 via a pipe 122. By directing compressed air above the nozzle 101 before culling commences, the seawater 123 in the inner casing 111 may be displaced so that, instead of trying to cut through seawater, the abrasive water jet 103 cuts in air, which is very much more efficient. A further pipe 121, which terminates a little lower in the casing 111, below the nozzle 101, allows excess pressure to be relieved. It also allows debris to be exhausted, particularly during start up. Alternatively, the pipe 121 may terminate higher in the casing, above the nozzle 101, and the pipe 122 may terminate lower in the casing 111, below the nozzle 101.

When oil and gas wells are sunk, the annulus between the outer casing 114 and the next adjacent inner casing 113 is typically cemented full. Typically, cement return ports are provided at the top of the annulus. However, the depth of the cement is considerable and whilst it is selling, slumping often occurs, which results in an un-cemented portion of the annulus underneath the wellhead.

Figure 2 shows casings 111 to 114 generally as shown in Figure 1, but with a wellhead 105 in position and cement return ports 151 at intervals around the wellhead. It may be seen that the intermediate annuli, between casings 111 and 112, and between casings 112 and 113, are effectively capped by the welihead 105. In Figures 2 to 4, the nozzle 101,jet 103, pipes 104, 121, 122 and seawater level 123 are omitted, in the interests of clarity. However, in use, those components would be present.

When cutting commences, the compressed air that is introduced as shown in Figure 1 displaces the water 123 but, as the cut penetrates the penultimate casing, a conduit is created that allows the compressed air to escape upwards through the cement return ports 151 in the wellhead 105. As the cut progresses circumferentially, with the nozzle 101 rotating about a vertical axis, the cut area becomes progressively larger, permitting more and more compressed air to escape.

Whilst at low operating seawater depths it is possible to pump large volumes of compressed air to compensate, as depths increase, the pressure of the compressed air has to be proportionally higher to compensate for the increased hyperbaric pressure. And as pressure demanded of commercially available air compressors increases, the volume of deliverable air decreases. By blocking the cement ports 151 of the wellhead 105, the compressed air is prevented from escaping and high-pressure, low-volume pumps become adequate for use.

In the embodiment of Figure 3, socks 161 are filled with a settable material and inserted in their unset state into cement ports 151. When the material sets, it prevents the general escape of compressed air. It is to be noted that perfect sealing is not required and, indeed, there is likely to be substantial marine growth around the cement ports 151 that has developed over the years since it was first installed. Thus, in the context of the specification, the term "seal" does not necessarily imply a perfect seal. In practice, a certain amount of leakage may be tolerated. It will typically be sufficient to provide sufficient sealing to retain the desired air pressure.

In the variation of Figure 4, a nozzle 171 directs into successive cement return ports 151 a rapid-setting, lighter-than-seawater cement 172, which is pumped into the annulus between the outer two casings 113, 114. When set, the cement 172 forms an effective barrier to inhibit the general escape of compressed air.

In this way, therefore, the cutting efficiency of the abrasive water jet 103 can be very much higher.

The pipes 104 and 121 will typically communicate with apparatus located at the surface of the water above the wellhead.

In an alternative arrangement, holes may be formed from inside the casings, and cement (or other sealant) injected through the holes into the annular space(s) between casings.

Any of the methods and apparatus for cutting an underwater component, as illustrated in and/or described with reference to any of Figures 1 to 4, may be combined with any of the methods and apparatus as illustrated in and/or described with reference to any of the subsequent figures.

Figure 5 shows a plugging device I incorporating a cover 2 to cover an aperture 7 leading to an annular space 8 between outer 9 and middle 10 casing walls of a wellhead. Magnets 3 attach the device to the outer casing 9, with a seal 4 (e.g. an annular 0-ring), a locating portion 5, and a handle 6 to allow an operator to manipulate the device I into position.

Figure 6 shows an alternative embodiment but without having a seal.

However, the locator 5 includes a resilient material 11, in a preferred form of a cone, to seal against the walls of the aperture. It will however be appreciated that the seal 4 could also be provided.

In figures 7a and 7b, the plugging device includes an expandable portion 12, such as a balloon, over a projecting portion 13 also serving to locate the device. An "inflation" aperture 14 passes through the cover 2 but does not pass through the expandable portion. The expandable portion may be Inflated" by introducing fluid under pressure through the aperture 14 which may be greater than the counter pressure acting against the expandable portion. However, pressure may be actively applied through that aperture, such as by pumping air or other medium, like a settable flowable material, into the expandable portion. The inflation aperture may thereafter be sealed to retain pressure therein or the material allowed to set firm/solid.

