WELL ABANDONMENT

The present invention relates in particular to apparatus for removing a section of tubular such as casing or lining from a well, and related methods.

In the oil and gas exploration and production industry, tubulars such as risers, casing or lining are used to carry fluids and equipment from deep underground to topside facilities for the processing of hydrocarbons. In subsea wells, it is typical to have a number of concentrically arranged casings, each of which extends progressively deeper in the well and upon which further casing/liner strings are suspended to reach the desired depth. Typically, the first casing section is a 30" (762mm) conductor which is cemented into the seabed. This conductor can extend from the seabed, typically through a rig structure to the topside facilities. Progressively smaller diameter casings e.g. 20" (508mm), 13 3/8" (340mm), 9 5/8" (244mm) are inserted and supported from casing hangers. A 7" (178mm) liner may then be suspended at the extremity of the innermost casing. Cement or grout is located in the annulus between concentric casings to stabilise the wellbore wall, for example by preventing collapse of formation material into the wellbore, and can help to control pressure and provide fluid containment in the wellbore during use of the wellbore, such as when drilling or producing oil or gas.

In various situations it can be desirable to remove a section of the tubing which has previously been installed at a well. Typically, this involves cutting the casing to produce a cut section of a manageable size, and then pulling on the cut section to remove it. Removal of casing or lining may be a necessary and important part of a plug and abandonment operation for abandoning a well, where there may typically be a need to remove the casing in order to provide a suitable plug for the well. For complete abandonment of a well, UK legislation requires the removal of the wellhead together with all casing to a depth of 3m below the seabed.

Previous approaches to removing sections of casing or lining in a wellbore have involved running a cutting tool and a pulling tool into the wellbore in separate trips for performing the cutting and pulling operations respectively. By performing these operations in separate trips however, such approaches can be inefficient and costly. Accordingly, combined casing cutting and pulling tools have been developed so that the cutting and pulling of the casing can be achieved on a single trip. Such a tool is the TRIDENT® System to the present Applicants, Ardyne Technologies Limited.

WO2017046613 describes a cutting and pulling tool which advantageously has a cutting tool which can be operated by rotation of the work string while the pulling tool is anchored to the inside wall of the casing section above the cut to hold the casing in tension and provide stability to the cutting action. The pulling tool may be considered as an anchor or spear. A prior art anchor mechanism for assisting in the cutting operation and, if required, for pulling the tubular is illustrated in Figures 1A and IB. The present invention is an improvement on this design. For completeness in understanding this invention, the prior art anchor mechanism of Figures 1A and IB will be reviewed to provide a framework to understand the present invention.

Figure 1A is a sectional view of an anchor mechanism 10 in a run-in configuration. The principle of operation of the anchor mechanism 10 is to force slips 12 up a slope or ramp 14 of a cone 16 so that the slips 12 move outwards and engage the tubular 30. The slips 12 are moved axially by a piston in the form of a sleeve 18 actuated by a hydraulic force being the fluid pressure against a face 20 of the sleeve 18. The sleeve 18 acts against a spring 22. The slips 12 are retained by an underside 24 of the sleeve 18. On pumping fluid through the central bore 26, by virtue of a restriction arranged in the bore below the anchor (not shown), fluid enters ports 28 to act against face 20. Sleeve 18 moves axially acting on the slips 12 forcing them up the ramp 14 of the cone 16. The slips 12 move radially outwards until their travel is limited by the underside 24 of the sleeve 18. As the slips 12 travel outwards they will engage the inner surface 32 of the tubular 30. This is as illustrated in Figure IB. In this example, the slips 12 engage 9 5/8" (244mm) tubular 30. To set the slips into the surface 32 of the tubular 30 an over pull would typically be applied which forces the cone 16 under the slips 12 to drive them further outwards to anchor onto the tubular 30. Such action means that the fluid through the bore 26 can be stopped or varied without activating or de-activating the slips 12. When the anchor mechanism requires to be unset, weight is set down on the mechanism 10, so as to move the cone 16 away from the slips 12, the release of support coupled with the bias on the spring 22 releases the slips 12 from contact on the inner surface 32 of the tubular 30. The slips 12 are drawn back and the anchor mechanism can be moved and reset elsewhere.

