DOWNHOLE DRAG REDUCTION APPARATUS

Abstract

A downhole drag reducing apparatus 18 comprises a mandrel 20 and a bearing sleeve 24 mounted on the mandrel 20, such that the mandrel 20 and bearing sleeve 24 are rotatable relative to each other. The bearing sleeve 24 defines a bore wall engaging surface. The apparatus 18 comprises a reciprocating piston 23 mounted within a piston housing 25 to define a piston chamber 27. The apparatus 18 further comprises a rotary valve assembly 29 operated by relative rotation between the mandrel 20 and the bearing sleeve 24 to cyclically pressurise and depressurise the piston chamber 27 to provide reciprocating movement of the reciprocating piston 23 and the generation of vibration within the apparatus 18.

Claims

1. A downhole drag reducing apparatus, comprising: a mandrel; a bearing sleeve mounted on the mandrel such that the mandrel and bearing sleeve are rotatable relative to each other, the bearing sleeve defining a bore wall engaging surface; a reciprocating piston mounted within a piston housing to define a piston chamber; and a rotary valve assembly operated by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise the piston chamber to provide reciprocating movement of the reciprocating piston and the generation of vibration within the apparatus.

2. The downhole apparatus of claim 1, wherein the rotary valve assembly comprises a valve inlet and a valve exhaust, wherein the rotary valve assembly is operated between a pressure configuration and an exhaust configuration, wherein in the pressure configuration, the piston chamber is in pressure communication with the valve inlet and isolated from the valve exhaust, and in the exhaust configuration, the piston chamber is isolated from the valve inlet and in pressure communication with the valve exhaust.

3. (canceled)

4. The downhole apparatus of claim 2, comprising a rotary valve member operatively associated with the valve inlet and the valve exhaust, wherein the rotary valve member is configured to provide selective communication between at least one of the valve inlet and valve exhaust and the pressure chamber.

5. (canceled)

6. The downhole apparatus of claim 4, wherein the rotary valve member is rotatably fixed with respect to the bearing sleeve and comprises one or more flow passageways.

7. The downhole apparatus of claim 2, wherein the mandrel comprises the valve inlet and valve exhaust, and wherein the valve inlet and the valve exhaust are rotatable with respect to the one or more flow passageways.

8. The downhole apparatus of claim 4, wherein the rotary valve member is configured to selectively block or obturate at least one the valve inlet and valve exhaust.

9. The downhole apparatus of claim 7, wherein the mandrel comprises one or more flow passages extending through a wall thereof, the flow passages defining or communicating with the valve inlet.

10. The downhole apparatus of claim 7, wherein the mandrel comprises one or more flow channels defining or communicating with the valve exhaust.

11. The downhole apparatus of claim 1, comprising a retaining shoulder rotabably fixed with respect to the mandrel and located axially between the rotary valve assembly and the piston chamber.

12. The downhole apparatus of claim 11, wherein the retaining shoulder comprises one or more flow paths permitting pressure communication between at least one of the valve inlet and valve exhaust and the pressure chamber.

13. The downhole apparatus of claim 12, wherein the one or more flow paths comprises a plurality of flow paths, and the plurality of flow paths are circumferentially distributed.

14. The downhole apparatus of claim 1, wherein the reciprocating piston travels in a first axial direction and a second axial direction within the piston chamber, and the apparatus comprises a biasing arrangement biasing the reciprocating piston in the second axial direction, wherein pressurisation of the piston chamber urges the reciprocating piston to move in the first axial direction.

15. (canceled)

16. The downhole apparatus of claim 14, wherein the biasing arrangement acts on the reciprocating piston with a force sufficient to move the reciprocating piston in the second axial direction to depressurise the piston chamber.

17. The downhole apparatus of claim 14, wherein the biasing arrangement comprises one or more springs circumferentially arranged with respect to the reciprocating piston.

18. (canceled)

19. The downhole apparatus of claim 11, wherein the reciprocating piston comprises a first impact surface for impacting a second impact surface formed on an impact shoulder provided on or formed with the mandrel, and the reciprocating piston comprises a third impact surface for impacting a fourth impact surface formed on the retaining shoulder, and wherein impact of the first and third impact surfaces and/or impact of the second and fourth impact surfaces generates vibration within the apparatus.

