WO2006095160A1

WO2006095160A1 - Tree plug

Info

Publication number: WO2006095160A1
Application number: PCT/GB2006/000815
Authority: WO
Inventors: David Glen Martin; Roland Marcel Van Dort; Ewan Sinclair
Original assignee: Caledyne Ltd
Current assignee: Caledyne Ltd
Priority date: 2005-03-09
Filing date: 2006-03-07
Publication date: 2006-09-14
Anticipated expiration: 2007-09-09
Also published as: GB2439666B; GB0718131D0; GB0504837D0; GB2439666A

Abstract

The invention relates to a tree plug for a tree of the type used in the oil and gas exploration and production industry. The invention also relates to a plug and to a lock adapted to be located in various types of downhole tubing, and to a method of plugging a tree bore. In one embodiment, a tree plug (16,17) for a tree (10) having a main tree bore (14) is disclosed, the tree plug having a plug body (22) , a number of locking members (24) mounted for radial movement relative to the plug body between retracted and extended positions, a radially expandable seal element (26) , and a mandrel (28) . The mandrel is translated relative to the plug body to urge the locking- members to the extended position, and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the plug within the tree bore.

Description

TREE PLUG

The present invention relates to a tree plug and to a method of plugging a tree bore. In particular, but not exclusively, the present invention relates to a tree plug for a tree of the type used in the oil and gas exploration and production industry. The present invention also relates to a plug and a lock adapted to be located in various types of downhole tubing.

In the oil and gas exploration and production industry, well fluids are recovered from a well through a Christmas tree located on a wellhead of the well, which controls flow of the well fluids. In offshore environments, these Christmas trees are located on the seabed, and a number of trees on different wells may be coupled to a subsea pumping station and thus to a rig or other production facility, enabling access to a number of wells from a single rig. Christmas trees of this type are typically referred to as horizontal subsea trees, and have an outlet extending from a side of the tree.

In use during production of well fluids through a horizontal tree, tree plugs or crown plugs are generally installed in a main bore of the tree in pairs to close an upper end of the bore, and well fluids are recovered through the side outlet, controlled by appropriate valves. Conventional tree plugs typically comprise some form of locking member for locking the plug relative to the tree and an arrangement of seals, which abut an inner seal surface of the tree bore and thus seal the bore when the plug is set. When it is desired to carry out an intervention procedure, the crown plugs are removed to allow access to the wellbore through the tree.

To prevent damage to the seal surface of the tree bore, for ^'example, by passage of tools or rotation of a drill string within the bore, a thin sleeve is located in the tree bore. This both narrows the bore diameter and must be removed before a plug can be set.

In a similar fashion, it is often necessary to locate a tubing or tool string within existing borehole casing or liner, for example, to carry out completion or workover activities. US Patent No. 5,865,255 (assigned to Halliburton Energy Services, Inc.) discloses a nipple and associated lock mandrel for locating tubing strings in subterranean wells. The nipple is coupled to a tubing string for transport and positioning within the well and includes an internal bore at least as great as a 'drift' diameter of a tubing string to which it is attached. A latch profile is formed in a tubular body of the nipple and a seal bore is formed above the profile, the diameter of the seal bore being greater than the diameter of the main parts of the body internal bore. In this fashion, the seal bore is protected from direct exposure to axial fluid flow through the internal bore. A lock mandrel is configured for use with the nipple and includes a radially outwardly extendable circumferential seal and a number of keys which are radially slidingly received in an opening of an outer housing of the lock mandrel, to engage a latch profile in the nipple.

In use, an expander sleeve of the lock mandrel is displaced axially downwardly to urge the seal radially outwardly and to prevent the keys from disengaging the latch profile in the nipple. However, the lock mandrel of US 5,865,255 requires the keys to be radially outwardly biased in order to successfully engage the latch profile. During running of the lock mandrel into the tubing, the keys contact the internal surface of the tubing with a likelihood of damage to one or both of the keys and the tubing, and in particular to the seal bore of the nipple.

Furthermore, as the expander sleeve of the lock mandrel is urged downwardly to set the seal and hold the keys radially outwardly in engagement with the latch profile, forces exerted on the lock mandrel by fluid flow through the tubing string will tend to cause the expander sleeve to move axially upwardly, thereby releasing the lock.

Another lock is disclosed in European Patent Publication No. 0298683 (Petroline Wireline Services Limited) . EP 0298683 discloses a downhole lock assembly having a lock mandrel with a number of keys or dogs which are urged outwardly to set the tool. The lock mandrel is set on a no-go shoulder in downhole tubing and by jarring down to shear setting pins, fingers on an inner mandrel are allowed to retract so that the inner mandrel may move upwardly to expand the lock-out keys. However, the downhole lock of EP 0298683 has a relatively complex mechanism for setting the dogs which requires precise alignment of finger ends of the inner mandrel with grooves in an outer body in order that the lock is correctly set. Furthermore, the lock mandrel is not sealed to the respective tubing.

