Abstract:
A fluid flow actuated downhole tool is configurable in at least a first tool configuration and a second tool configuration. The tool comprises a tubular housing and an activating sleeve, the housing being adapted to catch the sleeve when the sleeve is dropped from surface and the engagement of the sleeve with the housing permitting actuation of the tool between the first and second tool configurations. A flow restriction is provided for permitting fluid flow actuation of the tool when the activating sleeve has been caught in the body.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/031,219 filed Jan. 15, 2002, which is a 371 of PCT/GB00/02712 filed Jul. 14, 2000, which claims priority of United Kingdom Patent Application 9916513.6 filed Jul. 15, 1999. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a downhole tool which is actuatable between at least two tool configurations. In particular, but not exclusively, the present invention relates to a downhole tool comprising a bypass tool for location in a borehole of a well, wherein the bypass tool is actuatable between a closed configuration and an open configuration in response to the flow of fluid through the borehole.  
       BACKGROUND OF THE INVENTION  
       [0003]     Bypass tools are typically disposed within a borehole of, for example, an oil well, for selectively allowing fluid communication between a bore defined by a tubular string disposed in the borehole, and an annulus defined between an outer wall of the tubing string and an inner wall of the borehole. Typical known assemblies are often complex, comprising many interconnected components, and often require, for example, multiple fluid pressure cycles of fluid in the borehole to actuate the bypass tool between two or more distinct tool configurations.  
         [0004]     It is amongst the objects of the present invention to obviate or mitigate at least one of the foregoing disadvantages.  
       SUMMARY OF THE INVENTION  
       [0005]     According to the present invention there is provided a fluid flow actuated downhole tool being configurable in at least a first tool configuration and a second tool configuration, the tool comprising: 
        a tubular housing;     an activating sleeve, the housing being adapted to catch the sleeve when dropped from surface and then permitting actuation of the tool between the first and second tool configurations; and     flow restriction means for permitting fluid flow actuation of the tool when the activating sleeve has been caught in the body.        
 
         [0009]     The invention also relates to a method of operating a fluid flow actuated tool, the method comprising: 
        running the tool into a borehole in a tubular string;     circulating fluid through the string and the tool;     passing an activating sleeve into the string;     catching the sleeve in the tool; and     circulating fluid through the string, the sleeve and a flow restriction in the tool to actuate the tool.        
 
         [0015]     Thus, prior to the sleeve being caught in the tool, the tool is “dormant”, and may only be actuated after the sleeve is received in the tool.  
         [0016]     As noted above the sleeve is simply dropped into the string and is allowed to fall through the string, or may in addition also be carried into the string by circulating fluid.  
         [0017]     Unlike a ball or other flow occluding tool activating member, which will substantially occlude the string bore, the use of a tool activating sleeve allows fluid to continue to flow through the string and tool, and may permit access to the section of the bore below the tool. Also, the use of a sleeve allows fluid to be circulated while the sleeve is moving down through the string; unlike a ball or other flow-occluding device, the sleeve will not induce a large hydraulic shock on engaging the tool.  
         [0018]     The sleeve may define a flow restriction, such as a nozzle, which flow restriction permits or facilitates fluid actuation of the tool. Alternatively, the restriction may be defined by another part of the tool, which part is fixed before the sleeve is caught in the tool. Two or more axially spaced flow restrictions may be provided, allowing creation of a greater fluid pressure force without a significant restriction in bore diameter.  
         [0019]     The tool may be a bypass tool, preferably the tool being initially closed, and after the sleeve is caught in the tool the tool may be re-configured to permit flow between the tool bore and the surrounding annulus.  
         [0020]     Preferably, following activation of the tool by the sleeve, the tool may be repeatedly actuated between the first and second configurations.  
         [0021]     A further aspect of the invention relates to a method of operating a fluid flow actuated tool, the method comprising: 
        (a) running the tool into a borehole in or as a part of a tubular string;     (b) circulating fluid through the string and tool;     (c) passing an activating device into the tool;     (d) catching the device in the tool;     (e) circulating fluid through the string and the tool including the device, to actuate the tool; and     (f) repeating step (e) at least once.        
