Patent Publication Number: US-8973666-B2

Title: Running adapter

Description:
FIELD OF THE INVENTION 
     The present invention relates to plugs, particularly to plugs for sealing wellbores and christmas trees. 
     BACKGROUND OF THE INVENTION 
     Conventionally wellbores, and christmas trees associated with wellbores, have been sealed with plugs having three basic parts: an anchoring system, a sealing element and a setting system. 
     The first stage in setting a conventional plug is anchoring the plug in the wellbore. Anchoring systems for conventional wellhead plugs use a set of locking dogs, which engage a recessed profile in the wellbore or tree, or use a set of slips which “bite” the casing to hold the plug in place. 
     The seal is then set using a linear action setting mechanism to create a linear displacement to deform the seal element. The force required to create the seal is then locked in using a linear locking mechanism. In wellbore applications the seal is generally a metal-to-metal seal formed by swaging a metal ring element into the bore or onto a no-go shoulder. 
     To provide a seal capable of withstanding well pressures, the required setting force needs to be as high as the maximum force generated by the well pressure. 
     In recent years a number of high pressure, high temperature, high flow rate wells have been completed which have highlighted shortcomings in conventional designs of well bore plugs and tree plugs. For example, swaged seals can dislodge when exposed to the high pressure, temperature and vibration cycles of these wells, and the jarring action used to set the seal can damage the plug or the surrounding environment. 
     Additionally, linear locking mechanisms have a degree of backlash which in a high temperature, pressure and vibration cycle environment can lead to failure. 
     A further disadvantage of conventional plugs is the expansion achievable from the metal seal element is not sufficient to permit the plug to be run into the wellbore with adequate clearance between the plug and the wellbore to prevent a build-up of pressure in front of the plug, resisting the placement of the plug. This can be a particular problem when a number of plugs are to be located in series in a conduit, as a hydraulic lock can be formed between plugs. 
     It is an object of the present invention to obviate or mitigate at least one of the aforementioned disadvantages. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a body having a first section and a second section; 
     at least one seal element for creating a seal between the plug and a conduit, the at least one seal element being adapted to be energised by movement in a seal setting direction of the first body section relative to the second body section; and 
     seal locking means comprising a first portion and a second portion; 
     wherein as the at least one seal is energised, the seal locking means first portion is rotatable unidirectionally relative to the seal locking means second portion to take up the movement of the first body section relative to the second body section in the seal setting direction and prevent movement of the first body section relative to the second body section in a releasing direction, opposite the seal setting direction. 
     Preferably, the seal locking means is arranged along an arc centred on, and substantially perpendicular to, a longitudinal axis of the plug. 
     The use of unidirectional rotational locking means to take up movement of the first body section relative to the second body section, particularly when arranged along an arc centred on, and substantially perpendicular to, the longitudinal axis of the plug provides a tree plug in which the possibility of the seal element partially releasing due to backlash is minimised. 
     Movement of the first body section relative to the second body section covers situations in which the first body section is stationary and the second body section moves, the first body section moves and the second body section is stationary, or both body sections move. 
     Preferably, the plug is adapted to be connected to a running adapter. 
     Preferably, the at least one seal element is adapted to be energised by axially translating the first body section relative to the second body section in the setting direction. 
     Preferably, the plug is adapted to be set by the application of linear forces to one or both of the first body section and the second body section to axially translate the first body section relative to the second body section in the setting direction. 
     Alternatively, the at least one seal element is adapted to be energised by rotationally translating the first body section relative to the second body section in the setting direction. 
     Preferably, the first body section is a plug housing, or part of a plug housing. 
     Preferably, the second body section is a seal setting means. 
     Preferably, the running adapter is adapted to apply rotational forces to one or both of the housing and the seal setting means to rotationally translate the housing relative to the seal setting means in the setting direction. Rotation of the housing relative to the seal setting means may additionally cause the seal setting means to translate axially along the housing. 
     Preferably, rotation of the seal setting means causes the seal setting means to translate axially along the housing. 
     Preferably, the seal setting means translates axially along the housing by means of a threaded connection. 
     Preferably, the threaded connection comprises a first thread located on an external surface of the setting means and a complementary second thread located on an internal surface of the housing. 
     One or each of the first and second portions may be integral with one of the first and second body sections. 
     Preferably, the seal locking means first portion comprises at least one locking member which engages the seal locking means second portion. 
     Preferably, the/each locking member is biased against the seal locking means second portion. 
     Preferably, the/each locking member is located within a respective channel defined by the first portion. 
     Preferably, the/each channel is angled to an interface between the first and second portions. Most preferably, the angle between the/each channel and the interface between the first and second portions is an acute angle. The/each channel may be tapered. 
     Preferably, the locking member is biased by a spring. 
     Preferably, the/each locking member is a ball bearing. In use a ball bearing, located in an angled channel and biased against the second portion, will permit relative movement between the first and second portions in one direction, but not in the opposite direction. 
     Preferably, the seal locking means first portion is a locking nut. 
     Preferably, the locking nut is connected to the second body section by a threaded connection. Having the seal locking means arranged on an arc which is centred on and is substantially perpendicular to the longitudinal axis of the housing virtually eliminates backlash generally present when movement between the locking nut and the second body section is facilitated by a threaded connection. 
     Preferably, a first portion of the locking nut is split axially into a plurality locking nut sections. 
     Preferably, there are six locking nut sections. 
     Preferably, each of the locking nut sections is connected to a second locking nut portion. 
     Most preferably, each of the locking nut sections is permitted to move radially relative to the second locking nut portion. 
     Preferably, each of the locking nut sections is connected to the second locking nut portion by means of a dovetail connection. 
     Preferably, the seal locking means further comprises a retaining sleeve. 
     Preferably, the retaining sleeve prevents radial movement of the locking nut sections. 
     Preferably, the retaining sleeve is releasably connected to the locking nut. 
     Preferably, the retaining sleeve is releasably connected to the locking nut by means of at least one shear screw or pin. 
     In an alternative embodiment, the seal locking means is a first unidirectional latching means. 
     Preferably, the first unidirectional latching means is a seal ratchet, the seal ratchet comprising a set of seal ratchet teeth and at least one complementary seal ratchet pawl, the set of seal ratchet teeth being associated with one of the seal locking means portions, the at least one complementary seal ratchet pawl being associated with the other of the seal locking means portions. 
     Alternatively, the first unidirectional latching means comprises a first set latching teeth or castellations associated with one of the seal locking means portions and a second set of latching teeth or castellations associated with the other of the seal locking means portions. 
     Having a seal ratchet mechanism arranged on an arc which is centred on and is substantially perpendicular to the longitudinal axis of the housing virtually eliminates backlash generally present when movement between the seal setting means and the housing is facilitated by a threaded connection. For example, if the threaded connection had 10 threads per inch, the potential backlash with a linear body lock ring without the rotational ratchet mechanism would be 0.1 inches, however by utilising the seal ratchet mechanism described above with, for example, 36 teeth, the backlash is reduced to (0.1/36) inches or 0.0028 inches. 
     Preferably, the seal ratchet teeth are located on an internal surface of a portion of the first body section and the at least one seal ratchet pawl is located on a portion of the second body section. In this case the portion of the second body section may comprise an annular locking ring having a radially inner surface and a radially outer surface, the at least one seal pawl being located on the radially outer surface of the locking ring. 
     Alternatively, the seal ratchet teeth are located on an external surface of a portion of the second body section and the at least one seal ratchet pawl is located on a portion of the first body section. In this case the portion of the first body section may comprise an annular locking ring having a radially inner surface and a radially outer surface, the at least one seal pawl being located on the radially inner surface of the locking ring. 
