Cutting Device and Method

The present invention relates to a cutting device arranged to direct a jet of a cutting material on to an object to be cut, and to a corresponding method. The present invention also relates to a blowout preventer (BOP) and a well shutoff device arranged to direct a jet of a cutting material on to an object to be cut; a BOP assembly and well shutoff assembly incorporating a cutting device arranged to direct a jet of a cutting material on to an object to be cut; and to corresponding methods.

As is well known in the oil and gas exploration and production industry, a wellbore is typically drilled from surface in order to gain access to subterranean hydrocarbon containing formations, for recovering well fluids to surface. A metal casing is installed in the drilled wellbore and cemented in place, to support the drilled formations; to permit recovery of well fluids to surface; and to permit passage of further tubing and/or tool strings downhole .

In more detail, a wellbore is typically formed by drilling to a first depth, and then installing a first casing in the drilled section and cementing the casing in place. The wellbore is subsequently extended to a second depth and a second, smaller diameter casing installed in the extended section and also cemented in place. This process is repeated as necessary until the wellbore has been extended to a producing formation. Alternatively, a liner may be installed, extending from the base or ^λ shoe' of the lowermost casing section to the producing formation. The well is then completed in preparation for recovery of well fluids to surface.

It is necessary to carefully control the flow of fluids from the well during all phases of exploration and production, and thus during construction and completion of the well, and during subsequent production of well fluids. During the construction phase and in the event that an intervention procedure is to be carried out, this is achieved using a blowout preventer (BOP) .

In an offshore environment, a BOP is usually installed on a wellhead on the seabed, although it is also known to provide surface BOPs. Onshore, BOPs are provided on the wellhead at surface. In each case, the BOPs include a number of shear rams and seal rams, and tubing and tools are run into the wellbore through the BOP. In the event of an emergency situation arising, such as extreme weather conditions offshore requiring a rig to urgently move off-station, or when an uncontrolled flow of well fluids into the wellbore occurs which could lead to a blowout, the shear rams of the BOP are actuated to shear the tubing and/or tool strings

extending through the BOP, to shut-in the well. Gripper rams below the shear rams hold the cut section of tubing or tool string to prevent them falling into the well. Following shearing and, if necessary, remedial action to recover control of the well, fluid communication with the sheared section of tubing can be re-established.

Current BOPs are relatively large, heavy and complex pieces of equipment. This leads to disadvantages, particularly the weight that the BOP adds to the surface casing and conductor used to support the wellhead. This is in part due to a requirement for the BOP to carry relatively large shear rams for shearing tubing extending through the BOP; it will be understood that such tubing may comprise production tubing, drill tubing, coiled tubing or tubing forming part of a tool string run into the wellbore, as well as larger diameter tubing such as casing and liner. The shear rams must therefore be capable of cutting relatively large diameter and wall thickness tubings.

Furthermore, the shear rams of current BOPs typically operate to cut and shear a tubing or the like located between the rams by first compressing and crimping the tubing within the BOP bore, before cutting through the compressed tubing. This causes significant resultant deformation in the remaining portion of the tubing located within the BOP bore, which can hamper re-entry into the well. In particular, it is necessary to mill away the remaining damaged tubing section to provide full access into the wellbore, which is time-consuming and costly, and generates swarf/shavings which is difficult to clean-out and which can hamper performance of the BOP.

An alternative method and apparatus for drilling and completing a well is disclosed in International Patent Application no. PCT/GB2005/002885 (WO2006/010906) in the name of the current inventor. According to WO2006/010906, a subsea shutoff device including ball gripping mechanisms is latched to a template at surface. Drilled in casing is suspended from the device using the gripping mechanism, and the device is run to a sea floor on a casing string. The casing string is drilled into place and converted into a riser, and this is all done on a single trip, to speed up the operation and reduce risk. The casing string is captured and sealed within the subsea shutoff device after installation and space out of a surface BOP. In the event that an emergency ^' disconnection is required, dual shear rams in the device cut the casing and seal in the well. The system then allows a conventional subsea reconnection of the riser.

Whilst the system and method disclosed in WO2006/010906 offers significant advantages over the conventional methods and apparatus described above, the shutoff device requires large shear rams, in a similar fashion to conventional BOPs.

