Patent Publication Number: US-6039129-A

Title: Locking system for a firing mechanism of a downhole tool

Description:
FIELD OF THE INVENTION 
     The present invention relates to a locking system and, in particular, but not exclusively, to a locking system for a firing mechanism of a tool used in a system for in situ replacement of cutting means for a ground drill to prevent premature firing thereof. 
     BACKGROUND OF THE INVENTION 
     A system for in situ replacement of cutting means for a ground drill is described in Applicant&#39;s International application no. PCT/AU94/00322 (WO 94/29567), the contents of which are incorporated herein by way of reference. 
     The system in WO 94/29567 comprises a drive sub which is adapted for connection to a lower end of a core barrel attached to a drill pipe; a tool for installing and retracting drill bit segments from the drive sub; and, an insert or bit locking sleeve for selectively locking the bit segments into seats provided about the inner circumferential surface of an end of the drive sub and subsequently releasing the bit segments for those seats. The tool includes a main body portion and a sleeve slidably mounted thereon. Installation latch dogs provided in the tool extend from apertures or slots cut in the sleeve so as to engage the bit locking sleeve and force it into an installation position in which it locks the bit segments in a cutting position about the drive sub. The tool further includes retrieval latch dogs which can extend from different slots provided in the sleeve for engaging the bit locking sleeve and pulling it upwardly into a retrieval position in which the bit segments can be retrieved from the drive sub. 
     A slidable cradle extends from a lower end of the tool for carrying the bit segments to and from the drive sub. When installing the bit segments, the cradle is extended from the lower end or head of the tool against the bias of a spring. Bit segments are held by rubber bands about the cradle with one end abutting a stop provided at one of the cradle and an opposite end bearing against the head of the tool. When the tool is lowered into the ground drill (comprising the combination of the drill tube, core barrel and drive sub) and reaches a predetermined position within the drive sub (that being the point of engagement with the bit locking sleeve) the sleeve is caused to move relative to the main body of the tool which in turn releases a set of pins holding the spring about the cradle in compression. The spring expands, retracting the cradle into the main body of the tool. This causes an upper end of the bit segment to slide along the head of the tool so as to extend laterally of the outer periphery of the tool. The bit locking sleeve is simultaneously pushed by the tool so as to catch the ends of and move inside the drill bit segments thereby expanding the drill bit segments to the inner diameter of the drive sub and locking the drill bit segments in the cutting position. 
     When lowering the tool into the ground drill the tool is initially placed within a transport sleeve which acts to compress the installation latch dogs to prevent catching on internal surfaces of the drill tube prior to entering a core barrel and the drive sub. A landing ring is provided between the core barrel and drill tube of a diameter which prevents further progress of the transport sleeve but allows the tool to pass therethrough. The transport sleeve sits on the landing ring and, after installation or retrieval of the cutting means again carries the tool once pulled from beneath the landing ring to the surface. 
     Field trials of the above system have proved very successful. Nevertheless, it is thought that there is a potential for various problems to arise under extreme operational conditions. 
     One such potential problem is the release of the cradle spring pins (when installing bit segments in the ground drill) prior to the bit locking sleeve being able to move behind the bit segments and lock them into the cutting position. 
     SUMMARY OF THE INVENTION 
     The present invention was developed with a view to substantially preventing the premature firing of the above tool. To this end, it is an object of the present invention to provide a locking system for a tool having a main body and a sleeve slidably mounted thereon, which can selectively lock the sleeve and main body against relative sliding motion. 
     According to the present invention there is provided a locking system for selectively locking a linearly slidable sleeve to a main body of a tool, said tool adapted to travel within a conduit, said locking system adapted to cooperate with en inner surface of said conduit and comprising: 
     first and second contiguous recesses formed in said main body, said first recess being of greater depth than said second recess; 
     an opening formed in or associated with said sleeve and locatable over said first or second recess; and 
     a locking member adapted for capture within said a first recess and to extend through said opening for abutment with said inner surface, said locking member being movable between a first position in which said locking member resides in said first recess and locks said sleeve against sliding relative to said main body, and a second position in which said locking member can escape to said second recess to allow said sleeve to slide relative to said main body, said locking member being movable from said first position to said second position in response to a variation in the internal diameter of said inner surface of said conduit. 
     In one embodiment said first recess is in the form of a through hole formed in said main body. Further said second recess is in the form of a channel spaced above the level of said hole. Preferably said channel is spaced above said hole by a discrete step. Preferably said locking system further comprises biasing means for lifting said locking member from said first recess to the level of said second recess when said tool passes through said portion of said conduit. 
     Preferably said biasing means comprises a spring adapted to expand into said hole. 
     Preferably said biasing means further comprises a cup-like member disposed in an inverted manner ini said hole so that one end of said spring is located within said cup-like member and said locking element sits on said cup-like member in said first recess. 
     In an alternate embodiment said first recess is in the form of a cavity of a shape complementary to a first portion of the exterior surface of said locking element. Further, said second recess is of a shape complementary to a second portion of the exterior surface of said locking element, said second portion being smaller in area than said first portion. Preferably said second recess is spaced above the level of said first recess. Preferably said locking system further comprises a sloping ramp extending from said first recess to said second recess whereby when said tool passes through said portion of said conduit, and said sleeve is retarded relative to said main body, said sleeve can push said locking element along said ramp to said second recess. 