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

According to a first aspect of the present invention,^' there is provided a tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the plug within the tree bore.

The invention therefore provides a tree plug including a movable mandrel which may serve both for urging the locking member to the extended position, and for exerting an axial force on the seal element. In this fashion, the mandrel may cause the tree plug to be both locked and sealed within the tree bore. This is facilitated, in part, by the provision of a seal element which is radially expandable by exertion of an axial force, such as a compressive load, on the seal element. It will be understood that this load may be exerted on the seal element by the mandrel during movement of the mandrel relative to the plug body.

The mandrel may be axially movable within the plug body, and it will be understood that references herein to the mandrel being axially movable are relative to a longitudinal axis of the plug body.

Preferably the mandrel is upwardly movable relative to the plug body for urging the locking member to the extended position and for exerting the axial force on the seal element. It will be understood that reference herein to the term 'upward' are in a vertical sense, but that where the tree bore is deviated or disposed at an angle from the vertical, the term upward will be understood to mean in a direction along the tree bore towards the surface and thus may be from a downstream to an upstream location relative to a direction of flow of well fluids through the tree to surface.

The plug may comprise a pressure equalisation assembly for selectively equalising pressure between an interior and an exterior of the plug. By facilitating pressure equalisation across the plug, the plug may be safely removed by ensuring that a large differential pressure does not exist between the interior and exterior of the plug.

The pressure equalisation assembly may comprise a pressure equalisation passage, channel or the like for facilitating pressure equalisation. The passage may be normally or initially closed and may be adapted to be selectively opened for pressure equalisation.

The equalisation assembly may comprise a pressure equalisation mandrel, tube, body or the like mounted for movement with respect to the plug body for equalising pressure between the plug interior and exterior. The pressure equalisation mandrel may be adapted to selectively open the passage on movement of the mandrel with respect to the plug body. In embodiments of the invention, the passage may be formed in the plug body and an^' internal end of the passage may be closed by an internal wall of the plug body, or by a shoulder, boss, nub, valve, cap or the like. The pressure equalisation mandrel may be adapted to shear, cut or abrade the internal wall or the like on movement of the mandrel with respect to the plug body, to open the passage, and may include an appropriate cutting surface or element.

The pressure equalisation mandrel may be dimensioned and located within the main mandrel such that when it is desired to release the plug, the pressure equalisation mandrel is moved to equalise pressure between the interior and exterior of the plug before the main mandrel is moved to release the locking member and the seal element. This may ensure that pressure between the interior and exterior of the plug is equalised prior to release of the plug from the tree bore.

The plug may comprise a locking mechanism for restraining the mandrel, and thus for restraining the locking member in the extended position and the seal element in the radially expanded position. The locking mechanism may be adapted to permit movement of the mandrel in a first axial direction with respect to the plug body and to restrain movement of the mandrel in a second, opposite axial direction. In this fashion, movement of the mandrel with respect to the plug body in a first direction (to set the locking member and the seal) may be permitted and, following such movement, the locking mechanism may restrain the mandrel against return movement, to thereby restrain the locking member and the seal element in the respective extended and expanded positions. The locking mechanism may comprise a lock or latch ring, which is preferably a one-piece ring, but may be comprised of a plurality of segments or the like, the lock or latch ring serving for restraining movement of the mandrel relative to the plug body. The lock ring may be provided between the mandrel and the plug body, and may comprise a plurality of locking teeth or protrusions, such as ratchet teeth, which may be adapted to engage corresponding teeth on the mandrel. It will be understood that interengagement of the respective lock ring and mandrel ratchet teeth may serve for restraining the mandrel against movement. The ratchet teeth of the lock ring and the mandrel may be shaped to permit movement of the mandrel with respect to the plug body in said first direction and to restrain the mandrel against movement in said second direction.

The lock or latch ring may form an inner ring and the locking mechanism may also comprise an outer lock or latch ring, preferably comprising a plurality of locking segments, but alternatively comprising a one-piece ring. The outer locking segments may be adapted to be located between the inner lock ring and the plug body. The inner and outer lock rings may be adapted to co-operate to selectively restrain the mandrel against movement with respect to the plug body. For example, the segments may define a thread, a plurality of ratchet teeth or other protrusions for engaging a corresponding thread on an outer surface of the inner lock ring. Interengagement between the outer locking threads and the threads on the inner lock ring, and between the inner lock ring ratchet teeth and the mandrel ratchet teeth may serve for together restraining the mandrel against movement with respect to the plug body. This may be achieved by selectively restraining the outer lock ring against ^'axial movement with respect to the plug body.

