Abstract:
Disclosed herein is a device that relates to an actuator comprising a plurality of individual piston chambers pressurized by a common pressure source. A plurality of pistons are in operable communication with the piston chambers, the pistons each being in operable communication with an actuatable device, such that the plurality of pistons simultaneously act upon the actuatable device upon application of pressure to the plurality of piston chambers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to G.B. provisional application, 0515072.7, filed Jul. 22, 2005, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a downhole tool for use in operating or actuating a further tool. In particular, but not exclusively, the present invention relates to a downhole setting tool for use in setting a further tool, such as a bridge plug tool. 
   BACKGROUND OF THE INVENTION 
   Many downhole well bore tools require to be activated when located downhole at the required location or depth. There are many systems available, which may be utilized to perform such actuation, and may include downhole motors, piston arrangements or the like. However, it is sometimes the case that such systems require to be powered or carefully monitored and controlled from surface level to ensure reliable and correct operation. These therefore require relatively complex arrangements of conduits and power cables and the like to be run from surface level to the required depth. 
   Simplified arrangements, therefore, of downhole tool actuation are desirable in the art. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Disclosed herein is a device that relates to an actuator. The actuator comprising, a plurality of individual piston chambers pressurized by a common pressure source, and a plurality of pistons in operable communication with the piston chambers, the pistons each being in operable communication with an actuatable device, such that the plurality of pistons simultaneously act upon the actuatable device upon application of pressure to the plurality of piston chambers. 
   Further disclosed is a device that relates to a downhole actuation tool. The tool comprising, an inner member having a fluid passage therein, the passage being selectively communicable with a fluid pressure source. The tool further comprising, an outer member slidable relative to and fluidically sealed with the inner member, wherein the outer member defines a piston face, and the piston face is in fluid communication with the passage. The piston face and a face of an adjacent additional inner member define a fluid chamber to which pressure is applyable to slidably move the outer member relative to the inner member. 
   Further disclosed herein relates to a downhole actuation tool. The tool comprising, an inner member defining a central bore extending therein from one end thereof, wherein the central bore is in selective fluid communication with a fluid source. Further comprising an outer member relatively slidably mounted about the inner member, and a plurality of piston chambers defined between the inner and outer members. The plurality of piston chambers are in fluid communication with the central bore of the inner member and in relative sliding movement with the inner and outer members in response to fluid communication between the fluid source and the plurality of piston chambers. 
   Further disclosed relates to a downhole actuation tool. The tool comprising, a first tool portion having an inner member defining a central bore extending from one end thereof, wherein the central bore is configurable to be in selective fluid communication with a fluid source. An outer member relatively slidably mounted about the inner member and a plurality of piston chambers defined between the inner member and the outer member. The piston chambers are in fluid communication with the central bore of the inner member such that fluid from the fluid source is communicable with the piston chambers to urge relative sliding movement of the inner and outer members. And a second tool portion comprising a valve body configurable to selectively permit fluid communication of fluid from the fluid source with the central bore of the inner member. 