     Preferably said locking system further comprises a replaceable wear ring fitted about a lower end of said opening and arranged to contact said locking member for pushing said locking member from said first recess to said second recess. 
     Preferably said locking member and said opening are relatively shaped and dimensioned so that said locking member cannot wholly pass through said opening. 
     Preferably said locking member comprises a ball bearing. 
     Preferably said locking member is one of a plurality of locking members disposed circumferentially about said tool. 
     According to another aspect of the present invention there is provided a tool for transporting a cutting means to and from a ground drill to enable in situ replacement of said cutting means, said ground drill defining a conduit provided with an inner surface with which said tool can operatively cooperate, said inner surface including a length in which the internal diameter thereof varies, said tool comprising: 
     a main body; 
     a sleeve slidably mounted on said main body portion; 
     a locking system in accordance with the first aspect of the present invention for selectively locking said sleeve against sliding motion relative to said main body. 
     Preferably said tool further comprises a cradle extending from an end of said tool for carrying said cutting means to said ground drill, said cradle being held by locking means against a first bias means to extend from said lower end of said tool, wherein said locking means can be released by sliding movement of said sleeve relative to said main body to fire said carrier means causing a retraction of said carrier means into said main body; and, 
     whereby in use, said cradle is prevented from firing when said locking member is in said first position, and can be fired when said locking member is in said second position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment 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 longitudinal side view of a system for in situ replacement of cutting means for a ground drill. 
     FIGS. 1a, 1b, 1c and 1d are longitudinal section side views taken on lines a--a, b--b, c--c and d--d on FIG. 1 of a system for in situ replacement of a cutting means for a ground drill in a state prior to the cutting means being engaged in the ground drill and including an embodiment of a locking system for a tool which transports the cutting means through the ground drill; 
     FIGS. 2a, 2b, 2c and 2d are sectional views of the system for in situ replacement of cutting means in the ground drill but with the longitudinal-section being in a plane rotated 90° to that of FIGS. 1a, 1b, 1c and 1d; 
     FIGS. 3a, 3b, 3c and 3d are longitudinal sectional side views of the system for in situ replacement of cutting means in a ground drill in the same plane as shown in FIGS. 1a, 1b, 1c and 1d but with the system in a second state where the cutting means are locked to the ground drill; 
     FIGS. 4a, 4b, 4c and 4d are views of the system shown in FIGS. 3a, 3b, 3c and 3d but in a sectional plane rotated 90° to that of FIGS. 3a, 3b, 3c and 3d; 
     FIG. 5 is a perspective view of a tool incorporated in the system for in situ replacement of cutting means in a ground drill and incorporating the locking system shown in FIGS. 1 to 4; 
     FIG. 6 is a longitudinal-sectional view of a drive sub incorporated in the system for in situ replacement of cutting means in a ground drill; 
     FIG. 7 is a longitudinal-sectional view of a bit locking sleeve of the system for in situ replacement of a cutting means shown in FIGS. 1-4; 
     FIG. 8 is a longitudinal-sectional view of the bit locking sleeve of FIG. 7 disposed within the drive sub of FIG. 6; 
     FIG. 9 is an enlarged longitudinal-sectional view of the locking system prior to the tool passing through a landing ring of the ground drill; 
     FIG. 10 illustrates the locking system of FIG. 9 after the tool passes through the landing ring; 
     FIG. 11 is a view of section E--E of the tool shown in FIG. 1; 
     FIG. 12A is a longitudinal section view of a part of the tool showing a second embodiment of the locking system when in a first state; and, 
     FIG. 12B is a view of the locking system shown in FIG. 12A when in a second state. 
     FIG. 13 is an isometric view of a push ring incorporated in the locking system shown in FIGS. 12A and 12B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, embodiments of the locking system in accordance with this invention will be described in relation to a complete system for the in situ replacement of cutting means for a ground drill. However, it is to be understood that the locking system is not limited only to use in a system for the in situ replacement of cutting means in a ground drill. 
     Referring to the accompanying drawings, and, in particular, to FIGS. 1-7, it can be seen that a system 10 for the in situ replacement of cutting means for a ground drill comprises a number of separate but interactive components including a drive sub 12 (refer in particular to FIG. 6) adapted for connection to a lower end of a core barrel 26 (shown in FIGS. 9 and 10); a installation and retrieval tool 14 (refer in particular to FIG. 5) which is dimensioned to travel through the ground drill for carrying cutting means in the form of drill bit segments 16 (refer in particular to FIGS. 2a, 3a and 4a) to and from the drive sub 12; and, a substantially cylindrical bit locking sleeve 18 (refer in particular to FIG. 7) which is slidably retained within the drive sub 12 between an installation position (shown in FIGS. 3a to 3d and 4a to 4d) in which the locking sleeve retains the bit segments 16 in a cutting position at the end of the drive sub 12 and, a retrieval position (shown in FIGS. 1a to 1d and 2a to 2d) in which the locking sleeve 18 is disposed above the end of drive sub 12 to allow the release of the bit segments 16. 