In the mechanism 10, a single piston in the form of the sleeve 18 acts against the slips 12 to move them uniformly radially outwards. Thus each slip travels at the same rate over the same distance. It is assumed that the mechanism 10 is on a central axis of the tubular 30 and the inner surface 32 is circular in cross-section. In this way, the slips 12 will all touch the inside surface 32 at the same time and grip to the same degree, thus maintaining the centralised position of the mechanism 10 when anchored which should centralise the cutting tool.

A disadvantage of this mechanism 10 exists when the inner surface is not cylindrical. As the tubular 30 has been in use for the life of the well, the inner surface may have become corroded or have deposits from scale or other debris. Further due to compression, the tubular 30 may have buckled and now provide a non-circular cross-section. Additionally, it is known in the construction of some tubing, particularly the 30" (762mm) conductor, to roll and weld plate which creates a longitudinal seam weld on the inner surface 32. Figure 1C shows the effect of the mechanism 10 being set in a tubular 30 where a seam weld 34 causes a discontinuity on the inner surface 32. In the prior art, when the sleeve 18 acts on the slips 12, if a slip 12a meets the weld 34, then the mechanism 10 is moved off axis as the slipl2a continues to expand radially outwards until the opposing slip 12c contacts the inner surface 32. Continued expansion, causes effective gripping by only two slips 12a, c and setting of the mechanism 10 and tool string off-axis. Besides reducing stability, the blades on the cutting tool may not cut through the entire tubular 30 at the weld 34 preventing pulling of the tubular or the lifting capacity of the spear could be severely compromised as potentially only a proportion of the slips would be load bearing. Figure 1C shows the effect with four slips 12a-d, but the effects will be apparent with any number of slips. It is therefore an object of the present invention to provide apparatus for anchoring a string at a well for the purpose of cutting a tubular which obviates or mitigates at least some of the disadvantages of the prior art.

It is a further object of the present invention to provide a method for anchoring a string at a well and cutting a tubular which obviates or mitigates at least some of the disadvantages of the prior art.

According to a first aspect of the present invention there is provided apparatus for anchoring a string at a well for the purpose of cutting a tubular, including an anchor mechanism, the anchor mechanism comprising : a tubular body having a central bore between an inlet and a first outlet, the inlet and first outlet being adapted for connection in a drill string to be run into the tubular;

a cone arranged around the tubular body, the cone having an outer surface including a slope;

a plurality of selectively operable slips, each slip having an outer surface configured to grip an inner surface of the tubular and an inner surface matching the slope of the cone they are arranged upon; and

a plurality of pistons, there being a piston for each slip, each piston being operable to act to move each slip independently over the cone between a retracted configuration and an extended configuration wherein the outer surface of each of the slips contacts the inner surface of the tubing. By having each slip operable by a separate piston, the slips can move non-uniformly over the cone so that if one slip meets an obstruction, the others can continue to move radially outwards to contact the inner surface of the tubular. In this way, the apparatus is centralised within the tubular when the anchor is set.

Preferably, the tubular body is rotatable relative to the cone. In this way the tubular body is rotatable relative to the slips. Advantageously the apparatus can be used to stabilise the drill string while tools below can be operated by rotation of the drill string. Preferably, a bearing is located between a base of the cone and a ledge on the tool body. In this way, with the slips set, the tool body and drill string to which it is attached can be rotated within the static cone.

Preferably, each piston comprises a piston rod contained within a cylinder arranged co-linearly with a central axis of the apparatus. Each piston rod is arranged to act on a face of each slip. The piston may be mechanically or hydraulically operated. In an embodiment, the piston is hydraulically operated by action of fluid from the central bore. This allows the slips to be moved remotely by pumping fluid from surface above a pre-set flow rate threshold. Each piston rod may be configured to move in response to fluid pressure acting on an end face of the piston rod.

Preferably, each piston includes biasing means to hold each slip in the retracted configuration. In an embodiment, the biasing means is a spring arranged in the cylinder to act against the piston rod. Advantageously, the flow rate threshold may be set by changing the spring force acting on the piston rod. This allows other tools on the string to be activated by fluid pressure in the central bore also.