The outer lock ring may be selectively radially moveable in order to permit movement of the mandrel with respect to the plug body; radial movement of the outer ring may cause disengagement of the outer and inner ring threads, thereby facilitating movement of the mandrel, which may carry the inner lock ring during its movement. The outer ring, in particular, the segments may be resilient or sprung and may at least initially be held in a stressed configuration until such time as the segments are permitted to radially move relative to the mandrel. When such radial movement of the segments is permitted, the segments may spring Outwardly to release the inner lock ring and allow movement of the mandrel.

The plug body may comprise a main plug body portion and a second portion or sub coupled to the main plug body portion and selectively moveable with respect thereto. The sub may define an internal recess or area for receiving the outer lock ring segments, to facilitate radial movement of the segments and thus release the mandrel when the sub is moved axially relative to the main plug body portion to align the segments with the recess. In this fashion, the segments can be arranged to move radially to release the mandrel by translating the sub relative to the main plug body portion.

Preferably the seal element is a generally annular element and may be substantially ring-shaped. The seal element is preferably also adapted to be axially compressed and thus shortened in length when an axial force is exerted on the seal element by the mandrel, thus urging the seal element outwardly into sealing contact with the tree bore. Further preferably, the seal element is adapted to increase in radial width when an axial force is exerted thereon by the mandrel.

The seal element may be provided as an integral part of the plug body, or as a separate component adapted to be coupled to or mounted on the plug body.

The seal element may have a wall of a reduced (undeformed) wall thickness in a region of the seal element wall to be deformed into sealing contact with the tree bore, relative to a remainder of the seal element. Accordingly, the seal element may be influenced to deform radially outwardly in a desired location. For example, an axially central region of the seal element may be of a wall thickness which is reduced relative to regions of the seal element spaced in first and/or second axial directions away from said central region. The seal element may define a zone of deformation where the element is adapted to be urged into sealing contact with the tree bore.

The seal element may be adapted to provide a seal between a first and a second surface which may be between an internal surface of the tree bore and an external surface of the plug, and the element may comprise: an inner deformable surface, an outer deformable surface, the inner and outer surfaces being joined at opposed first and second ends providing a body defining an interior volume entirely bounded by the said surfaces and said ends, a deformable filler material within said interior volume, wherein the element deforms in a predetermined manner to seal between the first and second surfaces when said ends are brought toward each other. The seal element may therefore comprise a seal element as defined in the applicant's International Patent Application No. PCT/GB2004/003631 (WO2005/022012 ) .

Preferably, the locking member and the seal element are adapted to engage corresponding lock and seal recesses provided in the tree bore and which may be provided in a wall of a tubing forming the tree bore. By providing a seal recess in which the seal element is received, it is not necessary to provide a protective sleeve in the tree bore and, furthermore, recessing the seal surface reduces or avoids fluid flow damage to the seal surface, in use.

Preferably, the plug comprises a plurality of locking members, which may comprise locking dogs, keys or the like located in radially extending recesses or apertures in the plug body. The locking member may comprise a cam surface or ramp adapted to co-operate with a corresponding surface on the mandrel, such that the mandrel may urge the locking member radially outwardly on translation of the mandrel relative to the plug body.

According to a second aspect of the present invention, there is provided a tree comprising: a main tree bore and a tree plug located in the tree bore and sealed relative to the bore, the tree plug comprising a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially extendable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position to engage a latch recess in the tree bore and to exert an axial force on the seal element to radially expand the seal element to engage a seal recess in the tree bore, to thereby lock and seal the plug within the tree.

According to a third aspect of the present invention, there is provided a downhole plug adapted to be located in downhole tubing, the plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the plug within the downhole tubing. According to a fourth aspect of the present invention, there is provided a downhole lock adapted to be located in downhole tubing, the downhole lock comprising: a main body; at least one locking member mounted for radial movement relative to the main body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the main body to urge the locking member to the extended position and to exert an axial force on the seal element 'to radially expand the seal element, to thereby lock and seal the downhole lock within the downhole tubing.

It will be understood that the principles and features of the tree plug defined in the first aspect of the present invention may be provided in a downhole plug or lock for alternative uses, such as for locating a tubing or tool string within a casing or liner.

According to a fifth aspect of the present invention there is provided a method of plugging a tree bore comprising the steps of: locating a tree plug within the tree bore; translating a mandrel of the tree plug relative to a body of the plug, to urge a locking member mounted for radial movement relative to the plug body from a retracted to an extended position to cause the locking member to engage a latch recess in the tree bore, and to exert an axial force on a seal element of the plug, to radially expand the seal element into sealing abutment with a seal recess in the tree bore, to thereby lock and seal the plug within the tree .

