EP2288471B1

EP2288471B1 - Method of creating an underwater cutting zone, and related plugging devices and methods

Info

Publication number: EP2288471B1
Application number: EP09726842A
Authority: EP
Inventors: Jonathon Edwards; Alexender Burns; David White; Stuart Tennant
Original assignee: Helix Well Ops SEA Pty Ltd; Well Ops UK Ltd
Current assignee: WELL OPS UK Ltd; Helix Well Ops SEA Pty Ltd
Priority date: 2008-04-05
Filing date: 2009-03-30
Publication date: 2012-07-04
Anticipated expiration: 2029-03-30
Also published as: EP2288471A1; AU2009233524A8; AU2009233524B2; WO2009122203A1; GB2458786B; AU2009233524B8; GB2458786A; GB0905334D0; AU2009233524A1

Abstract

A string of concentric casings (111) to (114) 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) to cut through the casings (111) to (114) such that their upper parts, and any associated wellhead, can be removed. Air is delivered at high pressure into the inner casing (111) via pipe 122therebyto displace seawater (123) in the inner casing (111) so that, instead of trying to cut through seawater, the abrasive water jet (103) cuts in air, which is very much more efficient. In Figure (3), socks (161) are filled with a settable material and inserted intheir unset state into cement ports (151). When the material sets, it prevents the general escape of compressed air. Various other plugging devices are also disclosed.

Description

TECHNICAL FIELD

The present invention relates to cutting environments, plugging devices and methods for plugging apertures in abandoned subsea wellheads for removal thereof from casing strings.

BACKGROUND

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 wellhead 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 wellheads.
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 therethrough.
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 wellhead 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.
Document US 5381631 discloses a method and a system for creating a cutting zone in an underwater structure, the system comprising:

an underwater structure that comprises a plurality of components disposed one inside another ;
a pipe that passes through an end of the underwater structure;
a plugging means in the innermost component ; and
means arranged to introduce a gas under pressure through said pipe into the component, thereby to displace liquid within said structure from the locality of a required cutting zone;
whereby said plugging means restricts escape of the gas through said component, from within the underwater structure.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method of creating a cutting zone in an underwater structure that comprises a plurality of components disposed one inside another to define therebetween an annular gap that communicates with an aperture in the outermost component, the method including the steps of:

passing a pipe through an end of the underwater structure;
sealing said end of the underwater structure;
applying a plugging means to the vicinity of said aperture in the outermost component;
and introducing a gas under pressure through said pipe, thereby to displace liquid within said structure from the locality of a required cutting zone:
- whereby said plugging means restricts escape of the gas through said annular gap and aperture, from within 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 said aperture. Alternatively, or in addition, plugging may include covering the said aperture with a cover device, such as a plate, cover or cap.
Plugging may also or alternatively include locating a plugging device over and/or in the said 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 said aperture. For example, opposed mechanical means (such as cones) may be brought towards each other to expand a resilient means (such as a flexible container portion in the form of 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.
A flexible container portion may be expanded by introduction therein of a flowable settable material and/or gas.
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 the pressure of the gas.
Alternatively or in addition to attaching a plugging device to the underwater 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 a 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 gas.
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 settable medium may comprise a cement, preferably a lighter than water cement which is preferably rapid setting.
The plugging step may include introducing a low-temperature medium to the vicinity of the said aperture, thereby to cause a medium in the vicinity of the aperture to solidify.
Preferably, pressure relief means is provided to relieve pressure within the underwater structure to be cut.
A pressure relief pipe may be passed through said end of the underwater structure, to allow relief of pressure within said structure.
Other pressure relief means may be provided, such as a pressure relief "valve", 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 structure to be cut.
The underwater structure may comprise a string of casings connected to a subsea wellhead.
The outermost component may have a plurality of apertures, the method including the step of plugging at least one of said apertures.
A method as above may include the further steps of passing a conduit through said end of the underwater structure and delivering via the conduit a cutting fluid at high pressure at the cutting zone to cut the structure.
Preferably, the cutting fluid forms a jet directed by a nozzle upwardly or downwardly, with respect to the horizontal. Preferably, said nozzle directs said jet at an angle in the range 5° to 15° with respect to horizontal.
In another aspect, the invention provides a system for creating a cutting zone in an underwater structure, the system comprising:

an underwater structure that comprises a plurality of components disposed one inside another to define therebetween an annular gap that communicates with an aperture in the outermost component;
a pipe that passes through an end of the underwater structure; a seal at said end of the underwater structure;
a plugging means in the vicinity of said aperture in the outermost component; and means arranged to introduce a gas under pressure through said pipe, thereby to displace liquid within said structure from the locality of a required cutting zone:
- whereby said plugging means restricts escape of the gas through said annular gap and aperture, from within the underwater structure.