The use of chemical cutting technology for severing down- hole tubulars in wellbores has been described in several previous patent publications. For example US Patent no. 2,918,125 to Sweetman discloses a downhole chemical cutter which employs a cutting fluid that violently reacts with the object to be cut, generating extremely high temperatures sufficient to melt, cut or burn the tubular.

Under the Sweetman procedure, halogen fluorides are employed in jet streams which impinge on the downhole tubular to sever it. Examples of chemical cutting agents disclosed in Sweetman are fluorine and halogen fluorides including compounds such as chlorine trifluoride, chlorine monofluoride, bromine trifluoride and bromine pentafluoride . The cutting fluid is expelled from the tool through ports which extend radially from a central bore.

Further devices are disclosed in US Patent no. 4,619,318 to Terrell et al, where objects may be completely dissolved leaving no debris in the wellbore; US Patent no. 3,076,507 to Sweetman; US Patent no. 4,125,161 to Chammas; and US Patent no. 4,494,601 to Pratt et al.

Each of the above cutting devices are disclosed as operating from a retrievable tool which is internal to the pipe to be severed. These tools must be spaced out and anchored relative to the pipe to be cut. Accurately spacing out the tools is difficult and can lead to errors in location of the tools, with potentially serious consequences.

It is amongst the objects of embodiments of the invention to obviate or mitigate at least one of the foregoing disadvantages.

According to a first aspect of the present invention, there is provided a cutting device comprising: a housing having an internal bore; and at least one cutting torch coupled to the housing and arranged to direct a jet of a cutting material on to an

object located within the housing bore to thereby cut the object.

Providing a cutting device with a cutting torch arranged to cut an object located within the housing internal bore permits use of the device to cut a wide range of different objects, and offers significant advantages over current cutting devices. In particular, the cutting device of the invention permits cutting of tubing and the like of similar diameters and wall thicknesses to those which may be cut with BOP shear rams, but is of significantly lower weight in comparison to the BOP rams and associated operating equipment. Furthermore, the torch provides a clean cut without significant distortion of a remainder of the tubing, which is again in contrast to current BOP shear rams, which compress and distort a tubing during shearing, as described above.

Preferably, the cutting device is for use in cutting a downhole object, and may be a wellbore cutting device. The object may be tubing such as casing, liner, production tubing, drill tubing, coiled tubing, tubing forming part of a downhole tool or tool string, or parts or sections thereof.

The cutting device may be adapted to form part of a pressure containing system for containing fluid pressure. In particular, where the cutting device is a downhole cutting device, the cutting device may be adapted to form part of a pressure containing system for containing wellbore pressure and thus for containing wellbore fluids. The cutting device

housing may be adapted to retain fluid pressure and thus to prevent fluid egress from or past the housing.

In an embodiment of the invention, the cutting device housing may be adapted to be coupled to a BOP, in particular to a housing of a BOP. Alternatively, the cutting device housing may be defined by at least part of a housing of a BOP. The cutting torch of the cutting device may then perform primary cutting of tubing or the like extending through the BOP, with shear rams of the BOP optionally held in reserve; downrated for use in cutting smaller diameter/wall thickness tubing and the like (and thus reducing weight of the BOP) ; or replaced by the cutting torch.

In an alternative embodiment, the cutting device housing may be adapted to be coupled to a shutoff device, in particular to a housing of a shutoff device, such as that disclosed in WO2006/010906, the disclosure of which is incorporated herein by way of reference. Alternatively, the cutting device housing may be defined by at least part of a housing of a shutoff device. The cutting torch of the cutting device may then perform primary cutting of tubing or the like extending through the shutoff device.

The cutting device may be adapted to be selectively activated when it is desired to cut an object. The cutting device may be adapted to be selectively activated in response to detection of a condition indicative of an emergency situation. Such emergency situations may comprise loss of control of well fluids; or a requirement to shut-in

(seal off) a well urgently without pulling a tubing string or the like extending through the BOP.

Preferably, the cutting device comprises a plurality of cutting torches, which may be spaced around a circumference or perimeter of the housing. The cutting device may comprise a plurality of sets of cutting torches, each set comprising at least one cutting torch, and the sets may be spaced axially and/or circumferentially relative to or about the housing.

The or each cutting torch may be arranged to direct a jet of cutting material in a direction transverse to a main axis of the housing bore, and may be arranged to direct a jet of cutting material in a direction perpendicular to the bore main axis. The or each cutting torch may be radially arranged or spaced around the perimeter of the housing, and may be arranged in a plane perpendicular to the bore axis.

