Abstract:
A downhole apparatus such as multi-cycle circulating subs used during downhole drilling operations includes a piston ( 42 ) slidably mounted in a body ( 4 ) between positions in which at least one aperture ( 40 ) in the body is opened and closed. Movement of the piston ( 42 ) is controlled by a pin ( 86 ) secured to one of the body and a control member, and a control groove ( 52 ) in which a portion of the pin is received formed in the other of the body and control member. An arrangement of elements ( 32,76 ) respectively connected to the control member and body is such that, as the control member moves axially, increasing lengths of the elements locate adjacent one another so as to provide resistance to relative rotation in at least one direction of the control member and body. The relative rotation is rotation which presses the control member against the control groove. The risk of damage to the control pin is thereby reduced.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to downhole apparatus and particularly, but not exclusively, to multi-cycle circulating subs used during downhole drilling operations. 
   2. The Prior Art 
   It is often necessary in downhole drilling operations to bleed the flow of wellbore fluid down the drill string into the wellbore annulus. For example, this may be necessary where the desired fluid flow rate to drive a drilling tool is insufficient to carry all the drilled material up the annulus to the surface. In these circumstances, a circulating sub may be used to allow the flow rate required to remove the drilled material to be pumped into the annulus whilst maintaining the lower flow rate required at the drilling tool. 
   It is known to provide a circulating sub with an axially movable piston for opening and closing vent apertures. The vent apertures are provided in a body of the sub and allow wellbore fluid pumped downhole through a central bore of the sub to pass into the surrounding wellbore annulus. Opening and closing of the vent apertures by means of the piston is controlled by a pin and groove arrangement. The pin is located in one of the piston and body and is received within the groove provided in the other of the piston and body. The profile of the groove is such that axial movement of the piston results in rotation of the piston within the body. Furthermore, the extent of axial piston movement is limited by the groove profile. Thus, the piston may be moved axially downhole by means of a predetermined fluid flow rate and returned uphole by means of a biasing spring so as to cycle the piston into a position wherein the control groove permits subsequent movement of the piston from a vent aperture closed position to a vent aperture open position. 
   A problem associated with the aforementioned prior art means for controlling the piston results from the helical compression spring generally used to bias the piston uphole. As the piston is pressed downhole by a fluid flow so as to compress the spring, there is a tendency for the spring to grip the piston and apply a rotational force thereto. This rotational force can often be in opposition to the control groove and pin. For example, in a movement of a piston from a vent aperture closed position to a vent aperture open position, a control groove will typically have a profile which is intended to allow for axial piston movement without any rotation of the piston relative to the body. In these circumstances, it is known for the rotational force applied by the spring to undesirably shear the control pin within the control groove. 
   SUMMARY OF THE INVENTION 
   The present invention provides apparatus for selectively providing fluid communication between the interior of a downhole assembly and the exterior thereof, such apparatus including a body incorporating a wall provided with at least one aperture extending therethrough; a piston having a longitudinal bore extending therethrough and being slidably mounted in the body so as to be movable between a first position relative to the body preventing fluid communication between the bore of the piston and the exterior of the body via the or each aperture and a second position relative to the body permitting fluid communication between the bore of the piston and the exterior of the body via the or each aperture; and controlling means for controlling the movement of the piston between the first and second positions, the controlling means comprising: a control member slidable in the body and movable by fluid pressure in the body in a first axial direction relative to the body; a spring biasing the control member in an opposite axial direction of the body; a pin secured to one of the body and the control member; and a control groove in which a portion of the pin is received formed in the other of the body and the control member, the control groove being shaped to limit axial displacement of the control member generated by pressure variations in the body such that only after a predetermined number of movements of the control member to a first axial position is the control member able to move to a second axial position so as to displace the piston from one of the first and second piston positions to the other of the first and second piston positions; characterised in that the controlling means further comprises a first element connected to the control member so as to prevent relative rotation between the first element and the control member, and a second element connected to the body so as to prevent relative rotation between the second element and the body, wherein the arrangement of said elements is such that, as the control member moves from said first axial position to said second axial position, increasing lengths of said elements locate adjacent one another so as to provide resistance to relative rotation, in at least one direction, of the control member and body, said relative rotation being relative rotation which presses the control pin against the control groove. 