 
         [0028]     Preferably, the activating device is a sleeve, which may define a restriction or nozzle, incorporate a rupture disc, or contain an extrudable or soluble material.  
         [0029]     The activation for the tool may be achieved by releasing a coupling to permit relative movement of parts of the tool, which coupling may be, for example, a shear coupling or a sprung coupling.  
         [0030]     Another aspect of the invention relates to a method of actuating a downhole tool, the method comprising: 
        running a tool into a borehole in a tubular string;     circulating fluid through the string and tool;     locating an activating device in the string; and     circulating fluid through the string and tool as the device travels down through the string, as the device engages the tool, and following engagement of the device and the tool.        
 
         [0035]     This method is particularly useful in drilling or circulating operations, as there is no requirement to stop fluid circulation as the device moves through the string and then engages the tool, such that drilling or circulation may continue with the device in the string with a fluid flow rate sufficient to entrain drill cutting and carry them to surface, or to allow continuation of some other fluid circulation-related activity. This contrasts with conventional methods, in which it is necessary to stop or at least substantially reduce circulation to prevent the occurrence of a hydraulic shock on the activating device, typically in the form of a steel ball, engaging the tool. Such a hydraulic shock would result in damage to the ball and tool, and possibly also to the string itself.  
         [0036]     The activating device may be a sleeve, such that the device restricts fluid flow to a limited extent but does not occlude the string bore.  
         [0037]     A still further aspect of the present invention provides a downhole tool for disposition in a borehole of a well, the tool being configurable in at least a first and a second tool configuration, the tool comprising: 
        a tubular housing for running into a borehole on a tubing string;     a tubular sleeve assembly for disposition within the tubular housing and axially movable therein and including fluid responsive means for actuating the tool between said first and second tool configurations; and     means for maintaining said sleeve assembly in a selected one of said first and second tool configurations.        
 
         [0041]     Thus the present invention allows a downhole tool to be disposed in a borehole, which tool may be actuated between two or more tool configurations by supplying fluid to the tool in the borehole and by varying the flow rate of the fluid through the tool.  
         [0042]     Preferably, the downhole tool is a bypass tool. The bypass tool may be in a closed configuration in the first tool configuration and an open configuration in the second tool configuration. The tubular housing may form part of a liner, casing, or drill string or any other tubing string for disposition in the borehole.  
         [0043]     The tubular housing of the bypass tool may comprise at least one bypass port extending through a wall of the housing. The at least one bypass port may extend radially through the wall of the housing. The sleeve assembly may be axially movable to selectively move to the open configuration, to allow fluid communication between the housing interior wall, and an annulus defined by an outer face of the housing wall and the borehole wall.  
         [0044]     The fluid responsive means may include a flow restriction, such that flow of fluid induces a pressure differential, and therefore a fluid pressure force, across the restriction. Alternatively, said means may define a differential piston with, for example, one piston face experiencing internal housing pressure and another face experiencing annulus pressure, such that an increase in internal pressure will actuate the tool.  
         [0045]     The tubular sleeve assembly may comprise a control sleeve and a flow restriction within the control sleeve for restricting the flow of fluid through the control sleeve. Preferably, the restriction is defined by an insert which may be dropped or lowered from the surface into the tubing string and may travel through the string and engage the control sleeve. Fluid flow through the flow restriction creates a force acting axially across the flow restriction, and thus on the control sleeve, urging the sleeve assembly to move axially. Alternatively, the flow restriction may be integral with the control sleeve. The flow restriction may comprise an annular, radially inwardly extending ring defining a nozzle.  
         [0046]     The maintaining means may comprise a releasable connection, such as one or more sprung dogs, keys or a shear connection, such as one or more shear pins, for engaging the control sleeve and maintaining it in a selected one of said first and second tool configurations.  
         [0047]     The bypass tool may further comprise a flow restriction-engaging insert, such as a nozzle, dart, sleeve or ball, for engaging the flow restriction, although as noted above in other embodiments the insert may itself provide the flow restriction. Thus, in response to pressurization of the fluid in the tubing string above the insert, a pressure force acting across the insert may be caused to urge the tubular sleeve assembly axially downwardly to release the connection, and in addition or alternatively to actuate the tool. The flow restriction engaging insert may be injected into the tubing string at the surface and may travel through the string bore to engage the flow restriction. When the insert is a ball, preferably the ball is deformable to allow the ball to be forced through the flow restriction in response to an increase in the pressure of the fluid in the tubing string above the ball.  
