A downhole apparatus which can be used as multi-cycle circulating subs utilized in downhole drilling operations, includes a piston (242) slidably mounted in a body between positions in which at least one aperture in the body is opened and closed. Movement of the piston (242) is controlled by a pin (86), secured to one of the body and a control member, and a control groove (52) formed in the other of the body and control member for receiving a portion of the pin. An arrangement of elements (232, 276) respectively connected to the control member and body is such as to normally resist axial movement of the control member from a first axial position to a second axial position. A spring (44) is located in a chamber for biasing the piston (242). The chamber is vented by an opening in the body.

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 bypass or partially bypass 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 is that there can be a tendency for the control pin to become damaged within the control groove as a result of axial and rotational forces acting on the piston. These forces can shear the control pin within the control groove.

In addressing this problem, our UK patent application number 0116472.2 provides apparatus comprising a piston slidably mounted in a body between positions in which at least one aperture in the body is opened and closed. Movement of the piston is controlled by one or more pins (secured to one of the body and a control member) and a control groove (formed in the other of the body and control member) in which a portion of the or each pin is received. An arrangement of elements respectively connected to the control member and body is such that, as the control member moves axially, 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. The relative rotation is a rotation which presses the control member against the control groove. The elements are also arranged to limit axial movement of the control member. The apparatus thereby provides means by which the risk of damage to the control pin is reduced.

The present invention provides apparatus for selectively providing fluid communication between the interior of a downhole assembly and the exterior thereof said apparatus comprising: 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 bodes 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; wherein 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 element is such that, in the first axial position of the control member, the first and second elements normally abut one another so as to resist axial movement of the control member toward the second axial position, said elements locating offset relative to one another so as to allow movement of the control member to the second axial position only after a predetermined number of movements of the control member to the first axial position; and wherein the spring is located in a chamber defined between the control member and the body, and at least one vent opening is provided in the body for venting fluid located in the chamber to the exterior of the body. The arrangement of said elements may be 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, movement of the control member past the first axial position is normally prevented by an abutment of the first and second elements and, as a consequence, an undesirable application of axial pressure by the control groove on the control pin may be avoided. Also, 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, the elements are better able to resist relative rotation due to the increasingly long lengths of element portions located adjacent one another. Also, since the spring chamber may be exposed to wellbore fluid pressure, a resultant fluid pressure may be applied to the control member which, in use, reduces the risk of an accidental cycling of the control pin within the control groove.

Ideally, at least one vent opening is provided in the control member for venting fluid located in the chamber to the exterior of the body. The or each vent opening in the control member or the body may also be occluded so as to prevent a passage of fluid therethrough. The or each occluded vent opening may be occluded with a removable plug. Thus, the spring chamber can be vented to the piston bore or wellbore annulus depending on which set of vent openings are occluded.

It is also desirable for the axial movement of the piston to be limited by one or more stop shoulders provided on the body. A first shoulder may limit axial movement of the piston in a first direction. A second shoulder may limit axial movement of the piston in a second direction opposite to said first direction. In this way, the application of axial thrust forces to the or each pin with the piston in the uppermost and lowermost positions may be avoided.

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 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. The said elements may be offset angularly. It is also preferable for the arrangement of the first and second elements to be such that, when said elements are 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 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. Also, the or each aperture may be arranged so that wellbore fluid flowing in use through the or each aperture from the interior of the apparatus is directed in a direction having a component parallel to the longitudinal axis of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first tool shown inFIGS. 1 to 4of the accompanying drawings is a multi-cycle circulating sub2defined by a plurality of internal parts mounted within a substantially cylindrical body4. The body4is defined by three cylindrical members6,8,10threadedly connected to one another so as to define an elongate bore12. The first body member6is threadedly connected to an uphole end of the second body member8so as to provide a downwardly facing internal shoulder14. The third body member10is threadedly connected to a downhole end of the second body member8so as to define an upwardly facing shoulder16. An upper end18of the first body member6is provided with an internal screw thread20whilst a lower end22of the third body member10is provided with an external screw thread24so as to facilitate attachment of the circulating sub2to adjacent components of a downhole string.