     Referring to FIG. 6, it can be seen that the drive sub 12 is composed of a lower section 20 and an upper section 22 which are threadingly coupled together. An upper end of section 22 is provided with a screw thread 24 for threadingly engage the core barrel 26. Moving in a downward direction from threaded end 24, it can be seen that inner circumferential surface 28 of the drive sub 12 is provided with a sequence of contiguous portions of differing diameter. Specifically, the inner circumferential surface 28 includes a first section 30 of a first diameter; a contiguous second section 32 of greater diameter; and a contiguous third section 34 of yet greater diameter. Step 35 is formed on the surface 28 at the boundary between the first section 30 and the second section 32. Section 34 extends to the end of the section 22 of the drive sub which, as previously mentioned, is threaded to lower section 20. Following the third portion 34 of the inner circumferential surface 28, is a fourth portion 36 of yet greater diameter which includes the screw thread for the section 20 of the drive sub 12 enabling connection with the section 22. Contiguous with a fourth portion 36 is a fifth portion 38 of smaller diameter than portion 36 but greater diameter than portion 34. Contiguous fifth portion 38 is contiguous with a stepped up (ie greater diameter) sixth portion 40. The inner surface 28 is next provided with a seventh portion 42 which is a step wise smaller diameter than the sixth portion 40. Contiguous with a seventh portion 42 is a tapered eighth portion 44 which progressively increases in diameter leading to ninth portion 46 which is of constant diameter and extends for a major length of section 20 and leads to a sequence of flat and tapered surfaces shown generally as item 48 which form part of a seat 50 for the bit segments 16. The seat 50 includes a circumferential land 49 for engaging the bit segments 16 and is completed by a series of circumferentially spaced drive lugs 52 provided about inner circumferential surface 28 at a lower most end of the drive sub 12. A series of circumferentially spaced apart splines 54 are bolted about the ninth portion 46 of the inner circumferential surface 28 of the drive sub 12. 
     The locking sleeve 18 (refer FIGS. 7 and 8) is in the form of a tube having a pair of peaks 56 (only one of which is shown) at an upper end 58. The peaks 56 are spaced apart and lead to a land 60 disposed therebetween. The outer surface of the upper most part of peaks 56 is tapered radially inwardly so that that portion of the peaks 56 is spaced from the inner circumferential surface 28 (refer FIG. 2c). A first circumferential recess 62 is formed about the outer surface of the locking sleeve 18 below the land 60. Spaced from the recess 62 is a second circumferential recess 64 again formed about the outer surface of the locking sleeve 18. A pair of opposing slots 66 are cut through the locking sleeve 18 and extend in the direction of the length of the locking sleeve 18. The slots 66 are located below the second recess 64. Lower end 68 of the locking sleeve 18 is provided about its outer surface with a series of splines 70 and recess 72 which engage the splines 54 of the drive sub 12 to guide the travel of the locking sleeve 18. More particularly, each spline 70 is disposed between adjacent splines 54 with each spline 54 able to ride within a corresponding recess 72. This arrangement allows the locking sleeve 18 to slide along the inner circumferential surface 28 but prevents rotation of the locking sleeve. 
     The tool 14 comprises a main body portion 74 and an outer sleeve 76 slidably mounted on the main body 74. An upper end of the main body 74 is threadingly connected via coupling 78 to a pivotal spear point 80. The spear point 80 facilitates coupling of the tool 14 to a running line (not shown). The main body 74 is itself composed of a first portion 82 and a second portion or head 84 which, as will be explained in greater detail below, are retractably coupled together. 
     Housed within a cavity 86 of the main body 74 is a latching mechanism 88 known as &#34;installation latch dogs&#34;. The installation latch dogs 88 essentially comprise a pair of arms 90 which are pivotally coupled together at one end by a pin 92 and biased by a spring 94 at an opposite end so as to extend from the outer surface 96 of the tool. Opposite ends of the pin 92 pass through respective slots 98 formed in the main body 74 and into diametrically opposed holes 100 formed in the sleeve 76. This provides a slidable connection between the sleeve 76 and main body 74 as, when sleeve 76 moves longitudinally relative to the main body 74, the pin 92 is able to slide within slots 98. Pin 92 is held in place by a snap ring 102 which is disposed within a circumferential recess 104 formed about the outer periphery of the sleeve 76. To assist in locating the snap ring 102 about the pin 92 opposite ends of the pin are also provided with grooves 106 within which the snap ring 102 can sit. Snap ring 102 is basically in the form of a metal wire ring which is resiliently expandable. 
     The end of the arms 90 which extend from the cavities 86 are provided with a planar latching face 108 for engaging the lands 60 of the locking sleeve 18. A central part of the spring 94 is wound about a stud 110 which resides wholly within the main body 74 and held at its opposite ends in diametrically opposed slots 112. 