         [0048]     In an alternative arrangement, the tubular insert may be adapted to release the connection on engaging the control sleeve.  
         [0049]     Preferably, the downhole tool further comprises indexing means for selectively allowing actuation of the tool between said first and second tool configurations. The indexing means may comprise a cam arrangement such as a groove, slot or other profile extending around an outer circumference of the tubular sleeve assembly, and a cam follower such as a pin extending radially inwardly from an inner surface of the housing for engaging the groove. Of course, in alternative arrangements the groove or the like may be defined by the housing, and the pin or the like mounted on the sleeve assembly. In still further arrangements, the indexing means may be provided between different parts of the sleeve assembly. The pin and groove may cooperate to rotate the tubular sleeve assembly, or at least a part of the assembly, when it is moved axially. Conveniently, the groove defines first and second axial pin rest positions. Preferably, the groove defines a plurality of first and second axial pin rest positions. The first axial pin rest position may correspond to a valve open configuration and the second axial pin rest position may correspond to a valve closed configuration. The groove may further define a plurality of third axial pin rest positions for allowing actuation of the tool to an intermediate configuration between said first and second tool configurations, and which intermediate position may provide a further tool function, or may correspond to the function provided by one of the first or second tool configurations. The third axial pin rest positions may be provided between second axial pin rest positions. Thus the groove and pin may allow the tool to be disposed in the intermediate configuration alternatively when the pressure in the borehole is increased.  
         [0050]     The maintaining means may further or alternatively comprise a spring for applying a force upon the sleeve assembly. The spring may be a fluid spring or a compression or tension spring. Preferably, the spring is disposed in an annular cavity between the housing and the sleeve assembly, to impart an upward force upon the sleeve assembly, to maintain it in a closed configuration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0052]      FIG. 1A  is a longitudinal cross-sectional view of a downhole tool in accordance with an embodiment of the present invention;  
         [0053]      FIG. 1B  is a schematic illustration of a pin and groove arrangement forming part of the downhole tool of  FIG. 1A ;  
         [0054]      FIG. 2  is a longitudinal cross-sectional view of a downhole tool in accordance with an alternative embodiment of the present invention;  
         [0055]      FIG. 3  is a longitudinal cross-sectional view of a downhole tool in accordance with a further embodiment of the present invention;  
         [0056]      FIG. 4A  is a longitudinal sectional view of a downhole tool in accordance with another embodiment of the present invention;  
         [0057]      FIG. 4B  is a schematic illustration of a pin and groove arrangement forming part of the tool of  FIG. 4A ;  
         [0058]      FIG. 5  is an enlarged view of part of the tool of  FIG. 4A ; and  
         [0059]      FIG. 6  is a further enlarged sectional view on line  6 - 6  of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]     Referring firstly to  FIG. 1 , there is shown a longitudinal cross-sectional view of a downhole tool in accordance with an embodiment of the present invention, the downhole tool indicated generally by reference numeral  10 . The downhole tool  10  forms part of a drill string (not shown) run into a borehole (not shown) of an oil well, and is coupled at its upper and lower ends to sequential sections of drill string tubing via threaded joints, in a fashion known in the art.  
         [0061]     The downhole tool  10  shown in  FIG. 1A  is a bypass tool comprising a tubular outer housing  12 , a tubular bypass sleeve  14 , a tubular flow restriction insert  16 , a bypass sleeve spring  18  and a pin and groove assembly indicated generally by reference numeral  19 .  
         [0062]     Those of skill in the art will understand that the tool  10  will be provided with a variety of appropriate seals, however in the interest of brevity the individual seals will not be identified and described.  