In figures 8a and 8b, a resilient material 15 may be expanded within an aperture to be plugged. This is achieved in the embodiment shown by bringing towards each other a pair of cones 16a, 16b, which may themselves form a locator similar to locator 5. This may be achieved by rotating the handle 6 to operate a helical screw mechanism, thereby drawing together the cones to expand the resilient material. Alternatively, mechanically driven means may be utilised, such as a hydraulic or pneumatic actuator. It will be appreciated that spring operated actuating means may be adopted, whereby the device is retained in an unactuated state until placed in position and then the spring mechanism released to plug the aperture.

Figures 9 to 16 show further embodiments of plugging devices.

In the plugging device 20 of Figure 9, a medium with high absorbent properties, such as an soil spill pad" 21, is soaked in a material that expands on contact with water or when mixed, which may be a hydrophilic hydro-expanding urethane. The medium is shaped and sized for a particular wellhead cement overflow port to be blocked. The soaked absorbent pad 211s then placed in a waterproof membrane 22. Optionally, a weight and/or a small magnet 23 is placed in the bottom of the membrane 22, which is then sealed around the pad 21.

The plugging device 20 is then run subsea and placed into a wellhead cement overflow port to be plugged. The purpose of the weight is to make the plugging device 20 neutral to negatively buoyant and the magnet assists in keeping the plugging device 20 in place in the port to be plugged, due to magnetic attraction between the magnet 23 and ferromagnetic material in the vicinity of the port.

Once the plugging device 20 is securely fitted in to the cement overflow port, the waterproof membrane 22 is punctured, allowing the expanding material to react with the seawater. Once expanded, the material fills the port to be plugged and becomes set in position.

The plugging device 30 of Figure 10 comprises a hydraulic ram 31 having rubber stops 32 fitted respectively to the cylinder 33 and piston 34, at opposite ends of the ram 31. In use, the plugging device 30 is placed between a wellhead structure and a port to be plugged. Most wellheads have a surrounding frame structure that provides a convenient bracing point. Upon positioning and extending the ram 31, one of the rubber stops 32 plugs the port and the other reacts against the frame, thereby jamming the respective rubber stop 32 in the port to create a seal. The rubber stops may be of hemispherical or any other convenient shape.

In Figure 11, a plugging device 40 comprises a tool with multiple (here shown with four) independent chambers 41 for applying chemicals in various ratios subsea. Chemicals that react with one another to form a sealing compound are placed in the respective chambers and ejected hydraulically to pass through an in-line static mixer 42 where they react together as they are ejected from the tool. A set of pistons 43 is arranged to be actuated which may be by a hydraulic cylinder or a remotely operated underwater vehicle (ROV) in order to eject the chemicals from the tool. The tool is designed primarily for blocking cement overflow ports and slots (as can be found on older wellhead systems). In Figure 11, two chemical components A and B are shown in equal proportions in the chambers 41. This may be particularly suitable for two-part hydrophilic hydro-expanding urethanes. However, any number of chemicals may be mixed in desired ratios by suitable choice of the number of chambers.

Figure 12 shows a similar plugging device 50 which has only two chambers. Figure 13 is similar to Figure 12, but shows a plugging device 60 with a flexible expanding membrane 43 at the output of the static mixer 42, in order to contain the sealing compound while it sets.

The plugging device 70 shown in Figure 14 comprises a compressible plug 71 (e.g. of rubber), which is constrained between a pair of washers 72. A screwthreaded rod 73 passes through the centre of the plug and washers and has a wing nut 74 and regular nut 75 engaged at opposite ends. As the nuts 74, are tightened, the rubber plug 71 is compressed to deform radially outwardly, thereby to engage a port to be plugged. In a subsea environment, the nuts 74, 75 may be tightened by an ROy. The nuts 74 and/or 75 may be replaced by a bolt with captive nut or other convenient arrangement to enable the assembly to be tightened reliably from one end.

Preferably, the plug 71 has two separate internal spaces 76, 77 which are sealed from one another and contain respective chemical components A and B (e.g. of a two-part resin) and also a rotatable blade 78. As the assembly is tightened, the blade 78 rotates to rupture an internal dividing wall so that the two chemical components A and B are mixed together to react, expand and then set, thereby locking the plugging device 70 firmly in place in a port to be plugged.

In Figure 15, a plugging device 80 comprises a rubber toroid 81 which is expanded radially outwardly by two cones 82 that are urged together by a hydraulic actuator 83. As with other embodiments, the toroid 81 may contain two or more chemical components that become mixed together as the toroid is expanded such that they react, expand and then set, thereby locking the plugging device 80 firmly in place in a port to be plugged.

In Figure 16, liquid nitrogen 181 is delivered through pipes 180 to the vicinity of ports 151 in order to freeze the water locally 182 and thereby block the ports 151. The pipes 180 may extend into the outer annulus between the outermost casings. Since seawater is generally cold at typical wellhead depths, the water may remain frozen long enough for a cutting operation to be completed.

