DOWNHOLE EQUIPMENT

Abstract: A downhole casing back-off assembly for disconnecting first and second casing sections within a wellbore comprises a circulating sub (900), a by-pass valve (800), an upper slack joint (200), a swivel joint (300), an upper anchor (400), a clutch assembly (500), a torque generator (600), a lower slack joint (700) and a lower anchor (400'). The torque generator is hydraulically operated and includes first and second shafts which are interconnected by a helical spline such that, as the shafts are pulled together, e.g. by hydraulic pressure, the shafts will rotate relative to one another. The clutch assembly (500) transmits rotational force in one rotational direction only, thereby allowing the torque generating tool to be operated repeatedly to produce the required total relative rotation between the casing sections to be disconnected.

Description: DOWNHOLE EQUIPMENT This invention relates to downhole equipment, that is, equipment used downhole in a wellbore in connection with the recovery of hydrocarbonaceous fuel reserves. More specifically the invention relates to a downhole anchor, a downhole torque generator, a downhole clutch assembly, a downhole swivel joint and a n overshot tool; the invention also relates to a downhole casing back-off assembly incoφorating one or more of these items. According to one aspect of the present invention, there is provided a downhole anchor, for use in a wellbore, comprising an elongate housing, gripping means movable relative to said housing between a release position and a gripping position in which the gripping means can engage an internal surface of the wellbore, and a hydraulic chamber within said housing in fluid communication with a surface of the gripping element, whereby the application of pressure to a hydraulic fluid within said hydraulic chamber causes said gripping means to move from the release position to the gripping position. Thus, the anchor according to the invention achieves its gripping function by hydraulic means, rather than by the hydraulic-mechanical means of known anchors, that is, the hydraulic force acts directly on the gripping means without any intermediate mechanical linkage. By means of this arrangement, the hydraulic fluid can directly push the gripping means into engagement with the interior surface of the wellbore, without the use of any intermediate linkage or connection. It will be understood that when the wellbore is cased, then the internal surface of the wellboεe will comprise the internal surface of the casing Advantageously, the arrangement is such that the application of pressure to said hydraulic fluid causes the gπpping means to move radially relative to the axis of the housing, without any concomitant axial movement of the gπpping means The housing would usually be arranged with its longitudinal axis substantially parallel to that of the wellbore Preferably the gπpping means compπses a plurality of gπpping elements arranged for movement radially within the wellbore Two or more gnpping elements may be spaced along the longitudinal axis of the housing. In addition, or instead, two or more gπpping elements may be arranged about the longitudinal axis of said housing In the preferred embodiment a set of three gripping elements are arranged about the longitudinal axis of the housing at an angle of 120° to one another, it is prefeπed that two of said sets of three gπpping elements are provided, the two sets being spaced from each other along the longitudinal axis of the housing When two sets of three gπpping elements are provided, it is preferred that the second set is arranged about the longitudinal axis of the housing with a staggered conformation relative to the first set, the angle between the two sets preferably being 60° It is desirable that biasing means is provided to bias the gnpping means towards the release position. The biasing means is preferably a leaf spπng. It is most desirable each gπpping element is secured to a pair of leaf springs. Each gripping element preferably comprises a blade and at least one piston, the blade having at least one serrated outer edge portion adapted to grip the intemal surface of the wellbore. The blade may be provided with a recess for receiving each piston. The hydraulic pressure preferably acts on a surface of the blade within each recess to push the blade away from the housing and away from the or each piston. Usually, the blade and the or each piston would be arranged so that the blade is pushed away from the top of the or each piston. Thus, in this configuration, the blade acts as a hydraulic cylinder. According to another aspect of the invention there is provided a downhole torque generator, for use in a wellbore, comprising an elongate first shaft, and an elongate second shaft, each shaft having a bore extending longitudinally therethrough, wherein the bore of the first shaft is in fluid communication with the bore of the second shaft, and the first shaft is coupled to the second shaft by a torque generating connection, whereby the application of an axial tension force and/or an axial compression force to said shafts causes said first and second shafts to rotate relative to one another about said torque generating connection. Preferably the first shaft is provided in the form of a tubular housing mounted around the second shaft by the torque generating connection, and the second shaft acts as a drive shaft. It is also preferred that the torsion is generated by an axial compression force, which is produced by hydraulic pressure within the bores of the shafts. Preferably the torque generating connection comprises a helical spline provided on one of said shafts (preferably the second shaft), which is received in a corresponding helical recess in the other of said shafts (preferably the first shaft). In the preferred arrangement, if the shafts are pulled together, they will rotate relatively in one direction and will move axially away from one another, and if the shafts are pushed together, they will rotate relatively in an opposite direction and will move axially towards one another: the shafts can move relatively between a retracted position, in which the combined length of the shafts is at a minimum; and an extended position, in which the combined length of the shafts is at a maximum. Stop means can be provided to limit movement of the shafts beyond the retracted and the extended positions. It is preferred that at least a portion of the second shaft is received within the first shaft Desirably, biasing means is provided for biasing the shafts to the retracted position An annular chamber may be provided between the intenor of the first shaft and the exteπor of the portion of the second shaft within the first shaft, and the biasing means may be disposed in this annular chamber It is particularly preferred that the biasing means is a stack of disc spπngs Preferably a piston is provided within the bore of the first shaft, said piston being adapted to apply an axial compressive force against the biasing means The piston may be attached to the second shaft by means of a screw thread If a hydraulic fluid is pumped through the bore of the first shaft to the bore of the second shaft, and through the bore of the second shaft, then the first and second shafts will remain in the retracted position, because there will be no relative rotational movement, due to the force of the biasing means If the bore of the second shaft is blocked - or, more usually, a blockage is created downstream of the second shaft - then the pressure will build up in the bore, and will apply a force on the piston This force will eventually overcome the biasing force of the biasing means, and cause the first and second shaft to rotate relatively, thereby moving them to the extended pos ition If the fluid pressure is removed, the biasing means will force the first and second shafts back to the retracted position The onset of relative rotational movement could also be achieved by selecting the relative size of the bores m the shafts to produce a pressure drop to act on the piston to overcome the force of the biasing means This could be earned out for example, by reducing the size of the bore in the second shaft According to another aspect of the invention there is provided a downhole clutch assembly, for use in a wellbore, compπsing an elongate dnve shaft and an elongate dnven shaft, each shaft having a bore extending longitudinally therethrough, the bore of the dπve shaft being in fluid communication with the bore of the dnven shaft, and coupling means for coupling the dnve shaft to the driven shaft, said couphng means being such that rotation of the dπve shaft in a first direction causes the driven shaft to rotate with the drive shaft, and rotation of the drive shaft in a direction opposite to the first direction does not cause the driven shaft to rotate with the drive shaft. Preferably the coupling means compπses. a collar on the dnven shaft, said collar being axially movable along the driven shaft and being rotationally fixed relative thereto; a plurality of detentes on the collar for engagement with a plurality of cooperating detenjes on the dnve shaft; and biasing means for biasing the detentes on the collar into engagement with the detentes on the drive shaft, wherein the detentes and biasing force are such that when the dnve shaft is rotated in said first direction, the detentes are held in engagement, whereby rotational movement of the dnve shaft is imparted to the dnven shaft through the collar, and when the dnve shaft i s rotated in a direction opposite to the first direction, the collar moves axially away from the dnve shaft, and the detentes of the collar disengage from the detentes on the d rive shaft, whereby rotational movement of the dπve shaft is not imparted to the collar This can be achieved by suitable shaping of the detentes in one direction the detentes are angled relative to one another so that upon rotational movement they slide over one another and push the collar axially against the force of the spnng It will be appreciated that the collar could, instead, be provided on the dnve shaft, m which case the detentes on the dπve shaft would, instead, be provided on the dnven shaft According to another aspect of the invention there is provided a downhole swivel joint, for use in a wellbore, compπsing a first elongate connector portion and a second elongate connector portion rotatably secured to the first elongate connector portion, wherein the first and second connector portions are each provided w th a bore extending longitudinally therethrough, the bore of the first connector portion being in fluid communication with the bore of the second connector portion, and the first and second connector portions are each provided with means to connect additional equipment thereto. Preferably the second connector portion, which is arranged lowermost, in use, is provided with a shoulder adapted to engage a surface within the wellbore to prevent further movement of said swivel joint down the wellbore. Desirably, the first elongate portion is rotatably secured to the second rotatable portion by means of a set of axial thrust discs. The means to connect additional equipment is typically a screw thread The shoulder is advantageously used to abut against the top of a well casing, in order to prevent further do vnward movement of the swivel joint relative to the well casing According to another aspect of the invention there is provided an overshot tool for retrieving downhole equipment, comprising an overshot shaft adapted to be secured to other downhole equipment; an overshot housing extending over at least part of the overshot shaft, gnpping means for gnpping the downhole equipment to be retneved, the overshot housing having locking means adapted to lock the gnpping means in a gnpping position, and the overshot housing being axially movable relative to the shaft from a release position, in which the locking means does not lock the gnpping means in the gπpping position and a locking position in which the gnpping means does lock the gripping means in the gnpping position; and a release member axially movable relative to the overshot shaft and housing, said release member being movable into contact with the overshot housing to move the overshot housing to the release position The overshot tool is also known in the art as a fishing tool A wide variety of downhole equipment can be retneved with the overshot tool- it is particularly well adapted to removing casing from a well, as part of a back-off assembly. The overshot shaft preferably has a central bore extending axially of the shaft. Preferably means is provided to prevent rotational movement between the overshot shaft and the overshot housing. This means desirably comprises a plurality of interlocking splines provided on an outer surface of the overshot shaft and on an inner surface of the overshot housing. This arrangement enables torque to be