According to a sixth aspect of the present invention, there is provided a tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be^' translated upwardly relative to the plug body to urge the locking member to the extended position and to radially expand the seal element, to thereby lock and seal the plug within the tree bore.

According to a seventh aspect of the present invention, there is provided a tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement _ relative to the plug body between retracted and extended positions; at least one radially expandable seal element; a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position and to radially expand the seal element, to thereby lock and seal the plug within the tree bore; and a locking mechanism comprising an inner lock ring having a set of inner protrusions for engaging corresponding outer protrusions on the mandrel, and an outer lock ring having a set of inner protrusions for engaging corresponding outer protrusions on the inner lock ring, interaction between the outer lock ring protrusions, the inner lock ring protrusions and the mandrel protrusions permitting movement of the mandrel in a first direction relative to the plug body and restraining the mandrel against movement in a second, opposite direction to thereby restrain the locking member in the extended position and the seal element in a radially expanded position; wherein the outer lock ring is selectively radially outwardly moveable to disengage the outer lock ring protrusions and the outer protrusions on the inner lock ^' ring, to permit movement of the mandrel in said second direction to release the plug.

It will be understood that the features of the tree plug of the sixth and seventh aspects of the present invention may equally be provided in a downhole plug or lock, in a similar fashion to the tree plug defined in the first aspect of the invention.

Also, further features of the invention of the second, third, fourth, sixth and seventh aspects are defined in relation to the first aspect of the invention.

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

Fig. 1 is a schematic, partial cross-sectional view of a subsea horizontal Christmas tree with two tree plugs located within the tree, in accordance with a preferred embodiment of the present invention; Fig. 2 is a detailed, longitudinal cross-sectional view of one of the tree plugs shown in Fig. 1;

Fig. 3 is an enlarged perspective view of part of the tree plug of Fig. 2 with a sub of a body of the plug removed, for clarity;

Figs. 4 and 5 are longitudinal half-sectional views of a seal element forming part of a tree plug in accordance with an alternative embodiment of the present invention, shown both prior to and subsequent to expansion;

Figs. -6 and 7 are longitudinal half-sectional views of seal elements forming parts of a tree plug in accordance with a further alternative embodiment of the present invention, again shown prior to and subsequent to expansion;

Fig. 8 is an enlarged perspective view of an inner lock ring forming part of a locking mechanism of the tree plug of Fig. 2;

Fig. 9 is an enlarged, perspective view of an outer lock ring forming part of the locking mechanism;

Figs. 10 and 11 are views of the tree plug of Fig. 2 at later stages of a process of running, setting, releasing and retrieving the plug; and

Fig. 12 is a longitudinal, half-sectional view of a tree plug in accordance with a further alternative embodiment of the present invention. Turning firstly to Fig. 1, there is shown a subsea horizontal Christmas tree, the Christmas tree indicated generally by reference numeral 10. The Christmas tree 10 is located on a wellhead 12 of an oil or gas well and serves, amongst other things, for controlling flow of well fluids. The Christmas tree 10 includes a main bore 14 and, when it is desired to provide a secure barrier in the tree 10 to close an upper end of the bore 14, lower and upper tree plugs 16, 17 are located within the bore 14, enabling production from the well to be directed through a side outlet 2 having valves 6, 8 for controlling fluid flow therethrough.

Fig. 2 is a longitudinal cross-sectional view of the lower tree plug 16, which is in accordance with a preferred embodiment of the present invention. It will be understood that the upper plug 17 is of similar structure and that the description of the plug 16 applies equally to the plug 17. The plug 16 is shown in Fig. 2 in a running position, and has been run into the tree 10 through tubing extending to a production rig (not shown) at the sea surface. This tubing has been coupled to the tree 10 for running-in the plug 16, and is not normally required to obtain production fluids from the well.

Part of a housing 20 of the tree 10 is shown in Fig. 2 and defines a portion of the tree main bore 14. The tree plug 16 comprises a plug body 22, and at least one locking member in the form of a locking dog or key 24, which is mounted for radial movement relative to the plug body 22 between retracted and extended positions. The plug 16 in-fact comprises a number of circumferentially spaced dogs 24 which are located in corresponding apertures 25 in the plug body 22 and the dog 24 shown in Fig. 2 is in the retracted position.

The plug 16 also comprises a radially expandable seal element 26 and a mandrel 28 which is adapted to be translated relative to the plug body 22 both to urge the locking dog 24 to the extended position, and to exert an axial force on the seal element 26 to radially expand the seal element, thereby locking and sealing the plug 16 within the tree bore 14. When it is desired to release the plug 16 from the tree bore 14, the mandrel 28 is returned downwardly, de^"-supporting the dogs 24, enabling retraction of the dogs and facilitating return of the seal 26 to the radially retracted or collapsed position shown in Fig. 2.