In addition to the cylindrical body members6,8,10as described above, the body4may be considered to also incorporate a cylindrical sleeve26located in the elongate bore12between the downwardly and upwardly facing shoulders14,16. The sleeve26has an external diameter substantially equal to the internal diameter of the second body member8. The external surface of the sleeve26is provided with two O-ring seals28for preventing axial fluid flow between said external surface and the internal surface of the second body member8. The arrangement of the sleeve26within the second body member8is such that the sleeve26may slide axially within the bore12. However, as will be explained hereinafter, such axial movement of the sleeve26occurs only during emergency conditions. During normal use of the circulating sub2, the cylindrical sleeve26is selectively retained in a predetermined axial position relative to the second body member8by means of a shear pin30. One or more shear pins may be provided.

At the downhole end of the sleeve26, three elements32integral with the sleeve26extend inwardly from the interior surface of the sleeve26(seeFIG. 3a) so as to provide three upwardly facing sleeve shoulders34. The elements32extend only a short distance into the bore12so as to maintain a circular fluid path38therepast. As will be understood from the following discussion, the number of elements32may be varied so as to alter the number of cycles required to translate the circulating sub between open and closed configurations. The elements32are equi-spaced about the longitudinal axis of the circulating sub2and define slots36therebetween extending in a longitudinal direction. The three elements32are identical to one another and, accordingly, the slots36are identical to one another and equi-spaced about the longitudinal axis of the circulating sub2.

The body4is provided with six apertures40extending radially through the wall thereof so as to allow fluid communication between the bore12and the exterior of the circulating sub. The apertures40lie in a single plane orientated perpendicularly to the longitudinal axis of the body4. More specifically, the apertures40are provided in the second body member8and sleeve26. The O-ring seals28are located uphole and downhole of the apertures40so as to prevent an ingress into the bore12of wellbore fluid located in the apertures40.

The body4houses a plurality of internal parts including a piston42and a helical compression spring44as principal components. The piston42has a generally cylindrical shape with the upper part46thereof having a greater outer diameter than the lower part48. The difference in diameter between the upper and lower parts46,48of the piston42provides a piston shoulder50(seeFIG. 2in particular). The external surface of the upper part46is circumscribed by a control groove52having the unwrapped profile shown inFIG. 1a. The control groove52is provided in a direction having a first component parallel to the apparatus axis so as to allow axial movement of the piston42, and a second component extending circumferentially so as to allow rotation of the piston42. The control groove52is thereby formed to produce rotary indexing of the piston42as the piston42moves axially.

An O-ring seal54and wear ring56are provided on the external surface of the piston42above the groove52. The piston42is also provided with a bore58having a sufficiently large diameter to allow the passage of wireline or coil tubing tools. It will be understood fromFIGS. 1 to 4that the external diameter of the piston upper part46is substantially equal to the internal diameter of the second body member8, that the external diameter of the piston lower part48is substantially equal to the internal diameter of the sleeve26, and that the diameter of the piston bore58is substantially equal to the diameter of the circular fluid path38past the three sleeve elements32. The dimensions of the piston42relative to the body4are such as to allow ready axial movement of the piston42within the body4.

The piston42is located in the bore12of the second body member8with the piston shoulder50positioned uphole of a spring shoulder60defined by the uphole end of the sleeve26. The compression spring44extends between the spring shoulder60and the piston shoulder50so as to bias the piston42in an uphole axial direction towards the first body member6. A bearing62is located between the spring44and the piston shoulder50so as to allow the piston42to rotate relative to the spring44more readily. Uphole displacement of the piston42is limited by the downwardly facing shoulder14. The body4and the piston42thereby form a piston spring chamber64which is sealed by means of the piston O-ring seal54or glyd ring and a further O-ring seal66or glyd ring mounted in the inner surface of an uphole portion of the sleeve26. The further seal66may be provided on the piston42. The axial movement of the piston42within the bore12is assisted by the provision of vent holes68which, when in use, vent the piston spring chamber64to the piston bore58. Four vent holes68are provided. The diameter of each vent hole68determines the degree of damping provided to the piston42. Increasing the diameter of a vent hole68decreases the damping. The rate of piston movement may be thereby controlled and axial drilling vibration and shock inputs counteracted.

