DOWNHOLE ASSEMBLY TO MITIGATE HIGH FREQUENCY TORSIONAL OSCILLATION, AND OSCILLATION MITIGATION TOOL SUITABLE FOR USE IN A DOWNHOLE ASSEMBLY

Abstract

The invention relates to a downhole assembly to mitigate high frequency torsional oscillation, in particular the high frequency torsional oscillation which can occur in a drilling operation for geothermal energy or for oil and gas. The downhole assembly (10) has a rotary steerable tool (18) and a high frequency torsional oscillation mitigation tool (20), the high frequency torsional oscillation mitigation tool having a one-way coupling (100), the high frequency torsional oscillation mitigation tool being located above the rotary steerable tool. The invention also relates to an oscillation mitigation tool (20) suitable for use in a downhole assembly. The high frequency torsional oscillation mitigation tool has a first end, a second end, and a one-way coupling (100) between the first end and the second end. The tool has a first connector (142) at the first end and a second connector (144) at the second end, the first and second connectors being configured for releasable connection to other downhole components in use. The one-way coupling has a first part (134) and a second part (140), the first part being connected to rotate with the first end and the second part being connected to rotate with the second end. The one-way coupling has an engaged condition in which the first part (142) and the second part (144) can rotate together whereby drill string rotation can be communicated to the drill bit. The one-way coupling has a disengaged condition in which the first part can rotate relative to the second part.

Claims

1-22. (canceled)

23. A downhole assembly including a rotary steerable tool and a high frequency torsional oscillation mitigation tool, the high frequency torsional oscillation mitigation tool having: a first end, a second end, and a one-way coupling between the first end and the second end, a first connector at the first end and a second connector at the second end, the first and second connectors being configured for releasable connection to other downhole components in use, the one-way coupling having a first part and a second part, the first part being connected to rotate with the first connector and the second part being connected to rotate with the second connector, the one-way coupling having an engaged condition in which the first part and the second part can rotate together and a disengaged condition in which the first part can rotate relative to the second part, the first part being a tubular shaft and the second part being a sleeve surrounding the tubular shaft, one of the sleeve and tubular shaft carrying a number of rollers of circular cross-section, each of the rollers being located in a recess of the sleeve or tubular shaft respectively, each recess having a floor which is inclined from a radially shallower end to a radially deeper end, and in which the roller projects from the recess when located at the radially shallower end, each of the rollers comprising a set of aligned roller elements, the high frequency torsional oscillation mitigation tool being located above the rotary steerable tool.

24. The downhole assembly according to claim 23 in which the rollers are carried by the sleeve and are located in recesses of the sleeve.

25. The downhole assembly according to claim 23 in which the high frequency torsional oscillation mitigation tool is a first high frequency torsional oscillation mitigation tool, and in which the downhole assembly also has a second high frequency torsional oscillation mitigation tool located below the rotary steerable tool, the second high frequency torsional oscillation mitigation tool also having a one-way coupling.

26. The downhole assembly according to claim 23 in which the high frequency torsional oscillation mitigation tool is releasably connected directly to the rotary steerable tool.

27. The downhole assembly according to claim 23 in which the first connector is one of a pin connector and a box connector.

28. The downhole assembly according to claim 23 in which the second connector is one of a pin connector and a box connector.

29. The downhole assembly according to claim 23 in which the tubular shaft is rigidly connected to the first connector and the sleeve is rigidly connected to the second connector.

30. The downhole assembly according to claim 23 in which there is a radial gap between the tubular shaft and the sleeve, and in which each of the rollers extends across the gap when located at the radially shallower end.

31. The downhole assembly according to claim 23 in which each of the rollers is engaged by a resilient biasing means, the resilient biasing means urging the roller towards the shallower end of its recess.

32. The downhole assembly according to claim 31 in which the resilient biasing means comprises a set of aligned shorter resilient biasing elements.

33. The downhole assembly according to claim 31 in which the resilient biasing means is a cantilever spring.

34. The downhole assembly according to claim 23 in which at least a part of the high frequency torsional oscillation mitigation tool is lubricated by a fluid which surrounds the high frequency torsional oscillation mitigation tool in use.