   Thus, in apparatus according to the present invention, as the control member moves from the first axial position to the second axial position and thereby displaces the piston into one of the first and second piston positions, elements connected to the control member and apparatus body locate adjacent one another so as to provide resistance to relative rotation of the control member and body. As a consequence, relative rotation which tends to press a control pin against the control groove can be resisted and damage to the control pin thereby avoided. The first and second elements may be arranged so as to allow relative rotation between the control member and body as may be permitted by the control groove profile. However, the elements do not allow rotation which will press the control pin and groove against each other to the extent that damage to the pin may occur. Furthermore, as the control member is moved from said first axial position to said second axial position, the elements locate adjacent one another to an increasing extent by virtue of said elements sliding over one another in a collapsing telescoping type of movement. Thus, as the control member moves towards the second axial position (with the spring tending to apply an increasing rotational force), the elements are better able to resist relative rotation due to the increasingly long lengths of element portions located adjacent one another. In the event that the spring applies a rotational force opposing the control groove and pin, adjacent lengths of elements abut one another and prevent the force transmitted between the control groove and control pin increasing to an unacceptable level. Since the rotational force applied by the spring (by virtue of its compression) acts in one direction only, the elements need only resist relative rotation in one direction. Accordingly, the elements need only locate adjacent one another along one edge (said edge extending in a generally axial direction so as to be capable of transmitting rotational force centered on the apparatus axis). 
   It is preferable for said first element to remain axially spaced from said second element until the control member is axially moved to the first axial position. The arrangement of the first and second elements may be such that said elements become angularly offset to one another, so as to permit axial movement of said elements past one another, only after said predetermined number of movements of the control member to the first axial position. It is also preferable for the arrangement of the first and second elements to be such that, when said elements are angularly offset so as to permit their axial movement past one another, the control pin is received in one of a plurality of portions of control groove allowing the control member to move to the second axial position. The arrangement of the first and second elements may also be such that, when said elements are angularly offset so as to permit their axial movement past one another, the control pin is received in a portion of control groove allowing the control member either to displace the piston in said first axial direction from the first piston position to the second piston position and then to a third piston position preventing fluid communication between the bore of the piston and the exterior of the body via the or each aperture, or to displace the piston in said first axial direction from the second piston position to the first piston position and then to a third piston position permitting fluid communication between the bore of the piston and the exterior of the body via the or each aperture. 
   The control groove may comprise a plurality of said portions allowing displacement of the piston to said third piston position. Movement of the control member in said first axial direction past the second axial position may be prevented by means of an abutment of the second element with the control member or a component connected thereto. The second element may also be releasably connected to the body. The second element may be releasably connected to the body by means of a shear pin. When in the second piston position, the piston may be located so as to seal a fluid pathway through the apparatus and thereby, in use, direct fluid flowing into said apparatus through the or each aperture. 
   Embodiments of the present invention will now be described with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional side view of a first embodiment of the present invention arranged in a first closed configuration; 
       FIG. 1   a  is a plan view of the unwrapped profile of a control groove located relative to a control pin as shown in  FIG. 1 ; 
       FIG. 2  is a cross-sectional side view of the first embodiment arranged in a second closed configuration with downhole movement of a sleeve restricted by the control groove and pin; 
       FIG. 3  is a cross-sectional side view of the first embodiment arranged in an open configuration; 
       FIG. 3   a  is a cross-sectional view taken along line  3 - 3  of  FIG. 3 ; 
       FIG. 4  is a cross-sectional side view of the first embodiment arranged in a third (emergency) closed configuration; 
       FIG. 5  is a cross-sectional side view of a second embodiment of the present invention arranged in a first closed configuration; 
       FIG. 5   a  is a plan view of the unwrapped profile of a control groove relative to a control pin as shown in  FIG. 5 ; 
       FIG. 6  is a cross-sectional side view of the second embodiment arranged in a second closed configuration with downhole movement of a sleeve restricted by the control groove and pin; 
       FIG. 7  is a cross-sectional side view of the second embodiment arranged in an open configuration; 