Preferably a first end of each slip is located under a lip of a connector to each piston rod. In this way, the radial travel of each slip is limited so that each slip is retained within the apparatus.

In an embodiment, each piston includes an emergency release arrangement so as to ensure retraction of each slip when the apparatus is moved back from the extended configuration to the retracted configuration. Preferably the arrangement comprises a release piston configured to act on the piston rod. A release piston rod may be attached to the piston rod so as to move therewith, the release piston rod having an end face of smaller surface area than that of the end face of the piston rod. Each release piston may be mechanically or hydraulically operated. In an embodiment, the release piston is hydraulically operated by action of fluid from the central bore. A port may be arranged from the central bore to the cylinder of the release piston and fluid is diverted from the central bore and each piston, by a drop ball locating in a seat in the central bore.

In an alternative embodiment, the biasing means to holds each slip in the extended configuration. In this way, the apparatus can be used as a spear to recover a cut section of tubular. In this embodiment, a locking arrangement is provided which holds the slips in the retracted configuration until sufficient fluid pressure is applied. In this way, the apparatus can be run in with the slips retracted. Preferably, the locking arrangement is resettable. Preferably, the locking arrangement is re-set by movement of the release piston. More preferably, the locking arrangement comprises a detent in the form of a collet.

In an embodiment, the apparatus includes a plurality of selectively operable centralisation pads, each pad having an outer surface configured to match an inner surface of the tubular and an inner surface matching the slope of the cone they are arranged upon; and

the plurality of pistons including a piston for each centralisation pad, each piston associated with a centralisation pad being operable to move each centralisation pad independently over the cone between a retracted configuration and an extended configuration wherein the outer surface of each of the centralisation pads is at drift to the inner surface of the tubular. Preferably the slope of the cone at each centralisation pad is steeper in gradient than the slope of the cone at each slip. In this way, the centralisation pads are deployed ahead of the slips and thus the apparatus is centralised prior to contact of the slips with the inner surface of the tubular.

Preferably, there are an equal number of slips to centralisation pads on the apparatus. More preferably, a centralisation pad is located between each slip and spaced equidistantly around a circumference of the tool body.

The remaining features of the pistons may be as described herein with respect to the pistons associated with the slips. According to a second aspect of the present invention there is provided a method for anchoring a string at a well and cutting a tubular, comprising the steps:

(a) mounting apparatus according to the first aspect on a drill string;

(b) mounting a cutting tool on the drill string;

(c) running the drill string in the wellbore and locating the cutting tool at a location at which the tubular is to be cut;

(d) setting the slips independently to anchor the apparatus to an inner surface of the tubular to be cut;

(e) cutting the tubular using the cutting tool;

(f) unsetting the slips to release the apparatus from the inner surface of the cut section of tubular. In this way, by independently operating the slips, the apparatus can better grip the tubular regardless of any discontinuities, deposits upon or eccentricity of the tubular to which they are being anchored. This is achieved by the ability of the slips to be expanded non-uniformly from the apparatus.

Preferably, the method includes the step of hydraulically actuating the pistons to contact the slips to the inner surface of the tubular. This allows the slips to be moved remotely by pumping fluid from surface above a pre-set flow rate threshold.

Preferably the method includes the step of applying an over pull to the anchor mechanism once the slips have contacted the inner surface of the tubular. This sets the anchor mechanism to prevent accidental release of the anchor mechanism. The tension or pulling force may wedge or lock the slips between the outer surface of the cone and the tubular or downhole tubular. By setting the anchor mechanism the fluid pressure may be reduced below the pre-set threshold flow rate or stopped without the anchor mechanism being deactivated.

Preferably, the anchor mechanism is unset i.e. deactivated by applying a downward force to the tool. This force will pull the cone away from the slips and then the spring will bias the slips back into the recess.

Preferably, the slips are arranged to rotate with respect to the drill string. In this way, the cutting tool may be operated via rotation of the drill string with the apparatus set against the tubular to provide stability to the cutting tool.

Optionally, the method may include radially extending centralising pads independently on the apparatus. In this way, the drill string can be centralised and with it the cutting tool before the cut is made.

Optionally, the method may include activating an emergency release arrangement to unset the apparatus and draw the slips away from the inner surface of the tubular in the event that they don't release.