35. The downhole assembly according to claim 23 in which at least a part of the high frequency torsional oscillation mitigation tool is lubricated by oil, in which the oil is isolated from a fluid which surrounds the high frequency torsional oscillation mitigation tool in use, and in which the high frequency torsional oscillation mitigation tool has a flow path for the fluid, the flow path including a flow restrictor.

36. The downhole assembly according to claim 35 having a pressure compensator, the pressure compensator being in communication with the flow path downstream of the flow restrictor.

37. The downhole assembly according to claim 23 having a locking mechanism for the one-way coupling.

38. The downhole assembly according to claim 37 in which the locking mechanism is actuated by a tensile force which exceeds a predetermined threshold.

39. The downhole assembly according to claim 37 in which the locking mechanism has a locked condition in which the relative rotation between the first part and the second part of the one-way coupling is restricted, and an unlocked condition in which relative rotation between the first part and the second part is permitted.

40. The downhole assembly according to claim 39 in which the locking mechanism is biased to its unlocked condition by a resilient biasing means.

41. The downhole assembly according to claim 37 in which the first part of the one-way coupling is movable axially relative to the second part of the one-way coupling, the relative axial movement transferring the locking mechanism between its unlocked and locked conditions.

42. The downhole assembly according to claim 37 in which the locking mechanism comprises a first set of gears and a cooperating second set of gears.

43. The downhole assembly according to claim 42 in which the first set of gears is rigidly connected to the first part of the one-way coupling and the second set of gears is rigidly connected to the second part of the one-way coupling.

Description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

[0063] FIG. 1 shows a schematic representation of a downhole assembly according to the invention, connected to a drill string;

[0064] FIG. 2 shows a longitudinal sectional view of a first embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0065] FIG. 3 shows a cross-section at Ill-Ill of the tool of FIG. 2;

[0066] FIG. 4 shows a side view of part of the resilient biasing means of the tool of FIGS. 2 and 3.

[0067] FIG. 5 shows a longitudinal sectional view of a second embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0068] FIG. 6 shows a cross-section at VI-VI of the tool of FIG. 5;

[0069] FIG. 7 shows a longitudinal sectional view of a third embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0070] FIG. 8 shows a cross-section at VIII-VIII of the tool of FIG. 7;

[0071] FIG. 9 shows a longitudinal sectional view of a fourth embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0072] FIG. 10 shows a cross-section at X-X of the tool of FIG. 9;

[0073] FIG. 11 shows a longitudinal sectional view of a fifth embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0074] FIG. 12 shows a cross-section of the one-way coupling of the tool of FIG. 11;

[0075] FIG. 13 shows an enlarged view of part of the cross-section of FIG. 12;

[0076] FIG. 14 shows a longitudinal sectional view of a sixth embodiment of high frequency torsional oscillation mitigation tool according to the invention;

[0077] FIG. 15 shows a sectional perspective view of part of the locking mechanism of the tool of FIG. 14, in the unlocking condition;

[0078] FIG. 16 shows a side view of part of the locking mechanism of the tool of FIG. 14, in the unlocking condition;

[0079] FIG. 17 shows a sectional perspective view of part of the locking mechanism of the tool of FIG. 14, in the locking condition; and

[0080] FIG. 18 shows a side view of part of the locking mechanism of the tool of FIG. 14, in the locking condition.

DETAILED DESCRIPTION

[0081] FIG. 1 shows a schematic representation of a downhole assembly 10 connected to a drill string 12 in a borehole 14. The downhole assembly comprises a drill bit 16, a rotary steerable tool 18 and a high frequency torsional oscillation mitigation tool 20. In FIG. 1 the borehole 14 is shown to be horizontal but that is not necessarily the case and the borehole could alternatively be vertical or at another angle.

[0082] In known fashion, the drill string 12 is connected to surface equipment (not shown), the surface equipment including means to rotate the drill string 12 and drill bit 16 in use. The drill string 12 is hollow whereby drilling fluid (mud) can be pumped down the borehole, the mud acting to lubricate the drill bit 16 and to carry drill cuttings back to the surface. The mud and entrained drill cuttings return to the surface along the annulus 22 surrounding the drill string.