       FIG. 7   a  is a cross-sectional view taken along line  7 - 7  of  FIG. 7 ; 
       FIG. 8  is a cross-sectional side view of the second embodiment arranged in a third (emergency) closed configuration; 
       FIG. 9  is a cross-sectional side view of a third embodiment of the present invention arranged in a first closed configuration with downhole movement of a sleeve restricted by a control groove and pin; 
       FIG. 9   a  is a plan view of the unwrapped profile of a control groove located relative to a control pin as shown in  FIG. 9 ; 
       FIG. 10  is a cross-sectional side view of the third embodiment arranged in a second closed configuration with downhole movement of the sleeve restricted by the control groove and pin, and with the angular position of the sleeve differing to that shown in  FIG. 9 ; 
       FIG. 11  is a cross-sectional side view of the third embodiment arranged in an open configuration; 
       FIG. 11   a  is a cross-sectional view taken along line  11 - 11  of  FIG. 11 ; and 
       FIG. 12  is a cross-sectional side view of the third embodiment arranged in an emergency closed configuration 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first embodiment shown in  FIGS. 1 to 4  of the accompanying drawings is a multi-cycle circulating sub  2  defined by a plurality of internal parts mounted within a substantially cylindrical body  4 . The body  4  is defined by three cylindrical members  6 ,  8 ,  10  threadedly connected to one another so as to define an elongate bore  12 . The first body member  6  is threadedly connected to an uphole end of the second body member  8  so as to provide a downwardly facing internal shoulder  14 . The third body member  10  is threadedly connected to a downhole end of the second body member  8  so as to define an upwardly facing shoulder  16 . An upper end  18  of the first body member  6  is provided with an internal screw thread  20  whilst a lower end  22  of the third body member  10  is provided with an external screw thread  24  so as to facilitate attachment of the circling sub  2  to adjacent components of a downhole string. 
   In addition to the cylindrical body members  6 ,  8 ,  10  as described above, the body  4  may be considered to also incorporate a cylindrical sleeve  26  located in the elongate bore  12  between the downwardly and upwardly facing shoulders  14 ,  16 . The sleeve  26  has an external diameter substantially equal to the internal diameter of the second body member  8 . The external surface of the sleeve  26  is provided with two O-ring seals  28  for preventing axial fluid flow between said external surface and the internal surface of the second body member  8 . The arrangement of the sleeve  26  within the second body member  8  is such that the sleeve  26  may slide axially within the bore  12 . However, as will be explained hereinafter, such axial movement of the sleeve  26  occurs only during emergency conditions. During normal use of the circulating sub  2 , the cylindrical sleeve  26  is selectively retained in a predetermined axial position relative to the second body member  8  by means of a shear pin  30 . One or more shear pins may be provided. 
   At the downhole end of the sleeve  26 , three elements  32  integral with the sleeve  26  extend inwardly from the interior surface of the sleeve  26  (see  FIG. 3   a ) so as to provide three upwardly facing sleeve shoulders  34 . The elements  32  extend only a short distance into the bore  12  so as to maintain a circular fluid path  38  therepast. As will be understood from the following discussion, the number of elements  32  may be varied so as to alter the number of cycles required to translate the circulating sub between open and closed configurations. The elements  32  are equi-spaced about the longitudinal axis of the circulating sub  2  and define slots  36  therebetween extending in a longitudinal direction. The three elements  32  are identical to one another and, accordingly, the slots  36  are identical to one another and equi-spaced about the longitudinal axis of the circulating sub  2 . 
   The body  4  is provided with six apertures  40  extending radially through the wall thereof so as to allow fluid communication between the bore  12  and the exterior of the circulating sub. The apertures  40  lie in a single plane orientated perpendicularly to the longitudinal axis of the body  4 . More specifically, the apertures  40  are provided in the second body member  8 . The sleeve  26  includes apertures  90  (see  FIG. 4 ). The O-ring seals  28  are located uphole and downhole of the apertures  40  so as to prevent an ingress into the bore  12  of wellbore fluid located in the apertures  40 . In the normal (non-emergency) configuration of the sub the apertures  40  of the body are aligned with apertures  90  provided in the sleeve  26 . 
   The body  4  houses a plurality of internal parts including a piston  42  and a helical compression spring  44  as principal components. The piston  42  has a generally cylindrical shape with the upper part  46  thereof having a greater outer diameter than the lower part  48 . The difference in diameter between the upper and lower parts  46 ,  48  of the piston  42  provides a piston shoulder  50  (see  FIG. 2  in particular). The external surface of the upper part  46  is circumscribed by a control groove  52  having the unwrapped profile shown in  FIG. 1   a . The control groove  52  is provided in a direction having a first component parallel to the apparatus axis so as to allow axial movement of the piston  42 , and a second component extending circumferentially so as to allow rotation of the piston  42 . The control groove  52  is thereby formed to produce rotary indexing of the piston  42  as the piston  42  moves axially. 