The or each cutting torch may have a torch main axis, and may be radially arranged around the housing such that the or each torch main axis intersects with the bore axis. This may provide an effective contact between the jet of cutting material and the surface of the object to be cut. Alternatively, the or each cutting torch may be arranged such that the torch main axis does not intersect with the bore axis. For example, the or each torch may be arranged to direct a jet of material in a direction along a tangent of an tubular object located in the bore.

The or each cutting torch may be a chemical cutting torch, and may be an incendiary cutting torch, in particular a

pyrotechnic, molten metal jet torch of a type such as the THERMOL™ CUTTING SYSTEM commercially available from the Jet Research Centre, Texas, USA and/or as disclosed in US Patent no. 5,372,069 (the disclosure of which is incorporated herein by way of reference) . The or each torch may comprise a chamber housing a flammable material which, upon ignition, may be adapted to generate the jet of cutting material. Suitable materials include pyronol pellets which are pressed from an intimate powder mixture of (1) nickel, (2) metal oxide, (3) a component selected from the group consisting of (a) aluminium and (b) a mixture of at least 50 weight percent aluminium and a metal that is magnesium, zirconium, bismuth, beryllium, boron, or mixtures thereof.

The or each torch may also comprise an igniter material for igniting the flammable material, which may be a loose pyrotechnic starter powder such as (1) magnesium powder, or an intimate mixture of (2) Mg and CuO, (3) Mg and Fe ₂ O ₃ , (4) Mg and Co ₃ O ₄ , (5) Al and CuO, or (6) Al and Fe ₂ O ₃ powders. The cutting material may be a reaction product of the flammable material and the igniter material, and may be a molten jet or stream of metals, metal oxides and gas.

The or each torch may also comprise a detonator for igniting the igniter material, to thereby ignite the flammable material. The detonator may be adapted to generate an electrical discharge to ignite the igniter material, and may be an electric bridge wire.

The or each torch may comprise a chamber housing the flammable material, and the chamber may be lined with a

graphite sleeve resistant to erosion by the flammable material (when ignited) .

The or each torch may comprise a nozzle, optionally of a graphite material, for directing the jet of cutting material on to the object. The nozzle may be shaped to distribute cutting material around a surface of the object to be cut, and may be shaped to generate a diverging jet of cutting material. Where the cutting device comprises a plurality of cutting torches, the torches may be arranged to provide a distribution of cutting material around a circumference or perimeter of the object to be cut. The torches may be arranged such that the jets of cutting material intersect. The nozzle may comprise a shield, such as a diaphragm, which is adapted to rupture upon exposure to cutting material (generated following activation of the torch) of a predetermined pressure and/or temperature.

The or each cutting torch may be an alternative chemical cutting torch, and may comprise an incendiary cutting fluid. Suitable fluids, methods and the like are disclosed in US2,918,125; US4,619,318; US3, 076, 507; US4, 125, 161; and/or ϋS4,494,601, the disclosures of which are incorporated herein by way of reference.

In a preferred embodiment, where the cutting device comprises a plurality of cutting torches, the torches are adapted to be activated simultaneously. This may facilitate a clean cut of the object, particularly where the object is under tensile load, as may be the case with a tubing string.

Preferably, the or each cutting torch is sealingly coupled to the housing, which may facilitate pressure containment for example, where a tube containing fluid is cut.

According to a second aspect of the present invention, there is provided a blowout preventer comprising: a housing having an internal bore; and at least one cutting torch coupled to the housing and arranged to direct a jet of a cutting material on to an object located within the housing bore to thereby cut the object.

According to a third aspect of the present invention, there is provided a well shutoff device comprising: a housing having an internal bore; and at least one cutting torch coupled to the housing and arranged to direct a jet of a cutting material on to an object located within the housing bore to thereby cut the object.

The blowout preventer (BOP) and well shutoff device housings and torches may be of similar structure to the cutting device housing and torch defined above. Accordingly, further features of the BOP and well shutoff device housings and torches are defined in relation to the first aspect of the invention.

The well shutoff device may comprise a gripping mechanism for selective engagement to casing when inserted in the throughbore; and one or more rams for selectively sealing the housing bore, according to the teachings of WO2006/010906.