[0083] In certain applications the downhole assembly may also contain a downhole (mud) motor which can provide at least some of the rotational force to the bit. The tool 20 may be located above or below the motor as desired.

[0084] A number of stabilizers will typically be provided along the length of the drill string 12 to centralise the drill string in the borehole 14. There may also be a near-bit stabilizer between the drill bit 16 and the rotary steerable tool 18 if desired, and perhaps also a stabilizer between the rotary steerable tool 18 and the high frequency torsional oscillation mitigation tool 20. It is expected, however, that in most applications the high-frequency torsional oscillation mitigation tool 20 will be connected directly to the rotary steerable tool and/or any additional measurement while drilling (MWD) or logging while drilling (LWD) equipment comprised in the downhole assembly.

[0085] The rotary steerable tool 18 may be constructed according to EP 1 024 245, although the invention is not limited to any particular rotary steerable tool. As above described, a rotary steerable tool can steer the drill bit 18 in a desired direction by forcing the rotating drill string away from the centre of the borehole 14 in a chosen direction.

[0086] The components which extend from the rotary steerable tool 18 to engage the borehole 14 and force the drill string 12 away from the centre of the borehole are not shown in FIG. 1 for simplicity (and because the many different specific componentries will be well-known to a skilled person).

[0087] The high frequency torsional oscillation mitigation tool 20 can carry stabilizer blades to centralise it in the borehole if desired. Alternatively, its radial position is determined by the rotary steerable tool 18, and/or by a stabilizer above (or below) the mitigation tool 20. It may in certain applications be desirable for the tool to carry stabilizer blades (or otherwise engage the borehole) as any surface of the tool which rubs against the borehole surface in use provides a frictional contact which causes drag and dissipates energy in the form of heat.

[0088] As above indicated, if desired the downhole assembly can include two high frequency torsional oscillation mitigation tools 20, i.e. with another tool 20 between the drill bit 16 and the rotary steerable tool 18.

[0089] Detailed structures of six embodiments of a high frequency torsional oscillation mitigation tool 120, 220, 320, 420, 520 and 620 are shown in the Figures; it will be understood that these embodiments are representative of the many different detailed structures which are encompassed by the present invention. All of the embodiments are represented in an orientation corresponding to that of FIG. 1, i.e. with the uphole end to the right and the downhole end to the left as drawn. It will be understood, however, that the orientation could be reversed without detriment to the invention. Also, whilst a pin connector is shown at the uphole end and a box connector is shown at the downhole end of each embodiment, the connector arrangement could be reversed, or alternatively the tool could have two pin connectors or two box connectors, as required in a particular application to connect to the other downhole components.

[0090] The high frequency torsional oscillation mitigation tool 120 of FIGS. 2 and 3 incorporates a freewheel clutch 100 as the one-way coupling. The freewheel clutch 100 comprises a number of rollers 130 of circular cross-section, each of which is located in an inclined recess 132 in the body or central shaft 134 of the tool. The central shaft 134 is tubular and has an internal conduit 136 by which the mud can flow to the drill bit 16. The shaft 134 is closely surrounded by a sleeve 140. There is a small radial gap between the shaft 134 and the sleeve 140 which is provided to minimise any sliding contact during relative rotation of the shaft and sleeve.

[0091] The recesses 132 are inclined from a radially deeper end to a radially shallower end (the rollers 130 are all located at the deeper end of their respective recess in FIG. 3). The incline in this embodiment is curved and concave but could alternatively be linear or curved and convex. It is arranged that the distance between the deeper end of each recess 132 and the sleeve 140 is slightly greater than the diameter of the rollers 130 whereas the distance between the shallower end of each recess 132 and the sleeve 140 is slightly smaller than the diameter of the rollers 130. Accordingly, when the shaft 134 rotates clockwise as viewed in FIG. 3, each roller is urged up the incline of its recess 132 towards the shallower end and moves into engagement with the sleeve 140. It is arranged that the engagement of all of the rollers 130 with the sleeve 140 is sufficient to lock the sleeve 140 to the shaft so that both components rotate together (i.e. the one-way coupling 100 is engaged).