   An O-ring seal  54  and wear ring  56  are provided on the external surface of the piston  42  above the groove  52 . The piston  42  is also provided with a bore  58  having a sufficiently large diameter to allow the passage of wireline or coil tubing tools. It will be understood from  FIGS. 1 to 4  that the external diameter of the piston upper part  46  is substantially equal to the internal diameter of the second body member  8 , that the external diameter of the piston lower part  48  is substantially equal to the internal diameter of the sleeve  26 , and that the diameter of the piston bore  58  is substantially equal to the diameter of the circular fluid path  38  past the three sleeve elements  32 . The dimensions of the piston  42  relative to the body  4  are such as to allow ready axial movement of the piston  42  within the body  4 . 
   The piston  42  is located in the bore  12  of the second body member  8  with the piston shoulder  50  positioned uphole of a spring shoulder  60  defined by the uphole end of the sleeve  26 . The compression spring  44  extends between the spring shoulder  60  and the piston shoulder  50  so as to bias the piston  42  in an uphole axial direction towards the first body member  6 . A bearing  62  is located between the spring  44  and the piston shoulder  50  so as to allow the piston  42  to rotate relative to the spring  44  more readily. Uphole displacement of the piston  42  is limited by the downwardly facing shoulder  14 . The body  4  and the piston  42  thereby form a piston spring chamber  64  which is sealed by means of the piston O-ring seal  54  and a further O-ring seal  66  mounted in the inner surface of an uphole portion of the sleeve  26 . For ease of assembly, the further seal  66  may be provided on the piston  42 . The axial movement of the piston  42  within the bore  12  is assisted by the provision of vent holes  68  which, when in use, vent the piston spring chamber  64  to the piston bore  58 . Four vent holes  68  are provided. The diameter of each vent hole  68  determines the degree of damping provided to the piston  42 . Increasing the diameter of a vent hole  68  decreases the damping. The rate of piston movement may be thereby controlled and drilling vibration counteracted. 
   As shown in  FIG. 1 , the length of the piston  42  is slightly less than the distance between the downwardly facing shoulder  14  and the three upwardly facing sleeve shoulders  34 . Nevertheless, the piston  42  has sufficient length to extend downwardly past the apertures  40  of the body  4  when located in abutment with the downwardly facing shoulder  14 . Two O-ring seals  70  located uphole and downhole of the body apertures  40  in the inner surface of the sleeve  26  prevent undesirable ingress of fluid in said apertures  40  into the circulating sub  2  between the sleeve  26  and piston  42 . Nevertheless, the piston  42  is provided with six flow ports  72  which may be aligned with the apertures  40  through axial displacement of the piston  42  so as to permit a flow of wellbore fluid between the annulus and the interior of the circulating sub  2 . More specifically, the flow ports  72  i.e. in a single plane orientated perpendicularly to the longitudinal axis of the piston  42 . The flow ports  72  extend radially through the walls of the piston  42  and are of a similar diameter to the apertures  40 . The arrangement of the flow ports  72  relative to the apertures  40  is such that, when the piston  42  is located in a closed position as shown in  FIGS. 1 and 2 , the flow ports  72  locate uphole of the apertures  40  and neighboring seals  70  so as to isolate the bore  12  from the annulus, whereas when the piston  42  is located in an open position as shown in  FIG. 3 , the flow ports  72  align with the apertures  40  and thereby provide a fluid pathway between the annulus and the bore  12 . 