transmitted from the overshot shaft to the overshot housing, and to the gripping means through the locking means. In this manner, the overshot tool can be used to rotate the downhole equipment to be retrieved. This is particularly useful when the downhole equipment to be retrieved needs to be rotated in order to remove it from other downhole equipment. Stop means may be provided to prevent movement of the housing beyond the locking and the release positions. The release member preferably comprises a tubular member having a diameter substantially equal to the diameter of the overshot housing. This enables an end of the release member to bear against an end of the overshot housing when it moves into contact with the overshot housing. In the preferred embodiment the release member is secured to downhole equipment separate from the rest of the overshot tool. According to another aspect of the invention, there is provided a downhole casing back-off assembly, for disconnecting a first and second casing section within a wellbore, comprising first anchor means adapted to fixedly engage the first casing section, second anchor means adapted to fixedly engage the second casing section, and torque generating means adapted to generate a torsional force between the first and second anchor means, whereby the first casing section is disconnected from the second casing section. Advantageously the downhole casing back-off assembly is in the form of an elongate mandrel having a bore extending longitudinally therethrough, the bore being adapted to receive hydraulic fluid which is used to operate the assembly. The anchor means and the torque generating means form part of the elongate mandrel. The torque generating means is ideally disposed between the first and second anchor means. Advantageously the first and second anchor means each comprises an elongate housing, gripping means movable relative to said housing between a release position and a gripping position in which the gripping means can engage an intemal surface of the wellbore, and a hydraulic chamber within said housing in fluid communication with a surface of the gripping element, whereby the application of pressure to a hydraulic fluid within said hydraulic chamber causes said gripping means to move from the release position to the gripping position. Thus, the anchor means achieves its gripping function by hydraulic means, rather than by the hydraulic-mechanical means of known anchors, that is, the hydraulic force acts directly on the gripping means without any intermediate mechanical linkage. By means of this arrangement, the hydraulic fluid can directly push the gripping means into engagement with the interior surface of the wellbore, without the use of any intermediate linkage or connection. Advantageously, the arrangement is such that the application of pressure to said hydraulic fluid causes the gripping means to move radially relative to the axis of the housing, without any concomitant axial movement of the gripping means. The housing would usually be aπanged with its longitudinal axis substantially parallel to that of the wellbore. Preferably the gripping means comprises a plurality of gripping elements arranged for movement radially within the wellbore. Two or more gripping elements may be spaced along the longitudinal axis of the housing. In addition, or instead, two or more gripping elements may be arranged about the longitudinal axis of said housing. In the preferred embodiment a set of three gripping elements are arranged about the longitudinal axis of the housing at an angle of 120° to one another; it is preferred that two of said sets of three gripping elements are provided, the two sets being spaced from each other along the longitudinal axis of the housing. When two sets of three gripping elements are provided, it is preferred that the second set is arranged about the longitudinal axis of the housing with a staggered conformation relative to the first set, the angle between the two sets preferably being 60°. It is desirable that biasing means is provided to bias the gripping means towards the release position. The biasing means is preferably a leaf spring. It is most desirable each gripping element is secured to a pair of leaf springs. Each gripping element preferably comprises a blade and at least one piston, the blade having at least one serrated outer edge portion adapted to grip the intemal surface of the well casing. The blade may be provided with a recess for receiving each piston The hydraulic pressure preferably acts on a surface of the blade withm each recess to push the blade away from the housing and away from the or each piston. Usually, the blade and the or each piston would be arranged so that the blade is pushed away from the top of the or each piston. Thus, in this configuration, the blade acts as a hydraulic cylinder Advantageously the downhole torque generator compπses first and second elongate shafts, each shaft having a bore extending longitudinally therethrough, wherein the bore of the first shaft is m fluid communication with the bore of the second shaft, and the first shaft is mounted on the second shaft by a torque generating connection, whereby the application of an axial tension force and/or an axial compression force to said shafts causes said first and second shafts to rotate relative to one another about said torque generating connection Preferably the first shaft is provided in the form of a tubular housing mounted around the second shaft by the torque generating connection, and the second shaft acts as a dnve shaft It is also preferred that the torsion is generated by an axial compression force, which is produced by hydraulic pressure within the bores of the shafts. Preferably the torque generating connection compnses a helical spline provided on one of said shafts (preferably the second shaft), which is received in a corresponding helical recess in the other of said shafts (preferably the housing). In the preferred arrangement, if the shafts are pulled together, they will rotate relatively in one direction and wall move axially away form one another, and if the shafts are pushed together, they will rotate relatively in an opposite direction and will move axially towards one another: the shafts can move relatively between a retracted position, in which the combined length of the shafts is at a minimum; and an extended position, in which the combined length of the shafts is at a maximum. Stop means can be provided to limit movement of the shafts beyond the retracted and the extended positions. It is preferred that at least a portion of the second shaft is received within the first shaft. Desirably, biasing means is provided for biasing the shafts to the retracted position. An annular chamber may be provided between the intenor of the first shaft and the extenor of the portion of the second shaft within the first shaft, and the biasing means may be disposed in this annular chamber. It is particularly preferred that the biasing means is a stack of disc spnngs Preferably a piston is provided within the bore of the first shaft, said piston being adapted to apply an axial compressive force against the biasing means. The piston may be attached to the second shaft by means of a screw thread If a hydraulic fluid is pumped through the bore of the first shaft to the bore of the second shaft, and through the bore of the second shalt, then the first and second shafts will remain in the retracted position, because there will be no relative rotational movement, due to the force of the biasing means If the bore of the second shaft is blocked - or, more usually, a blockage is created downstream of the second shaft - then the pressure will build up in the bore, and will apply a force on the piston This force will eventually overcome the biasing force of the biasing means, and cause the first and second shaft to rotate relatively, thereby moving them to the extended position. If the fluid pressure is removed, the biasing means will force the first and second shafts back to the retracted position. The onset of relative rotational movement could also be achieved by selecting the relative size of the bores in the shafts to produce a pressure drop to act on the piston to overcome the force of the biasing means This could be carried out, for example, by reducing the size of the bore in the second shaft Preferably, the casing back-off assembly further includes a downhole clutch assembly which is disposed between the torque generating means and the first anchor means, which allows the torque generating means to apply a torsional force to the first anchor means in one direction, and does not allow the torque generating -I I - means to apply a torsional force to the first anchor means in an opposite direction. The downhole clutch assembly preferably comprises an elongate drive shaft and an elongate driven shaft, the drive shaft and the driven shaft each shaft having a bore extending longitudinally therethrough, the bore of the drive shaft being in fluid communication with the bore of the driven shaft, and coupling means for coupling the drive shaft to the driven shaft, said coupling means being such that rotation of the dπve shaft in a first direction causes the dnven shaft to rotate with the drive shaft, and rotation of the drive shaft in a direction opposition to the first direction does not cause the dnven shaft to rotate with the drive shaft. In practice, the drive shaft is disposed at the end of the clutch device towards the torque generating means, and the dnven shaft is disposed at the end of the clutch device towards the first anchor means. Preferably the coupling means compnses. a collar on the dnven shaft, said collar being axially movable along the dnven shaft and being rotationally fixed relative thereto; a plurality of detentes on the collar for engagement with a plurality of cooperating detentes on the dnve shaft; and biasing means for biasing the detentes on the collar into engagement with the detentes on the dnve shaft; wherein the detentes and biasing force are such that when the dnve shaft is rotated in said first direction, the detentes are held in engagement, whereby rotational movement of the dπve shaft is imparted to the dnven shaft through the collar, and when the drive shaft is rotated in a direction opposite to the first direction, the collar moves axially away from the drive shaft, and the detentes of the collar disengage from the detentes on the drive shaft, whereby rotational movement of the drive shaft is not imparted to the collar. This can be achieved by suitable shaping of the detentes- in one direction the detentes are angled relative to one another so that upon rotational movement they slide over one another and push the collar axially against the force of the spring. It will be appreciated that the collar could, instead, be provided on the drive shaft, in which case the detentes on the drive shaft would, instead, be provided on the driven shaft. Desirably the casing back-off assembly further includes a downhole swivel joint, for use in a wellbore, comprising a first elongate connector portion and a second elongate connector portion rotatably secured to the first elongate connector portion, wherein the first and second connector portions are each provided with a bore extending longitudinally therethrough, the bore of the first connector portion being in fluid communication with the bore of the second connector portion, and the first and second connector portions are each provided with means to connect additional equipment thereto. Preferably the second connector portion, which is disposed lowermost, in use, is provided with a shoulder adapted to engage a surface within the wellbore to prevent further movement of said swivel joint down the wellbore. The shoulder provides an excellent means for locating the back-off assembly at the correct part of the casing after it has been cut at the correct position with a cutting assembly. Desirably, the first elongate portion is rotatably secured to the second rotatable portion by means of a set of axial thrust discs. The means to connect additional equipment is typically a screw thread. The shoulder is advantageously used to abut against the top of a well casing, in order to prevent further downward movement of the swivel joint relative to the well casing. Preferably the back-off assembly further includes an upper slack joint disposed above the swivel joint. The upper slack joint comprises first and second elongate shafts each having a bore extending longitudinally therethrough, the bore of the first shaft being in fluid communication with the bore of the second shaft. The first and second shafts are axially movable relative to one another between a retracted position, in which the total length of the lower slack joint is at a minimum, and an extended position, in which the total length of the lower slack joint is at a maximum. The purpose of the upper slack joint is to allow upward