   Further disclosed is a device that relates to a downhole tool string. The tool string comprising, a first tool comprising an actuatable member, and a second tool comprising, an inner member defining a central bore extending from one end thereof, wherein the central bore is configurable to be in selective fluid communication with a fluid source. An outer member relatively slidably mounted about the inner member and engagable with the actuatable member of the first tool. And a plurality of piston chambers defined between the inner and outer members, wherein the piston chambers are in fluid communication with the central bore of the inner member such that fluid from the fluid source is communicable with the piston chambers to cause relative sliding movement of the inner and outer members to actuate the actuatable member of the first tool. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a bridge plug tool in accordance with an embodiment of the present invention, shown in a retracted configuration; 
       FIG. 2  is a perspective view of the tool of  FIG. 1 , shown in an extended configuration; 
       FIGS. 3A to 3D  present a longitudinal sectional view of the tool of  FIG. 1 ; 
       FIGS. 4 to 7  are enlarged part sectional views of a ratchet arrangement of the tool of  FIG. 1 ; 
       FIGS. 8 and 9  are perspective views of the tool of  FIG. 1 , showing the tool being moved to a retracted configuration; 
       FIG. 10  is a longitudinal sectional view of a setting tool in accordance with an embodiment of an aspect of the present invention, wherein the setting tool is shown in an unstroked, first configuration; 
       FIG. 11  is a longitudinal sectional view of the tool of  FIG. 10 , shown in a stroked (setting), second configuration; 
       FIGS. 12 and 13  are enlarged part sectional views of a portion of the tool shown in broken outline in  FIGS. 10 and 11 ; 
       FIG. 14  is a longitudinal sectional view of a trigger tool for use in conjunction with the setting tool of  FIGS. 10 and 11 , wherein the trigger tool is shown in a locked, first configuration; 
       FIG. 15  is a longitudinal sectional view of the trigger tool of  FIG. 14 , shown in an unlocked (triggered), second configuration; and 
       FIGS. 16 and 17  are enlarged part sectional perspective views of the tool of  FIGS. 14 and 15 , shown in the first and second configurations respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference is first made to  FIGS. 1 and 2  of the drawings, which show perspective views of a downhole bridge plug tool, generally identified by reference numeral  10 . The tool  10  is shown located in a portion of a cased well bore  12 , and in  FIG. 1  is shown in a retracted, first configuration, and in  FIG. 2  is shown in an expanded, second configuration. 
   The tool  10  comprises an outer tool body  14  mounted on a tool mandrel  16 , and a number of extendable assemblies  18  mounted on an outer surface of the tool  10 . As shown, the extendable assemblies  18  are arranged in two axially spaced sets,  20 ,  22 , wherein each set  20 ,  22  comprises three extendable assemblies  18  circumferentially distributed about the outer surface of the tool  10 . The extendable assemblies  18  of the first set  20  are pivotally mounted between a first support portion  24  and a second support portion  26 , and the extendable assemblies  18  of the second set  22  are pivotally mounted between the second support portion  26  and a third support portion  28 . The first support portion  24  is fixed relative to the tool mandrel  16  and the second and third support portions  26 ,  28  are axially slidably mounted relative to the tool mandrel  16 . 
   The tool  10  further comprises an outer sleeve assembly  30  slidably mounted relative to the tool mandrel  16 , wherein a lower end  30   a  of the outer sleeve assembly  30  engages the third support portion  28 . In use, the sleeve assembly  30  is caused to move downwardly relative to the tool mandrel  16  towards the leading end nose  94  to transmit a force to the third support portion  28 , thus causing the second and third support portions  26 ,  28  to be displaced downwardly relative to the tool mandrel  16  to cause the extendable assemblies  18  to extend radially outwardly, as shown in  FIG. 2 , into engagement with the wall  32  of the bore  12 . In this configuration, the tool is advantageously secured within the bore  12  by the interference engagement created between the extendable assemblies  18  and bore wall. The outer sleeve assembly  30  may be caused to move downwardly relative to the tool mandrel  16  by an appropriate setting tool (not shown in  FIGS. 1 and 2 ), such as that shown in  FIGS. 10 to 13 , which is an embodiment of an aspect of the present invention. 
   The outer sleeve assembly  30  incorporates a sealing member  34  which is adapted to be moved between a retracted configuration, as shown in  FIG. 1 , and an extended or sealing configuration, as shown in  FIG. 2 . The arrangement is such that when the extendable assemblies  18  are engaged with the bore wall  32  to provide support, continued downward movement of the outer sleeve assembly  30  will cause the sealing member to be deformed radially outwardly and ultimately brought into sealing engagement with the bore wall  32 . Thus, the established seal may be utilized to prevent or at least minimize the transmission of fluids between upper and lower regions  36 ,  38  of the well bore  12 . 