The BOP and well shutoff device may comprise one or more: seal rams; blind rams; and/or shear rams, which may be provided in the housing. Thus the torch may be provided in the BOP and well shutoff device integrally with other seal, blind and/or shear rams. Where shear rams are provided, the shear rams may be suitably rated for shearing tubing and the like of any anticipated diameter and/or wall thickness that it is anticipated will be run in through the BOP/shutoff device. Thus the rams may be provided as a back-up to the torch. Alternatively, the shear rams may be rated for cutting tubing and the like of smaller diameter and/or wall thickness than the torch. This may permit the size of the shear rams to be reduced (compared to existing rams) and thus the weight of the BOP/shutoff device to be minimised. For example, the torch may be adapted to perform a primary shearing of tubing such as casing, liner or drill tubing, whilst the shear rams may be rated to shear tubing such as coiled tubing residing within the casing or the like which may not have been cut by the torch.

According to a fourth aspect of the present invention, there is provided a blowout preventer assembly comprising: a blowout preventer having a housing defining an internal bore; and a cutting device coupled to the blowout preventer; wherein the cutting device comprises a cutting device housing having an internal bore, the cutting device housing adapted to be coupled to the blowout preventer housing; and at least one cutting torch coupled to the cutting device housing and arranged to direct a jet of a cutting material on to an object located within and extending through the

blowout preventer and cutting device housing bores, to thereby cut the object.

According to a fifth aspect of the present invention, there is provided a well shutoff assembly comprising: a well shutoff device having a housing defining an internal bore; and a cutting device coupled to the well shutoff device; wherein the cutting device comprises a cutting device housing having an internal bore, the cutting device housing adapted to be coupled to the shutoff device housing; and at least one cutting torch coupled to the cutting device housing and arranged to direct a jet of a cutting material on to an object located within and extending through the shutoff device and cutting device housing bores, to thereby cut the object.

Further features of the cutting devices of the BOP assembly and the well shutoff assembly are defined above in relation to the first aspect of the invention.

Preferably, the cutting device is provided as a modular unit which may be releasably coupled to the BOP/well shutoff device. This permits the cutting device to be fitted to any desired, existing type of BOP or well shutoff device (such as that disclosed in WO2006/010906) utilising standard industry fittings.

The well shutoff device may comprise a gripping mechanism for selective engagement to casing when inserted in the throughbore; and one or more rams for selectively sealing

the housing bore, according to the teachings of WO2006/010906.

The BOP and well shutoff device may comprise one or more: seal rams; blind rams; and/or shear rams, which may be provided in the housing. Thus the torch may be provided in the BOP and well shutoff device integrally with other seal, blind and/or shear rams. The rams may be rated as described above .

According to a sixth aspect of the present invention, there is provided a method of cutting an object, the method comprising the steps of: locating an object to be cut within an internal bore of a cutting device housing; and selectively activating an at least one cutting torch coupled to the housing to direct a jet of a cutting material on to the object to thereby cut the object.

The method may be a method of cutting a downhole object, although the method may have equal utility in cutting other objects such as pipeline or other tubing, at surface or subsurface such as on a seabed.

The method may comprise cutting an object and sealing the housing so as to restrict fluid flow/egress from or through the housing. This may be achieved by coupling the housing into a pressure containing system. For example, the housing may be coupled to a BOP or provided as part of a BOP; or the housing may be coupled to a well shutoff device, or provided as part of a well shutoff device, such as that disclosed in WO2006/010906.

The cutting torch may be utilised to perform primary cutting of an object extending through the BOP/shutoff device. Shear rams of the BOP/shutoff device may be utilised to perform secondary shearing of an object remaining in the BOP/shutoff device following activation of the torch. For example, the torch may be activated to cut tubing such as a casing, liner or drill tubing extending through the BOP/shutoff device. Following cutting of such tubing, the shear rams may be activated to shear secondary tubing, such as coiled tubing which had been located within the casing or the like.

The cutting torch may be activated in response to detection of a condition indicative of an emergency situation. Such emergency situations may comprise loss of control of well fluids; or a requirement to shut-in (seal off) a well urgently without first pulling a tubing string or the like extending through the BOP.

The method may comprise activating a plurality of cutting torches, preferably simultaneously, to cut the object. The method may comprise directing a jet of cutting material in a direction transverse to a main axis of the housing bore, and may comprise directing the jet in a direction perpendicular to the bore main axis.