The method may include the step of removing the cut section of tubular by using the anchor mechanism as a spear.

The method may be used in tubular within the wellbore such as casing or may be used on tubing above the wellbore i.e. in the wellhead, above the seabed and in the rig structure. The method may also find application in pipelines.

In the description that follows, the drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results.

Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including," "comprising," "having," "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes.

All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein including (without limitations) components of the apparatus are understood to include plural forms thereof.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figures 1A and IB are longitudinal section views, and Figure 1C is a cross-sectional view, of a prior art anchor mechanism in run-in and set positions;

Figure 2 is a longitudinal section view of an anchor mechanism in a retracted configuration according to an embodiment of the present invention;

Figure 3 is a view along lines A-A' of Figure 2; Figure 4 is a longitudinal section view of the anchor mechanism of Figure 2 in an extended configuration according to a further embodiment of the present invention;

Figure 5 is a longitudinal section view of the anchor mechanism of Figure 2 in an extended configuration according to an embodiment of the present invention;

Figure 6 is a view along lines B-B' of Figure 5;

Figure 7 is a longitudinal section view of the anchor mechanism of Figure 2 in an extended configuration according to a yet further embodiment of the present invention;

Figures 8A to 8D provide schematic illustrations of a method of removing a section of tubular from a wellbore according to an embodiment of the present invention;

Figure 9 is a longitudinal section view of a portion of an anchor mechanism in a retracted configuration according to a further embodiment of the present invention; and

Figure 10 is a view along lines A-A' of Figure 9.

Reference is initially made to Figure 2 of the drawings which illustrates an anchor mechanism, generally indicated by reference numeral 110, according to an embodiment of the present invention. Like parts to those of Figure 1 have been given the same reference numeral with the addition of 100. Anchor mechanism 110 is formed of a one piece cylindrical body 36, having a central bore 126 providing a through passage with a fluid inlet 50 at an upper end 46 and a first fluid outlet 52 at a lower end 48. At the upper end 46 there is a box section 56 and at the lower end 48 there is a pin section 58, for connecting the anchor mechanism 110 into a drill string (not shown) as is known in the art. A difference as compared to Figure 1A of the prior art mechanism 10 is in the presence of a second cylindrical body 60 located around the first cylindrical body 36. The second cylindrical body 60 has at a first end 62, the cone 116 machined on an outer surface 64 thereof and at a second end 66 a plurality of piston housings 68. A single piston housing 68 is shown in Figure 2, but there will be a number of equidistantly spaced piston housings 68 arranged around the cylindrical body 60. In an embodiment, there are eight piston housings 68a-h, shown in the cross- sectional view in Figure 3. The second cylindrical body 60 is also of unitary construction as for the first cylindrical body 36. An end cap 70 provides closure to the piston housings 68a-h, being attached to the second cylindrical body 60 by screws 72.

Each piston housing 68 is a cylindrical bore 74 containing a piston rod 76. The piston rod 76 has a back plate 78 providing a back face 80. The back plate 78 is sized to fit and move within the bore 74, while being sealed thereto, by an o-ring 82. The piston rod 76 is stepped to provide a first annular face 84 against which a spring 122 can act and a second annular face 86 which limits the travel of the piston rod 76 from the bore 74, by contact of the face 86 with an inner wall 88 of the end 90 of the housing

68. It will be noted that there is a separate spring 122 in each piston housing 68 as compared to the prior art spring 22 shown in Figure 1. Each spring 122 is biased between the first annular face 84 and the inner wall 88.

The piston rod 76 is actuated in a similar manner as for the prior art, except in that a plurality of ports 128 now exist to connect the central bore 126 to each piston housing 68a-h. The piston rods 76 are thus driven by fluid from the central bore acting on the back face 80.

An aperture 92 through the end 90 of the housing 68 allows the piston rod 76 to exit the housing 68 and act upon an end face 94 of a slip 112. In an embodiment, there are four slips 112 spaced equidistantly around the cone 116 and located in box groove channels 95 as are known in the art. Each slip 112 is an elongate member having an outer surface 96 with a curvature to match that of the inner surface 32 of the tubular 30 in which it is intended to be set. The outer surface 96 is knurled, grooved or toothed to provide a suitable grip and bite into the inner surface 32 of the tubular 30 on contact. The inner surface 98 of each slip 112 is the reverse of the outer surface or ramp 114 of the cone 116 along the channel 95. This is done to provide a mating arrangement.