[0092] It will be appreciated that the torque to rotate the drill bit is significant and the rollers 130 must accommodate that torque when locked to the sleeve 140, ideally without any relative rotation. The number and diameter of the rollers 130 is limited by the radial space within which the shaft 134 and sleeve 140 must be accommodated. The axial length is not so limited, however, and in order to accommodate the torque the rollers 130 are typically of significant length, perhaps 400 mm to 600 mm in a typical application.

[0093] The uphole pin connector 142 is connected rigidly to the shaft 134 (and in this embodiment is integral with the shaft 134). The downhole box connector 144 is rigidly connected to the surrounding sleeve 140 so that clockwise rotation of the shaft 134 as viewed in FIG. 3 (i.e. in the first direction) is communicated from the drill string 12 (or motor) to the rotary steerable tool 18 and the drill bit 16.

[0094] A resilient biasing means 146 is located in each of the recesses 132 to bias the rollers 130 up the incline of the recess 132, i.e. to help to ensure that the one-way coupling 100 is locked against relative rotation in the first direction. The resilient biasing means minimise backlash and enable the rollers to lock in the engaged condition quickly.

[0095] FIG. 4 shows a side view of part of one embodiment of resilient biasing means 146, specifically a canted spring of metal. It will be understood that the canted spring can be flattened somewhat from the condition shown (i.e. compressed in the direction towards the bottom of the page) and will seek to return to the unstressed condition. It will also be understood that the canted spring can be made in any length, and in practical applications may substantially match the length of the rollers 130). Other resilient biasing means can alternatively be provided, including for example a folded metal (accordion) spring, or an elastomeric rod. Another alternative is a cantilever spring as shown in the embodiment of FIGS. 11-13, which could be used in the present embodiment instead of the canted spring (and vice versa).

[0096] When the shaft 134 rotates anti-clockwise as viewed in FIG. 3, each roller 130 is urged down the incline of its recess 132 towards the deeper end (with a force sufficient to overcome the resilient biasing means 146). The rollers 130 move out of engagement with the sleeve 140 and the sleeve does not rotate with the shaft 134. Relative rotation of the drill string 12 in this (second) direction is therefore not communicated to the rotary steerable tool 18 and the one-way coupling 100 is disengaged.

[0097] It is arranged that the one-way coupling is engaged during normal operation of the downhole assembly 10, i.e. with the drill string 12 rotating clockwise when viewed downhole towards the drill bit. The shaft 134 rotates with the drill string 12 in this first direction to communicate normal drilling rotation to the drill bit. Normal drilling rotation is also communicated to the rotary steerable tool 18 to permit the drill bit 16 to be steered in a chosen direction.

[0098] In the presence of high frequency torsional oscillation, the downhole assembly and part of the drill string will oscillate rapidly clockwise and anticlockwise at the resonant frequency/frequencies. During periods of reverse rotation (i.e. with the shaft 134 rotating anti-clockwise relative to the sleeve 140), the one-way coupling 100 disengages so that minimal torque and energy transfer occurs and subsequently the release of energy back into the downhole assembly (as part of the natural resonance phenomena) is reduced or prevented. The likelihood of damage to the downhole assembly is thereby reduced, which furthermore reduces the likelihood that damaging high frequency torsional oscillations will build up in the drill string 12.

[0099] The mitigation tool 120 of FIGS. 2 and 3 has two sets of axial bearings 150a and two sets of radial bearings 150r, one of each set to each side of a joint 152 of the tool housing. The joint 152 rigidly connects an end cap (which includes the sleeve 140) to the box connector 144 and is necessary for assembly of this embodiment of mitigation tool 120.

[0100] The sets of bearings 150a,r, and also the one-way coupling 100, are lubricated by oil which is isolated from the surrounding mud. A pressure equaliser or compensator 154 is provided, which can slide to balance the pressure of the oil with that of the surrounding mud, in known fashion. Other compensation systems could alternatively be used, e.g. a bladder or an array of smaller axial pistons in the toroidal space occupied by the illustrated balance piston.