As shown inFIG. 1, the length of the piston42is slightly less than the distance between the downwardly facing shoulder14and the three upwardly facing sleeve shoulders34. Nevertheless, the piston42has sufficient length to extend downwardly past the apertures40of the body4when located in abutment with the downwardly facing shoulder14. Two O-ring seals70or glyd rings located uphole and downhole of the body apertures40in the inner surface of the sleeve26prevent undesirable ingress of fluid in said apertures40into the circulating sub2between the sleeve26and piston42(i.e. prevents fluid leakage past the piston in the closed position). Nevertheless, the piston42is provided with six flow ports72which may be aligned with the apertures40through axial displacement of the piston42so as to permit a flow of wellbore fluid between the annulus and the interior of the circulating sub2. More specifically, the flow ports72i.e. in a single plane orientated perpendicularly to the longitudinal axis of the piston42. The flow ports72extend radially through the walls of the piston42and are of a similar diameter to the apertures40. The arrangement of the flow ports72relative to the apertures40is such that, when the piston42is located in a closed position as shown inFIGS. 1 and 2, the flow ports72locate uphole of the apertures40and neighbouring seals70so as to isolate the bore12from the annulus, whereas when the piston42is located in an open position as shown inFIG. 3, the flow ports72alien with the apertures40and thereby provide a fluid pathway between the annulus and the bore12.

The downhole end of the piston42is provided with three axially extending slots74(only two of which are visible in the accompanying drawings). The piston slots72extend through the full thickness of the piston wall and effectively provide three elements76downwardly projecting from the downhole end of the piston42. The three piston elements76are equi-spaced about the longitudinal axis of the circulating sub2and have a length and circumferential width substantially identical to that of the sleeve slots36. The relative sizes of the sleeve slots36and piston elements76are such that the piston elements76may align with and slide axially into the sleeve slots36. Clearly, the circumferential width of the sleeve elements32relative to the piston slot74are also such that, when aligned, the piston slots74may slide axially over the sleeve elements32. As with the piston elements76and sleeve slots36, the circumferential widths of the piston slots74and sleeve elements32are substantially equal. The purpose of this equality of circumferential widths is to ensure that, when the elements32,76are respectively engaged with the slots34,36, the relative rotation possible between the piston42and44is minimal. As will be understood from the following discussion, the purpose of the element/slot engagement is more specifically to prevent rotation of the piston42relative to the body4in one particular direction during movement of the piston42towards the open position shown inFIG. 3. Thus, an attempt by the piston42to rotate relative to the body4whilst the elements32,76and slots36,74are engaged will result in abutment of each sleeve element32with an adjacent piston element76at longitudinally extending edges thereof. Thus, in order to minimise possible relative rotation between the piston42and body4, 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 piston42begins movement towards the open position. The relative angular positions of the remaining longitudinally extending edges of the sleeve and piston elements32,76which 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 sub2.

As most clearly shown in the expanded view ofFIG. 1, a removable annular nozzle78is mounted in the piston bore58at an uphole end of the piston42. The nozzle78is secured against an upwardly facing shoulder80defined in the piston bore58with an annular retaining ring82. The retaining ring82is itself located in an annular groove provided in the piston bore58. Fluid flow between the nozzle78and piston42is prevented by means of an O-ring seal84. The purpose of the nozzle78is to provide a pressure drop in fluid flow passing through the piston bore58. The nozzle78may be selected so as to provide a desired restriction in the piston bore58and thereby initiate downhole axial movement of the piston42within the body4at a given flow rate of fluid through the circulating sub2.

A control pin86extends through the wall of the second body8so as to project into the bore12and locate in the control groove52. The control pin86is secured in position by means of a retaining plug88. One or more control pins may be provided. The shear pin30connecting the second body member8and sleeve member26also extends through an aperture through the wall of body member8and is retained in position by means of a retaining plug.