The feature of the underside 24 of the sleeve 18 in the prior art to limit radial travel of the slip 12, is maintained in the present invention. However, the underside 124 is now created by a plate 99 attached to each rod 76.

An additional feature is also provided on the anchor mechanism 110. Between the base 38 of the second cylindrical body 60 and a ledge 40 on the first cylindrical body 36 there is located a bearing 83. Bearing 83 is as known in the art and allows the tubular body 36 to rotate relative to the tubular body 60. Chevron seals 85 are also located between the bodies 36,60 which allow rotation therebetween while maintaining a seal around the ports 128. Thus, if the slips 112 are set, the slips 112 and the second cylindrical body 60 can remain stationary and the first cylindrical body 36 can rotate with the string. This allows the transmission of rotation through the anchor mechanism 110 when the anchor mechanism 110 is set.

A yet further feature is shown on the anchor mechanism 110. An emergency release system, generally indicated by reference numeral 42, is provided comprising a secondary piston 44 located in the piston housing 68 behind and attached to the main piston rod 76. There is a release system 42 at every piston housing 68a-h in the anchor mechanism 110. Piston 44 has a front actuating face 54 with a smaller surface area than the back actuating face 80 of the main piston rod 76. In this way, fluid coming through ports 43 to the release system 42, when the same fluid pressure is delivered to ports 128 to the main piston rod 76, will cause only the main piston rod 76 to be actuated. Ports 128 are closed when the emergency release is operated via a ball landing on a ball seat 45 in the central bore 126 between the ports 43,128. There is no spring present and the piston 44 is moved by fluid pressure against the front actuating face 54 which draws the main piston rod 76 back into the housing 68 so as to actively disengage the slips 112.

In an embodiment of the present invention a number of the slips 112 are replaced by centralising pads 51. In a preferred embodiment, there are four slips 112 and four centralising pads 51, with a centralising pad 51 arranged between each slip 112 and spaced equidistantly around the body 36. The centralising pads 51 are operated from an identical piston housing 68 arrangement as for the slips 112, with the exception that the piston rod 76 now includes a stop ring 53 at face 86 to limit the travel of the piston rod 76 out of the housing 68. In this way, the centralising pads are prevented from contacting the inner surface 32 of the tubular 30. The curvature on the outer surface 55 of centralising pads 51 and the distance of travel can be made to match the curvature of the tubular 30 so as to drift in the tubular 30 and centralise the apparatus 110. Additionally the ramp 114 of the cone 116 can be made steeper at the location of the centralising pads 51 as compared to the ramp 114 at the slips 112 so that the centralising pads extend first from the body 60. A centralising pad 51 and piston housing 68 arrangement is shown in Figure 4.

Figure 2 illustrates the anchor mechanism 110 of the present invention in a retracted configuration. This is the run-in position. The slips 112 are entirely located within the second body 60 of the anchor mechanism 110, so to prevent the outer gripping surface 96 contacting any tubular on run- in. The slips 112 are at the bottom of the ramp 114 and the piston rod 76 is held back in the piston housing 68 by virtue of the spring 122. This will be the same for all the slips 112 and centralising pads 51, if present. When the anchor mechanism 110 is located in tubular 30 and an anchor is required, the slips 112 can be set. This is achieved in an identical manner to the prior art anchor mechanism 10 by pumping fluid from surface through the central bore 126 to meet a restriction in the string at a lower location (not shown). Fluid under pressure will then enter the ports 128, 43. Due to the larger surface are of the back face 78, the piston rod 76 is moved towards the lower end 48, taking the secondary piston 44 with it and compressing the spring 122. The movement of the piston rod 76 acts on slips 112 and pads 51 forcing them up the ramps 114 of the cone 116. The centralising pads 51 will extend first due to the steeper slope of the ramp 114 at their locations but they will also be stopped before they all can contact the tubular 30. Any contact will move the apparatus 110 and centralise it as the centralising pads 51a-d take up their drift position. As the slips 112 travel up the ramps 114 independently of each other, if any slip 112 contacts a discontinuity on the wall of the tubular, such as a seam well on a conductor, this slip will be prevented from moving further, but all the other slips 112 can continue to travel outwards and will all grip the inner surface 32 of the tubular 30. A slip 112 in the extended configuration is shown in Figure 5. The centralising pad 51 in Figure 4 is shown in the extended configuration. Cross section through B-B' on these Figures is shown on Figure 6, for the case when a discontinuity, such as a seam weld or deposit 34 is present on the wall 32 of the tubular 30. Flere it is seen that the slip 112a has not extended as far as the other slips 112b-d, but that the apparatus 110 remains centralised in the tubular 30, and the remaining slips 112b-d provide firm gripping contact with the inner surface 32 of the tubular 30, in direct contrast to the prior art case of Figure 1C. A majority of the slips 112 have engaged the tubular 30. To set the slips 112 into the surface 32 of the tubular 30 an over pull is applied which forces the cone 116 under the slips 112 to drive them further outwards to anchor onto the tubular 30. This can be considered as an extended configuration.