     Preferably, the at least one seal element is a metal seal element. 
     Preferably, the at least one seal element is a stack of frusto-conical washers. Frusto-conical washers are also known as disc springs or Belleville Washers™ 
     Alternatively, the at least one seal element is a plurality of metal seals, or a combination of metal and plastic seals. 
     Preferably, at least one frusto-conical washer in the stack is adapted to form an independent metal-to-metal seal with a conduit from at least one other frusto-conical washer in the stack. 
     Preferably, the frusto-conical washers are steel. Most preferably, the frusto-conical washers are Inconel™. 
     Preferably, the frusto-conical washers are coated with silver. Most preferably, the silver coating is approximately 35 nm thick. 
     Preferably, adjacent washers or seals are separated by at least one layer of softer material. 
     Preferably, the softer material is polymeric. 
     Preferably, the at least one layer is a laminate of softer material. Most preferably, the at least one layer is a laminate of a number of softer materials. In one embodiment the laminate is a layer of PTFE sandwiched between layers of PEEK. 
     In a further alternative, the at least one seal element may be multiple metal seals of differing hardness. 
     Preferably, the at least one seal element is energised by compression. 
     Preferably, where the at least one seal element is a stack of frusto-conical washers, in the uncompressed state each washer is at an angle of 8° to the horizontal. 
     Preferably, where the at least one seal element is a stack of frusto-conical washers, when the at least one seal element is energised, the washers are not flattened. Most preferably, in the energised configuration, each frusto-conical washer is at an angle of 5° to the horizontal. Retaining a slight angle, assists in the recovery of the frusto-conical washers to their original shape when the seal is released. 
     A metal seal element is required for use in wellbores. A stack of frusto-conical washers is preferred because a high expansion ratio is achievable by compression of a frusto-conical washer, permitting the plug to be run into position without building up a significant head of pressure in front of the plug. A further advantage of the frusto conical washer is the expansion is an elastic expansion; the plug can be easily removed from the conduit by releasing the compression force on the washers, thereby reversing the expansion of the seal element sufficiently to permit removal of the plug from the conduit. Additionally because the stack of frusto-conical washers is a smaller diameter than the target seal prior to and after sealing there is no requirement for jarring and no damage is done to the seal bore in the conduit. As no jarring is required to set the seal, the plug can be set by a running adapter which uses e-line or slick-line. 
     The use of a softer material between adjacent washers allows a tight seal to be obtained in a damaged conduit. As the stack is compressed the softer material is squeezed radially outwards into engagement with the damaged conduit. 
     Preferably, in an uncompressed configuration, the stack of frusto-conical washers describes an external diameter less than that of the plug body. This arrangement means the plug can be run in without the seal element being damaged on the conduit. 
     Preferably, the first body section includes a shoulder. Most preferably, the shoulder extends outwardly from the first body section. 
     Preferably, the shoulder is adapted to engage a no-go in the conduit. Providing a shoulder permits the plug to be landed on a conduit no-go ensuring the plug is set in the correct location. 
     Preferably, the plug further includes at least one anchor for securing the plug to the conduit, and an anchor setting means, the at least one anchor being adapted to engage the conduit by movement in an anchor setting direction of one of the first or second body sections relative to the anchor setting means. 
     Preferably, the/each anchor is at least one dog which is adapted to engage a recess in the internal surface of a conduit. 
     Alternatively, the at least one anchor may be at least one slip which is adapted to engage the internal surface of a conduit. 
     Preferably, the at least one dog is adapted to be moved radially outward from the plug. 
     Preferably, the/each at least one dog is adapted to be moved radially outwards by an anchor ramp. 
     Preferably, the anchor ramp is adapted to engage the at least one dog and apply a radially outward force to the at least one dog. 
     Most preferably, the anchor ramp has a tapered surface for engaging a complementary tapered surface on the at least one dog such that movement of the anchor ramp in a setting direction will force the at least one dog radially outwards. 
     Preferably, the tapered surface of the anchor ramp has a variable taper. 
     Preferably, the anchor ramp is a sleeve. 
     Preferably, the/each dog has a surface adapted to engage a complementary surface in the recess. Most preferably, the complementary surfaces are adapted, once engaged, to convert the radially outward force into a downward force on the plug. This arrangement is especially useful when used in conjunction with a conduit no-go as the downward force will be resisted by the no-go, thereby securing the plug in place. 
     Preferably, the other of the first or second body sections and the anchor setting means are the same. Most preferably, the seal setting direction is the same as the anchor setting direction. 
     Preferably, the seal locking means is also an anchor locking means adapted to take up movement of the first body section relative to the anchor setting means in the setting direction and prevent the first or second body section moving relative to the anchor setting means in a releasing direction. 
     Preferably, in this case, the seal locking means second portion acts on the anchor ramp. 
     Alternatively, the second body section and the anchor setting means are different. 
     In this case, preferably, the first body section is a plug housing, and the second body section is a seal setting means. 
     Preferably, the at least one anchor is set by rotationally translating the anchor setting means relative to the housing. 
     Preferably, the anchor setting means and the housing are at least partially connected by second unidirectional latching means arranged along an arc centred on, and substantially perpendicular, to the longitudinal axis such that unidirectional rotational movement of one of the anchor setting means or the housing with respect to the other of the anchor setting means and the housing to set the at least one anchor with the conduit is permitted. 
     Preferably, the second unidirectional latching means is an anchor ratchet, the anchor ratchet comprising a set of anchor ratchet teeth and at least one complementary anchor ratchet pawl, the set of anchor ratchet teeth being associated with one of the anchor setting means or the housing, the at least one complementary anchor pawl being associated with the other of the anchor setting means or the housing. 
     Alternatively, the second unidirectional latching means comprises a set latching teeth or castellations associated with the anchor setting means and a set of latching teeth or castellations associated with the housing. 
     Preferably, rotation of the housing or the anchor setting means causes the anchor setting means to translate axially along the housing. Most preferably, rotation of the anchor setting means causes the anchor setting means to translate axially along the housing. The anchor setting means may translate axially along the housing by means of a threaded connection. 
     The anchor ratchet teeth may be located on an internal surface of a portion of the housing and the at least one anchor ratchet pawl located on a portion of the anchor setting means. In this case, the portion of the anchor setting means may comprise an annular locking ring having a radially inner surface and a radially outer surface, the at least one anchor pawl being located on the radially outer surface of the locking ring. 
     Alternatively, the anchor ratchet teeth may be located on an external surface of a portion of the anchor setting means and the at least one anchor ratchet pawl located on a portion of the housing. In this case, the portion of the housing may comprise an annular locking ring having a radially inner surface and a radially outer surface, the at least one anchor ratchet pawl being located on the radially inner surface of the locking ring. 
     Preferably, the direction of rotation for setting the at least one the anchor is opposite to the direction of rotation for setting the at least one seal. Having opposite directions of rotation for setting the at least one anchor and the at least one seal enables setting the plug to be a two stage process. 
     Preferably, to release the at least one seal element, the second body section is moveable relative to the at least one seal element. 
     Preferably, the second body section is moveable from a set position to a released position, such that in the set position, a seal is formed between the at least one seal element and the second body section and in the released position there is a flow path between the at least one seal element and the second body section. 
     Preferably, the flow path is provided by at least one groove defined by the second body section. 
     Preferably, the plug further comprises a flow path locking means to lock the plug in the released position. This arrangement prevents the seal element being inadvertently reset after an operator believes the seal element has been released. 