   The down hole end of the piston  42  is provided with three axially extending slots  74  (only two of which are visible in the accompanying drawings). The piston slots  72  extend through the full thickness of the piston wall and effectively provide three elements  76  downwardly projecting from the down hole end of the piston  42 . The three piston elements  76  are equi-spaced about the longitudinal axis of the circulating sub  2  and have a length and circumferential width substantially identical to that of the sleeve slots  36 . The relative sizes of the sleeve slots  36  and piston elements  76  are such that the piston elements  76  may align with and slide axially into the sleeve slots  36 . Clearly, the circumferential width of the sleeve elements  32  relative to the piston slot  74  are also such that, when aligned, the piston slots  74  may slide axially over the sleeve elements  32 . As with the piston elements  76  and sleeve slots  36 , the circumferential widths of the piston slots  74  and sleeve elements  32  are substantially equal. The purpose of this equality of circumferential widths is to ensure that, when the elements  32 ,  76  are respectively engaged with the slots  34 ,  36 , the relative rotation possible between the piston  42  and spring  44  is minimal. As will be understood from the following discussion, the purpose of the element/slot engagement is more specifically to prevent rotation of the piston  42  relative to the body  4  in one particular direction during movement of the piston  42  towards the open position shown in  FIG. 3 . Thus, an attempt by the piston  42  to rotate relative to the body  4  whilst the elements  32 ,  76  and slots  36 ,  74  are engaged will result in abutment of each sleeve element  32  with an adjacent piston element  76  at longitudinally extending edges thereof. Thus, in order to minimize possible relative rotation between the piston  42  and body  4 , it is important for the aforementioned abutting edges to be in abutment with one another or at least very close to one another as the piston  42  begins movement towards the open position. The relative angular positions of the remaining longitudinally extending edges of the sleeve and piston elements  32 ,  76  which do not tend to abut one another in use (due to the direction of relative piston/body rotation) are not critical. To this extent, equality of the element and slot circumferential width is not essential to the operation of the circulating sub  2 . 
   As most clearly shown in the expanded view of  FIG. 1 , a removable annular nozzle  78  is mounted in the piston bore  58  at an uphole end of the piston  42 . The nozzle  78  is secured against an upwardly facing shoulder  80  defined in the piston bore  58  with an annular retaining ring  82 . The retaining ring  82  is itself located in an annular groove provided in the piston bore  58 . Fluid flow between the nozzle  78  and piston  42  is prevented by means of an O-ring seal  84 . The purpose of the nozzle  78  is to provide a pressure drop in fluid flow passing through the piston bore  58 . The nozzle  78  may be selected so as to provide a desired restriction in the piston bore  58  and thereby initiate downhole axial movement of the piston  42  within the body  4  at a given flow rate of fluid through the circulating sub  2 . 
   A control pin  86  extends through the wall of the second body  8  so as to project into the bore  12  and locate in the control groove  52 . The control pin  86  is secured in position by means of a retaining plug  88 . One or more control pins may be provided. The shear pin  30  connecting the second body member  8  and sleeve member  26  also extends through an aperture through the wall of body member  8  and is retained in position by means of a retaining plug. 
   When in use, the multi-circulating sub  2  forms part of a downhole string through which well bore fluid may be pumped in order to operate equipment such as an anchor packer or a drilling tool, for example, a turbo drill or a positive displacement motor.  FIGS. 1 and 1   a  show the circulating sub  2  arranged with the piston  42  located in an inactivated closed position. In this inactivated position, the piston  42  is located in abutment with the downwardly facing shoulder  14  of the first body member  6 . The downhole end of the piston  42  (including the plurality of piston elements  32 ) is located uphole of the plurality of upwardly facing sleeve shoulders  34 . Furthermore, the control pin  86  is located at one of six inactivated groove positions X within the control groove  52 . The piston  42  will remain in the inactivated position until a predetermined flow of wellbore fluid through the circulating sub  2  is generated. As already indicated, the predetermined fluid flow may be adjusted by changing the dimensions of the nozzle  78 . Once the predetermined fluid flow is generated or exceeded, the piston  42  will attempt to move to the activated open position shown in  FIG. 3 . 
   However, the axial movement of the piston  42  is controlled by the interaction of the control pin  86  and the control groove  52 , and the piston  42  will be prevented from moving to the activated position unless the control pin  86  is located at one of three inactivated groove positions XX within the control groove  52  (see  FIG. 1   a ) immediately before the predetermined flow rate is produced. If the control pin  86  is not located at one of said three inactivated groove positions XX, then the axial movement of the piston  42  will be limited by the abutment of the control pin  86  against the side of the control groove  52  at one of three intermediate groove positions Y (see  FIG. 1   a ). Although displaced axially, no part of the piston  42  has moved downwardly past the upwardly facing sleeve shoulders  34  when the control pin  86  is located at any one of the intermediate groove position Y see  FIG. 2 ). With the control pin  86  located in an intermediate groove position Y, the downhole ends of the piston elements  76  are abutting (or, alternatively, spaced from) the sleeve shoulders  34 . The relative angular position of the piston  42  and sleeve  26  is such that the piston and sleeve elements  76 ,  32  do not align with the sleeve and piston slots  36 ,  74 . With the piston  42  located in either of the inactivated or intermediate positions shown in  FIGS. 1 and 2  respectively, the flow ports  72  remain uphole of the body apertures  40  and sealed therefrom by means of the adjacent O-ring seal  70 . Thus, a discharge of wellbore fluid from the sub  2  through the apertures  40  is prevented. 