20. The downhole apparatus of claim 19, wherein the first and third impact surfaces and/or the second and fourth impact surfaces do not impact one another, the reciprocating movement of the reciprocating piston being sufficient to generate vibration within the apparatus.

21. (canceled)

22. The downhole apparatus of claim 1, wherein the mandrel defines a circumferentially continuous inner bearing race, and wherein the bearing sleeve defines a circumferentially continuous outer bearing race, the bearing sleeve being mounted on the mandrel such that the outer bearing race of the bearing sleeve circumscribes the inner bearing race of the mandrel.

23. The downhole apparatus of claim 22, comprising a rolling bearing arrangement radially interposed between the inner bearing race of the mandrel and the outer bearing race of the bearing sleeve to permit the bearing sleeve and the mandrel to be rotatable relative to each other.

24. The downhole apparatus of claim 22, wherein at least one of the outer bearing race and inner bearing race comprises a bearing raceway for axially captivating the rolling bearing arrangement.

25. The downhole apparatus of claim 1, wherein the bearing sleeve comprises a varying outer diameter and includes a central region which defines an outer gauge diameter of the downhole drag reducing apparatus defining the bore wall engaging surface, and axial end regions which define a smaller diameter than the central region.

26. (canceled)

27. The downhole apparatus of claim 25, wherein the bearing sleeve comprises an extended section extending from the axial end region located adjacent the first axial shoulder, the extended section defining the piston housing.

28. A method for performing a wellbore operation, comprising engaging a bore wall with a bore wall engaging surface of a bearing sleeve, wherein the bearing sleeve is mounted on a mandrel; rotating the mandrel relative to the bearing sleeve while the bearing sleeve is engaged with the bore wall; operating a rotary valve assembly by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise a piston chamber to provide reciprocating movement of a reciprocating piston mounted within a piston housing defining the piston chamber; and generating vibration within the apparatus by the reciprocating movement of the reciprocating piston.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0167] These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0168] FIG. 1 is a diagrammatic illustration of a wellbore under construction using a drill string which includes multiple downhole drag reducing apparatus;

[0169] FIG. 2 is a longitudinal cross-sectional view of a downhole drag reducing apparatus;

[0170] FIG. 3 is a lateral cross-sectional view of the downhole drag reducing apparatus, taken along line A-A;

[0171] FIG. 4 is a lateral cross sectional view of the torque reducer of FIG. 2, taken along line B-B;

[0172] FIG. 5 is a perspective view of the downhole drag reducing apparatus of the FIG. 2;

[0173] FIG. 6 is a perspective view of a rotary valve member used in the downhole drag reducing apparatus of FIG. 2;

[0174] FIG. 7 is a lateral cross-sectional view of the rotary valve member of FIG. 6;

[0175] FIG. 8 is a perspective view of the a reciprocating piston used in the downhole drag reducing apparatus of FIG. 2;

[0176] FIG. 9 is a lateral cross-sectional view of the reciprocating piston of FIG. 8;

[0177] FIG. 10 is an enlarged longitudinal cross-sectional view of the downhole drag reducing apparatus of FIG. 2, in a pressure configuration;

[0178] FIG. 11 is a lateral cross-sectional view of the downhole drag reducing apparatus of FIG. 10, taken along line C-C;

[0179] FIG. 12 is a lateral cross-sectional view of the downhole drag reducing apparatus of FIG. 10, taken along line D-D;

[0180] FIG. 13 is an enlarged longitudinal cross-sectional view of the downhole drag reducing apparatus of FIG. 2, in an exhaust configuration;

[0181] FIG. 14 is a lateral cross-sectional view of the downhole drag reducing apparatus of FIG. 13, taken along line C-C;

[0182] FIG. 15 is a lateral cross-sectional view of the downhole drag reducing apparatus of FIG. 13, taken along line D-D;

[0183] FIG. 16 illustrates an alternative embodiment of a drag reducing apparatus; and

[0184] FIG. 17 illustrates the downhole drag reducing apparatus used in a retrieving operation.

DETAILED DESCRIPTION

[0185] FIG. 1 diagrammatically illustrates a wellbore 10 under construction using a drill string 12 which is rotated and advanced from a drilling rig 14 located at surface. The drilling rig 14 may be of any suitable form, including a land based rig, offshore rig such as a jack up, a semi-submersible platform, a drillship and/or the like. In the present example, the wellbore 10 is a deviated bore which includes a vertical bore section 10a, a build-up section 10b and a horizontal section 10c. However, the present disclosure extends to any form of wellbore. Further, although not illustrated the wellbore 10, in some sections, may be cased or lined, with or without cementing.