     Preferably, the plug is provided with a sealed reservoir for location below the at least one seal, the reservoir comprising a housing containing a body of air at a fixed pressure, such that the reservoir is adapted to collapse or rupture in response to a threshold external pressure being exceeded. 
     Such an arrangement assists in the situation where it is desired to set two plugs adjacent to each other. In such a situation, after the first plug is set, an increase in pressure in the space between the two plugs can occur. This increase in pressure will apply a load against the seal elements of each plug and may affect the integrity of the seal. Providing a sacrificial reservoir substantially mitigates this problem as the increased pressure, once it exceeds a predetermined threshold pressure will rupture or collapse the reservoir permitting a reduction in pressure overall. 
     Preferably, the body of air in the sealed reservoir is at substantially atmospheric pressure. 
     According to a second aspect of the present invention there is provided a running adapter for setting a plug in a conduit, the running adapter arranged to convert a rotary input force into a rotary and an axial output force. 
     Preferably, the rotary output force is provided separately from the axial output force. 
     In one embodiment, the adapter comprises an input mandrel, an output mandrel, an adapter casing, and a locking sleeve. 
     Preferably, the adapter is arranged such that rotation of the input mandrel causes axial movement of the output mandrel relative to the adapter casing, and causes rotational movement of the locking sleeve. 
     Preferably, the input mandrel is adapted to be connected to a rotary drive. 
     Preferably, the adapter casing is adapted to engage a plug first body section. 
     Preferably, the output mandrel is adapted to engage a plug second body section. 
     Preferably, the locking sleeve is adapted to engage a plug seal locking means. 
     Preferably, the locking sleeve is adapted to selectively engage the input mandrel. Most preferably, the locking sleeve is adapted to selectively rotate with the input mandrel. 
     Preferably, the running adapter further comprises a locking sleeve clutch to disengage the locking sleeve from the input mandrel. 
     Preferably, the adapter casing is connected to the input mandrel by a threaded connection. Using a threaded connection converts rotation of the input mandrel to axial movement of the adapter casing with respect to the input mandrel. 
     Preferably, the output mandrel is axially fixed to the input mandrel. Most preferably, the output mandrel is rotationally independent of the input mandrel. 
     Preferably, a bearing interface is provided between the input mandrel and the output mandrel. A bearing interface permits the input mandrel to rotate with respect to the output mandrel. 
     Preferably, the output mandrel includes a bearing surface. There is the possibility that the bearing interface between the input and output mandrels might fail, in this case the output mandrel would rotate. If the output mandrel is directly or indirectly attached to a plug second body section, a bearing surface will reduce the possibility of damage to the second body section. 
     Preferably, the running adapter further comprises a latch, the latch being adapted to be located, in use, between the output mandrel and a plug second body section. 
     In an alternative embodiment, the running adapter comprises a tubular member having a longitudinal axis, an outer surface and an inner surface, one of the outer surface or the inner surface adapted to engage a portion of a plug seal setting means to set an at least one plug seal element, 
     wherein the at least one plug seal element is set by rotation of the running adapter in a first direction about the longitudinal axis. 
     Preferably, the running adapter is adapted to disengage from the plug seal setting means when rotation about the longitudinal axis is in a direction opposite to the first direction. 
     Preferably, the inner surface of the tubular member is adapted to engage a portion of an external surface of the plug seal setting means. 
     Preferably, the inner surface of the tubular member is adapted to engage the plug seal setting means by means of at least one first engagement element, the at least one first engagement element adapted to engage with at least one first complementary notch in the portion of the external surface of the plug seal setting means to rotate the plug seal setting means. 
     Preferably, the at least one first engagement element is arranged only to engage the at least one first complementary notch when rotation is in the first direction. 
     The at least one first engagement element may be pivotally mounted in an at least one first recess in the tubular member, the at least one first engagement element being biased to a position in which the at least one first engagement element sits proud of the inner surface of the tubular member, such that when rotation is in the opposite direction the outer surface of the plug seal setting means depresses the at least one first engagement member into the at least one first tubular member recess. 
     The outer surface of the running adapter tubular member may be adapted to engage with a portion of the internal surface of an inner plug anchor setting means to set at least one plug anchor, wherein the at least one plug anchor is set by rotation of the running adapter in the opposite direction about the longitudinal axis. 
     Preferably, the running adapter is disengaged from the plug anchor setting means when rotation about the longitudinal axis is in the first direction. 
     Preferably, the outer surface of the tubular member is adapted to engage the plug anchor setting means by means of at least one second engagement element, the at least one second engagement element adapted to engage with at least one second complementary notch in the portion of the internal surface of the plug anchor setting means to rotate the anchor setting means. 
     Preferably, the at least one second engagement element is arranged only to engage the at least one second complementary notch when rotation is in the opposite direction. 
     The at least one second engagement element may be pivotally mounted in an at least one second recess in the tubular member, the at least one second engagement element being biased to a position in which the at least one second engagement element sits proud of the outer surface of the tubular member, such that when rotation is in the first direction the inner surface of the plug anchor setting means depresses the at least one second engagement member into the at least one second tubular member recess. 
     The adapter described in the alternative embodiment will set a plug by firstly setting the plug anchors by rotating the adapter in one direction, and then set the plug seal by rotating the adapter in the other direction. 
     According to a third aspect of the present invention there is provided a method of sealing a plug in a conduit, the method comprising the steps of: 
     disposing a plug in a conduit; 
     moving a plug first body section relative to a plug second body section in a setting direction to energise at least one seal element into a sealing engagement with the conduit; 
     unidirectionally rotating a seal locking means first portion relative to a seal locking means second portion to take up the movement of the plug first body section relative to the plug second body section, substantially preventing the plug first body section moving relative to the plug second body section in a releasing direction, opposite the setting direction. 
     Preferably, the seal locking means is arranged along an arc centred on, and substantially perpendicular, to a housing longitudinal axis. 
     In one embodiment, the step of moving a plug first body section relative to a plug second body section in a setting direction comprises applying a linear force to one or both of the plug first body section and/or the plug second body section to move the plug first body section axially relative to the plug second body section. 
     In an alternative embodiment, the step of moving a plug first body section relative to a plug second body section in a setting direction comprises rotating the plug second body section to rotationally translate the plug second body section relative to the first body section. 
     Preferably, following the step of disposing the plug in the conduit, the method comprises the additional steps of: 
     moving one of a plug&#39;s first or second body sections relative to a plug anchor setting means in a setting direction to energise at least one anchor into an anchored engagement with the conduit; 
     unidirectionally rotating an anchor locking means first portion relative to an anchor locking means second portion to take up the movement of said plug body section relative to the anchor setting means, substantially preventing said plug body section moving relative to the plug anchor setting means in a releasing direction, opposite the setting direction. 
     Preferably, the anchor locking means is arranged along an arc centred on, and substantially perpendicular, to a housing longitudinal axis. 
     Preferably, the anchor setting means is the same as the other of the plug&#39;s first or second body sections. 
     In one embodiment, the step of moving the plug&#39;s first or second body sections relative to a plug anchor setting means in a setting direction comprises applying a linear force to one or both of the plug body section and/or the plug anchor setting means to move the plug body section axially relative to the plug anchor setting means. 
     In an alternative embodiment, the step of moving a plug&#39;s first or second body sections relative to a plug anchor setting means in a setting direction comprises rotating a plug anchor setting means to rotationally translate the plug anchor setting means relative to the plug body section. 
     In this alternative embodiment the seal setting direction may be opposite the anchor setting direction. 