When in use, the multi-circulating sub2forms 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 1ashow the circulating sub2arranged with the piston42located in an inactivated closed position. In this inactivated position, the piston42is located in abutment with the downwardly facing shoulder14of the second body member8. The downhole end of the piston42(including the plurality of piston elements32) is located uphole of the plurality of upwardly facing sleeve shoulders34. Furthermore, the control pin86is located at one of six inactivated groove positions X within the control groove52. The piston42will remain in the inactivated position until a predetermined flow of wellbore fluid through the circulating sub2is generated. As already indicated, the predetermined fluid flow may be adjusted by changing the dimensions of the nozzle78. Once the predetermined fluid flow is generated or exceeded, the piston42will attempt to move to the activated open position shown inFIG. 3.

However, the axial movement of the piston42is controlled by the interaction of the control pin86and the control groove52, and the piston42will be prevented from moving to the activated position unless the control pin86is located at one of three inactivated groove positions XX within the control groove52(seeFIG. 1a) immediately before the predetermined flow rate is produced. If the control pin86is not located at one of said three inactivated groove positions A, then the axial movement of the piston42will result in the control pin86moving to one of three intermediate groove positions Y (seeFIG. 1a). Although displaced axially, no part of the piston42has moved downwardly past the upwardly facing sleeve shoulders34when the control pin86is located at any one of the intermediate groove position Y see (FIG. 2). With the control pin86located in an intermediate groove position Y, the downhole ends of the piston elements76are abutting the sleeve shoulders34. The relative angular position of the piston42and sleeve26is such that the piston and sleeve elements76,32do not align with the sleeve and piston slots36,74. With the piston42located in either of the inactivated or intermediate positions shown inFIGS. 1 and 2respectively, the flow ports72remain uphole of the body apertures40and sealed therefrom by means of the adjacent O-ring seal70. Thus, a discharge of wellbore fluid from the sub2through the apertures40is prevented.

When the control pin86is located in one of the aforementioned three inactivated positions XX within the control groove52immediately before the predetermined flow rate is generated or exceeded, the profile of the control groove52allows the piston elements76to move rotationally into alignment with the sleeve slots36and to then allow the piston42to move axially downhole without further rotation (seeFIGS. 3 and 3a). As the piston42moves downhole relative to the body4, the control pin86moves within the control groove52from position XX to one of three activated groove positions Z (seeFIG. 1a). With the control pin86located in one of the three activated groove positions Z, the flow ports72in the piston42align with the body apertures40so as to allow the discharge of wellbore fluid from the string into the surrounding wellbore annulus.

Also, with the circulating sub2arranged in the open configuration, the closed ends of the piston slots74abut the upwardly facing sleeve shoulders34.

Movement of the piston42is assisted by the four vent holes68which allow fluid to flow between the piston spring chamber64and the piston bore58as the piston42moves axially and varies the volume of the spring chamber64.

It will be understood that the piston and sleeve elements76,32must be arranged so as to align with the sleeve and piston slots36,74when the control pin86moves from the aforementioned inactivated positions XX to the activated groove positions Z. More importantly, the piston and sleeve elements76,32should be arranged relative to one another so that should the piston42attempt to rotate (perhaps under the action of fluid imbalance in the piston bore) in opposition to the control groove and pin, adjacent piston and sleeve elements76,32abut one another and prevent piston rotation. In this way, the application of undesirable forces on the control pin86is prevented. The risk of the control pin86becoming sheared and/or the piston42becoming jammed is thus reduced.

In order to move the control pin86from an intermediate groove position Y or activated groove position Z and move the piston32towards the inactivated position shown inFIG. 1, the rate of wellbore fluid flow through the circulating sub2is reduced below the predetermined rate so as to allow the compression spring44to relax and press the piston42into abutment with the first body member6. Movement of the circulating sub2from an open configuration to a closed configuration may be thereby readily achieved. However, circumstances may arise where the piston42becomes jammed in a downhole position (perhaps due to debris) to the extent that the uphole biasing force of the compression spring44is insufficient to release the piston42even when the flow rate is reduced to zero. A situation may therefore arise where closing of the circulating sub2becomes problematic.