     A second latching mechanism 114, known as &#34;retrieval latch dogs&#34; are also located within the cavity 86. The retrieval latch dogs 114 comprise a pair of arms 116 which are disposed in the same plane as arms 90 of the installation latch dogs but are orientated in the opposite direction. The arms 116 are pivotally coupled together at a lower end about a pin 118 which threadingly engages and is wholly disposed within the main body 74. An opposite end of each arm 116 is biased by spring 120 so as to move out of the cavity 86 toward contact with an inner surface of the locking sleeve 18. A central part of the spring 120 is wound about and retained by stud 122. Opposite ends of the stud 122 are held within diametrically opposes slots 124 formed in the main body 74. 
     The end of arm 116 opposite the pin 118 is provided with a latching face 125 for engaging respective slots 66 in the locking sleeve 18. Adjacent an end of the latching face 125 nearest the sleeve 76 is a bevelled face 126 which slopes away from the centre of the tool 74 in the direction toward pin 118. The bevelled face 126 then leads to a straight face 128 on the outer side of each arm 116 which in turn leads to a second bevelled face 130. 
     A releasable pin 132 is provided which can pass through both the arms 116 to lock the retrieval latch dogs 114 in a substantially compressed state so as to be disposed within the confines of the main body 74. Pin 132 is held in place by a snap ring 133. This pin is inserted when the tool 14 is used in an installation mode to install the bit segments 16 into the drive sub 12, and removed when the tool 14 is in a retrieval mode for retrieving the bit segments 16 from the drive sub 12. 
     The lower end of the first portion 82 of the main body 74 is formed with a tubular extension 134 which receives a spigot 136 extending from upper end of the second portion 84. A pin 138 extends transversely through the tubular extension 134 and resides within opposing slots 140 formed in the spigot 136 intermediate the length of the tubular extension 134. A pair of diametrically opposed holes 142 is formed in the tubular extension 134 for seating respective ball bearings 144. There is a stepped reduction in the internal diameter at the lower end of tubular extension 134 so as to form a cup-like structure 146. 
     A pair of diametrically opposed elongate slots 148 is formed in the spigot 136 below the holes 142. The slots 148 receive the ball bearings 144 but are of a width so as to allow only a portion of the ball bearings 144 to extend therethrough, preventing the ball bearings 144 from passing wholly therethrough. The elongation of slots 148 allows relative movement of the spigot 136 and tubular extension 134 to facilitate movement of the head 84 relative to the first portion 82 of the tool. 
     An upper portion 150 of the head 84 is of a substantially cylindrical shape but has peripheral longitudinal channels 152 (refer FIG. 5) provided along the side thereof for allowing the flow of liquid such as water and drilling mud. Adjacent the upper portion 150 is an intermediate portion 154 of constant but reduced diameter. Contiguous with the intermediate portion 154 is a bottom portion 156 of substantially frusto-conical shape which narrows in the downward direction. 
     A plurality of ramps 158 are disposed radially about the outer surface of the bottom portion 156 for seating an upper end 160 of the bit segments 16. Each ramp 158 is bound by opposing side walls 162 between which the upper ends 160 of the bits segments 16, lie. Longitudinal channels 164 are also formed centrally of each ramp 158 to allow the flow of water and drilling mud. Similarly, channels 166 are formed between adjacent side walls 162 of adjacent ramps 158 again to allow for the flow of water and drilling mud. 
     As spring 168 is disposed about the spigot 136 and has an upper end seated in the cup-like structure 146 and a lower end bearing against an upper face 170 of the upper portion 150 of the head 84. The spring 168 is biased so as to push the head 84 and first portion 82 of the tool apart in a longitudinal direction. 
     Lower end 172 of the sleeve 76 is also biased in a direction so as to contact the face 170 on the head 84. This bias is provided by a coil spring 174 disposed about an upper portion of the main body 74 between the coupling 78 and an upper end 176 of the sleeve 76. 
     Cradle 178 passes through an axial hole 180 formed in the head 84 so that an upper portion of the cradle 178 is disposed within the spigot 136. The purpose of the cradle 178 is to hold the bit segments 16 during transport to and from the drive sub 12 and, when installing the bit segments 16, to expand the upper end 160 of the bit segments radially outwardly so that they can be collected by the locking sleeve 18. 
     A coil spring 182 surrounds an upper end of the cradle 178 disposed within the spigot 136. The spring 182 is retained on the cradle 178 by a washer 184 fixed to the cradle 178 by a bolt 186. When the tool 14 is being used to install bit segments 16 into the drive sub 12 (as shown in FIGS. 1a to 1d and 2a to 2d) the cradle 178 is extended from the head 84 so as to compress the spring 182. Spring 182 is held in compression by the ball bearings 144 which engage an upper surface of the washer 184 through the longitudinal slots 148. 
     Disk-like flange 188 extending in a plane transverse to the axis of the tool 14 is attached by a nut 190 to the bottom end of the cradle 178. An upper face of the flange 188 acts as a bearing face for cutting face 192 formed at a lower end of the bit segments 16. The bit segments 16 are held circumferentially about the cradle 178 by three elastic bands 194 extending around the cradle 178 about the outer surfaces of the bit segments. 