     According to a fourth aspect of the present invention there is provided a system for sealing a conduit, the system comprising a plug according to the first aspect of the present invention and a running adapter according to the second aspect of the present invention. 
     According to a fifth aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a first body section; 
     a second body section having an energising portion and a de-energising portion; and 
     at least one seal element for creating a seal between the plug and a conduit, the at least one seal element being energised and de-energised by movement of the first body section relative to the second body section; 
     such that to energise the seal, the energising portion of the second body section is engaged with the at least one seal element and to de-energise the seal the de-energising portion of the second body section is engaged with the at least one seal element, the de-energising portion defining a fluid flow path around the at least one seal element. 
     Provision of a de-energising portion permits, when the plug is sealed in a conduit, pressure equalisation across the seal element, which prevents the possibility of the plug being blown up the conduit by pressure trapped below the plug. 
     According to a sixth aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a body having a first body section and a second body section; and 
     at least one seal element for creating a seal between the plug and the conduit, the at least one seal element being set by relative movement between the first body section and the second body section; 
     wherein the at least one seal element comprises at least one frusto-conical washer. 
     According to a seventh aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a tubular housing having a longitudinal axis; and 
     at least one circular seal element for creating a seal between the plug and a conduit, the at least one seal element being moveable between a de-energised configuration and an energised configuration; 
     wherein, in the de-energised configuration, the at least one seal element is describes a circumference less than the circumference of the housing. 
     Providing a seal element which in a de-energised configuration describes a circumference less than the circumference of the housing means there is no requirement for jarring to locate the plug in a conduit and, accordingly, no damage is done to the conduit bore during location. 
     According to an eighth aspect of the present invention there is provided a method of anchoring a plug in a conduit, the method comprising the steps of: 
     lowering a plug into a conduit until a portion of the plug engages a no-go located on a surface of the conduit preventing further movement of the plug in an axially downward direction; radially expanding at least one anchor into an at least one complementary recess in the conduit; 
     engaging a first surface of the/each anchor with a first surface of the/each recess, the/each first anchor surface and first recess surface being arranged to apply an axial load on the plug in the direction of the plug portion. 
     Preferably, the plug portion is a shoulder. 
     According to a ninth aspect of the present invention there is provided a method of retrieving a plug from a conduit, the method comprising the steps of: 
     de-energising at least one seal element, the at least one seal element forming a seal between the plug and a conduit; 
     releasing at least one plug anchor, the plug anchor anchoring the plug with respect to the conduit; and 
     retrieving the plug to surface. 
     De-energising the seal element prior to releasing the plug anchors permits pressure equalisation across the seal element and prevents the possibility of the plug being blown up the conduit by pressure trapped below the plug when the anchors are released. 
     Preferably, the step of de-energising the at least one seal element comprises creating a fluid flow path across the at least one seal element. 
     Preferably, the step of creating a fluid flow path across the at least one seal element comprises moving a plug body portion relative to the at least one seal element. 
     Most preferably, the plug body portion has a de-energising region defining a fluid flow path for location behind the at least one seal element. 
     According to a tenth aspect of the present invention there is provided a sealed reservoir for location between a pair of adjacent seals, the reservoir comprising a housing containing a body of air at a fixed pressure; 
     wherein the reservoir is adapted to collapse or rupture in response to a threshold external pressure being exceeded. 
     According to an eleventh aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a housing having a longitudinal axis; 
     at least one seal element for creating a seal between the plug and the conduit; 
     seal setting means for setting the at least one seal element by rotationally translating one of the seal setting means or the housing with respect to the other of the seal setting means or the housing such that the at least one seal element is compressed into a sealing engagement with the conduit, the seal setting means and the housing being at least partially connected by first unidirectional latching means arranged along an arc centred on, and substantially perpendicular, to the longitudinal axis such that unidirectional rotational movement of one of the seal setting means or the housing with respect to the other of the seal setting means and the housing to compress the at least one seal element is permitted to set the seal. 
     According to a twelfth aspect of the present invention there is provided a running adapter for setting a plug in a conduit, the running adapter comprising a tubular member having a longitudinal axis, an outer surface and an inner surface, one of the outer surface or the inner surface adapted to engage a portion of plug seal setting means to set at least one plug seal element, 
     wherein the at least one plug seal element is set by rotation of the running adapter in a first direction about the longitudinal axis. 
     According to a thirteenth aspect of the present invention there is provided a method of sealing a plug in a conduit, the method comprising the steps of: 
     disposing a plug in the conduit, the plug having a housing and a longitudinal axis; 
     rotating a plug seal setting means in a first direction to rotationally translate either of the plug seal setting means or the housing with respect to the other of the plug seal setting means or the housing such that an at least one seal element is compressed into a sealing engagement with the conduit, 
     maintaining the sealing engagement by providing a seal ratchet arranged along an arc centred on, and substantially perpendicular, to the longitudinal axis, the seal ratchet comprising a set of seal ratchet teeth and at least one complementary seal ratchet pawl, the set of seal ratchet teeth being associated with one of the seal setting means or the housing, the at least one complementary seal ratchet pawl being associated with the other of the seal setting means or the housing. 
     According to a fourteenth aspect of the present invention there is provided a plug for sealing a conduit, the plug comprising: 
     a housing having a longitudinal axis; 
     at least one seal element for creating a seal between the plug and the conduit; 
     at least one anchor for anchoring the plug to a conduit; 
     anchor setting means for setting the anchor by rotationally translating one of the anchor setting means or the housing with respect to the other of the anchor setting means or the housing; and 
     an anchor ratchet, the anchor ratchet comprising a set of anchor ratchet teeth and at least one complementary anchor ratchet pawl, the set of anchor ratchet teeth being associated with one of the anchor setting means or the housing, the at least one complementary anchor pawl being associated with the other of the anchor setting means or the housing, 
     wherein the anchor ratchet is arranged along an arc centred on, and substantially perpendicular, to the longitudinal axis. 
     It will be understood that any of the preferred or alternative features of one aspect of the invention are equally applicable to a different aspect of the invention. 
     By virtue of the present invention a plug is provided from which backlash is substantially reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying figures in which: 
         FIG. 1  is a cut away side view of a plug, for sealing a conduit, and a running adapter for setting the plug in the conduit in accordance with a first embodiment of the present invention; 
         FIG. 2  is an enlarged cut away side view of section A of  FIG. 1 , showing the plug and part of the running adapter; 
         FIG. 3  is an enlarged cut away side view of section B of  FIG. 1 , showing part of the running adapter; 
         FIG. 4  is an enlarged cut away side view of section C of  FIG. 1 , showing part of the running adapter; 
         FIG. 5  is a cut away side view of the plug of  FIG. 1  in a conduit prior to the anchoring dogs being set; 
         FIG. 6  is a cut away side view of the plug of  FIG. 1  in the conduit after the anchoring dogs have been set and prior to the seal element being set; 
         FIG. 7  is a cut away side view of the plug of  FIG. 1  in the conduit after the anchoring dogs and the seal element have been set; 
         FIG. 8  is a cut away side view of the plug of  FIG. 1  in the conduit showing the retaining sleeve disengaged from the locking nut; 
         FIG. 9  is a cut away side view of the plug of  FIG. 1  in the conduit showing the seal element released; 
         FIG. 10  is a cut away side view of the plug of  FIG. 1  in the conduit showing the anchoring dogs released; 
         FIG. 11  is a perspective view of a locking nut; 
         FIG. 12  is a representation of part of the locking nut; 
         FIG. 13  is a perspective view of one of the first conical washers of the seal element; 
         FIG. 14  is a schematic cut away side view of part of a stack of frusto-conical washers in an uncompressed configuration; 
         FIG. 15  is a schematic cut away side view of part of a stack of frusto-conical washers in an compressed configuration; 
         FIG. 16  is a perspective view of part of the plug mandrel; 
         FIG. 17  is a perspective view of a plug for sealing a conduit in accordance with a second embodiment of the present invention; 
         FIG. 18  is a sectional view of the plug of  FIG. 1  taken through line A-A on  FIG. 17 ; 
         FIG. 19  is a sectional view taken through line B-B on  FIG. 18 ; 
         FIG. 20   a  is a perspective view of the first rotary lock ring of  FIG. 18 ; 
         FIG. 20   b  is a plan view of the second rotary lock ring of  FIG. 18 ; 
         FIG. 20   c  is an enlarged view of part of the first rotary lock ring of  FIG. 20   a.    