[0186] The drill string 12 may be formed of multiple jointed drill pipe and collars and includes a drilling bottom hole assembly (BHA) 16 which may incorporate conventional equipment such as a drill bit, stabilisers, measurement while drilling (MWD) equipment, directional drilling equipment and/or the like. The drill string 12 further includes multiple drag reducing apparatus 18 distributed along its length. As will be described in more detail below, the drag reducing apparatus comprises a sleeve rotatably mounted on a mandrel which is rotatably connected to the drill string 12, such that when the sleeve engages a wall of the wellbore 10, the mandrel, and the drill string 12, may more freely rotate. The relative rotation between the sleeve and mandrel also causes reciprocation of a reciprocating piston which generates a vibration effect, which may also assist in reducing the effect of drag forces, for example by more readily breaking static friction, by lowering dynamic friction, etc.

[0187] A longitudinal cross-sectional view of a drag reducing apparatus 18 used in the example of FIG. 1 is illustrated in FIG. 2.

[0188] The drag reducing apparatus 18 comprises a mandrel 20 and a bearing sleeve 24 mounted on the mandrel 20 such that the mandrel 20 and bearing sleeve 24 are rotatable relative to each other. The bearing sleeve 24 defines a bore wall engaging surface. The drag reducing apparatus 18 further comprises a reciprocating piston 23 mounted within a piston housing 25 to define a piston chamber 27. As discussed in more detail below, a rotary valve assembly 29 is operated by relative rotation between the mandrel 20 and the bearing sleeve 24 to cyclically pressurise and depressurise the piston chamber 27 to provide reciprocating movement of the reciprocating piston 23 and the generation of vibration within the apparatus 18.

[0189] In this example, the piston housing 25 is formed as a unitary component with the bearing sleeve 24. Therefore, hereinafter, unless stated or implied otherwise, any reference to the “bearing sleeve 24” should be construed as including the bearing sleeve 24 and the piston housing 25.

[0190] The drag reducing apparatus 18 may function to minimise friction between the drill string 12 and the wall of the wellbore 10 (which may be open-hole or cased/lined). During downhole use in a wellbore, engagement with the bore wall will cause the bearing sleeve 24 to be rotationally held. Thus, in this situation rotation of the mandrel will produce relative rotation between mandrel 20 and the bearing sleeve 24, thereby operating the rotary valve assembly 29 and providing reciprocating movement of the reciprocating piston 23, as described in more detail below. Such reciprocating movement of the reciprocating piston 23 generates vibration within the apparatus 18, which may assist in overcoming the friction, both static and dynamic friction, experienced when the apparatus 18 is engaged with the bore wall. Moreover, the relative rotation between the mandrel 20 and the bearing sleeve 24 may function to minimise friction between the work string 12, when rotated, and the bore wall of the wellbore 10, thus reducing the drag torque experience by the work string 12. In this respect, the bearing sleeve 24 may isolate the mandrel 20 from drag torque interactions with the bore wall, to thus maximise energy transfer between the drilling rig 14 and the drilling BHA 16. This may in turn reduce the drive torque requirements at the surface. The reduced friction effect may be of particular benefit in the build-up and horizontal sections 10a, 10c of the wellbore, and may permit further extended reach wellbores to be formed.

[0191] The mandrel 20 defines an inner bearing race 22, and the bearing sleeve 24 defines a corresponding outer bearing race 26. The bearing sleeve 24 is mounted on the mandrel 20 such that the inner and outer bearing races 22, 26 are aligned. A rolling bearing arrangement 28 is radially interposed between the mandrel 20 and the bearing sleeve 24 to permit relative rotation therebetween.

[0192] The mandrel 20 includes opposing end connectors 30, 32 (in the form of pin and box type connectors) for facilitating connection to the drill string 12. The mandrel 20 defines an axial throughbore 31.

[0193] The mandrel 20 and the bearing sleeve 24 are each of unitary construction such that the inner and outer bearing races 22, 26 are circumferentially continuous. However, it should be recognised that in some examples the mandrel and/or bearing sleeve 24 may be formed of multiple connected components while still allowing the bearing races 22, 26 to be circumferentially continuous. For example, one or both of the mandrel 20 and the bearing sleeve 24 may comprise separate interconnected axial portions, wherein the inner and/or outer bearing races 22, 26 are defined by a unitary axial component.