         [0063]     The tubular outer housing  12  includes flow ports  20  extending radially through a wall  22  of the housing  12 , and spaced circumferentially around the housing  12 . For clarity, only two such ports  20  are shown in  FIG. 1A , however it will be appreciated that any suitable number of such flow ports  20  may be provided in the housing  12 . The housing  12  has an inner face  24  and the internal diameter of the housing  12  defined by the inner face  24  varies along the length of the housing  12  from top to bottom. In particular, an upper portion  26  of the housing  12  is of a first general internal diameter, whilst a lower portion  28  of the housing  12  is of a smaller, second general internal diameter. This enables the housing  12 , in conjunction with the tubular bypass sleeve  14 , to define an annular cavity  30  in which the bypass sleeve spring  18  is located, as will be described in more detail below.  
         [0064]     The tubular bypass sleeve  14  includes flow ports  32 , and is axially movable within the housing  12 , to enable the flow ports  20  of the housing  12  and the flow ports  32  of the sleeve  14  to be aligned. This allows communication between an internal tool bore  34  and an annulus defined between an outer face  36  of the housing  12  and the borehole wall.  
         [0065]     The bypass sleeve spring  18  is a compression spring and is disposed in the cavity  30  between a washer  38  and a radially outwardly extending shoulder  40  of the bypass sleeve  14 . In the position shown in  FIG. 1A , the bypass sleeve spring  18  maintains the bypass sleeve  14  in a closed configuration wherein an upper end  42  of the bypass sleeve  14  is disposed adjacent to the upper end of the housing  12 .  
         [0066]     When it is desired to move the bypass sleeve  14  axially downwardly against the force of the bypass sleeve spring  18 , to align the flow ports  20  and  32 , the tubular flow restriction insert  16  is inserted into the drill string at the surface and carried down the internal string bore  34  until it engages the bypass sleeve  14  as shown in  FIG. 1A . The flow restriction insert  16  includes annular, radially inwardly extending shoulders  43  and  45 , which define first and second restrictions respectively. These restrictions to the flow of fluid through the internal bore  34  are such that, when fluid flows through the flow restriction insert  16 , a pressure differential is created across each restriction and a downward axial force is imparted upon the flow restriction insert  16  by the flowing fluid. Until the insert  16  is located in the sleeve  14 , the tool  10  is effectively dominant, as changes in fluid flow rate or pressure in the bore  34  will have no effect on the sleeve position.  
         [0067]     The flow rate of the fluid through the string and tool is increased until the force upon the flow restriction insert  16  becomes sufficiently large to overcome the force imparted upon the bypass sleeve  14  by the bypass sleeve spring  18 . The flow restriction insert  16  and the bypass sleeve  14  then move axially downwardly, compressing the spring  18  until the bypass sleeve  14  reaches the end of its travel, wherein a lower end  44  is disposed adjacent to the lower end of the housing  12 . The flow ports  20  and  32  are then aligned, allowing fluid communication between the internal bore  34  and the annulus bore. This may allow operations such as a “clean-up” operation to be carried out, wherein drill cuttings or the like lying in sections of the borehole may be entrained with and carried back to the surface by the fluid flowing through the aligned bypass ports  32  and  20 .  
         [0068]     When it is desired to move the bypass sleeve  14  back to the closed configuration shown in  FIG. 1A , the flow rate of the fluid flowing through the internal bore  34  is reduced, until the fluid pressure force applied by the fluid upon the bypass sleeve  14  and the flow restriction insert  16  drops below the force imparted upon the bypass sleeve  14  by the spring  18 . The bypass sleeve  14  is then moved axially upwardly by the spring  18  acting against the shoulder  40  of the bypass sleeve  14 .  
         [0069]     Referring now to  FIG. 1B , there is shown a schematic illustration of the pin and groove arrangement  19  shown in  FIG. 1A . The arrangement  19  includes an annular circumferential extending groove  46  and a pin  48 , though for clarity the illustrated portion of the groove  46  is shown as a planar groove. The groove  46  is notched or corrugated and defines a number of first pin rest positions  50   a  and  50   b , a number of second pin rest positions  52 , and a number of third pin rest positions  54 . The second and third pin rest positions  52  and  54  are spaced alternately around the circumference of the bypass sleeve  14 . The pin  48  is shown in  FIG. 1B  in one of the first pin rest positions  50   a  where the bypass sleeve  14  is in the closed configuration of  FIG. 1A .  