         FIG. 21  is a perspective view of a plug running adapter for setting a plug in a conduit in accordance with a second embodiment of the present invention; 
         FIG. 22  is a sectional view taken through line C-C on  FIG. 21 ; and 
         FIG. 23 , comprising  FIGS. 23   a  to  23   d  is a schematic of the plug of  FIG. 17  being set in a wellbore. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring firstly to  FIG. 1  there is shown a cut away side view of a plug, generally indicated by reference numeral  10  for sealing a conduit (not shown), and a running adapter  12  for setting the plug  10  in the conduit. 
     As can be seen from  FIG. 1  the plug and running adapter  10 , 12  had been divided into three sections indicated as “A”, “B”, and “C”, each of these sections is shown in  FIGS. 2 ,  3  and  4  respectively. 
     Referring to  FIG. 2 , an enlarged cut away side view of section A of  FIG. 1 , showing the plug  10  and part of the running adapter  12 . 
     The plug  10  includes a housing  14 , divided in to a moveable upper housing section  15  and a fixed lower housing section  17 . The plug  10  also includes a seal setting means  16  in the form of a plug mandrel  18  and a seal element  20  in the form of a stack of frusto-conical washers  22 . 
     The plug  10  includes eight anchoring dogs  34  for anchoring the plug  10  in the conduit (not shown). The dogs  34  are axially restrained by the lower housing section  17  but are permitted to move radially outwards from the housing  14  through a series of openings  36 . 
     The dogs  34  are moved radially outwards through the apertures  36  by the upper housing section  15 , specifically, by the action of a housing ramp  54 . 
     The plug  10  further comprises a seal and anchor locking means  24  comprising a locking nut  26 , a spacer sleeve  28 , and a retaining sleeve  30 . The retaining sleeve  30  is releasably fixed to the locking nut  26  by means of a number of sheer screws  32 , of which one is indicated. The locking nut  26  is attached to the plug mandrel  18  by a threaded connection  27 , and the spacer sleeve acts on the housing upper portion  14 , specifically the housing ramp  54 . It will be understood that the spacer sleeve  28  could be part of the housing  14 . 
     The seal and anchor locking means  24  permits movement of the housing upper portion  15  relative to the mandrel  18  in a setting direction, that is a direction which the seal element  20  is energised, but not in a releasing direction, opposite the setting direction. 
     Referring to  FIGS. 3 and 4 , enlarged cut away side views of sections B and C of  FIG. 1  showing the running adapter  12 , the adapter  12  is arranged, in use with the plug  10 , to convert a rotary input force applied to an input mandrel  80  into a rotary and an axial output force for application to the plug  10 . The rotary output force is applied to the locking nut  26  by a locking sleeve  82 , and the axial output force is applied to the upper housing section  15  by a running adapter casing  86 , and to the plug mandrel  18  by an output mandrel  84 . 
     The setting of the plug  10 , by the plug and the running adapter  12  will now be described with reference to  FIGS. 1 to 4 , and  FIGS. 5 to 7 .  FIGS. 5 to 7  are cut-away views of the plug  10  being set in a conduit  90 . For clarity, the running adapter  12  is not shown in any of  FIGS. 5 to 7 . 
     The plug and adapter  10 , 12  are lowered into the conduit  90 , in this case the bore of a christmas tree. As can be seen from  FIG. 5 , the stack of washers  22  is arranged so that the washers  22  do not extend beyond the circumference housing lower section  17 . This permits the plug  10  to be run in to the conduit  90  without damaging the seal element  20 . 
     The plug  10  is run into the conduit  90  until a housing shoulder  92  engages a conduit no-go  94 , indicating the plug  10  has reached the correct location. At this point the adapter  12  can be activated and the plug  10  can be set. 
     A rotary force is applied to the running adapter input mandrel  80  by an external drive (not shown) via an external drive connector  350 . The external drive connector  350  is provided with a longitudinal slot  347 . The slot  347  accepts a spline key  340  that is fixed to the input mandrel  80  by pins  342 ,  341 . The running adapter input mandrel  80  has a threaded shaft to form a threaded connection  96  that engages a nut  343  within the running adaptor casing  96 . The threaded connection  96  has a pitch of 0.2 inches. Also illustrated in  FIG. 3  are key  330  and threaded connection  311 . 
     The running adapter casing  86  is locked to a motorised setting tool (not shown) connected to the running adapter  12 , preventing the casing  86  from rotating with the input mandrel  80 . However linear axial movement of the running adapter casing  86  is permitted. The threaded connection  96  is arranged such that rotational movement of the input mandrel  80 , in the absence of a resistance, would result in the input mandrel  80  moving the direction of arrow “X” ( FIG. 4 ), applying a pulling force on the output mandrel  80 , and the casing moving in the direction of arrow “Y”, that is pushing on the upper housing section  15 . When the mandrel  80  is prevented from uphole movement, the nut  343  and attached casing  86  moves downhole in direction Y. 
     There is however a resistance preventing the input mandrel  80  moving in the direction is arrow “X”. The input mandrel  80  is connected to a collar  98  ( FIG. 3 ), which is in turn connected to the output mandrel  84  via a number of shear screws  100 . A pair of roller bearings  102  permit the input mandrel  80  to rotate within the collar  98  whilst still transmitting axial pulling forces, applied by the input mandrel  80 , to the output mandrel  84 . The output mandrel  84  is in turn connected to the plug mandrel  18  by means of a collet  104  ( FIG. 2 ). The pulling force applied to the plug mandrel  18  by the input mandrel  80 , via the collar  98  and the output mandrel  84  is resisted by a set of shear set screws  106 . 
     The resistance of the shear set screws  106  prevents the input mandrel moving in the direction of arrow “X” and therefore the running adapter casing  86  moves in the direction of arrow “Y” and applies an axial “pushing” force on the upper housing section  15 . 
     Referring to  FIG. 5 , a cut away side view of a plug  10  in a conduit  90  prior to the dogs  34  being set, under the action of this force, the upper housing section  15  and the housing ramp  54  move in the direction of arrow “Y”, the ramp  54  engaging the dogs  34  and pushing them radially outwards through the housing apertures  36 . The dogs  34  move towards engagement with a complementary recess  108  in the conduit wall  110 . The ramp  54  defines a variable surface taper  55  having two sections  57  of shallow taper and two sections  59  of steep taper. The steep taper sections  59  are arranged to move the dogs  34  rapidly towards the conduit recess  108 , with the shallow taper sections  57  pushing the dogs  34  for the final stage of their travel into the recess  108  and into engagement with the conduit wall  110 . A shallow taper for the stage of the travel in which actual engagement occurs is preferred because a shallow taper maximises the radial force applied to the dogs  34  and assists in locking the plug  10  in the conduit  90 . Utilising the steep taper sections  59  for the initial expansion of the travel reduces the axial length of the ramp  54 . 