[0194] The provision of circumferentially continuous inner and outer bearing races 22, 26 may afford more stability and strength within both the mandrel 20 and the bearing sleeve 24, minimising failure modes associated with clamshell type devices and their corresponding connection means. For example, the full hoop strength of the bearing sleeve 24 and the mandrel 20 may be retained. Furthermore, the continuous construction of the bearing races 22, 26 may provide continuous running surfaces for the rolling bearing arrangement 28, resulting in more even load distribution. Also, the ability to provide a form of sealing between the mandrel 20 and the bearing sleeve 24, as described below, may be improved. However, in other examples, the inner and outer bearing races may not define a continuous structure (e.g., they could be of a clam shell design).

[0195] In the present example the inner bearing race 22 formed by the mandrel 20 comprises a plurality of axially arranged bearing raceways in the form of circumferential grooves 34. The rolling bearing arrangement 28 comprises a plurality of rolling elements 36 (e.g., rollers, balls, needles etc.) located within each circumferential groove 34, such that a plurality of circumferential arrays of rolling elements 36 is provided, with one circumferential array illustrated in FIG. 3, which is a lateral sectional view of FIG. 2 taken along line A-A. Such axially distributed arrays of rolling elements 36 may provide for increased load bearing capability and allow for more stable support of the bearing sleeve 24 about the mandrel 20.

[0196] The outer bearing race 26 formed by the bearing sleeve 24 does not include any bearing raceway but rather a cylindrical bearing surface 38, along which the rolling elements 36 may roll. This arrangement may permit a thinner walled bearing sleeve 24 to be provided, in that additional wall thickness to accommodate the formation of one or more raceways therein is not required. Further, by providing the raceway grooves 34 only in the inner bearing race 22 a degree of axial movement may be achieved between the mandrel 20 and the bearing sleeve 24. This might minimise axial loading applied on the rolling bearing elements 36 which may minimise rotational drag, increase operational longevity and the like.

[0197] A bearing cavity 40 is formed between the mandrel 20, bearing sleeve 24 and first and second axial seals 42, such that the rolling bearing arrangement 28 is provided within the bearing cavity. The bearing cavity 40 is configured to be at least partially (incompletely or completely) filled with a lubricant, such as oil, grease and/or the like. The bearing sleeve 24 comprises one or more lubricant ports 46 extending from an outer surface thereof to facilitate delivery (e.g., injection) of lubricant into the bearing cavity 40. The lubricant port(s) 46 may be sealed or sealable with a plug or equivalent structure.

[0198] In the present example the first and second seals 42 are the same, although this need not be the case. The first and second seals 42 each comprise a sealing member 44, which may have bi-directional sealing capabilities. The sealing members 44 are deformable and/or moveable, for example axially moveable, to accommodate volumetric changes of the lubricant within the bearing cavity 40 which may be caused by thermal expansion and contraction. By virtue of both the first and second seals 42 having deformable and/or moveable sealing members 44 the volumetric changes which can be accommodated may be maximised.

[0199] The deformable and/or moveable capability of the sealing members 44 also facilitates pressure balancing of the bearing cavity 40 with respect to an external location, such as an annulus region within a wellbore. Such pressure balancing may be achieved by virtue of the sealing members 44 being in pressure communication with both the bearing cavity 40 and the external region. In use, as the drag reducing apparatus 18 is deployed deeper into a wellbore the ambient hydrostatic pressure will increase, wherein the sealing members 40 function to allow the bearing cavity 40 to be maintained in pressure balance with the ambient hydrostatic pressure. As such, the pressure differential across the seals 42 may be minimised, which may facilitate improved sealing performance. When exposed to high hydrostatic (or other) pressures the risk of wellbore fluids entering the bearing cavity 40 under pressure is minimised, thus minimising the effect such wellbore fluids might have on components within the bearing cavity 40. This pressure balance effect during use may also provide a similar benefit when the drag reducing apparatus 18 is retrieved towards surface, in which case the hydrostatic pressure will reduce, such that the risk of lubricant being ejected under pressure is minimised.

[0200] The sealing members 44 may be formed of any material which suits the application. In some examples the sealing members 42 may comprise a polymeric material, such as PTFE, an elastomer and/or the like.