movement, which takes place as the first casing section is disconnected from the second casing section Preferably the back-off assembly is provided with an overshot tool, so that the casing can be backed off and retrieved from the well bore in a single trip. The overshot tool is also known in the art as a fishing tool. The overshot tool is preferably disposed between the upper slack joint and the swivel joint. In the preferred embodiment the overshot tool comprises: an overshot shaft adapted to be secured at one end to the upper slack joint and at the other end to the swivel joint; an overshot housing extending over at least part of the overshot shaft; gripping means for gripping the casing, the overshot housing having locking means adapted to lock the gripping means in a gripping position, and the overshot housing being axially movable relative to the shaft from a release position, in which the locking means does not lock the gripping means in the gripping position and a locking position in which the gripping means does lock the gripping means in the gripping position; and a release member axially movable relative to the overshot shaft and housing, said release member being movable into contact with the overshot housing to move the overshot housing to the release position. Preferably means is provided to prevent rotational movement between the overshot shaft and the overshot housing. This means desirably comprises a plurality of interlocking splines provided on an outer surface of the overshot shaft and on an inner surface of the overshot housing. This arrangement enables torque to be transmitted from the overshot shaft (e.g. via the upper slack joint) to the overshot housing, and to the gripping means through the locking means. In this manner, the overshot tool can be used to rotate the casing that is being gripped. Preferably the overshot shaft is provided with a central bore extending axially of the shaft, said bore being in fluid communication with the bores of the upper slack joint and the swivel joint. Stop means may be provided to prevent movement of the housing beyond the locking and the release positions. The release member preferably comprises a tubular member having a diameter substantially equal to the diameter of the overshot housing. This enables an end of the release member to bear against an end of the overshot housing when it moves into contact with the overshot housing. In the preferred embodiment the release member is secured to the upper slack joint. It is especially preferred that the overshot shaft is connected to one of the first and second shafts of the upper slack joint, and the release member is connected to the other of the first and second shafts of the upper slack joint. In a preferred embodiment, the back-off assembly further includes a lower slack joint disposed between the torque generating means and the second anchor means. The lower slack joint comprises first and second elongate shafts each having a bore extending longitudinally therethrough, the bore of the first shaft being in fluid communication with the bore of the second shaft. The first and second shafts are axially movable relative to one another between a retracted position, in which the total length of the lower slack joint is at a minimum, and an extended position, in wh ich the total length of the lower slack joint is at a maximum. The purpose of the lower slack joint is to compensate for the increase in length of the torque generating means as the torque is generated, in order to prevent the anchor means being moved axially. The distance between the retracted and extended positions of the lower slack j oint is preferably substantially equal to the distance between the retracted and extended positions of the torque generating means. In the preferred aπangement the second shaft comprises an outer housing, and the first shaft extends within the outer housing. Desirably lubrication is provided between the first and second shafts of the lower slack joint, and the intemal hydraulic pressure is such as to bias the lower slack joint to the retracted position. When the first shaft of the lower slack joint is pushed downwardly by the torque generating means, the frictional force within the slack joint would tend to push the second anchor downwardly. However, the intemal hydraulic pressure tends to close the slack joint, and this creates an upward force on the outer housing that counterbalances the downward motion resisting a frictional force. The slack joint rotational drive can be typically a hexagon or a spline, and is preferably immersed in an oil bath. It is preferred that the first shaft of the lower slack joint is connected to the second shaft of the torque generating means, and the second shaft of the lower slack joint is connected to the housing of the second anchor means. Valve means is desirably provided in order to allow the filling of the equipment as it is run into the wellbore. The valve means can be secured to the second anchor means on the opposite side to the torque generating means. However, it is preferred that the valve means is secured above the upper slack joint below a circulating sub (the circulating sub is discussed in greater detail below). The valve means preferably comprises an elongate valve body, and a valve member within the valve body. The valve bodv is provided with a bore extending longitudinally therethrough, said bore being in fluid communication with the hydraulic chamber in the second anchor means. The valve body may also include at least one port extending from the bore of the valve body to the wellbore. The valve member is preferably movably arranged within the bore of the valve body. The valve member may be movable in response to fluid flow through the bore. The valve member can be secured to the valve body by a shear pin, so that it does not move relatively thereto until the fluid flow reaches a pre-determined amount - when the fluid flow reaches the predetermined amount, the shear pin shears, and the valve member begins to move relative to the valve body. The valve member moves to close the or each port, thereby preventing hydraulic fluid from leaving the valve body and entering the wellbore. This enables hydraulic pressure to build up within the back-off assembly. In a preferred embodiment the casing back-off assembly further includes an elongate circulating sub. If the valve means is disposed below the second anchor means, then the circulating sub may be disposed immediately above the upper slack joint. If the valve means is disposed above the upper slack joint (which is preferred) , then the circulating sub may be disposed immediately above the valve means. The circulating sub will be activated after the casing has been unscrewed, when it is desired to pull the back-off assembly and related equipment out of the wellbore. The circulating sub includes a bore extending longitudinally therethough in fluid communication with the bores in the other components of the casing back off assembly. The circulating sub also includes a radial bore in fluid communication with the longitudinal bore and with the intenor of the wellbore. However, a sealing member normally prevents fluid communication between the radial and longitudinal bores. This sealing member may be held in position by a sheanng pin, and the sheaπng pin may be broken by applying a sufficient downward force on the sealing member. This force may be applied, for example, by dropping a ball from the surface Reference is now made to the accompanying drawings, in which Figure IA is a schematic view of a wellbore within which is disposed a a first embodiment of a back-off assembly according to the invention, Figure 1 B is a schematic view of a wellbore within which is disposed a second embodiment of a back-off assembly according to the invention, Figure 2A is a cross-sectional view of an upper slack joint for use in a back-off assembly according to the invention, in an extended position, Figure 2B is a cross-sectional view of an upper slack j oint for use in a back-off assembly according to the invention, in a retracted position, Figure 3 is a cross-sectional view of a swivel joint lor use in a back-off assembly according to the invention, Figure 4 is a cross-sectional view of anchor means for use in a back-off assembly according to the invention, and showing gnpping means in a release position, Figure 4A is a view taken along lines A-A of Figure 4, on an enlarged scale relative to Figure 4, Figure 4B is a view taken along lines B-B of Figure 4, on an enlarged scale relative to Figure 4, Figure 4C is a view taken along lines C-C of Figure 4, on an enlarged scale relative to Figure 4, Figure 4D is a cross-sectional view of part of the anchor means shown in Figure 4, and illustrates a leaf spnng and a spπng block in the body; Figure 4E is an enlarged side view of the leaf spring, the spring block and retaining pins for the anchor means shown in Figure 4; Figure 4F is a view similar to Figure 4, showing the gripping means in a gripping position; Figure 5A is a cross-sectional view of a clutch assembly for use in a back¬ off assembly according to the invention, Figure 5B is a view along lines B-B of Figure 5A; Figure 5C shows a detent structure provided on the clutch assembly shown in Figure 5A; Figure 6A is a cross-sectional view of a torque generator for use in a back-off assembly according to the invention, in an extended position, Figure 6B is a cross-sectional view of a torque generator for use in a back-off assembly according to the invention, in a retracted position, Figure 7A is a cross-sectional view of a lower slack joint for use in a back¬ off assembly according to the invention, in an extended position, Figure 7B is a cross-sectional view of a lower slack joint for use in a back¬ off assembly according to the invention, in a retracted position, Figure 8 is a cross-sectional view of a valve means for use in a back-off assembly according to the invention, Figure 9 is a cross-sectional view of a circulating sub for use in a back-off assembly according to the invention, Figure 10 is a cross-sectional view of modification showing an upper slack joint, an overshot tool for removing a casing stump, and a swivel joint, Figure 1 1 A is a cross-sectional view showing the overshot tool of Figure 10 on an enlarged scale Figure 1 1 B is a view along lines B-B of Figure 1 1 A; and Figure 1 1 C is a cross-sectional view showing the lower part of the overshot tool of Figure 1 1 A on an enlarged scale. Refemng to Figure IA, which is not to scale, there is shown a wellbore 10 extending through a hydrocarbonaceous fluid bearing formation 12. In the region of the formation 12, the internal surface of the wellbore 10 is lined with a casing 14. The casing 14 has a first casing section 14a (or casing stump), uppermost in the wellbore 10, and a second casing section 14b, lower than the first casing section 14a. The first and second casing sections 14a and 14b are usually connected by a collar 14c, which has a screw thread (not shown) that engages corresponding screw threads on the sections 14a and 14b (not shown), whereby the sections 14a and 14b can be joined. The first casing section 14a has an upper edge 14d. A drill stnng 16 extends from the surface 18 to the formation 12, and cames a first embodiment of a casing back-off assembly generally designated 20 at the end thereof. The back-off assembly 20 compnses, in sequence from top to bottom, a circulating sub 900, a by-pass valve means 800 (Figure 8), an upper slack joint 200 (Figures 2A and 2B), a swivel joint 300 (Figure 3), an upper anchor 400 (Figures 4, 4A, 4B, 4C, 4D, 4E and 4F), a clutch assembly 500 (Figures 5A, 5B and 5C), a torque generator 600 (Figures 6A and 6B), a lower slack joint 700 (Figures 7A and 7B), and a lower anchor 400' (Figure 4). A blanking cap (not shown) is provided at the lower end of the lower anchor 400'. Figure I B illustrates a second embodiment of a back-off assembly 20' in the toπnation 12. Figure I B is identical to Figure I A - except that the by-pass valve means 800 is disposed below the lower anchor 400' - and like reference numerals have been used to refer to like parts. The first embodiment shown in Figure 1 A is preferred over that shown in Figure 1 B because: there is less weight hanging on the shaft of the torque generator 600; there is less hydraulic pressure acting on the torque generator 600 prior to the by-pass valve 900 closing; and there is less likelihood of the anchor blades of the anchors 400 and 400' being driven out when the assembly 20 is pulled out of the wellbore 10. In the following description it will be assumed that the by-pass valve means 800 is located in the position shown in Figure IA. Referring to Figures 2A and 2B, the upper slack joint 200 comprises a first shaft 202 and a second shaft in the form of an outer housing 204. The first shaft 202 has an internal bore 206, which is in fluid communication with an intemal bore 208 of the outer