     The movement of the upper housing section  15  relative to the plug mandrel  18  is taken up by the locking nut  26 , which engages the plug mandrel  18  by means of the threaded connection  27 . As the upper housing section  15  moves relative to the plug mandrel  18 , the locking nut  26  is rotated by the running adapter locking sleeve  82  relative to the spacer sleeve  28  via the interlocking castellations  360  provided on the nut  26  and the sleeve  82 . This rotation is unidirectional preventing relative movement of the mandrel  18  with respect to the upper housing section  15  in the opposite direction, which, if permitted, would release the seal element  20 . 
     The locking sleeve  82  is connected to the input mandrel  80  by a clutch  112  ( FIG. 4 ). As the input mandrel  80  rotates the locking sleeve  82  rotates, however if the locking sleeve  82  encounters sufficient resistance, the clutch  112  slips and the rotation of the locking sleeve  82  stops. The pitch of the threaded connection  27  between the locking nut  26  and the plug mandrel  18  is 0.25 inches, compared to the pitch of the threaded connection between the input mandrel  80  and the adapter casing  86  of 0.2 inches. This difference in the two pitches means that for every revolution of the input mandrel  18 , the adapter casing  86 , and hence the upper housing section, will move 0.2 inches, and the locking nut will move 0.25 inches. However as the locking nut  26  is acts on upper housing section  15  via the spacer sleeve  28 , the full movement of the locking nut  26  per revolution of the input mandrel  18  is not permitted and sufficient resistance is generated on the locking sleeve  82  to slip the clutch  112 . 
     The locking sleeve  82 , however, applies a continual rotational force to the locking nut  26  and as soon as there is further movement of the upper housing section  15  relative to the plug mandrel  18 , the locking nut  26  will take up this movement. 
     Referring now to  FIG. 11 , a perspective view of the locking nut  26 , it can be seen the locking nut comprises a first locking nut portion  56  and a second locking nut portion  58 . The first locking nut portion  56  comprises six axial sections  60 , each axial section  60  being attached to the second locking nut portion  58  by means of a dovetail connection  62 . The internal surfaces  61  of the six axial sections  60 , when assembled, define one half of the threaded connection  27 . The dovetail connections  62  permit the axial sections  60  to move in a radial direction relative to the second locking nut portion  58  but not in an axial direction. 
     When the seal locking means  24  is assembled the axial sections  60  are prevented from moving radially outwards by the retaining sleeve  30 . 
     The locking nut  26  also includes unidirectional locking device  64 . The arrangement of each locking device  64  can be seen more clearly in  FIG. 12 . Each locking device  64  comprises a ball bearing  66  located in a channel  68  having an internal surface  74 . The ball bearing  66  is mounted on a spring  70  which pushes against the ball bearing  66 , forcing the ball bearing  66  out of the channel  68 . 
     As the locking nut  26  rotates with respect to the spacer sleeve  28 , the ball bearing  66  is pressed against the spacer sleeve surface  72 . If the locking nut  26  is moving relative to the spacer sleeve  28  in the direction of arrow “A”, the ball bearing is pushed back up the channel  68 , however if a force is applied to the locking nut  26  in the direction of arrow “B”, then the ball bearing  66  is drawn out of the channel  68  and wedges between the sleeve surface  72  and the channel surface  74 , preventing further movement in the direction of arrow “B”. As the interface between the locking nut  26  and the spacer sleeve  28  is located on an arc centred on, and substantially perpendicular to, the longitudinal axis of the adapter, backlash is minimised. For example if the locking nut moved 1/20 of a revolution in the direction of arrow “B”, this would result in axial movement in the release direction of (0.25× 1/20) inches, that is 0.0125 inches. The motorised setting tool (not shown) records the torque versus turn profile of the locking nut  26 . This information is transmitted live by e-line (not shown) from the adapter  12  and compared with the expected profile in order to confirm proper setting of the plug. 
     Referring now to  FIG. 6 , a cut away side view of the plug  10  in the conduit  90  after the anchoring dogs  34  have been set and prior to the seal element  20  being set, the dogs  34  have engaged the recess  108 , particularly, a first dog surface region  112  has engaged a first recess surface region  114 . This arrangement imparts a downward force on the plug  10  which is resisted by the interaction between the plug shoulder  92  and the conduit no-go  94 , with the result that the plug  10  is firmly locked in the conduit  90 . With the dogs  34  fully set, the upper housing section  15  can not move any further in the direction of arrow “Y”. 
     Once the plug  10  is firmly locked in position, the seal element  20  can be set. This is achieved by increasing the rotary force on the input mandrel  80 . 
     Referring to  FIG. 7 , a cut away side view of the plug  10  in the conduit  90  after the anchoring dogs and the seal element  20  have been set, the force is increased on the input mandrel  80  until the shear screws  106  shear, permitting the input mandrel  80 , and hence the plug mandrel  18 , to move in the direction of arrow “X”. 
     The seal element  20  is located in a seal recess  116  defined by the plug mandrel  18  and the lower housing section  17 . As the plug mandrel  18  moves upwards, that is in the direction of arrow “X”, the seal recess  116  reduces in size, compressing the seal element  20  into engagement with the conduit  90 . 
     The movement of the plug mandrel  18  relative to the housing  14  is taken up by the locking nut  26 , which is driven by the running adapter locking sleeve  82 , in the same way as described previously. 
     As discussed earlier, the seal element  20  is a stack of frusto-conical washers  22 . Referring to  FIG. 13  there is shown a perspective view of one of the first conical washers  22 . Each frusto-conical washer  22  is made from Inconel steel and coated in a layer of silver 35 microns thick. The washer inner edge  44  defines an aperture through which the plug mandrel  18  passes and when the seal is set this inner edge  44  is adapted to sealingly engage the mandrel  18 . The outer washer edge  46 , when the seal element  20  is energised, is adapted to form a seal with a conduit, each washer  22  in the stack forming an independent seal from every other washer  22 . 
     As can be seen from  FIG. 14 , a schematic cut away side view of part of a stack of frusto-conical washers  22  in an uncompressed configuration, between each washer  22  there is a laminate of softer material  48 . This laminate  48  is made up of a central layer  50  of PEEK sandwiched between two layers  52  of PTFE. As the stack of washers  22  is energised, by being compressed by relative movement between the housing  14  and the plug mandrel  18 , the laminate  48  is squeezed radially inwards, forming a seal with the plug mandrel  18 , and radially outwards, forming a seal with the conduit  90 .  FIG. 15  shows a schematic cut away side view of part of a stack of frusto-conical washers  22  in a compressed, or set, configuration 
     As can be seen from  FIG. 15  the laminate of softer material  48  is squeezed beyond the edges of the washers  22 , and assists in forming a seal if the conduit  90  is not entirely smooth; the softer material spreading into any voids or inconsistencies in the surface of the conduit  90 . 
     It will be noted from  FIG. 15  that even when fully compressed each washer  22  is not completely flattened. In the uncompressed state the angle of each washer to the horizontal, indicated as angle θ on  FIGS. 14 and 15 , is 8° to the horizontal. In the compressed, or set, configuration angle θ is 5°. The retention of a slight angle to the horizontal assists the seal element in recovering back to the uncompressed configuration when the compression force is removed. 
     Referring back to  FIG. 7 , in the leading end  38  of the plug  10  is a reservoir  40 . The reservoir  40  is sealed from the surrounding environment and contains a body of air at a pressure of 1 bar. A reservoir cap  42  is provided which seals the reservoir  40  and is adapted to rupture at a given threshold pressure. The purpose of the reservoir  40  is to reduce pressure on the seal element  20  in the event that a volume of air becomes trapped and pressurised below the plug  10 . A volume of air may get trapped if, for example, it is decided to set two plugs  10  in series. 