Preferably, such a system is adapted to perform a method according to any of the preceding aspects of the invention.
The term "sealing" used within the specification means total or partial sealing as required. That is sealing sufficient to create/maintain displacement of liquid from the locality of the required cutting zone.

BRIEF DESCRIPTION OF THE DRAWINGS

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 wellhead 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.

DESCRIPTION OF PREFERRED EMBODIMENTS

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 cutting jet 103 is inclined downwardly at a small angle to the horizontal - about 10° 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 cutting 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 setting, 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 wellhead 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 1 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 O-ring), a locating portion 5, and a handle 6 to allow an operator to manipulate the device 1 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 "oil 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 21 is 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, 75 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 ROV. 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 cutting subsea casing strings, the methods and apparatus may be adapted to cut other components underwater.

Claims

A method of creating a cutting zone in an underwater structure that comprises a plurality of components (111-114) disposed one inside another to define therebetween an annular gap that communicates with an aperture (151) in the outermost component (114), the method including the steps of :
a. passing a pipe (122) through an end of the underwater structure;

b. sealing said end of the underwater structure;

c. applying a plugging means (161) to the vicinity of said aperture (151) in the outermost component (114); and

d. introducing a gas under pressure through said pipe (122), thereby to displace liquid (123) within said structure from the locality of a required cutting zone;
whereby said plugging means (161) restricts escape of the gas through said annular gap and aperture (151), from within the underwater structure.
A method according to claim 1, including the step of using a locating means (5) to assist in correct positioning of said plugging means (1) with respect to said aperture (7).
A method according to claim 1 or 2, including expanding a portion (12) of said plugging means within and/or against said aperture.
A method according to claim 1, 2 or 3, wherein said plugging means comprises a plugging device (1) that is retained to the structure at least partly by magnetism (3) and/or adhesive.
A method according to claim 1, wherein the plugging means includes a settable medium (172) that is introduced into said aperture (151) and allowed to set.
A method according to claim 5, wherein the settable medium (172) is a lighter than water cement.
A method according to claim 1, wherein said plugging means includes a low-temperature medium (181) that is introduced to the vicinity of said aperture (151), thereby to cause a medium (182) in the vicinity of the aperture to solidify.
A method according to any of the preceding claims, including pressure relief means (121) to relieve pressure within the underwater structure to be cut.
A method according to claim 8, wherein said pressure relief means comprises a valve configured to release or open at or above a required pressure.
A method according to claim 8 or 9, wherein said pressure relief means is provided at or adjacent the underwater structure to be cut.
A method according to any of the preceding claims, wherein said structure comprises a string of casings (111-114) connected to a subsea wellhead (105).
A method according to any of the preceding claims, wherein said outermost component (114) has a plurality of apertures (151) and the method includes the step of applying said plugging means (161) to the vicinity of at least one of said apertures (151).
A method according to any of the preceding claims, including the further steps of passing a conduit (104) through said end of the underwater structure and delivering via the conduit (104) a cutting fluid (103) at high pressure at the cutting zone to cut the structure.
A method according to claim 13, wherein the cutting fluid forms a jet (103) directed by a nozzle downwardly, with respect to the horizontal.
A system for creating a cutting zone in an underwater structure, the system comprising:
an underwater structure that comprises a plurality of components (111-114) disposed one inside another to define therebetween an annular gap that communicates with an aperture (151) in the outermost component (114);

a pipe (122) that passes through an end of the underwater structure;

a seal at said end of the underwater structure;

a plugging means (161) in the vicinity of said aperture (151) in the outermost component (114); and

means arranged to introduce a gas under pressure through said pipe (122), thereby to displace liquid (123) within said structure from the locality of a required cutting zone;

whereby said plugging means (161) restricts escape of the gas through said annular gap and aperture (151), from within the underwater structure.
A system according to claim 15 and arranged to perform a method according to any of claims 2 to 14.