         [0070]     When the flow restriction insert  16  has been located in the bypass sleeve  14 , and the flow rate of fluid through the internal bore  34  has been increased to counteract the force of the bypass sleeve spring  18 , the bypass sleeve  14  moves axially downwardly until the pin  48  engages the sloping face  56  of the groove  46 , which rotates the bypass sleeve  14 . The pin  48  then becomes engaged in a slot  58  and comes to rest in a second pin rest position  52 , where the bypass sleeve  14  is in the open configuration with the flow ports  20  and  32  aligned. When the flow rate of the fluid is reduced, the bypass sleeve spring  18  carries the bypass sleeve  14  axially upwardly, and the pin  48  moves over the surface of a sloping face  60  of the groove  46 , rotating the sleeve  14 , to one of the first pin rest positions  50   b.    
         [0071]     When the flow rate is again increased, the bypass sleeve  14  again moves axially downwardly. However, movement of the sleeve  14  is stayed when the pin  48  comes to rest in the third pin rest position  54 . Retention of the pin  48  in the third pin rest position  54  prevents the flow ports  20  and  32  from becoming aligned. This may be useful when, for example, it is desired to drill with drilling fluid flowing of an elevated rate but without opening the tool  10 . When the fluid flow rate is next reduced, the pin  48  comes to rest in a first pin rest position  50   a , whereupon subsequent increase of the fluid flow rate allows the bypass sleeve  14  to move fully axially downwardly, with the pin  48  engaged in the second pin rest position  52 . Thus alternate opening of the bypass sleeve  14  may be achieved.  
         [0072]     Referring now to  FIG. 2 , there is shown a longitudinal cross-sectional view of a downhole tool in accordance with an alternative embodiment of the present invention, indicated generally by reference numeral  110 . For ease of reference, like components with the downhole tool  10  of  FIG. 1A  share the same reference numerals incremented by 100. The downhole tool  110  comprises a tubular outer housing  112 , a tubular bypass sleeve  114 , a bypass sleeve spring  118  and a pin and groove arrangement  119 . Flow ports  120  extend through a wall  122  of the housing  112 , and the bypass sleeve  114  includes flow ports  132  which may be aligned with the flow ports  120  of the housing  112 , when the bypass sleeve  114  is moved axially downwardly, in a similar fashion to the bypass sleeve  14  of the downhole tool  10  of  FIG. 1A .  
         [0073]     The bypass sleeve spring  118  is disposed in an annular cavity  130  between a washer  138  and a shoulder  140  of the bypass sleeve  114 . However, the housing  112  includes shear pins  162  disposed in the wall  122 , which extend radially inwardly to engage the bypass sleeve  114 . These shear pins  162  initially maintain the bypass sleeve  114  in a closed configuration as shown in  FIG. 2 . Furthermore, the bypass sleeve  114  includes an annular, radially inwardly extending shoulder  164  which defines a flow restriction.  
         [0074]     When it is desired to move the bypass sleeve  114  to the open configuration, where the flow ports  120  and  132  are aligned, a deformable ball  166  is inserted into the string bore and travels down to the tool  110  through the string bore  134 . The ball  166  is carried in a fluid such as drilling mud through the internal bore  134 , and engages in the shoulder  164  of the bypass sleeve  114 . This effectively blocks the internal bore  134 . When the pressure of the fluid in the internal bore  134  above the tool  110  is increased, which may occur instantaneously on the ball  166  engaging the restriction  164 , this creates a considerable pressure force acting axially downwardly upon the ball  166  and thus upon the bypass sleeve  114 , which compresses the spring  118  and shears the pins  162 . This moves the bypass sleeve  114  to the open configuration.  
         [0075]     However, the internal bore  132  remains blocked by the ball  166 . A further increase of the pressure of the fluid above the ball  166 , or indeed a continuation of the hydraulic shock which created the initial force to shear the pins  162 , causes the ball  166  to deform, elastically or plastically, and to pass through the restriction created by the shoulder  164  of the bypass sleeve  114 , allowing fluid to flow through the bypass tool  110 , through the flow ports  132  and  120 , and into the annulus bore. A ball catcher may be provided (not shown) disposed in the part of the drill string tubing below the tool  110 , to catch the ball  166  when it has passed through the bypass sleeve  114 , or alternatively the ball may disintegrate or otherwise degrade.  