In the event that the circulating sub2becomes jammed in an open configuration, an attempt to move the circulating sub2to a closed configuration can be made by increasing the flow of fluid through the circulating sub2so as to shear the shear pin30and move the piston42, together with the sleeve26, downhole towards the third body member10. It is envisaged that a greater resultant force on the piston42can be generated by a flow of fluid downhole through the borehole12than by the compression spring44. Thus, it may well be possible to move a jammed piston42downhole by means of dynamic fluid pressure in circumstances where the compression spring44is unable to move the jammed piston42uphole. However, since downhole movement of the piston42is limited in the open configuration by means of the sleeve elements32(so as to ensure alignment of the body apertures40and the flow port72), further downhole movement of the piston42must be accompanied by a downhole movement of the sleeve26. The force applied by the fluid flow to the piston42must therefore be sufficient not only to release the piston42, but also to shear the shear pin30and thereby allow movement of the sleeve26. Once a sufficient force is generated to release the piston42and shear the shear pin30, the piston42and sleeve26move downhole to an emergency closed position. The profile of the control groove52is such as to allow the further downhole movement of the piston42. As shown inFIG. 4, the further downhole movement of the piston42is limited by abutment of the sleeve26with the upwardly facing shoulder16defined by the third body member10. In the emergency closed configuration, the portions90of the body apertures40defined by the sleeve26remain aligned with the flow port72but locate downhole of the portions22of the body apertures40defined by the second body member8. Also, in the emergency closed configuration, the control pin86locates in one of three extended groove positions ZZ.

Variations and modifications to the above described tool will be apparent to the reader skilled in the art. For example, the control groove52may have an alternative profile with a different number of inactivated, intermediate, activated and extended groove positions. The control groove52shown inFIG. 1ahas a profile which causes the piston42to rotate through 120° when moving axially between successive intermediate or activated groove positions Y, Z. The profile may be altered so that the piston42rotates through a different angle when moving between these positions (consequential alternation to the arrangement of piston and sleeve elements76,32may also be required as will be apparent to the skilled reader).

The circulating sub2shown inFIGS. 1 to 4may be regarded as a two-cycle circulating sub in that two cycles of pressurising the sub in order to move the piston42axially downhole must be undertaken before the sub2will be translated from a closed configuration into an open configuration. The number of cycles is determined not only by the profile of the control groove52, but also by the arrangement of the piston and sleeve element76,32. It will be understood that the number of cycles will be changed by altering the arrangement of the piston and sleeve elements76,32without necessarily altering the profile of the control groove52. This is because, although the activated groove positions Z of the control groove52may allow downhole movement of the piston42into an open position, piston movement to the open position will not be realised unless the piston and sleeve elements76,32align with the sleeve and piston slots36,74. Thus, a six-cycle circulating sub102is shown inFIGS. 5 to 8of the accompanying drawings, wherein the profile of the control groove is identical to that of the first tool. Indeed, the six-cycle circulating sub102differs from the two-cycle circulating sub2only in the arrangement of the piston and sleeve elements.

As can be seen most clearly fromFIG. 7a, the sleeve126and piston142of the second tool102each comprise merely a single element132,176having a semicircular shape. The piston element176is arranged relative to the control groove52and the sleeve element132so that the control pin86is 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 element176with the upwardly facing sleeve shoulder134defined by the sleeve element132. However, when the sleeve and piston elements134,176are 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 edges133,177of the sleeve elements132and piston elements176ensures rotation of the piston142relative to the second body member8in opposition to the control groove and pin is resisted. It will be understood therefore that the control groove52and sleeve/piston elements132,176combine 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 inFIG. 8, the six-cycle circulating sub102may be moved to an emergency closed configuration (as with the first tool2) by increasing the flow rate through the circulating sub102and shearing the shear pin30.

A third tool202is shown inFIGS. 9 to 12of the accompanying drawings. The third tool202is a six-cycle circulating sub differing from the second tool102only in the arrangement of the downhole portions of the second body member208, sleeve226and piston242. The arrangement of these components is such that, when the piston is in a closed position as shown inFIGS. 9 and 10(or an emergency closed position as shown inFIG. 12), wellbore fluid may flow through the interior of the circulating sub202as in the case of the first and second tools; however when the piston242is in an open position as shown inFIG. 11, the bore12through the circulating sub202is closed and all wellbore fluid flowing downhole through the circulating sub202is directed into the annulus by the body apertures40.