     An upper end of the tool 14 is provided with a locking system 196 (refer FIG. 2) for selectively locking the sleeve 76 to the main body 74 preventing relative sliding motion. The locking system 196 includes a pair of diametrically opposed first recesses in the form of through holes 198 formed in the main body 74. The holes 198 are designed to capture locking members in the form of balls 200. Disposed within the main body 74 is a biasing system 202 designed to act on the balls 200 so as to force them radially outwardly. The biasing system 202 comprises a pair of cups 204 which are dimensioned so as to be able to slide within the recesses 198 and which between them retain a spring 206. The cups 204 and spring 206 are in turn disposed within a cylindrical casing 208 which extends transversely across cavity 86 in the main body 74 coaxially with the recesses 198. The casing 208 substantially seals the spring 206 from drilling fluids within which the tool 14 operates. 
     A second recess in the form of channel 210 extends from each hole 198 longitudinally along the outer surface of the main body 74. The channels 210 provide a race within which the balls 200 may travel when they are able to escape their respective holes 198. The channels 210 are spaced by a discrete step above the level of their corresponding holes 198. 
     The locking system 196 also includes a pair of diametrically opposed openings 212 of a diameter less then the maximum diameter of the balls 200 and formed at an upper end of the sleeve 76. The balls 200 are biased by the biasing system 202 so as to extend through the openings 212 and bear against the inner circumferential surface 28 of the drive sub 12. 
     A second embodiment of the locking system 196&#39; is shown in FIGS. 12A and 12B. In this embodiment the biasing system 202 of the first embodiment shown in FIG. 2 is not required. 
     In this embodiment the first and second recesses 198&#39; and 210&#39; are both in the form of cavities formed in the main body 74. Each recess 198&#39; and 210&#39; is of a shape complementary to the shape of a portion of the exterior surface of balls 200 with the recess 198&#39; being of larger surface area and deeper than recess 210&#39;. The recesses are joined by a sloping ramp 211. Balls 200 can be pushed up and along ramp 211 contingent on there being an increase in the internal diameter of the drive sub 12. 
     A replaceable wear ring 213 is fitted about lower end of opening 212 in the sleeve 76 for contacting the ball 200 in order to push it up ramp 211 into the second recess 210&#39;. 
     A push ring 215 is located about the main body 74 on the side of the balls 200 opposite ring 213. Push ring 215 is acted upon by the spring 174 to push against the balls 200 and bias them toward recesses 198&#39;. Edge 217 of the push ring 215 which abuts the balls 200 is tapered so as to extend over a portion of each ball 200 to prevent them from dislodging from the tool 14 when not in the drill pipe or drive sub. In effect, the rings 215 and 213 between them, form opening 212&#39;. The opening 212&#39; is associated with the sleeve 76 to the extent that the opening 212&#39; moves with the sleeve 76. 
     The balls 200 and recesses 198&#39; are dimensioned so as to allow passage of the tool 14 through the landing ring 232 (which is shown in FIGS. 9 and 10). If for some reason there is some movement of the sleeve 76 relative to the body 74, the balls 200 will be pushed up along ramps 211. 
     However, the distance of travel along ramps 211 will be limited by the balls 200 contacting the inner surface of the drill pipe above the landing ring 232 or the inner surface of the drive sub 12 above portion 32. That is, limited sliding motion of the sleeve 76 and main body 74 can occur but not to the extent to allow firing of the cradle 178. 
     In order to increase the contact area between edge 217 and balls 200, the edge 217 can be provided with an arcuate cut-out 219 for each ball (refer FIG. 13). This further assists in preventing the balls 200 from falling out when the tool 14 is being handled above the ground. 
     When the locking system 196&#39; is within portion 30 of drive sub 12, the ball 200 is held within recess 198&#39;. The sleeve 76 is prevented from sliding relative to the main body 74 by the ball 200 which cannot move from recess 198&#39; any substantial distance because there is simply no or very limited physical space available for it to ride up ramp 211, as shown in FIG. 12A. However, when the locking system 196&#39; is moved to portion 32 (refer FIG. 12B) of the drive sub 12 which is of increased internal diameter the sleeve 76 is able to be push ball 200 along ramp 211 to recess 210&#39;. 
     Moving in the downward direction from the openings 212, the sleeve 76 is provided with a pair of diametrically opposed longitudinally extending slots 214 through which the arms 90 of the installation latch dogs 88 can extend. The arms 90 are biased to extend through the slots 214 by the spring 94. 
     As best seen in FIG. 9, a compression system 216 is provided about the sleeve 76 and slots 214 for releasably retaining the installation latch dogs 88 within the confines of the outer surface of the tool 14. The compression system 216 includes a ring-like member in the form of a snap ring 218 which is adapted for location about the installation latch dogs 88. The snap ring 218 is able to be pushed or moved between two spaced apart grooves 220 and 222 (see FIGS. 9 and 1C) formed circumferentially about the outer surface of the sleeve 76 and across the slots 214. The groove 220 takes the form of a substantially U-shaped channel having a substantially upright bank 224 at a side nearest the groove 104 and an opposing sloping bank 226 which is inclined away from groove 104. 
     Groove 222 is also in the form of a channel having a sloping bank 228 on the side nearest and sloping toward groove 220. An opposite side of the groove 220 has an upright bank 230. The groove 220 is deeper than groove 222. Also, the groove 220 is disposed about a portion of slots 214 through which the arms 90 do not extend while, groove 222 is disposed about a part of the slots 214 through which the arms 90 can extend. 