         [0076]     The pin and groove arrangement  119  includes a groove  146  and a pin  148  and functions in a similar manner to the pin and groove arrangement  19  shown in  FIG. 1B  and described above. This therefore allows subsequent opening and closing of the bypass sleeve  114  in response to variations in the fluid flow rate acting on the flow restriction  164 .  
         [0077]     Referring now to  FIG. 3 , there is shown a downhole tool in accordance with a further embodiment of the present invention, indicated generally by reference numeral  210 . For clarity, like components of the tool  210  with the tool  10  of  FIG. 1A  share the same reference numerals incremented by 200.  
         [0078]     The downhole tool  210  comprises a tubular outer housing  212 , a tubular bypass sleeve  214 , a bypass sleeve spring  218 , a pin and groove arrangement  219  and a tubular release sleeve  268 . The housing  212  includes flow ports  220  disposed in a wall  222  of the housing  212  and extending radially therethrough.  
         [0079]     The tubular bypass sleeve  214  includes flow ports  232  and is mounted in the housing  212  to define an annular cavity  230 , in which the spring  218  is disposed, between a washer  238  and a shoulder  240  of the housing  212 . Elastomeric O-ring type seals  270  and  272  respectively are provided in the wall  222  of the housing  212 , to seal the annular cavity  230  and isolate it from fluid in the internal tool bore  234 . Also, bleed holes  274  extend through the wall  222  of the housing  212 , to fluidly couple the annular cavity  230  with the annulus of the borehole in which the tool  210  is disposed. Thus fluid in the annular cavity  230  experiences the same pressure as fluid in the annulus.  
         [0080]     The bypass sleeve  214  includes openings  276  at its upper end  242 , for engaging spring-loaded locking dogs  278 , to retain the sleeve  214  in the closed configuration shown in  FIG. 3 , whereby the flow ports  220  and  232  are misaligned. This prevents fluid communication between the internal bore  234  and the annulus bore. As shown in  FIG. 3 , the leading end  280  of each locking dog  278  is chamfered. This allows the release sleeve  268  to be run into the borehole and located within the bypass sleeve  214  as shown in  FIG. 3 , wherein a radially outwardly extending shoulder  282  of the sleeve  268  engages the leading end  280  of each locking dog  278 . This compresses a spring  284  of each locking dog  278 , forcing each locking dog  278  radially outwardly such that only the chamfered leading end  280  protrudes into the apertures  276 .  
         [0081]     To actuate the tool  210  to an open configuration, the pressure of fluid flowing through the internal bore  234  is increased such that the differential pressure between the fluid in the internal bore  234  and the fluid in the annulus bore increases. As the seal  270  defines a larger diameter than the seal  272 , a net axially downward force is imparted upon the bypass sleeve  214  due to this differential pressure. This causes the actuating sleeve  268  and the bypass sleeve  214  to move axially downwardly. The locking dogs  278  are disengaged from the engaging apertures  276  of the bypass sleeve  214  by the bypass sleeve  214  passing over the chamfered leading end  280  of each locking dog  278 . This allows the flow ports  220  and  232  to be aligned, allowing fluid communication between the internal tool bore  234  and the annulus. When the pressure of the fluid in the internal bore  234  is reduced sufficiently such that the net force upon the bypass sleeve  214  falls below the restoring force of the spring  218 , the spring  218  returns the bypass sleeve  214  to the closed configuration shown in  FIG. 3 , by acting against the shoulder  240  of the housing  212 .  
         [0082]     The pin and groove arrangement  219  comprises a groove  246  and a pin  248  similar to the groove  46  and pin  48  of  FIG. 1B  and the tool  10  of  FIG. 1A . When the bypass sleeve  214  returns to the closed configuration of  FIG. 3 , the locking dogs  278  again engage the engaging holes  276  of the bypass sleeve  214  to retain the sleeve in the closed configuration, until the pressure of the fluid in the internal bore  234  is increased sufficiently to counteract the spring force  218  and force the locking dogs  278  radially outwardly.  