[0101] The mitigation tool 120 also has a mud flow restrictor as is common to downhole tools which are lubricated by oil. It will be appreciated that the pressure of the mud within the internal conduit 136 (i.e. upstream of the drill bit 16) is significantly greater than the pressure of the mud in the annulus 22 surrounding the tool (i.e. downstream of the drill bit). The tool necessarily includes seals to separate the mud from the oil lubricant and it is preferable that the seals are not required to withstand the pressure differential between the internal conduit 136 and the annulus 22. The mud flow restrictor is provided to reduce the pressure differential across the relevant parts of the tool and thereby reduce the likelihood of a seal failure.

[0102] It will be seen that mud can pass around the outside of an end nut 164 adjacent to the box connector 144. The mud flow restrictor 160 is located between the end nut 164 and a conduit 166. The restrictor 160 and conduit 166 together provide a controlled mud leak path from the internal conduit 136 to the annulus 22. The compensator 154 is located adjacent to the conduit 166 and the mud pressure upon the compensator is therefore approximately the same as the pressure in the annulus 22. The pressure of the lubricating oil can therefore be compensated to the annulus pressure, in known fashion. The mud flow restrictor 160, which in this embodiment comprises an inner and outer ring with a very small clearance, is used to manage the pressure drop and flow rate of the mud, in a similar fashion to a mud lubricated radial bearing.

[0103] It will be observed that in the embodiment of FIGS. 2-4 the recesses 132 are formed in the shaft 134 (and similarly in the embodiment of FIGS. 7-8). The embodiment of FIGS. 11-13 shows the opposite arrangement in which the recesses are formed in the surrounding sleeve 140. The two options are interchangeable in each embodiment.

[0104] The high frequency torsional oscillation mitigation tool 220 of FIGS. 5 and 6 differs from that of FIGS. 2 and 3 in using a one-way coupling in the form of a sprag clutch, the sprag clutch and bearings being lubricated by oil which is isolated from the mud. A suitable sprag clutch type is the cage freewheel SF available from Ringspann GmbH (see the website www.ringspann.com).

[0105] The sprag clutch 200 is shown in more detail in the cross-sectional view of FIG. 6. In known fashion, a set of sprags 230 are located in an annular gap between the shaft 234 and the sleeve 240. The sprags have a major dimension which is larger than the annular gap, and a minor dimension which is smaller than the annular gap. It will be understood that when the shaft 234 rotates relative to the sleeve in a chosen direction (in this embodiment the clockwise direction as viewed in FIG. 6), the sprags are driven to rotate so that their major axes are more closely aligned with the radial direction. The sprags 230 become wedged between the shaft 234 and sleeve 240 and can transmit torque from the shaft 234 to the sleeve 240. Relative rotation in the opposing direction, however (corresponding to anti-clockwise rotation of the shaft 234 as viewed in FIG. 6) causes the individual sprags 230 to rotate so that their major axes are less closely aligned with the radial direction (i.e. the sprags 230 rotate to an orientation with a smaller radial dimension). The sprags 230 will no longer be in driving contact with the shaft and/or sleeve and the sleeve 234 can rotate relative to the shaft 240. It will be seen that in this embodiment the uphole pin connector 242 is rigidly connected to the shaft 234 whereas the downhole box connector 244 is rigidly connected to the sleeve 240.

[0106] The embodiment of FIGS. 5 and 6 also uses sets of axial and radial bearings 250a,r. In this embodiment each set of bearings comprises needle roller bearings 250r for radial support, but could alternatively use plain bearings, or a combination thereof.

[0107] The detailed operation of the tool 220 of FIGS. 5 and 6 matches that of the tool 120 and will not be repeated.

[0108] The high frequency torsional oscillation mitigation tool 320 of FIGS. 7 and 8 differs from the embodiment of FIGS. 2 and 3 in using mud to lubricate the one-way coupling and the bearing structures.

[0109] The tool 320 has an internal conduit 336 for the passage of mud from the surface to the drill bit 16. Inlet conduits 338 connect the internal conduit 336 to a location between the one-way coupling 300 and the axial bearings 350a. The mud pressure differential between the internal conduit 336 and the annulus 22 causes some of the mud to flow from the inlet conduit 338 in an uphole direction past the one-way coupling 300 and the upper radial bearing 350r before passing to the annulus 22 surrounding the tool. The remainder flows in a downhole direction through the axial bearing 350a and out of the tool by way of the outlet conduits 366.