[0201] The first and second seals 42 each further comprise a wiper seal 46, for example a scarf cut wiper seal, adjacent an outer side (relative to the bearing cavity 40) of the associated sealing members 44. The wiper seals 46 may function in combination with the sealing members 44 in order to ensure a clean sealing surfaces on the mandrel 20 and bearing sleeve 24 are maintained.

[0202] In the present example the bearing sleeve 24 comprises a varying wall thickness between opposing axial ends thereof, wherein such a varying wall thickness is provided via variations in the outer surface of the bearing sleeve 24 such that the inner surface of the bearing sleeve 24 may comprise a constant or uniform profile. More specifically, a central axial region of the bearing sleeve 24 defines a region of increased wall thickness relative to the axial end regions such that the central region defines the maximum outer gauge diameter of the drag reducing apparatus 18. As such, this central region may define the bore wall engaging surface which engages a bore wall during use. Further, this central region also circumscribes the rolling bearing arrangement 28 such that the thicker wall section has increased load bearing capacity. In addition, the bore wall engaging surface comprises one or more protruding structures or fins 49 to engage with the wall of the bore. The protruding structures 49, when engaged with a bore wall, may assist in providing relative rotation between the mandrel 20 and the bearing sleeve 24. In other examples, the protruding structures 49 may comprise ribs, vanes, humps, posts, dimples and/or the like.

[0203] The thinner walled axial end regions of the bearing sleeve 24 are aligned with the first and second seals 42. That is, the thinner walled axial end regions circumscribe the first and second seals 42. These thinner walled end regions are less likely to engage a bore wall (in view of the thicker central region) such that radial forces imparted on the seals 42 may be minimised.

[0204] The bearing sleeve 24 may comprise an extended section extending from one of the axial ends thereof, the extended section forming the piston housing 25. The bearing sleeve 24 is held in place between first and second axial shoulders 50, 52 provided on the mandrel 20, wherein a wear bush 54 is interposed between the axial ends of the bearing sleeve 24 and a respective axial shoulder 50, 52.

[0205] In the present example, the first axial shoulder 50 is integrally formed on the mandrel via an annular stepped profile, whereas the second axial shoulder 52 is separately formed and securable on the mandrel 20. Such an arrangement may permit the bearing sleeve 24 to be slid over an end of the mandrel 20 (e.g. over connector 30) with the second axial load shoulder 52 subsequently being installed to retain the bearing sleeve 24 in place. However, in other examples the second axial load shoulder may also be integrally formed with the mandrel 20. For example, some or all of the drag reducing apparatus 18 may be formed by additive manufacturing techniques.

[0206] The second load shoulder 52 comprises or is formed on a retaining arrangement 58 which includes a retaining mount 60 axially and rotatably secured to the mandrel 20, and a retaining ring 62 threadedly secured over the retaining mount 60. Referring additionally to FIG. 4, which is a lateral cross-sectional view of FIG. 2 taken along line B-B, the retaining mount is formed of two half segments 60a, 60b which can be mounted around the outer surface of the mandrel 20 to form a mounting ring structure. The retaining mount segments 60a, 60b each include a radial tab portion 64 which are received within a circumferential groove 66 formed in an outer surface of the mandrel in order to axially fix the segments 60a, 60b to the mandrel 20. The retaining mount segments 60a, 60b also collectively define a faceted or non-round internal profile 68 which is mounted on a corresponding faceted or non-round outer profile 70 on a portion of the mandrel 20, to thus allow the segments 60a, 60b of the retaining mount 60 to be rotatably secured to the mandrel 20.

[0207] The retaining mount segments 60a, 60b collectively define an outer threaded surface of the retaining mount 60 which is engaged by an inner threaded surface of the retaining ring 62, wherein the retaining ring 62, once threaded onto the retaining mount 60 functions to retain the retaining mount segments 60a, 60b on the mandrel 20. The retaining mount segments 60a, 60b also collectively define a torque shoulder 72 against which the retaining ring 62 may be torqued or tightened.

[0208] The rotary valve assembly 29 comprises a valve inlet 74, a valve exhaust 76 and a rotary valve member 80 operatively associated with the valve inlet 74 and valve exhaust 76. The valve inlet 74 is defined by one or more flow passages 73 formed through the mandrel 20, which define inlet openings on an inside (circumferential) surface of the mandrel 20.