   A more detailed description of the tool  10  will now be given with reference to  FIG. 3  in which there is shown a longitudinal sectional view of the tool  10 , in the configuration of  FIG. 1 . For clarity, the tool  10  in  FIG. 3  is presented on  4  separate sheets, in  FIGS. 3A-3D . 
   An upper portion of the tool  10  is shown in  FIG. 3A , in which there is shown a portion of the outer sleeve assembly  30  mounted on the tool mandrel  16 . An end portion  16   a  of the mandrel  16  incorporates a threaded portion  40  for securing to a further tool, such as a setting tool, either directly or via a suitable connector. The outer sleeve assembly  30  comprises an outer sleeve load transfer sub  42  having an annular end face  44  against which a loading tool, such as a setting tool, may abut to transmit an axial force to the load transfer sub  42 , which force is ultimately transmitted to the third support portion  28  ( FIGS. 1 and 2 ) and seal portion  34  ( FIGS. 1 and 2 ) to reconfigure the tool  10 . Accordingly, when the tool  10  is reconfigured, the outer sleeve assembly  30  is moved downwardly, in the direction of arrow  46 , relative to the tool mandrel  16 . 
   The outer sleeve assembly  30  further comprises a ratchet arrangement, generally indicated by reference numeral  48 , adapted to freely permit movement of the sleeve assembly  30  in the direction of arrow  46  relative to the tool mandrel  16 , and to selectively permit relative movement of the outer sleeve assembly  30  and tool mandrel  16  in a direction opposite to arrow  46 . Thus, the ratchet arrangement  48  is adapted to temporarily lock the tool  10  in the extended configuration (shown in  FIG. 2 ). A detailed description of the ratchet arrangement  48  and its operation is provided hereinafter below. 
   Reference is now made to  FIG. 3B  in which the remaining portion of the outer sleeve assembly  30  is shown. As noted above, the assembly  30  comprises sealing member  34 , which is secured with the sleeve assembly  30  by threaded connections  50 ,  52 , and is supported by seal supports  54 ,  56 . The sealing member defines upper and lower annular notches  58 ,  60  in an outer surface thereof, and a central annular notch  62  in an inner surface thereof, such that when a predetermined axial load is imparted on the outer sleeve assembly  30 , the sealing member  34  deforms at the location of the notches  58 ,  60 ,  62  to provide the required seal extension. The sealing member may be of a form such as that described in applicant&#39;s co-pending international patent application, publication number WO 02/04783. 
   The third support portion  28  is secured to the lower end of the sealing member  34  via a threaded connector sleeve  64 . When the tool  10  is initially set in the retracted position, the third support portion  28  is secured to the tool mandrel  16  via one or more shear screws  66  which are adapted to be sheared when the outer sleeve assembly  30  is subjected to a predetermined axial load. Once the shear screws  66  have been sheared, the third support portion  28  may then be displaced axially relative to the tool body  16  by the outer sleeve assembly  30 , thus causing the extendable assemblies  18  to be extended radially outwardly. This arrangement assists to prevent unintentional extension of the extendable assemblies  18 , for example when running into a well bore. 
   In the embodiment shown, the axial force required to shear the shear screws  66  is less than that required to deform the sealing member  34 . Accordingly, any axial load applied to the outer sleeve assembly  30  will advantageously be transmitted by the sealing member  30  and applied to the third support portion  28  via the connector sleeve  64  in order to shear the shear screws  66 , and subsequently effect extension of the extendable assemblies  18 , without any deformation of the sealing member  34  occurring. Once the extendable assemblies  18  engage the wall of a bore, an increased reaction force will be achieved such that an increased force may be applied by the outer sleeve assembly  30  to effect deformation and activation of the sealing member  34 . Thus, the tool  10  is adapted to be located at the required bore depth, fixed in location by the extendable assemblies  18 , and then establish a seal via sealing member  34 . 