     Without the reservoir  40 , the increased pressure would apply a force on the plug  10  which may affect the integrity of the seal element  20 . With the reservoir  40 , before any damage can be done to the integrity of the seal, the cap  42  ruptures, with the effect of reducing the overall pressure of the air trapped below the plug  10 . 
       FIGS. 5 to 7  explained the setting of the plug  10  in the conduit  90 , the releasing and retrieval of the plug will now be described with reference to  FIGS. 1 to 4  and  FIGS. 8 to 10 . The releasing and retrieval of the plug  10  is achieved using conventional wireline techniques 
     The plug  10  is prevented from being removed from the conduit  90  by the locking means  24 , particularly because the plug mandrel  18  can not move in the release direction relative to the housing  14 . As previously discussed the locking nut  26  comprises a first portion  56  and a second portion  58 , the first portion  56  comprising six radially moveable sections  60 , which together define one half of the threaded connection  27  between the locking nut  26  and the plug mandrel  18 . The retaining sleeve  30  prevents radial movement of the locking nut sections  26 . 
     Referring to  FIG. 8 , a cut away side view of the plug  10  in the conduit  90  showing the retaining sleeve  30  disengaged from the locking nut  26 , sufficient force has been applied to the retaining sleeve  30  by a wireline controlled releasing tool (not shown) to overcome the shear screws  32  so the shear screws  32  are no longer securing the retaining sleeve  30  to the locking nut  26 , permitting the six moveable sections  60  to move radially outwardly and break the threaded connection  27  between the locking nut  26  and the plug mandrel  18 . 
     As the plug mandrel  18  is no longer locked relative to the housing  14 , the wireline controlled releasing tool can apply a force to the plug mandrel  18  to move the plug mandrel  18  in the release direction, that is in the direction of arrow “R” on  FIG. 8 . 
     As the plug mandrel  18  moves in the direction of arrow “R”, the compression force on the seal element  20  is removed and the seals are permitted to spring back to the uncompressed configuration, releasing the pressure below the seal element  20 . 
     Referring to  FIG. 9 , a cut away side view of the plug  10  in the conduit  90  showing the seal element  20  released, the plug mandrel  18  includes a grooved section  120  describing a number of grooves  122 . The grooves  122  can be seen more clearly on  FIG. 16 , a perspective view of part of the plug mandrel  18 . When the seal element  20  is set, the inner edge  44  of each washer  22  engages a non-grooved section  124  of the plug mandrel  18 , however as the plug mandrel  18  moves in the release direction the grooved section  122  is translates behind the seal element  20 , and a pressure equalising flow path is created around the seal element  20 . 
     To ensure the seal element  20  does not re-set, the plug mandrel  18  is also provided with a wickered surface  126  ( FIGS. 9 and 16 ) which engages with a complementary wickered element  128  ( FIG. 9 ), which is secured to the lower housing section  17  by a screw  130 . The engagement between the wickered surface  126  and the wickered element  128  is arranged to permit only uni-directional movement, thereby preventing the plug mandrel  18  moving and resetting the seal element  20 . The plug mandrel  18  is moved in the direction arrow “R” until the plug mandrel lug  132  engages the wickered element  128 , preventing further movement of the plug mandrel  18 . 
     With the seal between the plug  10  and the conduit  90  broken, the plug  10  can be safely removed from the conduit  90 , because the seal element  20  has been de-energised and pressure equalisation has occurred across the seal element  20 . The pressure equalisation prevents the possibility of the plug being blown up the conduit  90  by pressure trapped below the plug  10 . The wireline releasing tool is recovered to surface and a wireline pulling tool (not shown) is sent down to the plug  10  to engage the plug housing  14 . 
     Referring to  FIG. 10 , a cut away side view of the plug  10  in the conduit  90  showing the anchoring dogs  34  released. As the input mandrel  18  can not now move relative to the housing  14 , the upper housing section  15  moves in the direction of arrow “S” under the action of the wireline pulling tool. The housing ramp  54  moves away from the dogs  34  permitting the dogs  34  to retract into the housing  14  through the housing apertures  36 . 
     The plug  10  is now released from the conduit  90  and can be recovered to surface by the wireline pulling tool. 
     A second embodiment of the present invention will now be described with reference to  FIGS. 17-23 . 
     Referring firstly to  FIG. 17 , there is shown a perspective view of a plug, generally indicated by reference numeral  510 , for sealing a conduit in accordance with a second embodiment of the present invention. The plug  510  comprises a housing  512  having a longitudinal axis  514 . The plug  510  further includes a plurality of seal elements  516  for creating a seal between the plug  510  and the conduit (not shown). Within the housing  512  is a seal setting means  518  for setting the plurality of seal elements  516  by rotationally translating the seal setting means  518  with respect to the housing  512  such that the plurality of seal elements  516  are compressed into a sealing engagement with the conduit (not shown). The plug further includes an anchoring system  520  for securing the plug  10  in the conduit (not shown). The anchoring system  520  includes a dog expander ramp  528  (shown and discussed in connection with  FIG. 18 ) and a plurality of dogs  522 . The anchoring system  520  is set by anchor setting means  524 . Rotation of the anchor setting means  524  with respect to the housing  512  translates the anchor setting means  524  with respect to the housing  512  and forces the dogs  522 , through the dog expander ramp  528 , into engagement with recesses in the conduit (not shown). 
     These and additional elements of the plug  510  can be seen on  FIG. 18 , a sectional view of the plug  510  taken through line A-A on  FIG. 17 . As can be seen from  FIG. 18 , the anchor setting means  524  comprises a dog nut  526 , the anchoring system  520  comprises six dogs  522  and the housing  512  further comprises a dog expander ramp  528 . The dog nut  526  engages the housing  12  by means of a threaded connection  530 . As the dog nut  526  is rotated it translates to the right of  FIG. 18 . This translation acts on the dog expander ramp  528  which also moves to the right. The dog expander ramp  528  includes a leading surface  532  which engages a back surface  534  of the dogs  522 . Co-operation between the dog expander ramp leading surface  532  and the dog back surface  534  causes the dogs  522  to move outwards from the plug  510 , through apertures  521  in the housing  512 , in a direction perpendicular to the longitudinal axis  514 . 
     Referring now to  FIG. 19 , there is shown a sectional view through line B-B from  FIG. 18 . This shows that the dog expander ramp  528  is rotationally fixed to the housing  512  by means of a key  536 . Therefore as the dog expander ramp  528  translates to the right it does not rotate. 
     Referring back to  FIG. 18 , the plug  510  further includes an anchor ratchet  538 . The anchor ratchet  538  comprises a set of teeth or serrations (not shown) in the form of a buttress, located on an end surface  544  of the anchor setting means  524  in the form of a dog nut and three complementary anchor ratchet tangs (not shown on  FIG. 18 ) located on a first rotary lock ring  546  pinned to the dog expander ramp  520 . The engagement of the tangs and the teeth or serrations allows rotation in one direction but not the other as the tang prevents rotation in the opposite direction because it would lock against the buttress. 
     The first rotary lock ring  546  can be best seen in  FIG. 20   a  and  FIG. 20   c , a perspective view of the first rotary lock ring  546 . The first rotary lock ring  546  comprises three tangs  542  located on, and sitting proud of, an external surface  552  of the first rotary lock ring  546 . One of the tangs is also shown in enlarged detail on  FIG. 20   a . The tangs  542  are machined into the first rotary lock ring  546 , and are bent outwards such that edge  553  forms a ratchet with the serrated face  544  of a dog nut. 