According to a seventh aspect. of the present invention, there is provided a method of controlling a well, the method comprising the steps of:

providing a blowout preventer having a housing defining an internal bore and at least one cutting torch coupled to the housing; and on detecting a condition indicative of an emergency situation, activating the at least one cutting torch to direct a jet of a cutting material on to an object located within the housing bore to thereby cut the object, facilitating control of the well.

According to an eighth aspect of the present invention, there is provided a method of controlling a well, the method comprising the steps of: providing a blowout preventer having a housing defining an internal bore; coupling a housing of a cutting device to the blowout preventer housing; running an object into the well to carry out a downhole procedure; and on detecting a condition indicative of an emergency situation, activating an at least one cutting torch coupled to the cutting device housing, to direct a jet of a cutting material on to the object, to thereby cut the object and facilitate control of the well.

According to a ninth aspect of the present invention, there is provided a method of controlling a well, the method comprising the steps of: providing a well shutoff device having a housing defining an internal bore and at least one cutting torch coupled to the housing; and on detecting a condition indicative of an emergency situation, activating the at least one cutting torch to

direct a jet of a cutting material on to an object located within the housing bore to thereby cut the object, facilitating control of the well.

According to a tenth aspect of the present invention, there is provided a method of controlling a well, the method comprising the steps of: providing a well shutoff device having a housing defining an internal bore; coupling a housing of a cutting device to the well shutoff device housing; and on detecting a condition indicative of an emergency situation, activating an at least one cutting torch coupled to the cutting device housing, to direct a jet of a cutting material on to an object located within and extending through the well shutoff device and cutting device housing bores, to thereby cut the object and facilitate control of the well.

According to a still further aspect of the present invention, there is provided a means of chemical cutting whereby the cutting tool is radially arranged around a pipe to be severed and is positioned in a fixed location ready at all times for deployment where the pipe must be severed in an emergency.

Such a situation is envisaged where a pipe extends from a drilling rig or vessel through the seafloor where it may be cemented in place or otherwise fixed. In an emergency it is necessary to rapidly disconnect the vessel from the pipe.

Prior art also describes the use of underwater explosive cutting tools based on the use of shaped charges for pipe severance. These charges are formed as an annular ring around the pipe to be severed. The inherent disadvantages of explosives are the shock waves created. These problems can be exacerbated if operated in a confined space such as a wellhead or BOP stack. According to this invention the chemical cutting method operating in annular mode whereby the cutting jets are directed inwards overcomes many of the inherent issues of explosives in a confined space.

Further features of the methods of the seventh to tenth aspects of the invention are defined above in relation to the sixth aspect of the invention.

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

Figs 1 and 2 are perspective and plan views, respectively, of a cutting device in accordance with a preferred embodiment of the present invention;

Fig 3 is a view of the cutting device of Figs 1 and 2, taken in the direction of the arrow X of Fig 1;

Fig 4 is an enlarged, detailed view of part of the cutting device shown in area A of Fig 3;

Figs 5 to 7 are schematic illustrations of a blowout preventer (BOP) assembly, incorporating the cutting device

of Figs 1 to 4, in accordance with an embodiment of the present invention;

Fig 8 is a detailed cross-sectional view of the cutting device shown in Figs 1 to 4, taken along line Y-Y of Fig 3; and

Fig 9 is a schematic, partial longitudinal sectional view of a well shutoff assembly, incorporating the cutting device of Figs 1 to 4, in accordance with an embodiment of the present invention.

Turning firstly to Fig 1, there is shown a perspective view of a cutting device in accordance with a preferred embodiment of the present invention, the cutting device indicated generally by reference numeral 10. The cutting device 10 is also shown in the plan view of Fig 2, in Fig 3, which is a view taken in the direction of the arrow X of Fig 1, and in Fig 4, which is an enlarged detail view of part of the device 10 shown in the area A of Fig 3.

The cutting device 10 generally comprises a housing 12 having an internal bore 14 and at least one cutting torch 16 coupled to the housing 12. In the illustrated embodiment, the cutting device 10 includes six torches lβa to 16f which are arranged spaced around a circumference or perimeter of the housing 12. The torches lβa to 16f are each of similar construction and operated in a similar fashion, and it will be understood that further reference herein to the torch and its method of operation applies equally to the torches lβa to lβf.