   A collar  68  is mounted about the outer surface of the tool mandrel  16 , beneath the sealing member  34 . In use, when the sealing member  34  is being deformed, the seal supports  54 ,  56  will engage either side of the collar  68 , thus limiting the amount of deformation of the sealing member  34  which may be achieved. The collar  68  may be fixed to the tool mandrel  16 , or may be slidably mounted on the mandrel  16 . 
   The form of the extendable assemblies  18  will now be described with reference to  FIG. 3C , in which a longitudinal sectional view of a complete extendable assembly  18  of the second set  22  ( FIGS. 1 and 2 ) is shown, which extends between the third support portion  28  and second support portion  26 . As noted above, the second support portion  26  is slidably mounted relative to the tool mandrel  16  such that relative downward movement of the second support portion  26  will be achieved when the third support portion  28  is caused to move axially by the outer sleeve assembly  30 . The second support portion  26  will be caused to move at a slower rate of displacement than the third support portion  28  in order to establish relative movement therebetween. Also shown in  FIG. 3C  is a portion of an extendable assembly  18  of the first set  20  ( FIGS. 1 and 2 ), which extends between the second support portion  26  and the first support portion  24  ( FIG. 3D ). As previously noted, the first support portion  24  is fixed relative to the tool mandrel  16 . Accordingly, when the outer sleeve assembly  30  applies an axial force, relative downward movement of the second and third support portions  26 ,  28  with respect to the tool mandrel  16  will result in extension of the extendable assemblies  18 . 
   Each extendable assembly  18  comprises a central engaging member  70  supported between first and second connecting members  72 ,  74 . The outer surface  71  of the engaging member  70  is adapted to engage the wall surface of the bore within which the tool  10  is located. In the embodiment shown, the outer surface  71  of the engaging member comprises serrations  73  to aid the grip between the member  70  and bore wall. Alternatively, tungsten carbide inserts or the like may be utilized. 
   As shown in the complete example in  FIG. 3C , one end of the first connecting member  72  is pivotally coupled to the third support portion  28  about pivot axis  76 , and an opposite end of the first connecting member  72  is pivotally coupled to the engaging member  70  about pivot axis  78 . Similarly, one end of the second connecting member  74  is pivotally coupled to the engaging member  70  about pivot axis  80 , and an opposite end of the second connecting member  74  is pivotally coupled to the second support portion  26  about pivot axis  82 . The pivot axes  76 ,  78 ,  80 ,  82  are aligned parallel with each other, and are obliquely aligned and radially offset from the central longitudinal axis  84  of the tool  10 . 
   In the preferred arrangement shown in the Figures, pivot axes  76 ,  78  are laterally offset from each other relative to the central axis  86  of the first connecting member  72 . That is, pivot axis  76  is positioned closer to an inner surface  90  of the first connecting member  72  than pivot axis  78 . In a similar fashion, pivot axis  82  is positioned closer to the inner surface  92  of the second connection member  74  than axis  80 . This specific arrangement of the respective pairs of pivot axes  76 ,  78  and  80 ,  82  advantageously results in the transmission of an axial force, applied by the outer sleeve assembly  30 , between the offset pivot axes pairs at an oblique angle relative to the longitudinal axis  84  of the tool  10 , such that the engaging member  70  will consistently be moved radially outwardly. Arranging the pivot axes in the particular manner shown and described beneficially eliminates or at least minimizes the possibility of the engaging members  70  being forced in a radially inward direction which would cause the extendable assemblies  18  to become jammed, which may cause premature extension of the sealing member  34 . 
   The lower end of the tool  10  is shown in  FIG. 3D . A conical nose portion  94  is secured to the lower end of the tool mandrel  16  via a threaded connection  96 . The first support portion  24  is secured to the nose portion  94  via a threaded connector sleeve  98 , such that the first support portion  24  is at least axially fixed relative to the tool mandrel  16 . 
   The form and function of the ratchet arrangement  48 , initially shown in  FIG. 3A , will now be described in detail with reference to  FIGS. 4 to 7 . 