housing 204. An end of the first shaft 202 remote from the outer housing 204 is provided with a screw threaded recess 210 by means of which the first shaft 202 is connected to the dπll stnng 16. An end of the outer housing 204 remote from the first shaft 202 is provided with a screw threaded projection 252 by means of which the outer housing 204 is connected to the swivel joint 300. The upper slack joint 200 is shown in a retracted position in figure 2B a nd in an extended position in figure 2A In figure 2A the slack joint is 1 8 inches (0 46m) longer than in figure 2B The hydraulic sealing diameters are designed to ensure that there is a slight bias towaids the joint being closed with by internal pressure The shaft 202 compnses an upper shaft portion 212 secured to a lower shaft portion 214 by a screw thread. The outer housing 204 compnses an upper body portion 21 6, an mteπnediate body portion 2 18 and a lower body portion 220 the upper body portion 216 is connected to the intermediate body portion by a screw thread, and the inteπnediate body portion 21 8 is connected to the lower body portion 220 by a screw thread A sealing nng 222 is provided on the upper shaft portion 21 6 for sealing between the upper body portion 2 1 6 and the upper shaft portion 212 A sealing nng 234 is provided on the lower shaft portion 214 for sealing between the inteπnediate body portion 21 8 and the lower shaft portion 214 A sealing nng 224 is provided on the lower shaft portion 214 for sealing between the lower shaft portion 214 and the lower body portion 220. It will be appreciated that the sealing nngs 222, 224 and 234 are capable of withstanding the operating pressure of the back-off assembly 20 (and all subsequently described sealing nngs that are exposed to the operating pressure of the back-off assembly 20 are also capable of withstanding this operating pressure). The sealing rings 222, 224 and 234 (and the subsequently described sealing rings) do not prevent relative sliding movement (axial or rotational) between the two surfaces they seal. The sealing diameters of the three seals can be designed to provide a hydraulic bias for the slack joint 200 to cause it to close up when pressurised. This causes the swivel joint 300 to lift off the upper edge 14d as the first casing section 14a is backed off. An annular chamber 226 is defined between the upper shaft portion 212 and the intermediate body portion 218, and an annular chamber 228 is defined between the lower shaft portion 214 and the intermediate body portion 21 8. The annular chambers 226, and 228 are prevented from communicating with one another by the lower shaft portion 214 Ports 230 are provided in the upper shaft portion 2 12 to provide fluid communication between the bore 206 and the annular chamber 226 Ports 232 are provided in the lntennediate body portion 218 to provide fluid communication between the annular chamber 228 and the intenor of the wellbore 10 Referring to figure 3, the swivel joint 300 compπses a first elongate connector portion 302 and a second elongate connector portion 304 The first connector portion 302 is provided with a boie 306 extending longitudinally therethrough The bore 306 is in fluid communication with a bore 308, which extends longitudinally through the second connector portion 304 The first connector portion 302 is provided with a screw threaded recess 310 for connection wi th the screw threaded projection 252 of the upper slack joint 200 The second connector portion 304 is provided with a screw threaded portion 312 for connection with the upper anchor 400. The upper end of the second connector portion 304 is provided with a penpheral shoulder 31 6 which is dimensioned so that it can engage the upper edge 14d of the first casing section 14. The part of the swivel joint that, m use, is disposed beneath the shoulder 31 6 is dimensioned so that it can fit through the casing sections 14a and 14b, as shown in figure 1. The first connector portion 302 is rotatably mounted to the second connector portion 304 by means of a bearing arrangement generally designated 314. The beaπng arrangement comprises a plurality of plain axial thrust discs in a sealed oil bath. Although other bearing arrangements can be used (such as roller bearings, or ball bearings), the plain axial thrust discs provide the highest axial load capacity. The first connector portion 302 comprises upper and lower portions 31 8 and 320 respectively, which are connected by a screw thread. The lower connector portion 304 comprises upper and lower portions 322 and 324 respectively, which are connected by a screw thread. A sealing ring 326 is provided on the lower first connector portion for sealing between the lower first connector portion 320 and the lower second connector portion 324. A piston 328 is provided between the iower first connector portion 320 and the upper second connector portion 322 The piston 328 is disposed upwards of the beanng arrangement 314 A sealing nng 330 is provided on the piston 328 for sealing between the piston 328 and the upper second connector portion 322 A further sealing nng 332 is provided on the piston 328 for sealing between the lower first connector portion 320 and the piston 328 Refernng to figures 4, 4A, 4B, 4C, 4D, 4E and 4F, there is shown the upper anchor 400 which comprises an elongate housing 402 having a bore 404 extending longitudinally therethrough. One end of the housing 402 is provided with a screw threaded recess 406 which is connected to the screw threaded projection 312 of the swivel joint 300. The housing 402 is provided with a plurality of radially extending bores 408, which communicate at one end with the bore 404 Gnpping means is provided at the other end of the bores 408: the gripping means compnses a plurality of gripping elements 410 (which are not shown in figure 4D): each gnpping element comprises six piston elements 410a and a blade 410b; each blade 410b is provided with a serrated outer surface 410c. The blade 410b is provided with six cylindrical recesses 410d, which receive a respective one of the piston elements 410a. A surface 410e of each the recesses 410d sits on the top of a respective one of the piston elements 410a. A sealing ring 41 Of is provided on the blade 410b, in each recess 410d, for sealing between the blade 410a and the outer surface of the piston elements 410a. The base of each piston element 410a is clamped against the housing 402, and an O-ring seal 410g is provided on the base of each piston element 410a to seal between said base and the housing 402. Each piston element 410a has a bore 41 1 extending therethrough: the bore 41 1 of each piston element 410a has an inner portion 41 1a and an outer portion 41 lb. The outer portion 41 1 b of each bore 41 1 has a larger diameter than the inner portion 41 1a, thereby creating a shoulder 41 Oh on each piston element 410b. A PTFE washer 440 is provided on the shoulder 41 Oh. Each piston element 410a is clamped to the housing 402 bv a respective piston retaining bolt 442. Each piston retaining bolt 442 has a substantially circular cross-section and compπses an inner portion 442a and an outer portion 442b. The outer portion 442b has a greater diameter than the inner portion 442a, so that the bolt 442 has a T-shaped cross section The outer portion 442b of each bolt 442 bears against a respective one of the PTFE washers 440 to clamp each piston element 410b to the housing 402. The inner portion 442a of each bolt 442 extends within the bore 408 and is secured to the wall of the bore 408 with a screw thread Each bolt 442 can be tightened with an alien key, which can be fitted in a hex socket provided on the top of each bolt 442. Each bolt 442 is provided with a central bore 442c which communicates at one end with one of the boies 408, and communicates at the other end with the outer portion 41 l b of the bore 41 1 There are a total of six gnpping elements 410 provided on the anchor 400, each of which comprises six piston elements 410a and one blade 410b Three of the gripping elements 410 are arranged about the axis of the housing 402 at an angle of 120° to one another, as shown in Figure 4A. The other three gnpping elements 410 are also aπanged about the axis of the housing 402 at an angle of 120°, and are axially spaced from the first three gripping elements 410. The second set of three gripping elements 410 are aπanged 60° out of alignment with the first set of gripping elements 410, so that if the first set of gripping elements 410 were superimposed upon the second set of gripping elements 410 by moving it axially to the right in figure 4, then there would be an angle of 60° between each gripping element 410. It should be noted that this is not the arrangement actually shown in Figure 4; for clarity, in Figure 4 the gripping elements 410 of the first and second sets are shown at an angle of 0° to one another. The gripping elements 410 are movable between a release position , as shown in figure 4, and a gripping position as shown in Figure 4F . The direction of movement of the gripping elements 410, between the release position and the gnpping position, is entirely radial with respect to the longitudinal axis of the housing 402 The gripping elements 410 can be moved from the release position to the gripping position by the increase of pressure in the bores 408 Each gripping elements 4 1 0 is connected to a pair of leaf springs 414, which bias it towards the release position The end of the upper anchor 400 remote from the screw threaded recess 406, is provided with the screw threaded projection 452 for connection to the clutch assembly 500. Stop means is provided for limiting the movement of each of the gripping elements 410 The stop means is located between the six piston elements 410a, and compnses stop elements 41 6 provided on each gripping element 41 0, and three stop screw 418 secured to the housing 402. Each stop element 41 6 is provided wi th a stop shoulder 41 6a, and each stop screw 41 8 is provided with a stop surface 418a. The outward radial movement of the blade 410b of the gnpping element 410 is limited by the mutual engagement of the stop shoulders 416a and 418a. Each gripping element 410 is seated its own blade pocket 420, so there are a total of six blade pockets. Each blade pocket 420 is provided with a spnng recess 422 extending along the two longitudinal edges thereof. One of the leaf springs 41 4 is received in each spring recess 422 and extends along the longitudinal edges of the blade pocket 420. Each end of each leaf spring 414 abuts against an upper surface 422a of its spring recess 422, as shown in Figure 4D. A spring block 424 is secured to the middle portion of the blade 410b, along opposite edges thereof, as shown in Figure 4C. Each of the spring blocks 424 extends into one of the spring recesses 422 underneath the central portion of one of the leaf spπngs 414. The blade 410b is secured to the spring blocks 424 by retaining pins 426. This arrangement is shown in Figure 4E, where, for claπty, only the leaf spring 414, the spnng block 424 and the retaining pins 426 are shown. The retaining pins 426 are held in place by plugs 428 When the hydraulic pressure within the bores 408 is increased, an outward radial force presses against the surface 410e of the cylindrical recesses 410d This force acts to pusjh the blades 410b radially outwardly; the spnng blocks 424 are pushed with the blades 410b by virtue of their attachment to the blades 410b by the retaining pins 426 This exerts a force at the centre of the leaf spnngs 414, which is resisted by the spnng force in the leaf spnngs 414 because thev are held fixed at their ends by the surfaces 422a When the hydraulic force is greater than the spnng force of the leaf spnngs 414, the leaf spnngs 414 deform and the blades 410b move radially outwardly Refernng to figuies 5A, 5B and 5C, a clutch assembly 500 compnses an elongate dnve shaft in the fonn of a dnve housing 502 and an elongate dnven shaft in the foπn of a dnven housing 504 The dnve housing 502 is provided with a longitudinally extending bore 506 The bore 506 is in fluid communication with a bore 508, which extends longitudinally through the dnven housing 504 Couphng means is provided to couple the dnve housing 502 to the dnven housing 504 The coupling means compnses collar 510, biasing means in the foπn of a spnng 512, a plurality of detentes 514 on the collar 510, and a plurality of detentes 51 6 on the dnve housing 502 The collar 510 is mounted on the dnven housing 504 using an axial spline configuration - this prevents rotation of the collar 5 10 relative to the dnven housing 504 Figure 5B shows how splines 510a on the collar 5 10 interlock with splines 504a on the dnven housing 504 The collar 510 is movable axially relative to the dnven housing 504 The configuration of the detentes 514 and 51 6 is such that when the dnve housing 502 is rotated in one direction the detentes engage one another, aided by the spπng 512, thereby causing