     The compression system 216, and more particularly the snap ring 218 is adapted to cooperate with a substantially stepped surface provided inside the drill pipe. This stepped surface is provided by a conventional landing ring 232 which is screwed into the ground drill between the core barrel 26 and drill pipe 234. When the tool 14 is being lowered through the drill pipe to transport the bit segments 16 to the drive sub 12, the installation latch dogs 88 are initially held in a relatively compressed state by the snap ring 218 located within groove 222 to ensure that the tool can pass through the landing ring 232. As shown in FIG. 9, when the snap ring 218 is in groove 220, the latching faces 108 of the arms 90 are disposed within the outer surface of the tool 14 so that they cannot engage the landing ring 232. However, the snap ring 218 has an upper portion which sits proud of the outer surface of the tool 14 and is contacted by and temporarily held against the landing ring 232. Due to the momentum of the tool 14 it continues to move in a downward direction and the snap ring 218 is expanded radially outwardly against the sloping banks 228 as the tool continues its downward movement. When the snap ring 218 is knocked out of the groove 222, the arms 90 are able to expand from the slots 214 by action of the spring 94 (refer FIG. 10). With the tool continuing to move in the downward direction, the groove 220 eventually underlies the snap ring 218 and, due to the resilient expansion of the snap ring 218, it can then compress into the groove 220 as shown in FIG. 10. The groove 220 is of a depth such that when the snap ring 218 is located therein, it is able to pass through the landing ring 232. 
     A second pair of longitudinally extending slots 235 extending collinearly with and disposed below the slots 214 is provided in the sleeve 76 for allowing the retrieval latch dogs 114 to expand therethrough and contact the inner surface of the locking sleeve 18. An upper end of each slot 235 is provided with a bevel 236 formed between the radially inner and radially outer circumferential surfaces of the sleeve 76 which, when looking in the upward direction, slope in a mutually converging manner. 
     As will be explained in greater detail below, the combination of the slots 235 formed in the sleeve 76 and the spring 174 co-act to form a retraction system for retracting the retrieval latch dogs into the cavity 86 during extraction of the tool 14 after retrieving a set of bit segments 16 from the drive sub 12. 
     Below the slots 235 in the sleeve 76 is a pair of elongated holes 238 which allow access to the pin 138 for removal and installation. By removing the pin 138, the head 84 can be detached from the first portion 82 of the tool 14 for serving and maintenance. 
     A lower portion 240 of the sleeve 76 near the end 172 fits over the tubular extension 134 of the main body portion 82. An upper length 242 of the lower portion 240 has an internal diameter arranged so that when the upper length 242 is located over the holes 142, it pushes the ball bearings 144 through the underlying slots 148 so as to be able to contact the washer 184. However, a lower length 244 of the lower portion 240 has increased in the diameter so as to provide a gap 246 between the outer circumferential surface of tubular extension 134 and the inner circumferential surface of the lower length 244. As explained in greater detail below, when the sleeve 76 slides backwardly relative to the main body 74, the ball bearings 144 are able to move into the gaps 246 out of contact with the washer 184 to allow expansion of the spring 182 and subsequent retraction of the cradle 178 into the head 84. 
     FIG. 8 shows the locking sleeve 18 in an installation position. As previously mentioned the locking sleeve 18 can be moved between the retrieval position shown in FIGS. 1a to 1d and 2a to 2d and an installation position as shown in FIGS. 3a to 3d, 4a to 4d and 8, by the tool 14. As shown in FIG. 8 the locking sleeve 18 is held in the installation position by a snap ring 248 located in a void between the first recess 62 and the sixth portion 40 of the inner circumferential surface 28 of the drive sub 12. Snap ring 248 is always maintained within the sixth portion 40. When the locking sleeve 18 is pulled to the retrieval position by the tool 14, the snap ring 248 expands out of recess 62 and subsequently collapses into the second recess 64 holding the locking sleeve in this position until the tool 14 is again lowered to insert new bit segments 16, (as shown in FIGS. 1a to 1d and 2a to 2d). 
     A self centering system 249 for centering the tool 14 within the locking sleeve 18 as shown generally in FIG. 11. The self centering system is disposed circumferentially about the tool 14 in a transverse plane taken through upper portion 150 of the head 84. The self centering system is provided with a plurality, in this case four, centering elements in the form of ball bearings 250 equally spaced about the circumference of the tool 14. Each ball bearing 250 is seated in a corresponding cavity 252 formed about the periphery of the upper portion 150. The cavities 252 are closed by a threaded cap 254 which has a central opening through which a ball bearing 250 can extend. However, the diameter of the opening is less than the diameter of the ball bearing thereby preventing the ball bearing 250 from falling out of the cavity 252. Ball bearings 250 are resiliently retained within the cavities 252 by a pad of resilient material 256 disposed beneath each ball bearing so as to force the ball bearing radially outwardly. Due to the resilience of the pads 256, the ball bearings are able to move radially between a first position substantially flush with the outer surface of upper portion 150 and a second position tangential to an imaginary circle subscribed about the head 84 having a diameter greater than the inner diameter of the locking sleeve 18. The pads 256 are of a resilience such that when the tool 14 is within the sleeve 18 both lying in a horizontal plane, the pads can support the weight of the tool or at least the head of the tool to ensure substantial centering of the tool within the locking sleeve 18. 