[0209] The valve exhaust 76 is defined by one or more flow channels 77 (visible in FIG. 5) formed on the mandrel 20 and one or more outlet ports 78 formed through the bearing sleeve 24. The rotary valve member 80 is rotatably connected to the bearing sleeve 24 and operatively associated with the valve inlet 74 and valve exhaust 76 so as to selectively prevent communication with the piston chamber 27. The mandrel 20 includes a retaining shoulder 81 formed on a surface thereof (discussed in more detail below).

[0210] The reciprocating piston 23 is operatively connected to a spring arrangement 82 biasing the reciprocating piston 23 in the second axial direction B. The spring arrangement 82 is disposed in a spring chamber 84 of the mandrel 20 and may be partially disposed in one or more apertures 87 formed in an end of the reciprocating piston 23. The spring arrangement 82 is connected to and supported by an impact shoulder 90 formed on the mandrel 20. The spring coefficient or stiffness of the spring arrangement 82 may be selected in accordance with vibration requirements of the apparatus 18, such as frequency and force of vibration.

[0211] The bearing sleeve 24 is provided with one or more lateral ports arranged adjacent the spring chamber 84 to provide pressure balancing of the spring chamber 84 as the reciprocating piston 23 moves back and forth in the piston chamber 27 in the first and second axial directions A, B. Moreover, the piston chamber 27 is sealed by the provision of a dynamic seal arrangement 91 provided on the reciprocating piston 23 and configured to seal against the mandrel 20 and bearing sleeve 24.

[0212] The rotary valve assembly 29 is operated by relative rotation between the mandrel 20 and the bearing sleeve 24 to be cyclically reconfigured between a pressure configuration and an exhaust configuration. As described above, such relative rotation is beneficially provided by the bearing sleeve 24 when it is engaged with a wall of the wellbore.

[0213] In the pressure configuration, the piston chamber 27 is in pressure communication with the valve inlet 74, and isolated from the valve exhaust 76, to permit the piston 24 to move in the first axial direction A in accordance with the piston chamber 27 being pressurised via the valve inlet 74. In FIG. 2, the apparatus 18 is illustrated in the pressure configuration, with the reciprocating piston 23 being urged towards the impact shoulder 90.

[0214] In the exhaust configuration, the piston chamber 27 is isolated from the valve inlet 74 and in pressure communication with the valve exhaust 76 to permit the piston chamber 27 to be depressurised. To achieve this, the reciprocating piston 23 is urged by the spring arrangement 82 to move in the second axial direction B and to displace the pressure in the piston chamber 27 out via the valve exhaust 76. Such movement of the reciprocating piston 23 in the first and second axial directions A, B generates vibration within the apparatus 18.

[0215] In use, the apparatus 18 is used in combination with a pressure region P and an exhaust region E, wherein the valve inlet 74 communicates with the pressure region P and the valve exhaust 76 communicates with the exhaust region E, such that a pressure differential is applied across the rotary valve assembly 29. Specifically, the pressure within the pressure region P may be elevated above the pressure in the exhaust region E. In particular, the pressure within the pressure region P may be sufficient (for example sufficiently high) to pressurise the piston chamber 27 to cause the reciprocating piston 23 to overcome the biasing force of the spring arrangement 82 and move in the first axial direction A. Moreover, the pressure within the exhaust region E may be sufficient (for example sufficiently low) to permit the piston chamber 27 to be depressurised and the reciprocating piston 23 to move in the second axial direction B by the force provided by the spring arrangement 82 releasing potential energy stored from the down stroke of the reciprocating piston 23.

[0216] However, the present downhole drag reducing apparatus 18 may be configured to operate irrespective of the direction of the pressure differential applied across the rotary valve assembly 29. In one mode of operation, suggested above, the pressure of the pressure region P is higher than the pressure of the exhaust region E. However, should the pressure differential be reversed then what was previously the pressure region P becomes the exhaust region E, and vice versa, and what was previously the valve inlet becomes the valve exhaust, and vice versa.

[0217] Referring to FIG. 5, a perspective view of the downhole drag reducing apparatus 18 is illustrated, in which some components have been removed for clarity, such as the reciprocating piston 23, the rotary valve member 80, the bearing sleeve 24 and the roller bearing arrangement 28.