The torches 16a to 16f are each arranged to direct a jet of a cutting material on to an object located within the housing bore 14 to thereby cut the object. The torches lβa to 16f are each of a type commercially available from the Jet Research Centre, Texas, USA under the THERMOL CUTTING SYSTEM Trade Mark. Similar such torches are also disclosed in US Patent no. 5,372,069 the disclosure of which is incorporated herein by way of reference.

Whilst the cutting device 10 has a general utility in the cutting of any desired object which may be located in the housing bore 14, the cutting device 10 has a particular utility for cutting downhole objects, such as casing, liner, production tubing, drill tubing, downhole tools, tool or tubing strings as used in the oil and gas exploration and production industry.

One such particular utility of the cutting device 10 is illustrated in Figs 5 to 7, which are schematic illustrations of a blowout preventor (BOP) assembly 18 during various stages of operation. The assembly 18 incorporates a BOP 20 and the cutting device 10 shown in Figs 1 to 4.

The BOP 20 is of a conventional type comprising a number of sets of shear rams, seal rams and blind rams, as is known in the art. In the illustrated schematic view, the BOP 20 includes an upper pair of shear rams 22, upper blind rams 24, and two pairs of lower pipe or seal rams 25 and 27. At an upper end of the BOP assembly 18, a hub 26 is provided by which the assembly 18 is coupled to a marine riser 28 using a suitable connector 29. The riser 28 is used to run and

support the assembly 18 from a surface rig or the like (not shown) to a seabed 30, for connection to a wellhead 32.

As shown in Figs 1 to 4, the housing 12 includes upper and lower flanges 34 and 36. These flanges 34, 36 serve for coupling the cutting device 10 into the BOP assembly 18. As shown schematically in Figs 5 to 7, the cutting device 10 is coupled to a lower end 38 of a housing 40 of the BOP 20 using the upper flange 34, which mates with a corresponding flange (not shown) on the BOP housing 40, and is secured using suitable API connectors (not shown). The BOP assembly 18 also includes a housing 42 which carries the pipe rams 25 and 27 which, as will be described below, serve for sealing an annulus 46 around a section of casing 48 extending through the BOP assembly 18. The cutting device housing 12 is coupled to the lower BOP housing 42 via flange 36.

The BOP assembly 18 is shown in Fig 5 during construction of a wellbore 52. The wellbore 52 is shown at a stage when a conductor comprising a first casing section 54 has been driven into the seabed 30 and cemented in place, and following extension of the wellbore 52 to a second depth. The casing 48, which is a second casing of smaller diameter (typically 7 5/8") is shown during run-in to the wellbore 52 and location in the extended wellbore section. The casing 48 carries a hanger 49 for suspending the casing from and sealing the casing to the wellhead 32, and is suspended from a running string 50.

In the event of an emergency situation arising prior to hanging of the casing 48, it may become necessary to shut-in the well, that is to seal-off the wellbore 52. One such

emergency situation is when extreme weather conditions occur which would be likely to cause a rig located over the wellbore 52 to move off-station, which could lead to damage to the marine riser 28, casing 48, BOP assembly 18 and/or wellhead 32. If sufficient time is available and in the situation shown in Fig 5, the casing string 48 would be pulled from the wellbore 52 through the BOP assembly 18 and returned to surface, the shear and blind rams 22 and 24 closed, and the marine riser 28 disconnected from hub 26. However, if there is insufficient time to carry out these steps, it becomes necessary to activate the cutting device 10. This severs the casing string 48, permitting the portion of casing above the cut to be returned to surface. The BOP rams 22, 24 are then closed and the marine riser 48 disconnected. The rig may then be moved to a safe location, or may weather-out the storm without concerns about moving- off station.

The steps in the method of cutting the casing 48 are shown in Figs 6 and 7, and are as follows. When the emergency situation arises, the pipe rams 25 are first actuated (rams 27 providing a backup) to securely grip the casing 48 and to seal the annulus 46. This ensures that, following cutting of the casing 48, the portion of the casing below the cut is held and thus prevented from falling into the wellbore 52, and sealed. The torches 16a to 16f of the cutting device 10 are activated simultaneously and direct jets 56 of cutting material on to an outer surface 58 of the casing 48. These jets 56 of cutting material rapidly cut through the wall of the casing 48, forming a relatively clean cut without significant deformation of the portion of the casing 48 adjacent the cut remaining within the BOP assembly 18. The

casing 48 has then been split into an upper section 48a, and a lower section 48b which is held by the rams 25 and remains within the borehole 52. The section 48a is recovered to surface through the marine riser 28.