     Although not shown, a substantially identical centering system can be provided about the midlength of the tool 14. In this instance, slots will be required along the sleeve 76 in order to provide for the required relative sliding motion of the sleeve 76 and main body 74 during the operation of the tool 14. 
     As explained in greater detail below, when the tool 14 is used to retrieve bit segments 16 it is necessary to lock the cradle 178 in an extended position. This is achieved by removing pin 132 from the retrieval latch dogs and inserting it through cradle locking hole 260 formed through the intermediate section 154 of the head 84. The cradle 178 is also provided with a hole 262 for alignment with the locking hole 260 through which the pin 132 can pass. Pin 132 is held in place by the snap ring 133 placed about the outer periphery of the intermediate section 154. 
     The operation of the system 10 will now be described. 
     When initially installing segments 16 in the drive sub 12, the balls 200 are located within the recesses 198 (198&#39;), the cradle 178 extended from the head 84 so that the spring 182 is compressed and locked in a compressed state by the abutment of the ball bearings 144 with the washer 184, and the bit segments 16 loaded on the cradle 178 and held in place by the rubber bands 194. The installation latch dogs 88 held in a relatively compressed state by the snap ring 218 being disposed within the groove 222 (as shown in FIG. 9). As the retrieval latch dogs 114 play no part in the installation of the bit segments 16, they are also locked in a relatively compressed state by pin 132 and corresponding snap ring 133. The locking sleeve 18 is held in the retrieval position by snap ring 248 residing in a void between the second recess 64 and the sixth portion 40 of the inner circumferential surface 28 of the drive sub 12. The tool 14 is lowered through the drill pipe by a wire line attached to the spear point 80. The balls 200 are held within the recesses 198 (or 198&#39;) against the inner circumferential surface of the drill pipe, thereby locking the sleeve 76 against substantial sliding relative to the main body 74, this prevents accidental or premature firing of the cradle 178. (The locking systems 196, 196&#39; may allow a small degree of relative sliding motion, but not enough to enable the cradle 178 to fire prematurely.) 
     Referring to FIGS. 9 and 10, as the tool 14 passes through the landing ring 232, the snap ring 218 held initially within the groove 222 is pushed along the sleeve 76 to snap back into the groove 220. When in this groove, the snap ring 218 radially compresses so as to pass through the landing ring 232. The balls 200 are also able to pass through the landing ring 232 by being compressed further into their recesses 198 against the bias of the spring 206. 
     Latching faces 108 of the installation latch dogs 88 contact the peaks 56 of the locking sleeve 18 causing the tool 14 to rotate about its longitudinal axis. This correctly orientates the bit segments 16 with the seat 50 and in particular drive lugs 52. As the tool continues to move downwardly, but prior to engagement of the latching faces 108 with the lands 60 of the locking sleeve 18, the balls 200 enter the second portion 32 of the inner circumferential surface 28 of the drive sub 12. The second portion 32 has a greater inner diameter than portion 30 immediately above it, and therefore by action of the bias applied by spring 206, the balls 200 are lifted out of their recesses 198 by the spring 206. Indeed, the spring 206 pushes the cups 204 to a position so that the surface thereof immediately below each ball 200 is substantially coplanar with the channel 210. At this point, the sleeve 76 and main body 74 are decoupled to the extend that the sleeve 76 is now able to slide relative to the main body 74. 
     The tool 74 then continues its downward travel until the latching faces 108 engage the lands 60 of the locking sleeve 18. This contact causes the main body 74 to continue to move forward relative to the sleeve 76 compressing the spring 174. If the second embodiment of the locking system as shown in FIGS. 12A-13 is used, the balls 200 are pushed up ramp 211 into recess 210&#39; against the bias of spring 174 by impact of the installation latch dogs 88 with the lands 60 of the locking sleeve 18. The ball bearings 144 move into the gap 246 between the lower length 244 of the sleeve 76 and the outside of the cup-like structure 146 of the portion 82 (refer FIGS. 3 and 4). The ball bearings 144 can now be pushed radially outwardly by the backward bias supplied to the washer 284 by the compressed spring 182. This frees the spring 182 to expand retracting the cradle 178 into the head 84. As a result, upper ends 160 of the bit segments 16 slide along the ramps 158 of the head 84 so as to extend laterally from the tool. The ends 160 are collected by the lower end of the locking sleeve 18 which moves behind the bit segments 16 and spreads the bit segments radially outwardly. The locking sleeve 18 moves in this manner by virtue of the continued downward movement of the tool 14 which by its latch dogs 88 engage the locking sleeve 18 pushing it downwardly. 
     While the tool 14 is in the locking sleeve 18, or at least the head 84 is in the sleeve 18, the self-centering system 249 maintains the tool 14 substantially centered in the sleeve 18, irrespective of the inclination of the drive sub or locking sleeve 18. 