         [0083]     Reference is now made to  FIG. 4A  of the drawings, which illustrates a bypass tool  310  in accordance with another embodiment of the invention. The tool  310  is similar in some respects to the tool  210  or  FIG. 3 , and therefore common features of the tools  210 ,  310  will not be described again in any detail.  
         [0084]     The tool  310  comprises a housing  312 , a two-part bypass sleeve  314 , a flow restriction sleeve  316 , a pair of sleeve springs  318   a ,  318   b , and a sleeve movement controlling pin and groove assembly  319 .  
         [0085]     Unlike the previous illustrated tools, the tool  310  is illustrated in a configuration in which the tool  310  is experiencing elevated fluid flow therethrough, but the sleeve movement controlling assembly  319  has not transmitted the corresponding axial movement of the restriction sleeve  316  and the associated part of the bypass sleeve  314   a  to the other part of the sleeve  314   b  defining the flow ports  312 , as will be described below.  
         [0086]     The tool  310  is initially run in without the restriction sleeve  316 . As noted above, the bypass sleeve  314  is in two parts  314   a ,  314   b , coupled by the pin and groove arrangement  319 , the form of which is illustrated in  FIG. 4B  of the drawings. The upper sleeve part  314   a , which defines the groove  346 , is initially locked to the housing  312  by an arrangement of sprung dogs  378 , as illustrated in  FIG. 6  of the drawings. The dogs  378  are mounted in the sleeve  314   a  and are biased radially outwardly to engage recesses  376  in a sleeve  386  located on the housing  312  between a circlip  388  and a housing shoulder  390 . Four circumferentially spaced dogs are provided, and are adapted to be retracted by the radial movements of respective release pins  392  coupled to the dogs  378  by rocker arms  394 . In this position, the springs  318   a ,  318   b  which act on the respective sleeve parts  314   a ,  314   b , to urge the sleeve parts towards the closed position, are fully extended.  
         [0087]     In this initial configuration, the tool  310  is effectively dormant, and variations in fluid flow or pressure differentials will have no effect on the tool configuration. This allows the tool  310  to be effectively “ignored”, until the tool  310  is required. This is useful as it allows, for example, drilling operators to vary drilling mud flow rate and pressure, and to switch mud pumps on and off without any concern for the tool configuration.  
         [0088]     When it is desired to utilize the tool  310 , the sleeve  316  is placed in the drill string, and will be carried to the tool  310  in the drilling fluid. The presence of restrictions  343 ,  345  in the sleeve  316  facilitates the sleeve  316  being carried by the flow, however the relatively minor flow restriction created by the free-falling sleeve  316  allows the drilling operators to maintain drilling fluid flow at the normal drilling rate, such that drilling is not interrupted by the passage of the sleeve  316  through the string to the tool  310 .  
         [0089]     On reaching the tool location, the sleeve  316  engages the upper part of the bypass sleeve  314   a , and in doing so pushes the release pins  392  outwardly to disengage the sleeve  314   a  from the housing  312 . The engagement of the restriction sleeve  316  with the bypass sleeve  314   a  creates a restriction in the fluid pathway through the string, but not to the extent that a significant hydraulic shock is induced.  
         [0090]     Flow through the restrictions  343 ,  345  creates a differential pressure force across the sleeve  316  and, if the force is sufficient, the upper by-pass sleeve  314   a  will move downwards, compressing the spring  318   a . Further, depending on the position of the pin  348  in the groove  346 , the pressure force will be transferred to the lower bypass sleeve  314   b . If sufficient force is created, the sleeve  314   b  may be moved downwards, compressing the spring  318   b , and aligning the ports  332 ,  320 .  
         [0091]     By varying the drilling fluid flow rate through the tool  310 , it is thus possible to cycle the position of the sleeve parts  314   a ,  314   b , to selectively open or close the ports  332 ,  320 .  
         [0092]     If there comes a point in the drilling operation where the tool  310  is no longer required, the sleeve  316  may be retrieved by wireline or the like and using a fishing tool adapted to engage a profile  390  in the upper end of the sleeve  316 .  
         [0093]     Various modifications may be made to the foregoing embodiments within the scope of the present invention. For example, the downhole tool may be any tool capable of being actuated between first and second tool configurations.