   Reference is initially made to  FIG. 4  in which there is shown a part sectional view of the tool  10  in the region of the ratchet arrangement  48 . The outer sleeve assembly  30  comprises an outer sleeve or load transfer sub  42 , which as noted above is adapted to transfer a load applied from an external tool. The sub  42  is secured to an inner sleeve  100  via a grub screw  102 , and the inner sleeve  100  is also initially secured to an outer release sleeve  104  via a plurality of shear screws  106 . The outer release sleeve  104  is secured to the upper end of the sealing member  34  by the threaded connection  50 . Additionally, the outer release sleeve  104  is also secured to a ratchet mandrel  108  via a threaded connection  110 . Thus, the arrangement is such that during normal use of the tool a permanent connection is provided between the sub  42  and inner sleeve  100 , and a permanent connection is provided between the outer release sleeve  104 , sealing member  34  and ratchet mandrel  108 , while the inner sleeve  100  and outer release sleeve  104  are temporarily secured together by virtue of the shear screws  106 . 
   The ratchet mandrel  108  defines two diametrically opposed apertures  112  (only one shown) within which is located a ratchet component  114 , spacer element  116  and a ratchet reverser component  118 . The ratchet component  114  defines a ratchet profile on an inner surface thereof, which is adapted to engage and cooperate with a ratchet profile  120  on the outer surface of the tool mandrel  16 . The ratchet component  114  is removed in  FIG. 5  to clearly show the ratchet profile  120  of the tool mandrel  16 . Referring again to  FIG. 4 , when in use, the ratchet arrangement  48  will permit movement of the outer sleeve assembly  30  in the direction of arrow  46 . That is, the ratchet profiles on the ratchet component  114  and tool mandrel  16  will cooperate to ratchet the ratchet component  114  radially outwardly into an annular cavity  122  defined between the inner sleeve  100  and the ratchet mandrel  108 . However, when relative movement of the tool mandrel  16  and outer sleeve assembly  30  is attempted in the opposite direction to that indicated by arrow  46 , cooperation of the ratchet profiles on the tool mandrel  16  and ratchet component  114  will cause the outer sleeve assembly  30  and tool mandrel  16  to become axially locked together. 
   When it is required to reconfigure the tool  10  from the extended configuration to the retracted configuration, it is necessary to disengage the ratchet profiles of the ratchet component  114  and tool mandrel  16 . To achieve this, a tool (not shown) is coupled to the inner sleeve  100  via fishneck  123 , wherein the tool pulls on the inner sleeve  100  in the direction of arrow  124  shown in  FIG. 6 , reference to which is now made. The tool used to pull on the inner sleeve  100  may be the same setting tool used to position the extendable assemblies  18  and sealing member  34  into extended configurations. Alternatively, a different tool may be used. When a predetermined axial force is achieved by the tool pulling on the inner sleeve  100 , the shear screws  106  will shear, thus severing the connection between the inner sleeve  100  and the outer release sleeve  104 , permitting the inner sleeve  100  and load transfer sub  42  to be displaced upwardly in the direction of arrow  124 . Upward displacement of the inner sleeve  100  will be permitted until an annular face  126  of the inner sleeve  100  engages an annular face  128  of the outer release sleeve  104 . In this position, the ratchet reverser component  118  is no longer enveloped by the inner sleeve  100 . 