[0110] Mud also flows from the internal conduit 336 around an end nut 364 located at the box connector 344, past the lower radial bearing 350r and out to the annulus 22 through the outlet conduits 366.

[0111] Accordingly, a small proportion (typically between approx. 1% and approx. 5%) of the mud flowing along the internal conduit 336 is diverted to lubricate the one-way coupling 300 and the bearings 350a,r of the tool 320.

[0112] As seen in FIG. 8, in this embodiment the one-way coupling 300 is similar to that of the embodiment of FIGS. 2 and 3 but that is not necessarily the case and other embodiments can utilise different one-way couplings suitable for a mud-lubricated system.

[0113] The high frequency torsional oscillation mitigation tool 420 of FIGS. 9 and 10 differs from the embodiment of FIGS. 4 and 5 primarily in using mud to lubricate the bearing structures. Oil is used to lubricate the sprag clutch 400 as in the embodiment of FIGS. 5 and 6.

[0114] The tool 420 has an internal conduit 436 for the passage of mud from the surface to the drill bit 16. An inlet conduit 438 connects the internal conduit 436 to an upper radial bearing 450r. Some of this mud passes in an uphole direction to the annulus 22. The remainder passes in a downhole direction to outlet conduits 462 which allow the mud to flow into the annulus 22

[0115] Mud can also flow from the internal conduit 436 around an end nut 464 located at the box connector 444, through the lower radial bearing 450r and the axial bearing 450a. This mud lubricates the bearings 450r and 450a and passes through outlet conduits 466 to the surrounding annulus 22.

[0116] Accordingly, a small proportion of the mud flowing along the internal conduit 436 is diverted to lubricate the radial bearings 450r and the axial bearings 450a. The radial bearing 450r also acts as a mud flow restrictor so that the mud pressure acting upon the compensator is close to that of the annulus 22.

[0117] The one-way coupling (which in this embodiment is a sprag clutch 400) is isolated from the mud by a combination of rotating seals 470 and static seals 472 and by the pressure compensator 474.

[0118] FIGS. 11-13 shown another alternative embodiment of mitigation tool 520 according to the invention.

[0119] In this embodiment the recesses 532 are formed in the sleeve 540. The rollers 530 therefore move inwardly towards the shaft 534 to the engaged condition (and outwardly to the disengaged condition), as compared to the earlier embodiments. For a given tool diameter the recesses 532 and rollers 530 can therefore be located at a slightly greater radius, and spread over a slightly greater circumferential length, which can permit an increase in the number of rollers 530.

[0120] The resilient biasing means in this embodiment is a cantilever spring 546, the profile of which can be better seen in the enlarged view of FIG. 13. Each cantilever spring 546 sits in a pocket of the recess 532 and biases the roller 530 towards the shallower end of the recess.

[0121] The detailed form of the cantilever spring 546 can be varied from that shown to suit the particular application. It will be understood that it is only necessary for the tool manufacturer to determine a suitable profile and material for the cantilever springs 546 and they can be made to any required length, ideally to match the full length of the rollers 530.

[0122] It will be understood that the general form of the recesses 532 and the general operation of the rollers 530, are as described previously and will not be repeated.

[0123] The arrangement of the components of the mitigation tool 520 are modified somewhat as compared to the embodiment of FIGS. 2 and 3, which in certain applications can make the tool easier to manufacture and assemble/disassemble. The modifications can also make the tool more durable and reliable. The modifications are all optional and are shown to demonstrate some of the variety of options for the detailed structure which can be utilised in practice.

[0124] The mitigation tool 520 of FIGS. 11-13 has two sets of axial bearings 550a and three sets of radial bearings 550r. The sets of bearings 550a,r and the one-way coupling 500 are lubricated by oil which is isolated from the surrounding mud.

[0125] The mitigation tool 520 has a mud flow restrictor 560 and leak conduits 566. In this embodiment the conduits 566 are provided by a component 568 which is secured in the sleeve 540. The components 568 are of hardened steel or tungsten carbide and are removable; the components 568 can therefore be replaced when they become eroded due to mud flow through the conduits 566.