In certain circumstances, a tubing such as a coiled tubing (not shown) may have been located within the casing 48, for carrying out desired wellbore functions. The torches 16 are likely to also cut through such tubing. However, in the event that the tubing is not cut, the shear rams 22 of the BOP 40 shear the tubing 60 and hold the portion below the cut. The remaining, upper section is then recovered to surface.

The shear and blind rams 22 and 24 are then activated to close off and seal a main bore 62 of the BOP 20. After pressure testing to verify that the BOP 20 has shut-in the well, the marine riser 28 can then be disconnected from the hub 26, as shown in Fig 7, and the rig can move-off station.

When it is desired to reconnect to the well, the riser 28 is reconnected to the hub 26 and a pressure test carried out. If necessary, a heavy (dense, viscous) fluid is pumped down the riser 28, to re-establish control of the well. The rams 22 and 24 can then be reopened. The remaining casing section 48b is then fished from the well, facilitated by the clean cut provided by the cutting device 10. Following remedial work such as well cleanout (if necessary) , a fresh casing string (not shown), similar to the casing 48, is run- in, as shown in Fig 5, and located on the wellhead 32. Further downhole operations may then be carried out to complete the well.

Turning now to Fig 8, there is shown a detailed cross- sectional view of the cutting device 10 of Figs 1 to 4, taken along line Y-Y of Fig 3. The housing 12 includes a central hexagonal portion 64 having a number of outer faces 66a to 66f, each one corresponding to a respective torch 16a to 16f. A number of apertures 68a to 68f extend through the central portion 64 between the outer faces 66 and the internal bore 14 of the housing 12. As will be described, the torches 16 are each located extending along the apertures 68, such that the torches can be arranged to direct jets 56 of cutting material on to the casing 48.

Each torch 16 includes a torch housing 70 carrying a flange 72 by which the torch housings are bolted to the respective central portion faces 66. 0 ring seals are provided for sealing between the torch housings 70 and the central portion 64, which ensure that well pressure is contained by the cutting device 10, and thus provide a barrier to fluid egress.

Each torch housing 70 defines a chamber 76 carrying a graphite sleeve 78a and an igniter core 80. Pyronol material 82 in pellet form is provided in the graphite sleeve 78 around the igniter core 80. A detonator 84 is coupled to the igniter core 80 and serves for igniting the core and thus the pyronol material 82. A graphite nozzle 86 is provided at the internal end of each graphite sleeve 78, for directing the cutting material (molten metals, metal oxides and gas) generated on activation of the torches 16 on to the casing surface 58. The nozzles 86 are each shaped to form flat, generally fan-shaped jets 56 of cutting material,

which impinge upon the outer surface 58 of the casing 48. It will be noted that some of the components of the torches 16b to 16f have not been indicated by reference numerals in Fig 8; this is for ease of illustration. All components are, however, illustrated in the torch 16a, and it will be understood that like components are provided in each of the torches 16b to 16f.

As shown in Fig 8, the torches 16a to 16f are activated simultaneously by sending appropriate signals to activate the detonators 84. The detonators 84 ignite the core 80, which in turn ignites the pyronol material 82. The nozzle 86 includes a shield in the form of a membrane (not shown) , and when a sufficient pressure and temperature has built up in the torch chambers 76, the resultant cutting material burns through the membrane and is jetted through the nozzles 86. The arrangement of the torches 16a to 16f spaced around a perimeter of the housing 12 provides a good spread of cutting material over the outer circumference of the casing 48, which is quickly burnt through. This, in combination with the applied tensile load on the casing 48 serves to form a clean cut at the location where the cutting material is jetted on to the casing 48. The upper casing section 48a can then be retrieved to surface, as described above.

It will be understood that in an alternative embodiment of the invention, a BOP (not shown) may be provided having a housing which defines the cutting device housing, and thus provides a mounting for the cutting torches. For example, referring to Figs 5 to 7, the BOP housing 40 may be extended and may define the housing 12 of the cutting device 10.

Thus the torches may be provided integrally with a BOP, rather than in a separate, bolt-on module.