the dnven housing 504 to rotate with the drive housing 502 The detentes 514 and 516 and are also formed so that when the drive housing 502 is rotated in an opposite direction, the detentes 514 ride up over the detentes 516, thereby pushing the collar 510 axially against the force of the spπng 512, so that the dnven housing 504 does not rotate with dπve housing 502. This detent structure is shown in greater detail in Figure 5C The dπve housing 502 and the collar 510 have been "unwrapped" in Figure 5C in order to show the structure more clearly. Each detent 514 and 51 6 has one surface - 5 14a and 51 6a respectively - that is at an angle of 15° to the longitudinal axis of the clutch assembly 500, and another surface - 514b and 516b respectively - that is at an angle of 45 ° to the longitudinal axis of the clutch assembly 500 When a torque is applied to rotate the collar 5 10 and the dnve housing 502 relatively in one direction, the 45 ° detent surfaces 5 14b and 5 1 6b engage one another, and they can slide over one another, when a torque is apphed to rotate the collar 510 and the dπve housing 502 relatively in the opposite direction, the 15 ° detent surfaces 5 1 4a and 51 6a engage one another, but they cannot slide over one another, and serve to prevent relative rotation in this opposite direction The dnven housing 504 is provided with a screw threaded recess 5 1 8 which is connected to the screw threaded projection 452, the dnve housing 502 is provided with a screw threaded projection 520 for connection to the torque generator 600 The dnven housing 504 compnses an upper dnven housing portion 522 and a lower dnven housing portion 524, which are connected by a screw thread The drive housing 502 compnses an upper dnve housing portion 526 and a lower dnve housing portion 528, which are connected by a screw thread The collar 51 0 is provided on the lower dnven housing portion 524 A thrust coupling 534 is provided within the upper dnve housing portion 526, between the lower dnven housing portion 524 and the lower dπve housing portion 528 A thrust nng 532 is provided at the upper end of the thrust coupling 534, between the upper dnve housing portion 526 and the lower dnven housing portion 524. A thrust nng 536 is provided at the lower end of the thmst coupling 534 between the thrust coupling 534 and the lower dnve housing portion 528 A sealing nng 530 is provided on the thrust coupling 534 for sealing between the thrust coupling 534 and the upper dnve housing 526 Refemng to figures 6A and 6B, the torque generator 600 compnses a first elongate shaft in the form of a housing 602 and a second elongate shaft m the form of a dnve shaft , ,604 The housing 602 is provided with a bore 606 extending longitudinally therethrough The bore 606 is in fluid communication with a bore 608, which extends longitudinally through the dnve shaft 604 The housing 602 is provided with a screw threaded recess 622 which is connected to the screw threaded projection 520 The dnve shaft 604 is provided with a screw thieaded projection 61 for connection with a screw thieaded recess 710 on the lower slack joint 700 The dnve shaft 604 is provided with external helical splines 61 2 along part of the external surface thereof The helical splines 612 can be received in corresponding helical recesses (not shown) in the intenor surface of the housing 602 An annulus 61 4 is defined between part of the extenor surface of the dnve shaft 604 and part of the intenor surface of the housing 602 The annulus 614 contains biasing means in the foπn of a disc spnng stack 61 6 (the disc spnng stack 61 6 is not shown in figure 6B, for clanty) A piston 61 8 is provided at an upper end of the annulus 614, and is moveable axially within the bore 606 The piston 61 8 is provided with a bore 620, which is in connection with the bores 606 and 608, whereby fluid can flow through the bore 606 to the bore 608 via the bore 620 The torque generator 600 is moveable between a retracted position shown in figure 6B, and an extended position, shown in figure 6A If the bore 608 is blocked, then the application of an increasing pressure to the bore 606 will cause the piston 618 to move downwardly (l e to the nght in figure 6A) against the spnng force of the disc spnng stack 616 This will cause the dπve shaft 604 to rotate relative to the housing 602, by virtue of the helical splines 612 and their corresponding recesses. In this way, the torque generator 600 can be moved from the re tracted position to the extended position If the pressure in the bore 606 is removed, then the torque generator 600 will move back to the retracted position, by virtue of the biasing force of the spnng 61 6 against the piston 618 A sealing nng 624 is provided on the piston 618 for sealing between the piston 618 and the housing 602 An adjusting nng 626 is provided at the uppermost end of the disc spnng stack 616 Thmst washers 628 are provided at intervals along the disc spnng stack jb 16 A shoulder washer 630 is provided at the lower end of the disc spnng stack 616, and sets the lower position of the disc spnng stack 616 Tapered washers (ie shims) are provided within each pair of disc spnngs of the stack 61 6 to ensure that they cannot be compressed beyond 75% of the full compression capacity, this helps to prevent pennanent set and/or failure of the disc spnngs and to maintain the disc spnng deflection within an acceptable load range Referring to figures 7A and 7B, the lower slack joint 700 compnses a first elongate inner shaft 702 and a second elongate shaft in the form of an outer housing 704 The first elongate shaft 702 is provided with a bore 706 extending l ongitudinally therethrough The bore 706 is in fluid communication with the bore 708, which extends longitudinally through the housing 704 The first elongate shaft 702 is provided with a screw threaded recess 710 for connection to the screw threaded projection 610 The housing 704 is provided with a screw threaded pro j ection 7 12 for connection with the second anchor 400' The shaft 702 compnses an upper shaft portion 71 6, and inteπnediate shaft portion 71 8 and a lower shaft portion 720 the upper shaft portion 71 6 is secured to the intermediate shaft portion 71 8 by a screw thread, and the intermediate s haft portion 7 1 8 is secured to the lower shaft portion 720 by a screw thread The housing compnses six housing portions 722, 724, 726, 728, 730 and 732, each of which is connected to its adjacent housing portion by a screw thread An annular chamber 714 is defined between the upper shaft portion 71 6 and the housing portion 726, a compensating piston 734 is provided within the annular chamber 714, and divides the chamber 714 into two sub-chambers 714a and 714b. An annular chamber 736 is provided between the upper shaft portion 718 and the housing portion 724. An annular chamber 738 is provided between the intermediate shaft portion 718 and the housing portion 726; ports 740 are provided in the housing portion 726 and permit fluid communication between the annular chamber 738 and the interior of the wellbore 10. An annular chamber 742 is provided between the inteπnediate shaft portion 71 8 and the housing portion 730; ports 748 permit fluid communication between the bore 706 and the annular chamber 742 An annular chamber 744 is provided between the lower shaft portion 720 and the housing portion 730; ports 746 permit fluid communication between the annular chamber 744 and the intenor of the wellbore 10 A sealing nng 750 is provided on the upper shaft portion 71 6 for sealing between the upper shaft portion 716 and the housing portion 722 A sealing nng 752 is provided on the inteπnediate shaft portion 71 8 for sealing between the intermediate shaft portion 718 and the housing portion 71 8 A sealing nng 754 is provided on the lower shaft portion 720 for sealing between the lower shaft portion 720 and the housing portion 732 A sealing nng 756 is provided on the compensating piston 734 for sealing between the compensating piston 734 and the housing portion 726, and a sealing nng 758 is provided on the compensating piston 734 for sealing between the compensating piston 734 and the upper shaft portion 716 A sealing nng 760 is provided on the lower shaft portion 720 for sealing between the lower shaft portion 720 and the housing portion 730. The cross-sectional shape of part of the outer surface of the shaft 702 is hexagonal, and the cross-sectional shape of the inner surface of the housing 704 is hexagonal; this configuration serves to prevent relative rotational movement between the shaft 702 and the housing 704. More specifically, this configuration prevents relative rotational movement between the upper shaft portion 716 and the housing portion 724. The lower slack joint 700 is movable between an extended position, as shown in figure 7A, and a retracted position, as shown in figure 7B. When the lower slack joint 700 is in the extended position, the torque generator 600 will be in the retracted position, and when the lower slack joint 700 is in the retracted position, the torque connector 600 will be in the extended position. When the lower slack joint 700 is in the retracted position, the annular chambers 714a and 742 are at their maximum volumes, and the annular chambers 736, 738 and 744 are at their minimum volumes; and when the lower slack joint is in the extended posiϋon, the annular chambers 714a and 742 are at their minimum volumes, and the annular chambers 736, 738 and 744 are at their maximum volumes The sub-chamber 714b remains at the same volume regardless of the stroke position, ie, the sub-chamber 714b has the same volume when the lower slack joint 700 is in the extended position as when the lower slack joint 700 is in the retracted position The lower slack joint 700 is lubncated, and is provided with lubncating fluid in the annular chamber 714a and 736 Intemal hydraulic pressure in the bore 706 results in a force causing the housing 704 and the shaft 702 to be retracted The arrangement is such that when the first elongate shaft 702 is subjected to a downward force from the torque generator 600, the downward force on the housing 704 created by fnction between the hexagonal part of the shaft 702 and housing portion 724 is exactly matched by the upward force on the housing 704 created by the piessure of the fluid m the bore 706 communicating with the chamber 742 The anchor 400' is connected to the lower slack joint 700 The construction of the lower anchor 400' is preferably exactly the same as the construction of the upper anchor 400 Thus, the screw threaded recess 406 of the anchor 400' would be connected to the screw threaded projection 712, and the screw threaded projection 452 of the lower anchor 400' would be connected to the blanking cap, which is of conventional design Refernng to figure 8, the valve means 800 is shown. The valve means 800 comprises an elongate valve body 802 having a bore 804 extending longitudinally therethrough. The valve body 802 includes ports 806 which can communicate with the bore 804. A moveable valve member in the form of a piston 808 is provided within the valve body 802; the piston 808 is moveable axially relative to the valve body 802. The piston 808 is initially secured to the valve body 802 by means of a shearing pin 810. The piston 808 has a bore 812 extending longitudinally therethrough which is in fluid communication with the bore 804 The piston 808 is provided with a bore 814 which is in fluid communication with the bore 806 of the body 802. The valye body 802 compπses an upper body portion 81 8 and a lower body portion 820- the upper body portion 818 is secured to the lower body portion 820 by a screw thread The piston 808 is moveable within the lower body portion 820 Sealing nngs 824 and 826 are provided between the piston 808 and a valve sleeve 840, which is disposed within the lower body portion 820 Four O-nngs 842, 844, 846 and 848 act as static seals around the valve sleeve 840 and a seal housing 850 An internal filter 828 is provided in the bore 804 upwards of the piston 808 A filter 830 and is provided in the ports 806 A nozzle aπangement 832 is provided in the bore 812 of the piston 808 The upper half of the valve means 800 shown in figure 8 shows the piston 808 in a first position with the sheaπng pin 810 intact, and the lower half of figure 8 shows the piston 808 in a second position in which it is moved axially relative to the first position, and the sheanng pin 810 has been sheared The sheanng pin 810 can be sheared by increasing the fluid flow through the bores 804, 810, 81 4 and 806 Once the sheanng pin 810 has sheared, the piston 808 moves axially in a downward direction (ie to the πght in figure 8), and this takes the bore 814 out of alignment with the bore 806, so that fluid can no longer flow through the valve means 800. The valve means 800 is provided with a screw threaded recess 81 6 which is connected to a screw threaded projection 906 of the circulating sub 900. Referring to figure 