[0126] The compensator 554 is located adjacent to the components 568.

[0127] The securing nut 564 by which the sleeve 540 is secured to the shaft 542 is located inwardly of the compensator 554.

[0128] FIGS. 14-18 show a further modified mitigation tool 620 with additional (optional) functionality. Specifically, this embodiment has a locking mechanism in addition to the one-way coupling, which locking mechanism might be required for example if the drill bit or another part of the downhole assembly becomes stuck in the borehole.

[0129] It is not intended that the locking mechanism is actuated during normal operation, and it is expected that in many drilling operations the locking mechanism will never be required and the mitigation tool will operate as above described. However, in the event that the drill bit or another part of the downhole assembly becomes stuck the operator may wish to impart more torque to the drill string than the one-way coupling can accommodate, and the locking mechanism provides a predetermined torque capacity which can be significantly greater than that of the one-way coupling.

[0130] The mitigation tool 620 includes a one-way coupling 600, and which can correspond to the detailed structure of any of the embodiments described above. The one-way coupling 600 and its associated bearings, seals etc. can therefore be similar to any of the earlier embodiments and a detailed description of that part of the tool 620 will not be repeated. The difference with the previous mitigation tools is, however, that the tool, and in particular the one-way coupling, can accommodate axial movement of the central shaft 634 relative to the surrounding sleeve 640. The relative axial movement does not need to be large, and a range of relative axial movement of 6 mm to 15 mm is expected to be sufficient for most tools (and depending upon the size of the tool).

[0131] Alongside the one-way coupling 600 the tool 620 has resilient biasing means in the form of a stack of disc springs 670. The disc springs 670 bias the central shaft 634 axially relative to the sleeve 640, to a normal (unlocked) condition. The disc springs 670 are sufficiently strong that during normal drilling operations, and proper operation of the mitigation tool 620, the shaft 634 does not move axially relative to the sleeve and the locking mechanism remains unlocked.

[0132] It will be seen from FIGS. 15 and 16 that the tool 620 has two pairs of aligned dog gears 672 and 674. The dog gears 672 and 674 do not engage in normal operation and in particular the springs 670 act to bias (and keep) the gears apart. The dog gears 674 are rigidly connected to the central shaft 634 and the dog gears 672 are rigidly connected to the surrounding sleeve 640. Operation of the one-way coupling 600 is therefore accompanied by relative rotation of the gears 672, 674.

[0133] In the event that a part of the downhole assembly below the mitigation tool 620 become stuck in the borehole 14, the operator will apply an overpull to the drill string 12, i.e. pulling the uphole pin connector 642 towards the right as viewed. That overpull will cause the stack of disc springs 670 to compress and the gears 672, 674 to enmesh. The gears 672 and 674 have tapered or sloping surfaces to help ensure proper meshing of the gears.

[0134] The tool 620 therefore moves to the condition shown in FIGS. 17-18 in which the gears 672 and 674 are meshed. In this locked condition the rotation of the drill string 12 is communicated directly from the uphole pin connector 642 to the downhole box connector (and consequently to the rotary steerable tool and drill bit) and the one-way coupling is overridden or bypassed. The operator can seek to release the stuck component by way of the overpull alone, or in addition by rotating the drill string 12. The locking mechanism allows the operator to apply a torque up to the limit of the dog gears 672, 674, which in practical applications will far exceed the torque which the one-way coupling 600 can withstand.

[0135] As regards the bearings used in the various embodiments, it is generally understood that ball bearing type thrust bearings are generally more suitable for the larger tolerances and clearances which are typically necessary in a mud lubricated system. Conversely, plain bearing type bushes are generally more suitable as radial bearings in mud lubricated applications. It is not excluded that the bearings of certain of the embodiments could be used in other embodiments, depending on their positions relative to the oil/mud sealing members.

[0136] It will be seen that the sets of axial or thrust bearings 150a, 250a, 350a, 450a, 550a are all located below the respective one-way couplings 100, 200, 300, 400, 500. It is possible to provide the axial bearings (or additional sets of axial bearings) above the respective one-way couplings if desired, but that is not expected to be necessary in practice.