More specifically, the downhole portions of the sleeve226and piston242are arranged with an asymmetric configuration. The piston242defines a piston bore258having an upper portion coaxially arranged with the longitudinal axis of the circulating sub202and a lower portion located downhole of the flow ports72which extends downhole at an angle relative to the longitudinal axis of the circulating sub202. Accordingly, the downhole end of the piston bore258opens at a location offset from the longitudinal axis of the apparatus202. This offset location provides a downhole facing piston shoulder259extending inwardly into the bore12of the circulating sub202. A single piston element276extends downwardly from the shoulder259. The downhole end of the sleeve226has a reduced diameter defining a restricted bore227within an axis offset relative to the longitudinal axis of the circulating sub202. Uphole of the reduced diameter, the sleeve226is provided with four ports229which extend radially through the thickness of the sleeve226.

When in the closed configuration as shown inFIGS. 9 and 10, wellbore fluid may flow through the circulating sub202via the piston bore258, about the downwardly facing piston shoulder259and through the restricted sleeve bore227. InFIG. 9, the circulating sub202is shown with the piston242displaced downhole against the bias of the compression spring44by means of an appropriate flow rate of well bore fluid. Displacement of the piston242into an open position is prevented by abutment of the piston element276against a single sleeve element232defining the restricted bore227. The circulating sub202is shown inFIG. 10cycled to a further closed configuration with the piston242having been rotated within the second body member208. Again, movement of the piston242into the open position is prevented by abutment of the piston element276against the sleeve element232. However, with the circulating sub202cycled to the configuration shown inFIGS. 11 and 11a, it will be seen that the piston242has rotated sufficiently for the piston element276to alien with the restricted bore227(acting as a sleeve slot) allowing the piston242to move further downhole relative to the sleeve226. In so doing, the piston flow ports72align with the body apertures40(allowing flow to the annulus) and the downwardly facing piston shoulder259closes the restricted sleeve bore227(preventing fluid flow within the bore12downhole past the second body member208). Fluid flow through the four ports229is not possible in the open and closed piston positions ofFIGS. 9,10,11and11adue to the sealing of these ports by means of the second body member208.

As described with relation to the first and second tools, the third tool202may be moved to an emergency closed position in the event that the piston242becomes jammed and the biasing force of the compression spring44is insufficient to return the piston242to its original uphole position in abutment with the first body member6. Again, as described in relation to the first and second tools, the emergency closed configuration is achieved by increasing the flow of fluid through the bore12. The flow rate is increased until the downhole force applied to the piston242is sufficient to release the piston242and shear the shear pin30. The piston242and sleeve226are then moved downhole. Downhole movement of the piston242and sleeve226is limited by abutment of the sleeve226with the third body member10. Although the restricted sleeve bore227remains sealed by the downwardly facing piston shoulder259, flow through the bore12into the third body member10is permitted by means of the ports229provided in the sleeve226. Flow through the ports229is possible with the sleeve226abutting the third body member10by virtue of a circumferential recess231provided in the interior surface of the second body member208at a downhole portion thereof. More specifically, the recess231is located uphole of the third body member10and downhole of the four ports229when the sleeve226is located in a non-emergency position (i.e. when retained by the shear pin30as shown inFIGS. 9 to 11a). The circumferential recess231has sufficient downhole length for wellbore fluid to flow through the sleeve ports229, around and beneath the sleeve element232, and into the third body member10.

It will be understood that any of the above described tools may be moved to the emergency closed configuration by running means for closing the piston bore. For example, a ball or dart may be dropped or run on a wire line downhole through the apparatus so as to locate in the piston42,142,242and block the piston bore. The shear pin30will then shear and the apparatus will close. The ball or dart may then be recovered and circulation through the apparatus restored. Alternatively, a burst disc in the dart may be ruptured so as to allow circulation.