The plug 16 and tree 10 will now be described in more detail. The tree housing 20 includes a locking groove or recess 30 in which the dogs 24 are engaged when the plug 16 is actuated. In a similar fashion, the tree housing 20 includes a recessed seal bore or groove 32 for providing a metal-to-metal seal with the seal element 26, when the plug 16 is actuated. It will be understood, however, that the seal element may carry an elastomeric, plastic or rubber seal surface, if desired.

The plug body 22 includes a main portion 34 and an upper sub 36 which is coupled to the main portion 34 and is axialIy movable relative to the main portion 34 through a coupling 38, which is shown in more detail in the enlarged perspective view of Fig. 3, where the sub 36 has been removed, for clarity. The coupling 38 includes an annular ring 40 which is generally L-shaped in cross-section and which is threadably coupled to an end 42 of the sub 36. The ring 40 is also movable along a recess 44 formed in the plug main portion 34, permitting axial movement of the sub 36 relative to the main portion 34. However, a shoulder 46 on 'the main portion 34 prevents disengagement of the sub 36 from the main portion 34 by abutment with the ring 40.

The seal element 26 is threadably coupled at 47 to a lower end 48 of the plug main portion 34 and defines an ^" end cap 50 of the plug 16. However, it will be understood that the plug body 22 may form the end cap 50, with the seal element 26 mounted on the plug body.

The seal element 26 is shaped to radially outwardly expand when compressed by exertion of an axial force, which, as will be described, is generated by the mandrel 28. This brings a sealing surface 52 of the seal element 26 into engagement with the recessed seal bore 32 of the tree housing 20. The seal element 26 thus defines a zone or region of deformation 54 where the seal element 26 is deformed when compressed.

The seal element 26 shown in Fig. 2 is a one-piece member, however, the seal element may be of the type disclosed in the applicant's co-pending International Patent Application No. PCT/GB2004/003631 (WO2005/022012) , the disclosure of which is incorporated herein by way of reference. In accordance with the teachings of PCT/GB2004/003631, an alternative seal element 26a is shown in the enlarged, longitudinal half-sectional view of Fig. 4, and includes an inner sleeve 56 and an outer sleeve 58 which are joined at upper and lower ends 60, 62. A filler 64 is located between the sleeves 56, 58 and is thus entirely contained within the sealing element 26a and bounded on all sides by the metal sleeves 56, 58. Thus an interior volume 66 defined by the sleeves 56, 58 contains the filler 64. The filler 64 is of a deformable material and may be a plastic or elastomer.

The seal element 26a is shown in Fig. 5 after the ends 60, 62 have been brought together by compressing the seal element, and the filler 64 supports the seal 26a to prevent the seal from collapsing under applied pressure. Figs. 6 and 7 show a further alternative seal element, indicated generally by reference numeral 26b, both before and following compression and deformation.

The mandrel 28 is axialIy movably mounted relative to the plug body 22 and thus relative to main portion 34 and upper sub 36, and includes a cam surface 68 for urging the dogs 24 radially outwardly when the mandrel 28 is moved upwardly. Also, the mandrel 28 includes a lower flange or shoulder 70 which serves for compressing the seal element 26 when brought into abutment with a shoulder 72 on the seal element.

The plug 16 also includes a locking mechanism 74 which includes an inner lock ring 76, and an outer lock ring 78 comprising a number of ring segments 80. The inner and outer lock rings 76, 78 are shown in more detail in the enlarged perspective views of Figs. 8 and 9, respectively, as well as in Fig. 3. The inner lock ring 26 includes a set of inner protrusions in the form of ratchet teeth 82 and a set of outer protrusions formed by a thread 84. The ratchet teeth 82 engage corresponding ratchet teeth 86 on an outer surface of the mandrel 28 and, as will be described, serve for permitting movement of the mandrel 28 upwardly in a first direction A (Fig. 2) but selectively restrict movement in a second, downward direction B, (also Fig. 2) by co-operation with the outer ring segments 80. To facilitate this, the outer ring segments 80 include inner protrusions formed by a thread 88 which engage with the thread 84 on the inner ^'ring 76. In the position of Fig. 2, the outer ring 78 is axially held between the plug shoulder 46 and a shoulder 90 on the sub 36, and thus the ratchet teeth 86, 82 are engaged, and the threads 84, 88 are engaged to restrict movement of the mandrel 28 as described above.