   When the control pin  86  is located in one of the aforementioned three inactivated positions XX within the control groove  52  immediately before the predetermined flow rate is generated or exceeded, the profile of the control groove  52  allows the piston elements  76  to move rotationally into alignment with the sleeve slots  36  and to then allow the piston  42  to move axially downhole without further rotation (see  FIGS. 3 and 3   a ). As the piston  42  moves downhole relative to the body  4 , the control pin  86  moves within the control groove  52  from position XX to one of three activated groove positions Z (see  FIG. 1   a ). With the control pin  86  located in one of the three activated groove positions Z, the flow ports  72  in the piston  42  align with the body apertures  40  so as to allow the discharge of wellbore fluid from the string into the surrounding wellbore annulus. 
   Also, with the circulating sub  2  arranged in the open configuration, the closed ends of the piston slots  74  abut the upwardly facing sleeve shoulders  34 . 
   Movement of the piston  42  is assisted by the four vent holes  68  which allow fluid to flow between the piston spring chamber  64  and the piston bore  58  as the piston  42  moves axially and varies the volume of the spring chamber  64 . 
   It will be understood that the piston and sleeve elements  76 ,  32  must be arranged so as to align with the sleeve and piston slots  36 ,  74  when the control pin  86  moves from the aforementioned inactivated positions XX to the activated groove positions Z. More importantly, the piston and sleeve elements  76 ,  32  should be arranged relative to one another so that, should the piston  42  attempt to rotate (perhaps under the action of the spring  44 ) in opposition to the control groove and pin, adjacent piston and sleeve elements  76 ,  32  abut one another and prevent piston rotation. In this way, the application of undesirable forces on the control pin  86  is prevented. The risk of the control pin  86  becoming sheared and/or the piston  42  becoming jammed is thus reduced. It will be appreciated that, as the piston  42  is increasingly displaced downhole with an increasing tendency for compression of the spring  44  to apply undesirable rotational forces to the piston  42 , an increasing length of the piston and sleeve elements  76 ,  32  locate adjacent one another allowing the piston and sleeve elements  76 ,  32  to resist piston rotation with increasing effectiveness. 
   In order to move the control pin  86  from an intermediate groove position Y or activate groove position Z and move the piston  32  towards the inactivated position shown in  FIG. 1 , the rate of wellbore fluid flow through the circulating sub  2  is reduced below the predetermined rate so as to allow the compression spring  44  to relax and press the piston  42  into abutment with the first body member  6 . Movement of the circulating sub  2  from an open configuration to a closed configuration may be thereby readily achieved. However, circumstances may arise where the piston  42  becomes jammed in a downhole position to the extent that the uphole biasing force of the compression spring  44  is insufficient to release the piston  42  even when the flow rate is reduced to zero. A situation may therefore arise where closing of the circulating sub  2  becomes problematic. 
   In the event that the circulating sub  2  becomes jammed in an open configuration, an attempt to move the circulating sub  2  to a closed configuration can be made by increasing the flow of fluid through the circulating sub  2  so as to shear the shear pin  30  and move the piston  42 , together with the sleeve  26 , downhole towards the third body member  10 . It is envisaged that a greater resultant force on the piston  42  can be generated by a flow of fluid downhole through the borehole  12  than by the compression spring  44 . Thus, it may well be possible to move a jammed piston  42  downhole by means of dynamic fluid pressure in circumstances where the compression spring  44  is unable to move the jammed piston  42  uphole. However, since downhole movement of the piston  42  is limited in the open configuration by means of the sleeve elements  32  (so as to ensure alignment of the body apertures  40  and the flow port  72 ), further downhole movement of the piston  42  must be accompanied by a downhole movement of the sleeve  26 . The force applied by the fluid flow to the piston  42  must therefore be sufficient not only to release the piston  42 , but also to shear the shear pin  30  and thereby allow movement of the sleeve  26 . Once a sufficient force is generated to release the piston  42  and shear the shear pin  30 , the piston  42  and sleeve  26  move downhole to an emergency closed position. The profile of the control groove  52  is such as to allow the further downhole movement of the piston  42 . As shown in  FIG. 4 , the further downhole movement of the piston  42  is limited by abutment of the sleeve  26  with the upwardly facing shoulder  16  defined by the third body member  10 . In the emergency closed configuration, the portions  90  of the body apertures  40  defined by the sleeve  26  remain aligned with the flow port  72  but locate downhole of the portions  22  of the body apertures  40  defined by the second body member  8 . Also, in the emergency closed configuration, the control pin  86  locates in one of three extended groove positions ZZ. 