[0218] Referring to FIG. 6, the rotary valve member 80 comprises a series of circumferential teeth or grooves 82 for connection with the bearing sleeve 24 via a spline arrangement 83 (illustrated in FIG. 7). As such, when the apparatus 18 is engaged with a bore wall, the rotary valve member 80 is rotationally held with the bearing sleeve 24, such that rotation of the mandrel 20 produces relative rotation between the mandrel 20 and the rotary valve member 80. The rotary valve member 80 includes a tubular ring having first and second parts 80a, 80b, which form a clamshell design.

[0219] The rotary valve member 80 comprises a series of flow passageways 67 formed through a body 69 of the rotary valve member 80 for selectively permitting the valve inlet 74 and valve exhaust 76 to communicate with the piston chamber 27. During one phase of relative rotation (e.g. in the pressure configuration), the valve inlet 74 defines an open configuration in which pressure communication with the piston chamber 27 is permitted via the flow passageways 67 of the rotary valve member 80, and the valve exhaust 76 defines a closed configuration in which pressure communication with the piston chamber 27 is prevented by the body 69 of the rotary valve member 80, such that the piston chamber 27 may be pressurised. In another phase (e.g. at some point during the transition between the pressure configuration and the exhaust configuration), both the valve inlet 74 and the valve exhaust 76 may define a closed configuration in which pressure communication is prevented by the body 69 of the rotary valve member 80, such that pressure is contained in the piston chamber 27. In yet another phase (e.g. the exhaust configuration), the valve exhaust 76 defines an open configuration in which pressure communication with the piston chamber 27 is permitted via the flow passageways 67 of the inlet selectors 85, and the valve inlet 74 defines a closed configuration in which pressure communication with the piston chamber 27 is prevented by the body 69 of the rotary valve member 80, such that the piston chamber 27 may be depressurised.

[0220] As mentioned above, the mandrel 20 includes a retaining shoulder 81 formed on a surface thereof. The retaining shoulder 81 is axially interposed between the reciprocating piston 23 and the rotary valve member 80, and is configured to retain the rotary valve member 80 in place. The retaining shoulder 81 is in the form of a tubular ring having a series of one or more circumferentially spaced notches forming flow paths 86 for pressure to be communicated between the piston chamber 27 and the valve inlet 74 or the valve exhaust 76 (depending on the relative rotation of the mandrel 20 and rotary valve member 80). Interposed between each of the flow paths, the retaining shoulder 81 comprises one or more impact structures 71. The flow paths 86 are arranged in alignment with the valve inlet 74 and valve exhaust 76, such that when the valve inlet 74 or valve exhaust 76 are communicating with the rotary valve member 80, the flow paths 86 are configured to permit pressure to be communicated to or from the piston chamber 27. However, when neither of the valve inlet 74 and valve exhaust 76 are communicating with the rotary valve member 80 (i.e. at a point during the transition between the pressure and exhaust configurations), the impact structures 71 of the retaining shoulder 81 and the body 69 of the rotary valve member 80 may together enclose the piston chamber 27. In both the pressure and exhaust configurations, fluid pressure is communicated to or from the piston chamber 27 via the flow paths 86 of the retaining shoulder 81. In this example, the retaining shoulder 81 is integrally formed with the mandrel 20, and thus rotates together with the mandrel 20.

[0221] Referring to FIG. 8, the reciprocating piston 23 comprises first and second parts 100a, 100b, also forming a clamshell design. Such a design permits the reciprocating piston 23 to be mounted on the mandrel 20 between the retaining shoulder 81 and the impact shoulder 90. The reciprocating piston 23 comprises a first impact surface 100 (e.g. a hammer formed with or coupled to the reciprocating piston 23), which in use is arranged to impact a second impact surface or anvil formed on the impact shoulder 90. Moreover, the reciprocating piston 23 comprises a third impact surface 102, opposing the first impact surface 100, and configured to impact a fourth impact surface or anvil formed on the retaining shoulder 81 of the mandrel 20. In this regard, the retaining shoulder 81 may function as a second impact shoulder for the reciprocating piston 23 to impact.

[0222] In use, the rotary valve assembly 29 is operable by relative rotation between the mandrel 20 and the bearing sleeve 24 to be cyclically reconfigured between the pressure configuration and the exhaust configuration, so as to move the reciprocating piston 23 in one of the first and second axial directions A, B, said movement of the reciprocating piston 23 engaging the respective impact surfaces to generate vibration force within the apparatus 18. However, the apparatus 18 may be configured so that the impact surfaces do not engage, said movement of the reciprocating piston 23 itself being sufficient to generate vibration or agitation forces. For example, the stiffness of the spring arrangement 82 might be such that the third and fourth impact surfaces are prevented from impacting one another.