The plug 16 also includes a pressure equalisation assembly indicated generally by reference numeral 92, which serves for equalising pressure between an interior 94 and an exterior 96 of the plug 16. The assembly 92 includes a passage 98 in the plug end cap 50 which is initially closed by an annular shoulder or nub 100, and an equalisation mandrel 102 which is movable within the main mandrel 28 for selectively shearing the nub 100 to open the passage 98. To achieve this, the mandrel 102 includes a cutter 104 and when sufficient axial load is applied to the mandrel 102, the cutter 104 shears the nub 100, equalising pressure across the plug 16 through the passage 98. As will be described, this facilitates safe removal of the plug 16 from the tree 10, when it is desired to re-open the tree. Operation of the plug 16 in use will now be described in detail, with reference also to Figs. 10 and 11, which show the plug at later stages in a process of running, setting, releasing and retrieving the plug.

The plug 16 is run-in to the tree bore 14 on a running tool (not shown) in the configuration of Fig. 2, with the dogs 24 retracted and the seal 26 in a radially contracted position. This ensures that the dogs 24 and seal 26 do not run against the tubing 18 and indeed that they do not run against the walls of the locking groove 30 and recessed seal bore 32. The running tool engages a fishing neck 108 of the sub 36, and includes a spear that is designed to engage a similar fishing neck 110 of the mandrel 28. The plug 16 is run-in on the running tool and seated on a no-go 106 in the tree housing 20. The mandrel 28 is then moved upwardly relative to the plug body 22, using the running tool to exert a force through the fishing neck 110 that shears set screws 111. The mandrel 28 is thus moved upwardly to the position shown in Fig. 10 and the cam surface 68 urges the dogs 24 outwardly into engagement with the locking groove 30, thereby locking the plug 16 in the tree bore 14. Simultaneously, the shoulder 70 on the mandrel 28 is brought into abutment with the seal shoulder 72, axially compressing the seal element 26 and deforming it,- expanding it radially outwardly to sealingly abut the recessed seal bore 32, thereby sealing the plug 16 in the tree bore 14. This upward movement of the mandrel 28 is retained through the ratchet teeth 82, 86, which prevent return downward movement of the mandrel 28. The upper plug 17 is run, located and set in a similar fashion, and the plugs 16, 17 are thus located and sealed within the tree bore 14, providing a secure barrier, and the running tool is then returned to surface and the tubing released from the tree 10. Finally, a tree cap 117 is located on the tree 10, to provide an upper, outer barrier.

When it is desired to release the plug 16 from the tree bore 14, a release tool (not shown) is run-in and latched to the fishing neck 108 of the sub 36. The sub 36 is then jarred upwardly, shearing screws 112 that couple the plug main portion 34 and sub 36 together, and carrying the sub 36 upwardly to the position shown in Fig. 10. This brings the ring 40 into abutment with a shoulder 46 on the main portion 34, de-supporting the outer lock ring segments 80. The segments 80, which are resilient or sprung, then move radially outwardly into a recess 114 defined by the sub 36, bringing the threads 84, 88 out of engagement. This effectively releases the mandrel 28, enabling return downward movement of the mandrel 28 to release the dogs 24 and facilitate retraction of the seal element 26. This is achieved by latching the release tool to the fishing neck 110 of mandrel 28 and urging the mandrel downwardly to the position of Fig. 11, carrying the inner lock ring 76. The plug 14 may then be returned to surface, re- opening the tree bore 14.

In the preferred embodiments of the invention, pressure is first equalised across the plug 16 before the mandrel 28 is moved downwardly to release the dogs 24 and retract the seal element 26. This is achieved by providing the release tool with a spear which abuts a collar 116 of the inner mandrel 102 before engaging the main mandrel fishing neck 110. In this fashion, when the release tool is moved downwardly to engage the fishing neck 110, the inner mandrel 102 is urged downwardly to shear the nub 100, opening fluid communication between the plug interior 94 and exterior 96 through the passage 98, through a port 118 in the inner mandrel 102. Thus pressure is equalised before the main mandrel 28 can be engaged and moved downwardly.

Turning now to Fig. 12, a plug in accordance with an alternative embodiment of the present invention is shown, the plug indicated generally by reference numeral 16'. Like components of the plug 16' with the plug 16 of Figs. 2-11 share the same reference numerals with the addition of suffix' , and only the differences between the plugs 16' and 16 will be described herein.

The plug 16' includes an alternative inner pressure equalisation mandrel 102' which includes a collet 120 at an upper end having collet fingers 122. The collet fingers 122 are designed to snap into a collet recess or groove 124 in the main mandrel 128' to retain movement of the inner mandrel 102' following shearing of the nub 100'.

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

The principles of the tree plug described above may be applied equally in relation to a downhole lock or downhole plug for location and use in other downhole tubing, such as casing, liner or some other tubing string, and may for example serve for locating a string of tubing or a tool for conducting a completion or intervention operation.