   The present invention is not limited to the specific embodiment described above. Variations and alternatives will be apparent to the reader skilled in the art. For example, the control groove  52  may have an alternative profile with a different number of inactivated, intermediate, activated and extended groove positions. The control groove  52  shown in  FIG. 1   a  has a profile which causes the piston  42  to rotate through 120° when moving axially between successive intermediate or activated groove positions Y, Z. The profile may be altered so that the piston  42  rotates through a different angle when moving between these positions (consequential alternation to the arrangement of piston and sleeve elements  76 ,  32  may also be required as will be apparent to the skilled reader). 
   The circulating sub  2  shown in  FIGS. 1 to 4  may be regarded as a two-cycle circulating sub in that two cycles of pressurizing the sub in order to move the piston  42  axially downhole must be undertaken before the sub  2  will be translated from a closed configuration into an open configuration. The number of cycles is determined not only by the profile of the control groove  52 , but also by the arrangement of the piston and sleeve element  76 ,  32 . It will be understood that the number of cycles will be changed by altering the arrangement of the piston and sleeve elements  76 ,  32  without necessarily altering the profile of the control groove  52 . This is because, although the activated groove positions Z of the control groove  52  may allow downhole movement of the piston  42  into an open position, piston movement to the open position will not be realized unless the piston and sleeve elements  76 ,  32  align with the sleeve and piston slots  36 ,  74 . Thus, a six-cycle circulating sub  102  is shown in  FIGS. 5 to 8  of the accompanying drawings, wherein the profile of the control groove is identical to that of the first embodiment. Indeed, the six-cycle circulating sub  102  differs from the two-cycle circulating sub  2  only in the arrangement of the piston and sleeve elements. 
   As can be seen most clearly from  FIG. 7   a , the sleeve  126  and piston  142  of the second embodiment  102  each comprise merely a single element  132 ,  176  having a semicircular shape. The piston element  176  is arranged relative to the control groove  52  and the sleeve element  132  so that the control pin  86  is able to move to only one of the activated groove positions Z. Movement to the remaining two activated groove positions Z is prevented by abutment of the downhole end of the piston element  176  with the upwardly facing sleeve shoulder  134  defined by the sleeve element  132 . However, when the sleeve and piston elements  134 ,  176  are positioned relative to one another so as to allow movement of the control pin to an activated groove position Z, abutment of the longitudinally extending edges  133 ,  177  of the sleeve elements  132  and piston elements  176  ensures rotation of the piston  142  relative to the second body member  8  in opposition to the control groove and pin is resisted. It will be understood therefore that the control groove  52  and sleeve/piston elements  132 ,  176  combine to provide a six-cycle indexing mechanism. 
   In order to provide improved versatility, the elements provided on the sleeve and piston may be respectively detachable from the sleeve and piston. This may be achieved by defining the elements on a cylindrical portion which is screw threadedly engageable with the lower part of the sleeve or piston. In this way, the cycle characteristics of a circulating sub may be rapidly and conveniently altered. 
   As shown in  FIG. 8 , the six-cycle circulating sub  102  may be moved to an emergency closed configuration (as with the first embodiment  2 ) by increasing the flow rate through the circulating sub  102  and shearing the shear pin  30 . 
   A third embodiment  202  is shown in  FIGS. 9 to 12  of the accompanying drawings. The third embodiment  202  is a six-cycle circulating sub differing from the second embodiment  102  only in the arrangement of the downhole portions of the second body member  208 , sleeve  226  and piston  242 . The arrangement of these components is such that, when the piston is in a closed position as shown in  FIGS. 9 and 10  (or an emergency closed position as shown in  FIG. 12 ), wellbore fluid may flow through the interior of the circulating sub  202  as in the case of the first and second embodiments; however when the piston  242  is in an open position as shown in  FIG. 11 , the bore  12  through the circulating sub  202  is closed and all wellbore fluid flowing downhole through the circulating sub  202  is directed into the annulus by the body apertures  40 . 