   Reference is now made to  FIG. 7  of the drawings in which there is shown an enlarged view of the ratchet arrangement  48 , shown in a released position. When the inner sleeve  100  has been displaced to uncover the ratchet reverser component  118 , an axial force may be applied to the tool mandrel  16  to move the mandrel in the direction of arrow  130  relative to the outer sleeve assembly  30 . Movement of the tool mandrel  16  in this direction will translate the ratchet component  114  in the same direction by virtue of the engaging ratchet profiles  120  such that the spacer element  116  is forced under the ratchet reverser component  118  to displace the component  118  radially outwardly into the annular space  132  previously occupied by the inner sleeve  100 . Furthermore, movement of the ratchet component  114  in the direction of arrow  130  will cause the ratchet component  114  to be displaced radially outwardly of the aperture  112  by cooperation of engaging ramp profiles  134  on the ratchet component  114  and ratchet mandrel  108 , thus disengaging the ratchet profiles to permit the tool mandrel  16  to then be freely displaced in the direction of arrow  130  relative to the outer sleeve assembly  30  in order to move the extendable assemblies  18  and sealing member  34  towards a retracted configuration, as discussed below with reference to  FIGS. 8 and 9 . 
   Referring initially to  FIG. 8 , which is a part sectional side view of the tool  10 , when the ratchet arrangement  48  is released, downward movement of the tool mandrel  16  in the direction of arrow  130  relative to the outer sleeve assembly  30  will initially cause the extendable assemblies  18  to be moved to a retracted position. Once the assemblies  18  are fully retracted, further displacement of the tool mandrel  16  will cause the sealing member  34  to be retracted, as shown in the perspective view in  FIG. 9 . Once in this configuration, the tool may be retrieved to surface, where it may be reset, for example by replacing shear screws  66  ( FIG. 3B) and 106  ( FIG. 4 ). 
   As noted above, a setting tool may be utilized to move the tool  10  towards an extended configuration in which the extendable assemblies  18  and sealing member  34  are brought into engagement with a bore wall. A setting tool according to an embodiment of an aspect of the present invention, which is suitable for use with the tool  10 , will now be described, with reference to  FIGS. 10 to 13 . 
   Reference is first made to  FIG. 10  in which there is shown a longitudinal sectional view of a setting tool, generally identified by reference numeral  150 , shown located within a cased bore, which for convenience is identified by reference numeral  12 . The setting tool  150  comprises an inner member  152  and an outer member  154  slidably mounted on the inner member  152 . The inner member  152  is formed by threadably coupling together a plurality of inner modular sections  156  end to end, and similarly, the outer member  154  is formed by threadably coupling together a plurality of outer modular sections  158 . The lowermost inner modular section  156   a  is adapted to be secured to the upper end of the tool mandrel  16  of the bridge plug tool  10  described above. Additionally, the lowermost outer modular section  158   a  is adapted to be secured to the outer sleeve assembly  30  of the bridge plug tool  10 , either directly or preferably via an intermediate connecting sleeve (not shown). 
   The uppermost inner section  156   b  is adapted to be secured to a further downhole tool (not shown), such as a trigger tool used to actuate the setting tool  150 , via a connector  160  which is threadably coupled at one end to the inner module  156   b , and comprises a nipple portion  162  at the other end for engagement with the further downhole tool. A preferred example of a trigger tool for use in actuating the setting tool  150  of the present invention is described hereinafter with reference to  FIGS. 14 to 17 . 
   The inner member  152  defines a central bore  164  extending from an end face of the uppermost inner module  156   b  and terminating in the region of the lowermost inner module  156   a . The central bore  164  is in selective fluid communication with fluid contained with well bore  12  via fluid port  166  in the nipple portion  162  of the connector  160 . Selective fluid communication is achieved by the insertion and removal of a piston member (not shown) into and from the fluid port  166 , wherein the piston member forms part of a further downhole tool, an example of which is shown in  FIGS. 14 to 17 , which is described below. 
   The inner member  152  further defines a plurality of transverse bores  168  axially distributed along the length of the inner member  152 , wherein the bores  168  communicate with the central bore  164 . Each transverse bore  168  is aligned with a respective bore  170  formed in the outer member  154 , wherein the bores  170  are in fluid communication with respective piston chambers  172  defined between the inner and outer members  152 ,  154 . 