The seal element may be of any suitable shape and may be one-piece or comprised of a number of separate components. Even where the seal element is one-piece, the seal element may be made up of a number of sections or portions bonded or otherwise coupled together. In place of the expandable seal element described above, a more conventional type of seal element may be provided, such as an annular elastomeric, rubber or other seal which may be actuated in a similar manner, by compression^' using the mandrel of the plug. Alternatively, the mandrel may be arranged to exert a radially outwardly directed force upon the seal element of this alternative^' type, for example, by an arrangement of cam surfaces, ramps or the like.

It will be understood that the locking recess and recessed seal bore in the tree may be retro-fitted to the tree or provided in the tree as supplied.

A suitable, alternative locking arrangement may be provided such as an alternative locking mechanism, and an alternative set or type of locking dogs may be provided. Furthermore, any suitable number of seal elements and/or locking dogs may be provided, for example, the plug may comprise a plurality of seal elements of different types such as a seal element suitable for providing a metal-to- metal seal and an elastomeric, plastics or rubber seal or any desired combination thereof.

Claims

1. A tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the plug within the tree bore.

2. A tree plug as claimed in claim 1, wherein the mandrel is upwardly movable relative to the plug body for urging the locking member to the extended position and for exerting the axial force on the seal element.

3. A tree plug as claimed in either of claims 1 or 2, wherein the plug comprises a pressure equalisation assembly for selectively equalising pressure between an interior and an exterior of the plug.

4. A tree plug as claimed in claim 3, wherein the pressure equalisation assembly comprises a pressure equalisation passage for facilitating pressure equalisation.

5. A tree plug as claimed in claim 4, wherein the passage is normally closed and is adapted to be selectively opened for pressure equalisation.

6. A tree plug as claimed in any one of claims 3 to 5, wherein the equalisation assembly comprises a pressure equalisation mandrel mounted for movement with respect to the plug body, for equalising pressure between the plug interior and exterior.

7. A tree plug as claimed in claim 6, wherein the pressure equalisation mandrel is adapted to selectively open the passage on movement of the main mandrel with respect to the plug body.

8. A tree plug as claimed in claim 4, or any one of claims 5 to 7 when dependent on claim 4, wherein the passage is formed in the plug body and an internal end of the passage is closed by a shoulder on an internal wall of the plug body.

9. A tree plug as claimed in claim 8, wherein the pressure equalisation mandrel is adapted to shear the shoulder on movement of the pressure equalisation mandrel with respect to the plug body, to open the passage.

10. A tree plug as claimed in any one of claims 3 to 9, wherein the pressure equalisation mandrel is dimensioned and located within the main mandrel such that when it is desired to release the plug, the pressure equalisation mandrel is moved to equalise pressure between the interior and exterior of the plug before the main mandrel is moved to release the locking member and the seal element .

11. A tree plug as claimed in any preceding claim, wherein the plug comprises a locking mechanism for restraining the mandrel, and thus for restraining the locking member in the extended position and the seal element in the radially expanded position.

12. A tree plug as claimed in claim 11, wherein the locking mechanism is adapted to permit movement of the mandrel in a first axial direction with respect to the plug body and to restrain movement of the mandrel in a second, opposite axial direction.

13. A tree plug as claimed in either of claims 11 or 12, wherein the locking mechanism comprises a lock ring serving for restraining movement of the mandrel relative to the plug body.

14. A tree plug as claimed in claim 13, wherein the lock ring is a one-piece ring.

15. A tree plug as claimed in claim 13, wherein the lock ring comprises a plurality of lock ring segments.

16. A tree plug as claimed in any one of claims 13 to 15, wherein the lock ring is provided between the mandrel and the plug body, and comprises a plurality of ratchet teeth adapted to engage corresponding teeth on the mandrel.

17. A tree plug as claimed in claim 16, wherein the ratchet teeth of the lock ring and the mandrel are shaped to permit movement of the mandrel with respect to the plug body in a first axial direction and to restrain the mandrel against movement in a second axial direction.

18. A tree plug as claimed in any one of claims 13 to 17 , wherein the lock ring forms an inner ring and the locking mechanism also comprises an outer lock ring, the inner and outer lock rings adapted to co- operate to selectively restrain the mandrel against movement with respect to the plug body.

19. A tree plug as claimed in claim 18, wherein the outer lock ring comprises a plurality of locking segments, the outer locking segments adapted to be located between the inner lock ring and the plug body.

20. A tree plug as claimed in claim 19, wherein the outer locking segments define a thread for engaging a corresponding thread on an outer surface of the inner lock ring, interengagement between the outer locking threads and the threads on the inner lock ring, and between the inner lock ring ratchet teeth and the mandrel ratchet teeth serving for together restraining the mandrel against movement with respect to the plug body.

21. A tree plug as claimed in any one of claims 18 to 20, wherein the outer lock ring is selectively radially moveable in order to permit movement of the mandrel with respect to the plug body.