In use, one or more plugging devices are placed over respective apertures in the undersea structure, such as an abandoned wellhead. A low density medium, such as air is pumped into the structure to expel water and/or hydrocarbon bearing liquid from the structure. It will be appreciated that the apertures need not be completely sealed provided the air can be pumped into the space to be evacuated at a greater rate than is lost through leakage. In pumping air into the space the water etc is expelled under pressure. This creates an air filled environment which is of lower density than the previous water/hydrocarbon filled space, and therefore easier for cutting operations e.g. to sever the wellhead by reducing energy losses in the cutting jet.

Although the illustrated methods and apparatus are for culling subsea casing strings, the methods and apparatus may be adapted to cut other components underwater.

In this specification, the verb "comprise" and variations thereof denote non-exclusive inclusion. That is, use of the word "comprise" to include one feature or more, does not exclude the possibility of also including further features.

The reader's attention is directed to all and any priority documents identified in connection with this application and to all and any papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS: 1. A method of creating a cutting zone in an underwater structure, including; plugging at least one aperture in the underwater structure; introducing a low density medium into the structure to remove higher density medium from a required cutting zone; whereby the plugging restricts escape of the low density medium to enable a low density medium cutting zone to be created in the underwater structure.
2. A method according to claim 1, wherein plugging includes insertion of a mechanical device, or part thereof, into the at least one aperture.
3. A method according to claim 1, wherein plugging includes covering the at least one aperture with a cover device, such as a plate, cover or cap.
4. A method according to any one of the preceding claims, including locating the plugging device using a locating means to assist in correct positioning within the at least one aperture.
5. A method according to any one of the preceding claims including expanding a portion of the plugging device within and/or against the at least one aperture.
6. A method according to claim 5, including expanding a flexible container portion of the device to seal the aperture.
7. A method according to claim 6, wherein the flexible container portion is expanded by introduction therein of a flowable settable material and/or gas.
8. A method according to claim 5, wherein a resilient material is mechanically expanded within and/or against the aperture.
9. A method according to claim 8, wherein opposed mechanical means are brought towards each other to expand the resilient means.
10. A method according to any one of the preceding claims, wherein the device is actuated manually, hydraulically and/or pneumatically.
11. A method according to any one of the preceding claims wherein the device is retained to the structure by magnetism.
12. A method according to any one of claims I to 10 wherein the device is retained to the structure by adhesive or a combination of magnetism and adhesive.
13. A method according to claim 1, wherein the plugging step includes introducing a settable medium into the aperture and allowing the settable medium toset.
14. A method according to claim 13, wherein the settable medium comprises at least two components that are mixed in the plugging step to react with one another and thereby cause the settable medium to set.
15. A method according to claim 1, wherein said plugging step includes introducing a low-temperature medium to the vicinity of the aperture, thereby to cause a medium in the vicinity of the aperture to solidify.
16. A plugging device for an aperture in an underwater structure, including means to restrict escape of low density medium from a required cutting zone within the structure.
17. A plugging device according to claim 16, including a seal to enhance restriction of escape of the medium.
18. A plugging device according to claim 17, including a resilient portion which, in use, inserts into and/or over the aperture to create a seal in and/or over the aperture respectively.
19. A plugging device according to claim 18, wherein the resilient portion includes rubber or similar material.
20. A plugging device according to claim 18, including mechanical and/or fluid means to expand the resilient portion
21. A plugging device according to claim 20, wherein the expandable means includes an expandable container.
22. A method of cutting an underwater component that contains water and/or hydrocarbons, comprising: creating a seal in the component; introducing at least one conduit into the component; delivering pressurized gas into the component to displace at least some of the water and thereby create a cutting zone; and delivering via the at least one conduit a cutting fluid at high pressure at the cutting zone to cut the component.
23. A method according to claim 22, including the step of creating a cutting zone by a method according to any of claims I to 15.
24. A system for cutting an underwater component that contains water and/or hydrocarbons, the system comprising: at least one seal for the component; a gas delivery conduit to, in use, provide pressurized gas into the component for displacing water and/or hydrocarbons present in the component an abrasive fluid delivery conduit to, in use, deliver a cutting fluid therethrough into the component; and means for directing, in use, a jet of said cutting fluid at an area of said component from which the water and/or hydrocarbons has been displaced, thereby to cut said area by said jet.
25. A system according to claim 24, including a plugging device according to any of claims 16 to 21.
26. A method of creating a cutting zone in an underwater structure, substantially as hereinbefore described with reference to the accompanying drawings.
27. A plugging device substantially as hereinbefore described with reference to the accompanying drawings.
28. A method of cutting an underwater component, substantially as hereinbefore described with reference to the accompanying drawings.
29. A system for cutting an underwater component, substantially as hereinbefore described with reference to the accompanying drawings.