   In use, the port  166  is opened which will permit well bore fluid to enter the central bore  164 , and into the piston chambers  172  via respective aligned bores  168 ,  170 . The hydrostatic pressure of the well bore fluid will cause the piston chambers  172  to fill with well bore fluid, thus forcing the outer member  154  to move relative to the inner member  152  in the direction of arrow  174 , as shown in  FIG. 11 . Thus, this movement of the outer member  154  may be transmitted to the outer sleeve assembly  30  of the bridge plug tool  10  to reconfigure the bridge plug tool  10 . An enlarged view of a piston chamber  172  is shown in  FIG. 12  with the outer member  154  in a retracted position, and in  FIG. 13  with the outer member  154  in an extended position with the piston chamber  172  filled with well bore fluid communicated from the well bore via bores  164 ,  168  and  170 . 
   The bridge plug tool  10  and setting tool  150  advantageously may be secured together to form a tool string in accordance with an embodiment of an aspect of the present invention. 
   While the setting tool  150  has been described above for use in activating the bridge plug tool  10  of  FIGS. 1 to 9 , it should be understood that the setting tool  150  may be utilized with any other downhole tool that requires some form of mechanical actuation. 
   As noted above, the setting tool  150  may be actuated by a trigger tool which permits selective fluid communication between the well bore  12  and the central bore  164  in order to fill the piston chambers  172  with well bore fluid. A preferred form of trigger tool for use in actuating tool  150  will now be described, with reference to  FIGS. 14 to 17 . 
   Referring initially to  FIG. 14 , there is shown a longitudinal sectional view of a trigger tool, generally identified by reference numeral  180 , which may be utilized in conjunction with the setting tool  150  described above. The trigger tool  180  comprises an upper connector  182  for coupling the tool  180  to the lower end of a support (not shown), such as a tubing string, coiled tubing, wireline or the like. The upper connector  182  is coupled to a first tool body  184  via a threaded connection  186 , and the first tool body  184  is secured to a lower, second tool body  188  via threaded connection  190 . Mounted on the lower end of the second tool body  188  is a lower connector  192  adapted to be coupled to the connector  160  of the setting tool  150  via nipple  162  which is received in bore  194  in the lower connector  192 , and secured therein via grub screw  196 . It should be noted that in the embodiment shown, no fluid sealing is provided between the connector  160  of the setting tool  150  and the connector  192  of the trigger tool  180 , thus permitting the bore  194  to be exposed to well bore pressure. 
   Slidably mounted within the lower end of the second tool body  188  is a differential plug  198  comprising a piston portion  200 , wherein the piston portion  200  is adapted to be received within the port  166  in the connector  160  of the setting tool  150  in order to prevent fluid communication between the well bore  12  and central bore of tool  150 . Fluid sealing is achieved between the piston portion  200  and port  166  via a pair of O-ring seals  202  mounted on the piston portion  200 , whereas fluid sealing is achieved between the piston portion  200  and the second tool body  188  via a pair of O-ring seals  206 , also mounted on the piston portion  200 . To actuate the setting tool  150 , the differential plug  198  is permitted to move in the direction of arrow  204  under the action of the hydrostatic pressure of the well bore fluid acting across the differential piston between the O-ring seals  202 ,  206 , as described below. 
   Between the O-ring seals  202 ,  206 , the differential plug  198  defines two dissimilar piston areas that may be exposed to hydrostatic well bore pressure. That is, O-ring seals  202  are mounted on a first section  208  of the piston plug  200 , which defines a first diameter, whereas O-ring seals  206  are mounted on a second section  210 , which defines a second, larger diameter. Accordingly, the difference in piston area, in the presence of well bore pressure, exerts a force on the piston plug  200  which will bias the plug in the direction of arrow  204 . In order to ensure communication of well bore pressure with the first and second sections  208 ,  210  of the piston plug  200 , a plurality of slots  212  are provided around the outer surface of the connector  192 , wherein the slots  212  are aligned with an annular notch  214  and a number of bores  216  formed in the second tool body  188 , such that well bore fluid will be communicated to annular chamber  218 . 