With the mechanism 110 now fully anchored, the fluid through the bore 126 can be stopped or varied without activating or de-activating the slips 112. When the anchor mechanism requires to be unset, weight is set down on the mechanism 110, so as to move the cone 116 away from the slips 112, the release of support coupled with the bias on the spring 122 releases the slips 112 from contact on the inner surface 32 of the tubular 30. The slips 112a-d and centralising pads 51a-d are drawn back and the anchor mechanism 110 can be moved and reset elsewhere.

When unset, the anchor mechanism 110 returns to the retracted configuration, see Figure 2. However, if the slips 112 failed to retract when weight was set down, the emergency release system 42 can be used. A ball 47 is dropped from surface through the central bore 126 to land in the ball seat 45. This cuts off fluid flow to the lower ports 128 which operate the piston rod 76 and instead directs fluid to the secondary piston 44. The fluid acts against the fluid actuating face 54 to move the piston 44 back towards the upper end 46. As the pistons 44,76 are connected, this action draws the piston rod 76 back into the housing 68 and with it the slips 112/centralising pads 51. The drop ball 47 arrangement is shown in Figure 7 with the ball 47 in place prior to movement of the secondary piston 44. The emergency release 42 is can be optionally included and optionally used if required. A typical method for anchoring a string at a well and cutting a tubular using the anchor mechanism 110, will now be described with reference to Figures 8A-D. Anchor mechanism 110 is mounted on a drill string 100 above a cutting tool 102, as is known in the art. The drill string 100 is run into tubular 30 and the blades 104 of the cutting tool located at a desired position to make a cut in the tubular 30. In the example given, the tubular is a 30' (762mm) conductor which may be cut either in the wellbore or above the seabed in the rig structure. If used within the wellbore the inner casing strings have been removed. As such the presence of a packer above the anchor mechanism 110 is not required but may be added for cutting other tubular sections. Additionally, a bridge plug and/or drill bit to dress off a cement plug are also not required but could be added to the drill string 100 if desired. For this application a brush 106 and/or wiper 108 can be included on the drill string 100, preferably below the cutting tool 102 to condition the inner surface 32 of the tubular 30 prior to anchoring and cutting. A suitable arrangement is illustrated in Figure 8A having been run into the tubular 30. During run-in the inside surface 32 of the tubular 30 will have been wiped and cleaned of debris and other detritus which could affect the gripping action of the slips 112.

With the blades 104 located at a desired cutting position, see Figure 8B, the anchor mechanism is set as described herein before. A restriction in the bore of the cutting tool 102, is used to increase the pressure in the anchor mechanism to provide movement of the piston rods 76. Each slip 112 and centralising pad 51 will move independently, with the centralising pads 51 come out first to centralise the string 100 and then the slips 112 engaging the inside wall 32 of the tubular 30 unless their travel is hindered by a discontinuity such as a seam weld on the conductor.