9, the circulating sub 900 comprises an elongate body 902 having a bore 904 extending longitudinally therethrough. The body 902 has a screw threaded projection 906 which can be secured to the screw threaded recess 81 6 of the by-pass valve 800. The circulating sub 900 includes a bore 908, which extends radially from the bore 904. However, fluid communication between the bore 904 and the bore 908 is prevented by a valve member 910 which is secured to the body 902 by a shearing pm 912. Four sealing nngs 914 are provided on the valve member 910 for sealing between the valve member 910 and the body 902. If a sufficient force is applied to the valve member 910 to shear the s heanng pin 912 - for example, by dropping a ball (not shown) to seat 910a in the valve member 910 - then the valve member 910 can move downwardly (ie to the right in figure 9), which will provide fluid communication between the bore 904 and the bore 908 The bore 908 is in fluid communication with the wellbore 10 The operation of the back-off assembly will now be descnbed Pnor to running in the back-off assembly 20, a pipe cutter (not shown) is run in the wellbore 10, and the upper casing section 14a is cut at the correct position to create the upper edge 14d The back-off assembly 20 is then lowered into the well bore 10 by the dnll stnng 16 until the shoulder 31 6 of the swivel joint 300 engages the upper edge 14d of the first casing section 14a The weight of the equipment below the upper slack joint is set down on the upper edge 14d of the first casing section 14a - for an 8 inch (20 cm) outer diameter back-off assembly this weight would be approximately 1 1 ,500 lbs (5200 kg). The equipment is moved down a further 18 inches (0.46 m), which is the distance between the retracted and un retracted positions of the upper slack joint 200. As the upper slack joint 200 reaches the fully retracted position, the weight on the upper edge 14d will increase. It should be allowed to increase to about 30,000 lbs ( 13,600 kg) The equipment should then be raised about 9 inches (0.23 m), when the weight on the upper edge 14d should drop back to 1 1 ,500 lbs (5200 kg). The shoulder 31 6 of the swivel joint 300 is now set on the upper edge 14d of the first casing section 14a. The drilling mud is now flowed through the back-off assembly 20. It will flow through the bores 904, 804, 812, 814 and 806 to the well bore area 10. The flow rate is gradually brought up to 125 to 150 U.S. gallons per minute. The sheanng pin 810 will shear at this flow rate (depending upon the mud density) which will cause the piston 808 to move downwardly and block fluid communication between the bore 812 and the bore 816. This results in a pressure build up in the back-off assembly 20. The anchors 400 and 400' need an internal pressure of about 10 to 30 psi in order for the gripping elements 412 to overcome the force of the leaf spnngs 414 and move from the release position to the gnpping position The valve means 800 needs approximately 50 psi for the valve to close The valve closure rate is set to be low, in order to minimise the initial pressure surge It should be noted that the "pop off valve of the mud pump (not shown) on the surface 18 should be set to the maximum pressure value possible for a flow rate of 200 U S gallons per minute, to ensure that the valve 800 will close and to have the maximum pressure available for the back-off assembly 20, ideally, 5000 psi should be made available As the pressure increases, with the valve means 800 closed, the hydraulic anchors 400 and 400' will gnp the first casing section 1 4a and the second casing section 14b respectively The gnpping force will increase as the piessure incieases When a pressure of about 175-285 psi is reached the piston 618 of the torque generator 600 will be able to overcome the biasing force of the disc spnng stack 616. A pressure of 550-660 psi is needed to fully compress the disc spnng stack 616. Thus, when the pressure increases above 1 75-285 psi the force from the piston 61 8 will start to generate a torque causing the housing 602 of the torque generator 600 to rotate relative to the dnve shaft 604 the direction of rotation is anticlockwise looking downhole. As the pressure nses the gnp that the two anchors 400 and 400' have on the casing sections 14a and 14b should be greater that the torque generated, so that slippage of the anchors 400 and 400' should not occur. The connection between the first and second casing sections 14a and 14b will start to untorque at a particular pressure, which depends upon the make-up torque and the corrosion/mud within the threads. It is expected that a total pressure of 5,000 psi will be sufficient to unscrew most casing connections that will be encountered in the field. As the torque is generated, the torque generator 600 moves from the retracted position to the extended position, which results in an increase in length of about 18 inches (0.46 m). At the same time, the lower slack joint 700 moves from the extended position to the retracted position in order to compensate for the increase in the length of the torque generator 600. When the bore 706 is pressurised (for example to 5000 psi), the annular chamber 742 is also pressurised through the ports 748. The sealing ring 760 is, for example, 150 mm in diameter, while the sealing rings 752 and 754 are, for example, 95 mm in diameter. The high pressure acting on the sealing ring 754 - through the lowermost end of the bore 706 - will push the outer housing to the right (in Figures 7A and 7B) with a force of 54934 lbs (5000.PI. (95 2 )/(4x25.4 2 )). As the chamber 742 is pressurised the pressure will also act to push the outer housing 704 to the left with a force of 82020 lbs (500O.PI.( 15O 2 -95 2 )/(4x25.4 2 )). Thus, there is a net force to the left of 27086 lbs. The loweπnost housing section 732 is attached to the anchor 400', s o this net hydraulic force acts to pull down the inner shaft 702. This intemal pressure balances the axial frictional drag that is created between the inner shaft 702 and the outer housing 704 by virtue of the torsional force acting on the hexagonal cross-sections of the inner shaft 702 and the outer housing 704. This frictional force would otherwise tend to push the anchor 400' downwards, but it is compensated by the force imbalance within the slack joint 700. The lower anchor 400', the lower slack joint 700, and the drive shaft 604 of the torque generator 600 should not rotate during the torque generation. The housing 602 of the torque generator 600 should rotate, along with the driven housing 502 of the clutch assembly 500, the drive housing 504 of the clutch assembly 500, and the upper anchor 400. The upper casing section 14b should also rotate in order to unscrew it from the collar 14c. The lower connector portion 304 of the swivel connector 300 will rotate relative to the upper connector portion 302, by virtue of the bearing arrangement 314. When the torque generator 600 has reached the extended position, then the pressure should be reduced back to zero gauge pressure on the surface pump. This will cause the spring force o the spring 61 6 to push the piston 618 upwardly and move the torque generator 600 from the extended position to the retracted position . The torque generator 600 will reach the retracted position at a pressure of about 175-285 psi, and at this pressure the gripping members 412 of the anchors 400 and 400' will still be gripping the interior surface of the first and second casing sections 14a and 14b. During the pressure reduction, the movement of the torque generator 600 from the extended position to the retracted position will result in a torque in a direction opposite to the torque generated during the pressure increase. This could result in re-tightening the first casing section 1 a to the collar 14c. However, the clutch device 500 prevents this, because when the shaft 602 of the torque generator 600 rotates in the opposite direction during the pressure reduction, the drive shaft 502 of the clutch device 500 is not able to drive the driven shaft 504. When the pressure reaches zero gauge pressure at the surface, the first cycle of the back-off assembly will be complete, and the first casing section 14a should have been unscrewed by about half a turn The cycle should then be repeated until the first casing section 14a is slack, ie less than 550-660 psi is needed Typically this would take about 8 cycles, although this will vary with the type of casing thread. It will be appreciated that the maximum pressure needed during each cycle decreases, as the connection between the first casing section 14a and the collar 1 4c - or between the second casing section 14b and the collar 14c - becomes looser. It is possible to determine when the connection is loose, because only 550-660 psi will be needed to move the torque generator 600 from the retracted position to the extended position (ie as soon as only the full spring force of the spring stack 616 needs to be overcome). As the upper casing section 14a is unscrewed the housing 204 of the upper slack joint 200 will move upwardly relatively to the shaft 202, ie towards the retracted position A ball (not shown) is then dropped into the well bore, which lands on the seat 910a and causes the sheanng pin 912 to shear and move the valve member 910 downwardly, thereby providing fluid communication between the bore 904 and the intenor of the wellbore 10 The purpose of the circulating sub 900 is to ensure that the dnll stnng Λ 6 is removed in a dry condition - the fluid in the stnng 1 6 em pties via the bore 908 - and to prevent the anchors opening on removal form the well bore 10 After the first casing section 14a has been unscrewed, it can be removed from the w ell bore using a standard spear aπangement (not shown) In a modification, the back-off assembly could incoφorate a mechanically or hydraulically activated spear of appropnate design between the swivel joint 300 and the upper slack joint 200 In this modification, the swivel joint 300 would not need the external shoulder 31 6, the landing shoulder could, instead, be provided on the casing retneval spear Referπng to Figures 1 0, 1 I A, 1 1 B and 1 1 C a modification is shown incoφorating an overshot tool 950 between the upper slack joint 200 and the swivel joint 300 Many of the parts shown in Figures 10 and 1 1 are identical to the parts shown in Figures 2A, 2B and 3, and like parts have been designated with like reference numerals The upper slack joint 200 has been inverted so that the by-pass valve 800 above the upper slack joint 200 needs to have a box-box crossover to connect it to the projection 252 of the upper slack joint 200 The puφose of the overshot tool 950 is to enable the casing stump 14a to be removed from the wellbore 10 after it has been untorqued (but not disconnected) from the rest of the casing 14 The advantage of the overshot tool 950 is that the casing stump 14a can be disconnected and removed from the wellbore 10 in a single tπp, rather than in two tπps The overshot tool 950 includes a release member in the form of a bu per tube 974, which is fixedly secured with a screwthread to the body portion 21 6 of the upper slack joint 200, and an overshot shaft in the form of a splined sub 952, which is provided at one end with a threaded projection 954 by means of which the sub 952 is connected to the threaded recess 210 on the upper slack joint 200 The length of the bumper tube 974 is such that the stroke of the upper slack joint 200 is reduced from 1 8 inches (45 7 cm) to 16 inches (40 6 cm) The sub 952 is provided with a central axιal)y extending bore 976, which provided fluid communication between the bores 206 and 306 The other end of the splined sub 952 is provided with a threaded projection 956 by means of which the sub 952 is connected to the threaded recess 310 on the swivel joint 300 The overshot tool 950 further compnses a splined housing 958 which surrounds part of the splined sub 952 Part of the exteπor surface of the splined sub 952 is provided with splines 960, which extend axially of the sub 952 Part of the intenor surface of the spl ed housing 958 is provided with splines 962, which extend axially of the housing 958 The splines 960 and 962 interlock, wherebv the sub 952 can move axially relative to the housing 958, but cannot move rotationallv relative to the housing 958 The housing 958 is secured to an overshot bowl 964 The splined housing 958 and the bowl 964 together fonn an overshot housing Gnpping means in the fonn of a grapple 966 is disposed WT thing the oveshot bowl 964 The grapple 966 is movable axially within the bowl 964, but it prevented from moving beyond the end of the bowl 964 by a stop 964a disposed on the end of the bowl 964 A nng 968 is provided to impart rotational movement from the bowl 964 to the grapple 966 The nng 968 is of conventional design and may compnse, for example, a plain cylinder nng with a key or finger (not shown) extending downwardly into a slot machined into the wall of the grapple 966 and partially milled into the inside diameter of the bowl 964 The inner surface of the bowl 964 is provided with tapered surfaces 964b, created on a left-hand spiral thread, and the outer surface of the grapple 966 is provided with tapered surfaces 966a, also on a left-hand spiral thread, the tapered surfaces 964b are in contact with the tapered surfaces 966a. The surfaces 964b and 966a are configured such that, as the surfaces 964b move upwardly relative to the surfaces 966a, the surfaces 964b press inwardly on the surfaces 966a, thereby causing the grapple 966 to grip an object disposed within it. Movement of the surfaces 964b downwardly relative to the surfaces 966a relaxes the pressure on the grapple 966. A wicker thread is provided on the inside of the grapple 966, which is also a left-hand spiral; this aids release of the grapple 966 off the casing section bv rotating to the right, because the drill pipe being used is right-hand threaded. A stop ring 970 is screwed onto the outer surface of the sub 952, and a circlip 972 is disposed immediately above the stop ring 970 to prevent the stop ring 970 from unscrewing. The splined housing 958 is movable axially relative to the sub 952. The stop ring 970 defines one of the limits of movement of the splined housing 958, and an upper surface of the first elongate connector portion 302 defines the other limit of movement of the splined housing 958. In total, the splined housing can travel about 2 inches (5. 