   More specifically, the downhole portions of the sleeve  226  and piston  242  are arranged with an asymmetric configuration. The piston  242  defines a piston bore  258  having an upper portion coaxially arranged with the longitudinal axis of the circulating sub  202  and a lower portion located downhole of the flow ports  72  which extends downhole at an angle relative to the longitudinal axis of the circulating sub  202 . Accordingly, the downhole end of the piston bore  258  opens at a location offset from the longitudinal axis of the apparatus  202 . This offset location provides a downhole facing piston shoulder  259  extending inwardly into the bore  12  of the circulating sub  202 . A single piston element  276  extends downwardly from the shoulder  259 . The downhole end of the sleeve  226  has a reduced diameter defining a restricted bore  227  within an axis offset relative to the longitudinal axis of the circulating sub  202 . Uphole of the reduced diameter, the sleeve  226  is provided with four ports  229  which extend radially through the thickness of the sleeve  226 . 
   When in the closed configuration as shown in  FIGS. 9 and 10 , wellbore fluid may flow through the circulating sub  202  via the piston bore  258 , about the downwardly facing piston shoulder  259  and through the restricted sleeve bore  227 . In  FIG. 9 , the circulating sub  202  is shown with the piston  242  displaced downhole against the bias of the compression spring  44  by means of an appropriate flow rate of well bore fluid. Displacement of the piston  242  into an open position is prevented by abutment of the piston element  276  against a single sleeve element  232  defining the restricted bore  227 . The circulating sub  202  is shown in  FIG. 10  cycled to a further closed configuration with the piston  242  having been rotated within the second body member  208 . Again, movement of the piston  242  into the open position is prevented by abutment of the piston element  276  against the sleeve element  232 . However, with the circulating sub  202  cycled to the configuration shown in  FIGS. 11 and 11   a , it will be seen that the piston  242  has rotated sufficiently for the piston element  276  to align with the restricted bore  227  (acting as a sleeve slot) allowing the piston  242  to move further downhole relative to the sleeve  226 . In so doing, the piston flow ports  72  align with the body apertures  40  (allowing flow to the annulus) and the downwardly facing piston shoulder  259  closes the restricted sleeve bore  227  (preventing fluid flow within the bore  12  downhole past the second body member  208 ). Fluid flow through the four ports  229  is not possible in the open and closed piston positions of  FIGS. 9 ,  10 ,  11  and  11   a  due to the sealing of these ports by means of the second body member  208 . 
   As described with relation to the first and second embodiments, the third embodiment  202  may be moved to an emergency closed position in the event that the piston  242  becomes jammed and the biasing force of the compression spring  44  is insufficient to return the piston  242  to its original uphole position in abutment with the first body member  6 . Again, as described in relation to the first and second embodiments, the emergency closed configuration is achieved by increasing the flow of fluid through the bore  12 . The flow rate is increased until the downhole force applied to the piston  242  is sufficient to release the piston  242  and shear the shear pin  30 . The piston  242  and sleeve  226  are then moved downhole. Downhole movement of the piston  242  and sleeve  226  is limited by abutment of the sleeve  226  with the third body member  10 . Although the restricted sleeve bore  227  remains sealed by the downwardly facing piston shoulder  259 , flow through the bore  12  into the third body member  10  is permitted by means of the ports  229  provided in the sleeve  226 . Flow through the ports  229  is possible with the sleeve  226  abutting the third body member  10  by virtue of a circumferential recess  231  provided in the interior surface of the second body member  208  at a downhole portion thereof. More specifically, the recess  231  is located uphole of the third body member  10  and downhole of the four ports  229  when the sleeve  226  is located in a non-emergency position (i.e. when retained by the shear pin  30  as shown in  FIGS. 9 to 11   a ). The circumferential recess  231  has sufficient downhole length for wellbore fluid to flow through the sleeve ports  229 , around and beneath the sleeve element  232 , and into the third body member  10 . 
   Finally, it will be understood that any of the above described embodiments may be moved to the emergency closed configuration by running means for closing the piston bore. For example, a dart may be run on a wire line downhole through the apparatus so as to locate in the piston  42 ,  142 ,  242  and block the piston bore. The shear pin  30  will then shear and the apparatus will close. The dart may then be recovered and circulation through the apparatus restored.