With anchor mechanism 110 set and the string 100 centralised, lifting the string 100 can place the tubular 30 in tension in preparation for the cut. The blades 104 can now be activated and will be stabilised by virtue of the anchor mechanism 110 to cut through the tubular 30. Fluid can be circulated through the cut to lubricate the blades 104 and flush material out from behind the tubular 30. This is as illustrated in Figure 8C. Advantageously, the cutting tool 102 can be operated by rotation of the drill string 100 from surface. The drill string 100 rotates through the stationary anchor mechanism 110 via the bearings 83. Once the cut is made the string 30 can be pulled again to determine if the cut has been successful. In smaller size tubular cut under tension there will be an automatic signal at surface when the cut is achieved as the tubular will part. As the conductor has an increased weight due to its dimensions, such a signal may not be detectable. Thus it is better to check that the string 100, with the cut tubular section attached via the anchor mechanism 110, can be raised to verify the cut. By setting the anchor mechanism 110 above the cutting tool 102, the cutting tool is both centralised and stabilised in the tubular. Once cut, the cut section of tubular 30 can be removed, if desired, by known means which may include unsetting and resetting the anchor mechanism 110 at a top of the tubular 30 to be used as a spear, to leave the tubular stub 109 in the well 107. Referring now to Figures 9 and 10 of the drawings there is illustrates a portion of an anchor mechanism, generally indicated by reference numeral 210, according to an embodiment of the present invention. Like parts to those of Figures 1 and 2 have been given the same reference numeral with the addition of 100.

Anchor mechanism 210 is formed as for anchor mechanism 110, with the addition of a locking arrangement 21. The second cylindrical body 160 with the machined cone 216 is shown with a single piston housing 168 having a piston rod 176 arranged to act on a slip 212. As for the earlier embodiments there will be a number of piston housings arranged equidistantly around the unitary body 60. Each piston housing 168 is a cylindrical bore 174 containing the piston rod 176. In this arrangement the back plate 178 is now placed around the piston rod 176 so that the spring 222 is now located behind the plate 178 and acts on the back face 180 biased against a wall 23 23 of the piston housing 168 beside the port 228. In this way the piston rod 176 is now biased out of the housing 168 and moving the slip 212 up the cone 216. Movement of the piston rod 176 is prevented by the locking arrangement 21. The end cap 70 is now replaced by a housing 25 through which is arranged a collet support 27 connected to an end of the secondary piston 44 with a collet ring 31 located around the support 27. Fixed in the housing 25 is the collet 33 comprising eight dogs 35a-h, shown in Figure 10, which engage the collet ring 31. As illustrated in Figure 9, the collet 33 prevents movement of the piston rod 176 until sufficient fluid force is placed on the back face 180 of the piston rod 176 via the port 228, to cause the dogs 35a-h to pass over the collet ring 31. The spring 222 then biases the slips 212 in the extended configuration and can be used to raise cut sections of tubular. To reset the locking arrangement 21, the emergency release system 142 is operated as described herein before with the dogs 35a-h now moving back over the collet ring 31 and the collet 33 being placed in the initial configuration. As for the earlier embodiments, this is preferably done hydraulically by use of a drop ball to block port 128 and direct fluid into the port 143. Flowever it can be achieved at surface by use of a bolt 37 attached to the collet support 27, arranged through the end cap 39 of the housing 25.

The principle advantage of the present invention is that it provides apparatus and method for anchoring a string at a well for the purpose of cutting a tubular in which the anchor mechanism has independently activated slips.

A further advantage of an embodiment of the present invention is that it provides apparatus and method for anchoring a string at a well for the purpose of cutting a tubular in which the anchor mechanism has a secondary release mechanism with active slip retraction.

A yet further advantage of an embodiment of the present invention is that it provides apparatus and method for anchoring a string at a well for the purpose of cutting a tubular in which a combination of slips and stabilisers are used to centralise the apparatus and cutting tool in the tubular with the stabilisers going to drift and the slips then gripping the tubular. It will be apparent to those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, while fluid pressure pistons are used to move the slips, the pistons may be actuated by alternative means such as mechanical or electrical. Also, while the terms 'upper' and 'lower' have been used these are relative and the invention finds use in deviated or horizontal wellbores. The string is referred to as a drill string but it will be appreciated that any string of tubular pipe may be used. While the method is described with reference to cutting the outermost tubular i.e. the conductor, the apparatus finds application in cutting a section of tubular of any diameter in a wellbore.