1 cm). The splined housing 958 is provided with slots 958a to allow fluid to exit on arriving up the annular gap between the back-off assembly 20' and the casing stump 14a. It should be noted that this feature is needed only if the back-off a s sembly 20' is used. It is not needed with the back-off assembly 20, where the by¬ pass valve 900 is located above the upper slack joint 200. A spring 980 is provided between the splined housing 958 and the splined sub 952. The spring 980 biases the splined housing 958 against the stop ring 970 as the overshot tool 950 is lowered into the well. This ensures that the bumper tube 974 can s trike the housing 958. It also ensures that a shoulder 958b of the splined housing 958 is always maintained at a position spaced from a shoulder 302a on the first elongate connector portion 302: this is important if it becomes necessary to release the grapple 966 from a casing section that cannot be recovered. When the grapple 966 is locked by pulling up the overshot bowl 964 (as described below), the spring 980 will be compressed to the position shown in Figure 1 IA. The operation of the overshot tool 950 will now be described. The back¬ off assembly 20 (or 20') is lowered from the surface in the usual manner so that the shoulder 316 comes into contact with the casing stump 14a. The splined housing 958, the bowl 964 and the grapple 966 move along the outside of the casing stump 14a. As the slack joint 200 moves to the completely closed position, the bumper tube 974 is forced downwardly into contact with the splined housing 958. After the bumper tube 974 contacts the housing 958, it pushes the housing 958 downwardly relative to the sub 952, and also pushes the bowl 964 (which is fixedly secured to the housing 958) downwardly, which ensures that the surfaces 964b do not press against the surfaces 966a, so that the grapple 966 does not grip the casing stump 14a. However, the grapple 966 will have frictional contact with the casing, even through it is free between the surfaces 964b and 966a. The back-off assembly 20 (or 20') is then pulled up 8 inches (20.3 cm) to its mid-stroke position, and the cycling of the back-off assembly 20 takes place in the manner described above. After 8- 10 cycles (ie after 8- 10 half rotations) the remaining torque required to complete the unscrewing of the casing stump 14a is relatively low (eg 5% of the initial break-out torque); at this time the overshot tool 950 can be activated by pulling the back-off assembly 20 (or 20') upwardly, so that the upper slack joint 200 is stroked to the extended position. The assembly can be further pulled up to draw the bowl 964 upwardly and cause the surfaces 964b to move upwardly relative to the surfaces 966a, since the grapple 966 has frictional contact with the casing. This causes the surfaces 964b to bear upon the surfaces 966a, and causes the grapple 966 to grip the casing stump 14a. When the grapple has been locked sufficiently to the casing stump 14a, left hand torque can be applied from the surface to complete the unscrewing of the casing stump 14a. This torque will be transmitted from the bowl 964 to the grapple 966 by the ring 968, through the key of finger of the ring 968. When the casing stump 14a has been completely unscrewed, the back-off assembly 20 (or 20') can be removed from the wellbore 10 along with the casing stump 14a, which will still be held in the grapple 966. The grapple 966 can be released in the event that the casing cannot be recovered If the casing threads are still engaged, a downward force is applied to the bumper tube 974, so that it stnkes the top of the housing 958 as the upper slack joint 200 closes This should break the freeze between the surfaces 966a and 964b The dnll stnng is then rotated to the nght (clockwise) As the stnng is rotated to the nght, the grapple 966 is jacked up the casing outer diameter, the torque being transmitted through the nng 968 The assembly should be inched upwards to compensate for the relative movement of the grapple 966, until it comes free The system should not be pressunsed or circulated dunng this phase The presence of the spnng 980 prevents the shoulder 958b of the housing 958 from being "bumped" down onto the shoulder 302a, this ensures that the force is transmitted to the overshot bowl 96-r Other aπangements, such as J-slots or shear pins mav be used to control the operation of the overshot tool 950 The particular method of operation descπbed above allows the too! weight to be transferred through the swivel to the casing If the wellbore 10 is not vertical then there is a possibility that the lower slack joint 700 will not be in the intended extended position after being lowered into th e wellbore In order to solve this problem a sheanng pin (not shown) can be provided on the lower slack joint 700 to hold it in the extended position when it has initially run into the wellbore 10 Dunng the first pressure cycle, the sheanng pin will shear so that the lower slack joint 700 can move to the retracted position

Claims: CLAIMS 1. A downhole casing back-off assembly, for disconnecting a first and second casing section within a wellbore, comprising first anchor means adapted to fixedly engage the first casing section, second anchor means adapted to fixedly engage the second casing section, and torque generating means adapted to generate a torsional force between the first and second anchor means, whereby the first casi ng section i.s disconnected from the second casing section. 2. A downhole casing back-off assembly as claimed in claim 1 , wherein the torque generating means is disposed between the first and second anchor means. 3. A downhole casing back-off assembly as claimed in claim 1 or claim 2, wherein a downhole clutch assembly is disposed between the torque generating means and the first anchor means, which allows the torque generating means to apply a torsional force to the first anchor means in one direction, and does not allow the torque generating means to apply a torsional force to the first anchor means in the opposite direction. 4. A downhole casing back-off assembly as claimed in any preceding claim, wherein a downhole swivel joint is disposed above the first anchor. 5. A downhole casing back-off assembly as claimed in any preceding claim, wherein an upper slack joint is disposed above the swivel joint. 6. A downhole casing back-off assembly as claimed in claim 5, wherein the upper slack joint comprises first and second elongate shafts each having a bore extending longitudinally therethrough, the bore of the first shaft being in fluid communication with the bore of the second shaft, and the first and second shafts being axially movable relative to one another between a retracted position, in which the total length of the Iower slack joint is at a minimum, and an extended position, in which the total length. 7. A downhole casing back-off assembly as claimed in claim 5 or 6, wherein an overshot tool is disposed between the upper slack joint and the swivel joint. 8. A downhole casing back-off assembly as claimed in any preceding claim, wherein an overshot tool is disposed above the first anchor. 9. A downhole casing back-off assembly as claimed in claim 8, wherein the release member of the overshot tool is secured to the upper slack joint. 10. A downhole casing back-off assembly as claimed in claim 9, wherein the overshot shaft is connected to one of the first and second shafts of the upper slack joint, and the release member is connected to the other of the first and second shafts of the upper slack joint. 11. A downhole casing back-off assembly as claimed in any preceding claim, wherein a lower slack joint is disposed between the torque generating means and the second anchor means. 12. A downhole casing back-off assembly as claimed in claim 11 , wherein the lower slack joint comprises first and second elongate shafts each having a bore extending longitudinally therethrough, the bore of the first shaft being in fluid communication with the bore of the second shaft, and the first and second shafts being axially movable relative to one another between a retracted position, in which the total length of the lower slack joint is at a minimum, and an extended position, in which the total length of the lower slack joint is at a maximum. 13. A downhole casing back-off assembly as claimed in claim 12, wherein the second shaft of the lower slack joint comprises an outer housing, and the first shaft of the lower slack joint extends within the outer housing. 14. A downhole casing back-off assembly as claimed in claim 13, wherein lubrication is provided between the first and second shafts of the lower slack joint, the internal hydraulic pressure biasing the lower slack joint to the retracted position. 15. A downhole casing back-off assembly as claimed in claim 14. wherein relative rotational movement between the first and second shafts of the lower slack joint is prevented by anti-rotation means. 16. A downhole casing back-off assembly as claimed in claim 15, wherein the anti-rotation means is immersed in an oil bath. 17. A downhole casing back-off assembly as claimed in claim 15 or 16, wherein the anti-rotation means is a spline. 18. A downhole casing back-off assembly as claimed in any one of claims 11 to 17, wherein the first shaft of the lower slack joint is connected to the torque generating means, and the second shaft of the lower slack joint is connected to the second anchor means. 19. A downhole casing back-off assembly as claimed in any preceding claim, wherein valve means is provided to allow the filling of the assembly with fluid as the assembly is run into the wellbore. 20. A downhole casing back-off assembly as claimed in claim 19. wherein the valve means compπses an elongate valve body, and a valve member within the valve body, the valve body being provided with a bore extending longitudinally therethrough, said bore being in fluid communication with the second anchor means. 21. A downhole casing back-off assembly as claimed in claim 20, wherein the valve body includes at least one port extending from the bore of the valve body to the wellbore. 22. A downhole casing back-off assembly as claimed in claim 20 or 21 , wherein the valve member is movably arranged within the bore of the valve body. 23. A downhole casing back-off assembly as claimed in claim 22, wherein the valve member is movable in response to fluid flow through the bore. 24. A downhole casing back-off assembly as claimed in claim 23, wherein the valve member can be secured to the valve body by a shear pin, or is spring loaded, such that the valve member does not move relatively thereto until the fluid flow reaches a pre-determined amount. 25. A downhole casing back-off assembly as claimed in any one of claims 19 to 24, wherein the valve means is disposed above the first anchor means. 26. A downhole casing back-off assembly as claimed in claim 25, wherein a circulating sub is disposed immediately above the valve means. 27. A downhole casing back-off assembly as claimed in claim 26, wherein the circulating sub comprises a bore extending longitudinally therethrough in fluid communication with the bores in the other components of the casing back-off assembly: a radial bore in fluid communication with the longitudinal bore and with the interior of the wellbore; and a sealing member normally preventing fluid communication between the radial and longitudinal bores, said sealing member being held in position by a shearing pin which may be broken by applying a sufficient downward force on the sealing member. 