     The first rotary lock ring  546  is centred on the longitudinal axis  514  of the plug  10  such that the anchor ratchet  538  is arranged along an arc centred on, and substantially perpendicular to the longitudinal axis  514 . 
     Referring back to  FIG. 18 , the plurality of seal elements  516  comprises a stack of fifteen frusto-conical washers  554 . Frusto-conical washers  554  are used because a high expansion ratio is achievable by compression of a frusto-conical washer permitting the plug  510  to be run into position within a conduit without building up a significant head of pressure in front of the plug  510 . The plug  510  is set by seal setting means  518  which comprises a two-part mandrel  556   a,b . The mandrel  556   a,b  is connected to the housing  512  by means of a threaded connection  558 . The threaded connection  558  is such that if the seal setting means  518  is rotated it translates to the left of  FIG. 17 , travelling along the threaded connection  558 . This motion compresses the frusto-conical washers  554  increasing the radius  560  defined by the frusto-conical washers  554  from the longitudinal axis  514 . As they expand, the frusto-conical washers  554  engage the wall of a conduit (not shown) and form a seal with the conduit. 
     Over compression of the frusto-conical washers  554  is prevented by stop  562  engaging with housing no-go  564 . 
     The plug  510  further includes a seal ratchet  580 . The seal ratchet  580  comprises a set of teeth (not shown) located on an external surface  582  of the mandrel  556  and six complementary seal ratchet tangs (not shown on  FIG. 18 ) located on a second rotary lock ring  584 . The second rotary lock ring  584  can be best seen in  FIG. 20   b , a plan view of the second rotary lock ring  584 . The second rotary lock ring  584  is secured to the housing no-go  564  by lugs  586 . The second rotary lock ring  584  comprises six tangs  588  located on, and sitting proud of, an internal surface  590  of the second rotary lock ring  584 . 
     The second rotary lock ring  584  is centred on the longitudinal axis  514  of the plug  510  such that the seal ratchet  580  is arranged along an arc centred on, and substantially perpendicular to the longitudinal axis  514 . 
     The setting of the plug  510  is a two stage process because the plug  510  is arranged such that rotation in one direction (here after referred to as direction X) will drive the dog nut  526  and set the dogs  522  in a conduit recess, and rotation in the opposite direction (hereafter referred to as direction Y) will drive the mandrel  556  and set the sealing element  516 . 
     Referring now to  FIG. 21 , there is shown a perspective view of a plug running adapter generally indicated by reference numeral  610  for setting the plug  510  in a conduit in accordance with a second embodiment of the present invention. The plug running adapter includes a housing  612 , and a tubular member  614  extending from the housing  612 . The tubular member  614  has a longitudinal axis  616 , an outer surface  618  and an inner surface  620 . The outer surface  618  is adapted to engage the anchor setting means  524  of the plug  510  and the inner surface  620  is adapted to engage the seal setting means  518  of the plug  510 . 
     Located on the inner surface  620  of the tubular member  614  are first engagement element  622  and located on the outer surface  618  of the tubular member  614  are second engagement elements  624 . The first and second engagement elements  622 , 624  can be best seen on  FIG. 22 , a sectional view taken through line C-C of  FIG. 21 . Each engagement element  622 , 624  is pivoted at one end about a pivot  626 . The first engagement element  622  are biased to sit proud of the internal surface  620  of the tubular member  614  and the second engagement elements  624  are biased to sit proud of the outer surface  618  of the tubular member  614 , as shown in  FIG. 22 . Associated with each of the first engagement elements  622  are first tubular member recesses  628  and associated with each of the second engagement elements  624  are second tubular member recesses  630 . 
     Referring to both  FIGS. 17 and 22  the anchor setting means  524  in the form of dog nut  526  have a number of second complimentary notches  640  in the internal surface  642  of the dog nut  526 . When the rotation of the running adapter  612  is in the direction X, the second engagement elements  624  engage the inner surface  644  of the second complimentary notches  640  thereby driving the dog nut  528 , and setting the dogs  522 . When the rotation of the running adapter is in direction Y, the inner surface  642  of the dog nut  528  depresses the second engagement  630  elements  624  into the second tubular member recesses  630 . 
     Continuing to refer to  FIGS. 17 and 22 , the first engagement elements  622  are adapted to engage with first complimentary notches  632  on the outer surface of the mandrel  556 . The complimentary notches  632  are separated by fingers  634 . The pivotal mounting of the first engagement elements  622  means that when the running adapter  610  is driven in direction Y, the first engagement elements  622  engage with the inner surface  636  of the first complimentary notches  632  thereby rotating the mandrel  556 , and setting the seal element  516 . When the direction of the running adapter is reversed, to direction X, the upper surface  638  of the fingers  634  press the first engagement elements  622  into the first tubular member recesses  628  such that there is no driving engagement between the running adapter  610  and the mandrel  556 . 
     Referring now to  FIG. 23 , comprising  FIGS. 23   a  to  23   d , there is shown a schematic of the plug  510  of  FIG. 17  being set in a wellbore  700 . 
     The plug  510  is shown in  FIG. 23   a  attached to the running adapter  610 , which in turn is suspended from a wireline cable  710 . The running adapter  610  includes a latch (not shown) which engages a recess  557  ( FIG. 18 ) in the inner surface of the mandrel  556 . In  FIG. 23   a , the plug/running adapter  510 , 610  is being run into the wellbore  700 . 
     When the plug  510  is in the correct position, shown in  FIG. 23   b  the dogs  522  are set in recesses  712 . The dogs are set, as described above, by rotating the tubular member  614  ( FIG. 21 ) of the running adapter  610  in a first direction. This rotation drives the dog nut  526  ( FIG. 18 ) towards the dogs  522 , which are moved into the position shown in  FIG. 23   b  by the action of the dog expander ramp  528  ( FIG. 18 ). 
     Once the dogs  522  have been set in the recesses  712 , and the plug  510  is correctly located in the wellbore  700 , the running adapter tubular member  614  is rotated in a second direction, which is opposite to the first direction. This rotation drives the two-part mandrel  556   a , 556   b , which in turn compresses the frusto-conical washers  714  into a sealing engagement with the wall  716  of the wellbore  700 , as shown in  FIG. 23   c . The plug  510  is now set in the wellbore  700 . 
     Finally the running adapter  610  is disconnected by shearing the running adapter latch (not shown) from the plug recess  557  ( FIG. 18 ). The adapter  610  is then withdrawn to surface. 
     Various modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, it will be understood that any suitable form of seal element may be used or slips may be used instead of the dogs described. For example, multiple metal seals could be used or, alternatively, a combination of metal and plastic seals where seal bore damage prevents an all metal seal arrangement from testing. Additionally, with regard to the first described embodiment, although a two trip releasing and recovery of the plug has been described, a single trip wireline tool could be used or the running adapter could be modified to retrieve the plug as well as set the plug. 
     Those of skill in the art will also recognise that the above described embodiment of the invention provides a plug in which backlash is substantially reduced. The use of a rotary lock mechanism substantially prevents any movement within the plug and is unaffected by vibration which can occur at the wellhead. Furthermore, from a simple rotational input the running adapter produces both rotational and axial force to set and seal the plug in the conduit. Because the running adapter delivers all the force required during setting and because the seal element is a smaller diameter than the diameter of the conduit at the point of sealing, there is no requirement for jarring and no damage is done to the conduit bore.