Turning now to Fig 9, there is shown a schematic, partial longitudinal sectional view of a well shutoff assembly in accordance with the teachings of WO 2006/010906, the disclosure of which is incorporated herein by way of reference, the shutoff assembly indicated generally by reference numeral 88.

The shutoff assembly 88 includes a shutoff device 89, template 90, a first casing string 92, and a second casing string 94 carrying a drillshoe 96 at a lower end. The shutoff device 89 also includes a housing 98 carrying two pairs of shear rams 100 and 102 and seals 103 for sealing the annulus around the casing 94. A gripping mechanism 104 having upper and lower gripping devices 106, 108 selectively engages with the second casing 94. The shutoff assembly 88 is used in the drilling and casing of a wellbore, such as the wellbore 52 shown in Figs 5 to 7 and, in accordance with the teachings of WO 2006/010906, is generally operated as follows. The shutoff device 89 is suspended from the casing string 94 using the upper gripping device 106, and is run from surface to the seabed 30. Fluid is then circulated through the casing 94 and the drillshoe 96 to jet the larger diameter casing 92 into the seabed 30. Once the template 90 has been installed in the seabed 30 and is self-supporting, the upper gripping device 106 is actuated to release the second casing string 94. Drilling using the shoe 96 then commences.

Following drilling and casing to a desired depth, the casing string 94 is cemented in place and the string 94 spaced-out and supported by a lower gripping device 108. The casing string 94 is then tensioned at surface using appropriate riser tensioners. The casing 94 is now converted into a riser, fixed at the seabed 30 and held in tension from the rig. The assembly 88, particularly the seals 103, is pressure tested and then ready for further downhole procedures to be carried out.

The shutoff assembly 88 incorporates the cutting device 10 of Figs 1 to 4, which is shown schematically in Fig 9. The cutting device 10 is bolted between the housing 98 and a lower housing 110 and, in a similar fashion to the BOP assembly 18, is pressure containing. In the event of an emergency situation arising following location and hanging of the casing 94 (such as extreme weather or a blowout where control of well fluids is lost), the torches 16 are activated to cut the second casing 94. The lower gripping device 108 holds the portion of the casing 94 below the cut, and the portion above the cut is retrieved to surface. The rams 102 are actuated to shut-in the well.

In a similar fashion to the BOP assembly 18 described above, the torches 16 provide a clean cut of the casing 94. When it is desired to reconnect to the shutoff assembly 88, a fresh casing string is run carrying a connector (not-shown) similar to the connector 29 of Figs 5 to 7. The connector engages with a re-entry hub 112 at an upper end of the assembly 88. The connector engages and seals to the hub 112, and also engages and seals to the fresh casing. After control of the well has been re-established, if required

(for example by pumping a heavy fluid down the casing string above the assembly 88), the rams 100, 102 can be reopened and fluid communication with the portion of the casing 94 residing within the wellbore 52 can be achieved. The clean cut of the casing 94 permits easy re-entry to the wellbore with further tubing and/or tool strings.

Various modifications may be made to the foregoing without departing from the spirit and scope of the present invention.

For example, the cutting device may be for use in cutting a wide range of types of downhole objects, including but not limited to liner, production tubing, drill tubing, tubing forming part of a downhole tool or tool string, or parts or sections thereof. The cutting torch of the cutting device may then perform primary cutting of tubing or the like extending through the BOP, with shear rams of the BOP optionally held in reserve; downrated for use in cutting smaller diameter/wall thickness tubing and the like (and thus reducing weight of the BOP) ; or replaced by the cutting torch.

The cutting device may be for cutting alternative objects, including other tubing such as pipeline, which may be provided at surface or subsea.

The cutting device may comprise a plurality of sets of cutting torches, each set comprising at least one cutting torch, and the sets may be spaced axially and/or circumferentially relative to or about the housing.

The or each cutting torch may be arranged such that the torch main axis does not intersect with the bore axis. For example, the or each torch may be arranged to direct a jet of material in a direction along a tangent of an tubular object located in the bore. It will be understood, however, that the or each torch may be provided in another suitable arrangement.

Other suitable types of cutting torch may be utilised, if desired. For example, cutting torches of alternative shapes and/or comprising alternative components may be provided. The torches may be chemical cutting torches operated according to alternative procedures, for example, using cutting fluids, and may be constructed and operated utilising the cutting techniques disclosed in one or more of US2,918,125; US4,619,318; US3,076,507; US4,125,161; and/or US4,494,601.