[0223] FIG. 10 illustrates an enlarged longitudinal cross-sectional view of the reciprocating piston 23 and rotary valve assembly 29. In FIG. 10, the apparatus 18 is shown in the pressure configuration, in which the relative rotation between the mandrel 20 and the bearing sleeve 24 is such that the flow passages 73 of the valve inlet 74 are aligned with the flow passageways 67 of the rotary valve member 80. In this configuration, pressure communication between the pressure region P is permitted with the piston chamber 27 via the flow passages 73. Therefore, the reciprocating piston 23 is urged in the first axial direction A by virtue of fluid pressure in the piston chamber 27. FIGS. 11 and 12 illustrate cross-sectional views of the apparatus 18 shown in FIG. 10, taken along lines C-C and D-D, respectively.

[0224] FIG. 13 illustrates the exhaust configuration, in which the mandrel 20 has been rotated relative to the bearing sleeve 24 such that the axial channels 77 are now aligned with the flow passageways 67 of the rotary valve member 80. In this configuration, pressure communication between the exhaust region E is permitted with the piston chamber 27 via the flow channels 77 and outlet ports 78. Therefore, the reciprocating piston 23 is urged in the second axial direction B by virtue of the potential energy stored in the spring arrangement 82 from the down stroke of the reciprocating piston 23 being released. FIGS. 14 and 15 illustrate cross-sectional views of the apparatus 18 shown in FIG. 10, taken along lines C-C and D-D, respectively.

[0225] FIG. 16 illustrates an alternative embodiment of a drag reducing apparatus 118, in which a first rotary valve assembly 29a and first reciprocating piston 23a (of the same description as above) are provided on one side of the rolling bearing arrangement 28 (of the same description as above) and a second rotary valve assembly 29b and second reciprocating piston 23b (of the same description as above) are provided on the other side of the rolling bearing arrangement 28.

[0226] The first rotary valve assembly 29a may be arranged in an out-of-phase relationship with the second rotary valve assembly 29b, so that each of the reciprocating pistons 23a, 23b simultaneously travel in the first axial directions A, B together. That is, when the valve inlet 74a of the first rotary valve assembly 29a is in pressure communication with its respective piston chamber 27a (i.e. when the valve inlet 74a is aligned with the flow passageways 67a of the first rotary valve member 80a), the valve inlet (not visible) of the second rotary valve assembly 29b is prevented from communicating with its respective piston chamber 27b. Likewise, when the valve exhaust 76b of the second the rotary valve assembly 29b is in pressure communication with its respective piston chamber 27b (i.e. when the valve exhaust 76b is aligned with the flow passageways 67b of the second rotary valve member 80b), the valve exhaust (not visible) of the first rotary valve assembly 29a is prevented from communicating with its respective piston chamber 27a. The provision of two reciprocating pistons 23a, 23b may provide for additional impacts in the first and second axial directions A, B, and therefore increased vibration forces within the apparatus 118.

[0227] The example above illustrates the downhole drag reducing apparatus 18 used in the process of drilling a wellbore 10. However, the downhole drag reducing apparatus 18 may be used in any number of other operations. For example, FIG. 17 illustrates the apparatus 18 (defined in accordance with the description above) used in a retrieving operation, where an object 200 is to be retrieved from the wellbore 210.

[0228] FIG. 17 diagrammatically illustrates a wellbore comprising a vertical bore section 210a and a build-up section 210b (however, the wellbore 210 may also comprise a horizontal section). A number of drag reducing apparatus 18 are disposed along a rotating string 212 providing an axial pulling force on the object 200 in the direction of arrow 204, in order to retrieve the object 200 from the wellbore 210. A swivel 202 may be located between the object 210 and the rotating string 212 so as not to impart rotational movement to the object 200 during the retrieving operation.

[0229] The apparatus 18 may function to minimise friction, in particular static friction, experienced by the apparatus 18 and/or rotating string 212 when engaged with a bore wall of the wellbore 210, thus improving the efficiency of the retrieving operation.

[0230] In other applications, for example, the apparatus 18 may be used with a run-in operation.