It has been found by the applicant that, although the tools described above in relation toFIGS. 1 to 12have beneficial operating characteristics, the performance of the tools can nevertheless be improved with certain modifications. These modifications are described below in relation to first and second embodiments of the present invention shown inFIG. 1316andFIG. 17respectively of the accompanying drawings. The first embodiment is an improved six-cycle circulating sub302. Apart from the modifications described below, the improved circulating sub302is identical to the third circulating sub202ofFIGS. 9 to 12and, accordingly, like reference numerals have been used to identify like components in the accompanying drawings.

A first modification comprised in the embodiment ofFIGS. 13 to 16is the provision of a second set of vent holes369to compliment the original set of vent holes368provided in the piston242. The two sets of vent holes368,369provide for the venting of fluid from the piston spring chamber. Axial movement of the piston242is thereby assisted. However, whereas the original set of vent holes368are provided in the piston242for venting of fluid from the piston spring chamber into the piston bore258, the second set of vent holes369are provided in the second body member208and thereby allow venting of fluid from the piston spring chamber to the exterior of the tool302(i.e. in use, to a wellbore annulus). Each set of vent holes368,369comprises four holes (although, for either set, an alternative number of holes may be provided).

One of the two sets of vent holes368,369should be occluded depending on particular operational requirements. In the arrangement shown inFIGS. 13 to 16, each hole369of the second set of vent holes is occluded with a NPT plug. Venting is therefore achieved via the original set of vent holes368. However, the original set of vent holes368may alternatively be occluded with NPT plugs with the second set of vent holes369being used to vent the piston spring chamber (as shown inFIGS. 14 to 16). With the second set of vent holes369open, the piston spring chamber becomes filled, in use, with wellbore fluid. The piston242is thereby exposed to wellbore fluid static pressure. This external fluid pressure will be less than the fluid pressure within the piston bore258when fluid is being pumped from the surface through the apparatus. With the annulus fluid pressure less than that in the piston bore258, the resultant axial force will act in a downhole direction and have a greater magnitude than if the spring chamber was vented to the piston. This can have the benefit of reducing a tendency for the piston242to undesirably cycle due to vibration. In other words, the pressure differential across the length of the piston will hold the piston in a half-down position so the apparatus remains in a closed configuration whilst a drilling operation is completed.

When the improved sub302is arranged so that the spring chamber is vented to the wellbore annulus, the flow rate required to move the piston will be lower than when the spring chamber is vented to the piston bore258. Also, when venting to the wellbore annulus, the improved sub302may be provided with a larger piston bore (or a piston nozzle having a larger bore). This can be advantageous since pressure losses across the sub302may be thereby reduced to allow increased pressure and greater system flow rates to be applied during drilling operations.

A high degree of axial vibration can occur when drilling hard rock formations and it is critical to drilling performance that the piston242is prevented from bouncing to such an extent that the body apertures40are opened. In venting the spring chamber to the annulus, the flow rate used during drilling will be considerably higher than the flow rate required to move the piston. In other words, during a drilling operation, the flow rate through the piston bore258will be sufficient to force the piston242downhole and retain the piston242in a closed position against uphole forces generated by axial vibration. In contrast, when the spring chamber is vented to the piston bore258, the additional flow rate, used during drilling, over that used to move the piston242, is reduced to an extent whereby there may be insufficient downhole force applied to the piston242to resist an uphole bouncing of the piston242. Regardless of whether the spring chamber is vented to the annulus or the piston bore, an undesirable bouncing of the piston242can be limited by reducing the cross-sectional area of the flow passage from the spring chamber. In this way, the ease with which fluid may flow into the spring chamber so as to allow an uphole movement of the piston242is limited. Piston movement is thereby provided with a degree of dampening. The cross-sectional area of the vent passage may be reduced by occluding one or more vent holes with a plug or partially occluding one or more vent holes with a plug having one or more apertures provided therein.