22. A tree plug as claimed in any one of claims 18 to 21, wherein the outer lock ring is resilient and at least initially held in a stressed configuration until permitted to radially move relative to the mandrel .

23. A tree plug as claimed in any preceding claim, wherein the plug body comprises a main plug body portion and a second portion coupled to the main plug body portion and selectively moveable with respect thereto.

24. A tree plug as claimed in claim 23, when dependent on claim 19, wherein the second portion defines an internal recess for receiving the outer lock ring segments, to facilitate radial movement of the segments and thus release of the mandrel when the sub is moved axially relative to the main plug body portion to align the segments with the recess.

25. A tree plug as claimed in any preceding claim, wherein the seal element is adapted to be axially compressed and thus shortened in length when an axial force is exerted on the seal element by the mandrel, thus urging the seal element outwardly into sealing contact with the tree bore.

26. A tree plug as claimed in any preceding clam, wherein the seal element has a wall of a reduced wall thickness in a region of the seal element to be deformed into sealing contact with the tree bore, relative to a remainder of the seal element .

27. A tree plug as claimed in claim 26, wherein an axially central region of the seal element is of a wall thickness which is reduced relative to regions of the seal element spaced from said central region.

28. A tree plug as claimed in any preceding claim, wherein the seal element defines a zone of deformation where the element is adapted to be urged into sealing contact with the tree bore.

29. A tree plug as claimed in any preceding claim, wherein the seal element comprises: an inner deformable surface, an outer deformable surface, the inner and outer surfaces being joined at opposed first and second ends providing a body defining an interior volume entirely bounded by the said surfaces and said ends, a deformable filler material within said interior volume, wherein the element deforms in a predetermined manner to seal between the first and second surfaces when said ends are brought toward each other.

30. A tree plug as claimed in any preceding claim, wherein the locking member and the seal element are adapted to engage corresponding lock and seal recesses provided in the tree bore.

31. A tree plug as claimed in any preceding claim, wherein the plug comprises a plurality of locking members located in radially extending recesses in the plug body.

32. A tree plug as claimed in any preceding claim, wherein the locking member comprises a cam surface adapted to co-operate with a corresponding surface on the mandrel, such that the mandrel urges the locking member radially outwardly on translation of the mandrel relative to the plug body.

33. A tree comprising: a main tree bore and a tree plug located in the tree bore and sealed relative to the bore, the tree plug comprising a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially extendable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position to engage a latch recess in the tree bore and to exert an axial force on the seal element to radially expand the seal element to engage a seal recess in the tree bore, to thereby lock and seal the plug within the tree.

34. A downhole plug adapted to be located in downhole tubing, the plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended ' position and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the plug within the downhole tubing .

35. A downhole lock adapted to be located in downhole tubing, the downhole lock comprising: a main body; at least one locking member mounted for radial movement relative to the main body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated relative to the main body to urge the locking member to the extended position and to exert an axial force on the seal element to radially expand the seal element, to thereby lock and seal the downhole lock within the downhole tubing.

36. A method of plugging a tree bore comprising the steps of: locating a tree plug within the tree bore; translating a mandrel of the tree plug relative to a body of the plug, to urge a locking member mounted for radial movement relative to the plug body from a retracted to an extended position to cause the locking member to engage a latch recess in the tree bore, and to exert an axial force on a seal element of the plug to radially expand the seal element into sealing abutment with a seal recess in the tree bore, to thereby lock and seal the plug within the tree .

37. A tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; and a mandrel adapted to be translated upwardly relative to the plug body to urge the locking member to the extended position and to radially expand the seal element, to thereby lock and seal the plug within the tree bore.

38. A tree plug for a tree having a main tree bore, the tree plug comprising: a plug body; at least one locking member mounted for radial movement relative to the plug body between retracted and extended positions; at least one radially expandable seal element; a mandrel adapted to be translated relative to the plug body to urge the locking member to the extended position and to radially expand the seal element, to thereby lock and seal the plug within the tree bore; and a locking mechanism comprising an inner lock ring having a set of inner protrusions for engaging corresponding outer protrusions on the mandrel, and an outer lock ring having a set of inner protrusions for engaging corresponding outer protrusions on the inner lock ring, interaction between the outer lock ring protrusions, the inner lock ring protrusions and the mandrel protrusions permitting movement of the mandrel in a first direction relative to the plug body and restraining the mandrel against movement in a second, opposite direction to thereby restrain the locking member in the extended position and the seal element in a radially expanded position; wherein the outer lock ring is selectively radially outwardly moveable to disengage the outer lock ring protrusions and the outer protrusions on the inner lock ring, to permit movement of the mandrel in said second direction to release the plug.