     The bit segments 16 engage the seating land 49 preventing any further downward movement thereof. The head 84 of the tool is prevented from falling at the bottom of the drive sub 12 by virtue of abutment with a stop in the form of a radially inner surface of the bit segments 16. However, the first portion 82 of the main body 74 is still able to travel a short distance due to the nature of the coupling between the head 84 and the first portion 82. As seen most clearly in FIGS. 1b and 2b, a gap exists between the surface 170 and the end of the cup-like structure 146. The first portion 82 is able to continue moving in the downward direction by a distance equal to that gap. In effect, the head 84 retracts into the first portion 82. This retraction allows the tool 14 and in particular, the first portion 82 to push the locking sleeve 18 fully home onto a landing seat formed by the inner surfaces of the bit finger 16. 
     With the bit segments 16 now installed in the cutting position, the tool 14 can be pulled upwardly and retracted from the drive sub 12 and drill string. 
     In order to retrieve the segments 16 for replacement, the snap ring 133 and pin 132 which maintain the retrieval latch dogs 114 in a compressed state are removed. This allows the retrieval latch dogs 114 to move in an outward direction in compliance with the bias supplied by the spring 120. However, the pin 132 is now reinserted into the cradle locking hole 260 so as to lock the cradle 178 in a fully extended position. Of course, as it is now desired to retrieve the bit segments 16, no bit segments are initially located onto the cradle 178 when lowering the tool 14 into the drill pipe. The remaining configuration of the tool is the same as for when installing the bit segments 16. 
     As the tool is passed through the landing ring 232, the snap ring 218 is moved from groove 222 to groove 220 allowing the installation latch dogs to extend from the slots 214. Again, the installation latch dogs 88 contact the peaks 56 causing the tool 14 to rotate so as to correctly orientate the bit 84 and cradle 176 to receive the bit segments. Additionally, when the balls 200 enter the second portion 32 of the inner surface of the drive sub 12, they are moved out of their respective recesses 198 (or 198&#39;) and are able to then ride along the channels 210 (or into recesses 210&#39;) facilitating relative sliding motion of the sleeve 76 and main body 74. 
     When the tool 14 has bottomed out with the head 84 abutting the inner surfaces of the bit fingers 16, the retrieval latch dogs 114 extend through slots 235 in the sleeve 76 and into the slots 66 of the locking sleeve 18. In this configuration, the bevelled face 126 of each arm 116 also bears against the ninth portion 46 of the inner circumferential surface of the drive sub 12. 
     As the tool 14 is now pulled upwardly by a wire line attached to the spear point 80, the latching faces 125 engaged in the slots 66 pull the locking sleeve 18 upwardly thereby releasing the bit segments 16. The bit segments 16 collapse onto the cradle 18 by action of the rubber bands 194. 
     In order to now fully withdraw the tool 14 and bit segments 16, the retrieval latch dogs 114 must now be disengaged from the slots 66 of the locking sleeve 18. This is achieved by a retraction system which includes the inner surface 28 of the drive sub 12 as well as the slots 234 of the sleeve 76. In particular, as the tool 14 is being dragged upwardly, the bevelled faces 126 and flat faces 128 contact the sloping ninth portion 44 of the inner surface of the drive sub 12 which pushes the arms inwardly toward each other. At the same time, the spring 174 is pushing the sleeve 76 in a downward direction. The arms 116 are pushed inwardly by the sloping ninth portion 44 inner surface of the drive sub 12 to an extent such that the bevelled faces 126 can be bought into contact with the bevels 236 at the top of the slots 234. The force of the spring 174 and the relative configuration of the bevelled face 126 and bevels 236 pushes the sleeve 76 over the retrieval latch dogs disengaging them from the locking sleeve 18. 
     If for some reason the sleeve 76 cannot be pushed by the spring 174 alone over the retrieval latch dogs, upon continued upward pull on the tool 14, the balls 200 engage the step 35 at the boundary between the first and second surface portions 30 and 32 of the drive sub 12 and maintain the sleeve 76 in a static position while rolling along channels 214. Accordingly, the force of the pull on the tool 14 is transmitted to the sleeve 76 to push it over the retrieval latch dogs 114. The balls 200 then collapse into their recesses 198 (or 198&#39;) compressing the spring 206 so as to allow full retraction of the tool 14. 
     The tool can then be withdrawn from the drill string, the bit segments 16 taken off the cradle and a fresh set of drill bits 16 loaded on to the cradle for installation into the drive sub. 
     Now that embodiments of the locking system 196 and 196 have been described in detail it will be apparent to those skilled in the relevant arts that numerous modifications and variations may be made without departing from the basic inventive concepts. For example, while the locking elements used in this embodiment may be shown as being balls 200, they may take the form of other elements which have opposite bearing faces to allow sliding motion along the circumferential surface 28 of the drive sub and along the channels 210. Also, the biasing system 202 may include a biasing element other than a spring, such as a pad of resilient material. Further, any number of balls 200 can be disposed about the circumference of the tool 14 
     All such modifications and variations are deemed to be within the scope of the present invention the nature of which is to be determined from the foregoing description and the appended claims.