28. A downhole anchor, for use in a wellbore, comprising an elongate housing, at least two gripping members movable relative to said housing between a release position and a gripping position in which the gripping member engages an internal surface of the wellbore, and a hydraulic gallery within said housing in fluid communication with a surface of the gripping member, whereby the application of pressure lo a hydraulic fluid within said hydraulic gallery causes said gripping members to move from the release position to the gripping position. 29. A downhole anchor as claimed in claim 28, wherein the application of pressure to said hydraulic fluid causes the gripping member to move radially relative to the axis of the housing, without any concomitant axial movement of the gripping means. 30. A downhole anchor according to claim 1 or claim 2 wherein each gripping member defines a plurality of hydraulic cylinders, and each hydraulic cylinder receives a piston which is fixed relative to the housing whereby each piston and cylinder defines a respective hydraulic chamber, all the hydrauliέ chambers being connected to the hydraul ic gal lery. 31. A downhole anchor according to claim 30 wherein each gripping member defines at least three and preferably si x hydraulic cvl inders. 32. A downhole anchor according to claim 30 or claim 31 wherein each piston is releasably secured to the housing by a retaining bolt which provides a hydraulic connection between the hydraulic gallery and the respective hydraulic chamber. 33. A downhole anchor as claimed in any one of claims 28 to 32, wherein biasing means, preferably a leaf spring, is provided to bias the gripping member towards the release position. 34. A downhole anchor as claimed in any of claims 28 to 33, wherein two or more gripping members are arranged around the longitudinal axis of said housing. 35. A downhole anchor as claimed in claim 34, wherein three gripping members are arranged as a set about the longitudinal axis of the housing at an angle of 120° to one another and two of said sets of three gripping elements are provided, the two sets being spaced from each other along the longitudinal axis of the housing. 36. A downhole anchor as claimed in claim 35, wherein the second set is arranged about the longitudinal axis of the housing with a staggered conformation relative to the first set. 37. A downhole anchor as claimed in claim 36, wherein the second set is staggered relative to the first set at an angle of 60°. 38. A downhole anchor as claimed in any one of claims 32 to 37, wherein each gripping member is secured to a pair of leaf springs. 39. A downhole anchor as claimed in claim 38, wherein the leaf springs are housed within recesses defined by the housing. 40. A downhole anchor as claimed in claim 39, wherein the recesses are substantially closed by the gripping members. 41 . A downhole torque generator, for use in a wellbore, comprising an elongate first shaft, and an elongate second shaft, each shaft having a bore extending longitudinally therethrough, wherein the bore of the first shaft is in fluid communication with the bore of the second shaft, and the first shaft is coupled to the second shaft by a torque generating connection, whereby the application of an axial force to said shafts causes said first and second shafts to rotate relative to one another about said torque generating connection. 42. A downhole torque generator as claimed in claim 41 , wherein the first shaft is provided in the form of a tubular housing mounted around the second shaft by the torque generating connection. 43. A downhole torque generator as claimed in claims 41 or claim 42, wherein the torsion is generated by an axial compression force which is produced by hydraulic pressure within the bores of the shafts. 44. A downhole torque generator as claimed in any one of claims 41 to 42, wherein the torque generating connection comprises a hel ical spline provided on o e of said shafts (preferably the second shaft) , which is received in a corresponding helical recess in the other of said shafts (preferably the first shaft) , such that, if the shafts are pulled together, they will rotate relatively in one direction and will move axially away from one another, and if the shafts are pushed together, they will rotate relatively in an opposite direction and will move axially towards one another: the shafts can move relatively between a retracted position, in which the combined length of the shafts is at a minimum; and an extended position, in which the combined length of the shafts is at a maximum. 45. A downhole torque generator as claimed in claim 44, wherein at least a portion of the second shaft is received within the first shaft. 46. A downhole torque generator as claimed in claim 45, wherein stop means is provided to limit movement of the shafts beyond the retracted and the extended positions. 47. A downhole torque generator as claimed in claim 46, wherein biasing means is provided for biasing the shafts to the retracted position. 48. A downhole torque generator as claimed in claim 47, wherem an annular chamber is provided between the interior of the first shaft and the exterior of the portion of the second shaft within the first shaft, the biasing means being disposed in this annular chamber. 49. A downhole torque generator as claimed in claim 48, wherein the biasing means is a stack of disc springs. 50. A downhole torque generator as claimed in claim 49. wherein a piston is provided within the bore of the first shaft, said piston being adapted to apply an axial compressive force against the biasing means, by hydraulic pressure. 51 . A downhole torque generator as claimed in claim 50. wherein the piston is attached to the second shaft by means of a screw thread. 52. A downhole clutch assembly, for use in a wellbore, comprising an elongate drive shaft and an elongate driven shaft, each shaft having a bore extending longitudinally therethrough, the bore of the drive shaft being in fluid communication with the bore of the driven shaft, and coupling means for coupling the drive shaft to the driven shaft, said coupling means being such that rotation of the drive shaft in a first direction causes the driven shaft to rotate with the drive shaft, and rotation of the drive shaft in a direction opposite to the first direction does not cause the driven shaft to rotate with the drive shaft. 53. A downhole clutch assembly as claimed in claim 52, wherein the coupling means comprises: a collar on the driven shaft, said collar being axially movable along the driven shaft and being rotationally fixed relative thereto; a plurality of detentes on the collar for engagement with a plurality of cooperating detentes on the drive shaft; and biasing means for biasing the detentes on the collar into engagement with the detentes on the drive shaft; wherein the detentes and biasing force are such that when the drive shaft is rotated in said first direction, the detentes are held in engagement, whereby rotational movement of the drive shaft is imparted to the driven shaft through the collar, and when the drive shaft is rotated in a direction opposite to the first direction, the collar moves axially away from the drive shaft, and the detentes of the collar disengage from the detentes on the drive shaft, whereby rotational movement of the drive shaft is not imparted to the collar. 54. A downhole clutch assembly as claimed in claim 52, wherein the coupling means comprises: a collar on the drive shaft, said collar being axially movable along the drive shaft and being rotationally fixed relative thereto; a plurality of detentes on the collar for engagement with a plurality of cooperating detentes on the driven shaft; and biasing means for biasing the detentes on the collar into engagement with the detentes on the driven shaft; wherein the detentes and biasing force are such that when the drive shaft is rotated in said first direction, the detentes are held in engagement, whereby rotational movement of the drive shaft is imparted to the driven shaft through the collar, and when the drive shaft is rotated in a direction opposite to the first direction, the collar moves axially away from the driven shaft, and the detentes of the collar disengage from the detentes on the drive shaft, whereby rotational movement of the drive shaft is not imparted to the collar. 55. A downhole clutch assembly as claimed in claim 53 or 54, wherein in one direction the detentes are angled relative to one another so that upon rotational movement they slide over one another and push the collar axially against the force of the spring. 56. A downhole swivel joint, for use in a wellbore, comprising a first elongate connector portion and a second elongate connector portion rotatably secured to the first elongate connector portion, wherein the first and second connector portions are each provided with a bore extending longitudinally therethrough, the bore of the first connector portion being in fluid communication wilh the bore of the second connector portion, and the first and second connector portions are each provided with means to connect additional equipment thereto. 57. A downhole swivel joint as claimed in claim 56, wherein the second connector portion, which is arranged lowermost, in use, is provided with a shoulder adapted to engage a surface within the wellbore to prevent further movement of said swivel joint down the wellbore. 58. A downhole swivel joint as claimed in claim 56 or 57, wherein, the first elongate portion is rotatably secured to the second rotatable portion by means of a set of axial thrust discs, or other type of ball or roller bearings. 59. An overshot tool for retrieving downhole equipment, comprising an overshot shaft adapted to be secured to other downhole equipment; an overshot housing extending over at least part of the overshot shaft; gripping means for gripping the downhole equipment to be retrieved, the overshot housing having locking means adapted to lock the gripping means in a gripping position, and the overshot housing being axially movable relative to the shaft from a release position, in which the locking means does not lock the gripping means in the gripping position and a locking position in which the locking means does lock the gπpping means in the gripping position; and a release member axially movable relative to the overshot shaft and housing, said release member being movable into contact with the overshot housing to move the overshot housing to the release position. 60. An overshot tool as claimed in claim 59, wherein the overshot shaft has a central bore extending axially of the shaft. 61 . An oversnot tool as claimed in claim 59 or 60, wherein means is provided to prevent rotational movement between the overshot shaft and the overshot housing. 62. An overshot tool as claimed in claim 61 , wherein the means provided to prevent rotational movement comprises a plurality of interlocking splines provided on an outer surface of the overshot shaft and on an inner surface of the overshot housing. 63. An overshot tool as claimed in any one of claims 59 to 62, wherein stop means is provided to prevent movement of the housing beyond the locking and the release positions. 64. An overshot tool as claimed in any one of claims 59 to 63, wherein the release member comprises a tubular member having a diameter substantially equal to the diameter of the overshot housing. 65. An overshot tool as claimed in any one of claims 59 to 64, wherein the release member is secured to downhole equipment separate from the rest of the overshot tool. 66. A downhole casing back-off assembly as claimed in any of claims 1 to 27, wherein the first and second anchor means are downhole anchors according to any one of claims 28 to 40. 67. A downhole casing back-off assembly as claimed in any of claims 1 to 27 or claim 66, wherein the torque generating means is a downhole torque generator according to any one of claims 41 to 51 . 68. A downhole casing back-off assembly as claimed in any of claims 1 to 27 or claim 66 or claim 67, wherein the downhole clutch assembly is a downhole clutch assembly according to any one of claims 52 to 55. 69. A downhole casing back-off assembly as claimed in any of claims 1 to 27 or 66 to 68, wherein the downhole swivel joint is a downhole swivel joint according to any one of claims 55 to 57. 70. A downhole casing back-off assembly as claimed in any of claims 1 to 27 or 66 to 69, wherein the overshot tool is an overshot tool according to any one of claims 58 to 64.

Inventors: MCGARIAN BRUCE, TULLOCH RORY MCGRAE, MCGARIAN, BRUCE, TULLOCH, RORY, MCGRAE
Applicants: MCGARIAN BRUCE, RED BARON OIL TOOLS RENTAL, TULLOCH RORY MCGRAE, MCGARIAN, BRUCE, THE RED BARON (OIL TOOLS RENTAL) LIMITED, TULLOCH, RORY, MCGRAE
Application year: 1996
Main classification: E21B17
IPC codes: E21B4, E21B17, E21B23, E21B31