With particular regard to the expanded partial views shown inFIGS. 14 to 16, it will be seen that the improved sub302comprises modifications to the arrangement of O-ring seals located between the second body member208, the sleeve226, and the piston242. The improved sub302includes an additional O-ring seal380between the piston242and the sleeve226, and a further additional O-ring seal382between the sleeve226and the second body member208. The first additional O-ring seal380ensures that fluid within the piston bore258does not leak to the wellbore annulus via the flow ports72and second set of vent holes369. The second additional O-ring seal382ensures that wellbore fluid cannot flow between the second body member208and the sleeve226via the body apertures40. This is of particular importance when the second set of vent holes369are occluded so as to prevent ingress of wellbore fluid into the piston spring chamber. Without the second additional O-ring seals382, wellbore fluid would flow into the piston spring chamber when the sub302is arranged in the emergency closed position.

A third additional O-ring seal384is provided between the sleeve226and the second body member208so that, when in the emergency closed position, wellbore fluid is prevented from accessing the flow ports72in the piston242via the body apertures40. A fourth additional O-ring seal386is located between the sleeve226and the second body member208so as to assist in ensuring that fluid flows between the piston bore258and the wellbore annulus via the flow ports72and body apertures40without undesirable leakage between the sleeve226and the second body member208. Also, a PTFE bearing support ring388is mounted on the sleeve226so as to assist in relative rotation between the sleeve226and the piston242. A yet further modification is the provision of a location rim (i.e. an annular recess) on the uphole end of the sleeve226for receiving the downhole end of the spring. The spring location rim is not apparent in the enclosed drawings. It will be understood that any of the aforementioned O-ring seals may be replaced with or types of static seal.

Although the improvements shown inFIGS. 13 to 16have been described as modifications to the third tool shown inFIGS. 9 to 12, the described modifications may also be advantageously applied to the tools shown inFIGS. 1 to 8. Indeed, the second embodiment of the present invention shown inFIG. 17is a modification of the tool shown inFIGS. 5 to 8of the accompanying drawings. Apart from the modifications described below, the improved circulating sub402ofFIG. 17is substantially identical to the circulating sub102ofFIGS. 5 to 8and, accordingly, lice reference numerals have been used to identify like components in the accompanying drawings.

As with the first embodiment302, the second embodiment402comprises two sets of vent holes468,469for venting the piston spring chamber. The set of vent holes469provided in the body comprises a single vent hole. Each hole of the second set of vent holes468is occluded with a NPT plug. Also, the body of the sub402is provided with a set of apertures440for allowing fluid communication between the bore of the sub and the exterior thereof. Each aperture440is provided as a fluid passageway arranged to direct fluid (flowing therethrough from the sub bore) in an uphole direction. To this end, each fluid passageway440has a longitudinal axis orientated at an acute angle to and in the same plane as the longitudinal axis of the sub402. Each passageway440is also provided with a nozzle441. The plurality of flow ports472provided in the piston142communicate with the single body aperture440by means of an annular fluid communication groove443. The annular groove443is provided in the interior surface of the body. The uphole orientation of the body aperture440results in an uphole flow of annulus fluid being boosted by fluid exiting the body aperture440with an uphole flow component.

A further modification provided to the sub402is the provision of three sets of stabiliser blades445immediately downhole of the body aperture440. Furthermore, the sleeve426may be provided in two components so as to ease manufacture. The two components of the sleeve426may be pinned or screw threadedly engaged with one another. The first embodiment shown inFIGS. 13 to 16may also be provided with a multi-piece sleeve to assist with manufacture.

Furthermore, as already mentioned in relation to the tools ofFIGS. 1 to 12, a dart may be run so as to block the piston bore and allow a sufficient build up of pressure to move a tool into the emergency closed configuration. The first embodiment of the present invention is shown inFIG. 16located in the emergency closed configuration with a dart390blocking the piston bore258. The dart390is shown in greater detail inFIG. 13wherein it can be seen that the dart comprises a through bore392occluded at an uphole end thereof by a burst disc394. The use of such a dart390allows fluid to be pumped through the sub302once the sub302has been moved to the emergency closed position. This is achieved by increasing the fluid pressure within the sub302so as to rupture the burst disc394and thereby allow access to the dart through bore392. The pressure required to rupture the burst disc394will be greater than that required to shear the pin30.

The present invention is not limited to the specific embodiments described above. Variations and modifications will be apparent to the reader skilled in the art.