   The trigger tool  180  comprises a releasable locking arrangement adapted to maintain the differential plug  198  in the position shown in  FIG. 14 , in order to maintain the piston portion  200  sealed within the port  166  of the setting tool  150 . When required, the locking arrangement is released thus permitting movement of the plug  198  by well bore pressure to open port  166  in tool  150 . 
   The locking arrangement comprises a primary lever  220 , which is shown in a locked position in  FIG. 14 , wherein a face  222  of the primary lever  220  engages and restrains the plug  198  from stroking. The primary lever  220  engages a first rolling lever  224  of a linear gear train  226 , wherein the linear gear train  226  is locked by a locking lever  228 , in which the locking lever  228  engages and is secured between the final rolling lever  230  of the linear gear train  226  and a locking trip nut  232 . The locking trip nut  232  is threadably mounted on a lead screw  234 , which is adapted to be driven by a wind-up clock mechanism  236  via a torque coupling  238 . To unlock the locking arrangement, the lead screw  234  is rotated to move the locking trip nut  232  in the direction of arrow  204 , such that the locking lever  228  is free to pivot in a clockwise direction about pivot axis  240 , as shown in  FIG. 15 . Thus, when the locking lever  228  is disengaged from the locking trip nut  232 , the pressure force acting on the differential plug  198  will cause the plug to move in the direction of arrow  204  causing the primary lever  220  to pivot in an anti-clockwise direction about pivot axis  242 . The primary lever  220  will apply a force on the first rolling lever  224  of the linear gear train  226 , which will be transmitted through to the final rolling lever  230  and ultimately to the locking lever  228  which will be caused to pivot in a clockwise direction. The linear gear train  226  advantageously reduces the force applied on the locking lever  228  and locking trip nut  232  by the external fluid pressure force acting on the plug  198 . Otherwise, the force applied would be too great to be overcome by the torque of the wind-up mechanism  236 , thus preventing the release of the primary lever  220  to permit movement of the plug  198 . 
   An enlarged part sectional perspective view of the locking arrangement is shown in  FIG. 16 , in which the arrangement is shown in a locked configuration, and in  FIG. 17  in which the arrangement is shown in an unlocked configuration. The locking trip nut  232  comprises a pair of arms  244 , which extends into respective elongate guide slots  246  (only one shown) which prevent rotation of the nut  232  as the lead screw  234  is rotated. Additionally, the locking lever  228  comprises a pair of parallel arm  248  which permit engagement with an underside of the locking trip nut  232 , while preventing interference with the lead screw  234  when the locking lever  228  is permitted to pivot clockwise about pivot axis  240 . 
   While the trigger tool  180  has been described above for use with the setting tool  150  shown in  FIGS. 10 to 13 , it should be understood that the tool  180  may be used with any other suitable downhole tool that requires a form of mechanical actuation. 
   It should be understood that the embodiments described above are merely exemplary and that various variations may be made without departing from the scope of the invention. For example, any number of extendable assemblies  18  may be provided with the bridge plug tool  10 , and additionally any number of sealing members  34  may be incorporated. 
   Additionally, the setting tool  150  may comprise any number of piston chambers  172 . Further, the connector  160  may be integrally formed with inner member  152 . Furthermore, the tool  150  may be adapted to be coupled to any other suitable tool or tools, and is not limited for use with the bridge plug tool  10  and trigger tool  180  described above. In this regard, any suitable form of connector  160  may be utilized. Additionally, the tool  150  is adapted to be actuated by the hydrostatic pressure of the well bore fluid. However, the tool  150  may be supplied with fluid under pressure from surface level via a suitable conduit. 
   The trigger tool  180  may incorporate a suitable mechanical drive means, such as an electric motor, in place of the wind-up clock mechanism  236 . Additionally, any suitable connector may be utilized in place of the connector  192 , depending on the form of tool with which the trigger tool  180  is intended to be used. 
   While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.