WO2012069858A2

WO2012069858A2 - Downhole drilling tool and bearing assembly

Info

Publication number: WO2012069858A2
Application number: PCT/GB2011/052348
Authority: WO
Inventors: Philip Head
Original assignee: Philip Head
Priority date: 2010-11-26
Filing date: 2011-11-28
Publication date: 2012-05-31
Also published as: WO2012069858A3; GB201020098D0

Abstract

A downhole drilling or reaming tool comprises a drive assembly including a reciprocating hydraulically powered drive element arranged in a housing which is fixed to the lower end of a tubing string, for example, a drillstring or a casing string. The drive element and other parts of the drive assembly may be made from a durable material or alternatively from a sacrificial material which is drilled or otherwise removed after installation of the casing string, leaving the housing in position. The drive element may drive two counter-rotating elements whereby the tool is torque balanced and avoids applying torque to the tubing string. The drive element may comprise a reciprocating mass which imparts percussive action to the or each rotating element and cutting head. In a further aspect the invention provides a more convenient bearing assembly and method of manufacture wherein a plurality of bearing balls are retained in holes in a cylindrical sleeve by deformation of marginal portions of the holes.

Description

Downhole drilling tool and bearing assembly

This invention relates in certain aspects to drilling and reaming tools for use in boreholes, and in another aspect to bearing assemblies for use between concentric cylindrical bearing surfaces.

It is often difficult to deploy a casing string to the full depth (i.e. axial length) of a previously drilled borehole, particularly in a deviated or horizontal borehole, due to obstructions resulting from loose material, deformation or partial collapse of the borehole, and the like. While installing a casing string in a long deviated or horizontal well section, debris in front of the casing shoe can hold up the installation process and prevent the casing being pushed all the way to its total depth. Its length can be so long that it is not possible to turn the casing from surface without exceeding its torque capability.

US 7849927 discloses a reaming apparatus comprising a sacrificial rotary drive assembly and cutting structure. The drive assembly is mounted inside a more durable housing attached to the lower end of a casing string and rotated by drilling fluid pumped from the surface during deployment of the casing in a previously drilled borehole so as to remove obstructing debris. The apparatus is cemented in situ together with the casing, following which the sacrificial parts are entirely removed, for example by drilling them out, so that subsequently the borehole can be deepened and the next casing string installed through the remaining housing.

Since withdrawal of the casing string after an unsuccessful attempt at deployment is not only difficult and costly but also likely to exacerbate any collapse or similar obstruction, it is important that such apparatus is capable of performing sufficient work to enable the casing string to reach the target depth. This can be difficult to achieve in view of the necessarily short lifespan of the apparatus, due to the brittle or malleable materials comprising its sacrificial portion and in particular the bearings of the turbine or other rotary drive assembly. Means may be provided for compensating for rapid bearing wear, which however add complexity to the design. Another problem lies in the necessity of locking the rotary drive assembly before it can subsequently be drilled out. If the locking means fails, then the sacrificial drive assembly may rotate when it is engaged by a conventional cutting head lowered down the cemented casing string, making it very difficult to remove.

In a first aspect the present invention has as its object the provision of a sacrificial drilling or reaming apparatus which mitigates at least some of the above mentioned problems.

In another aspect the invention sets out to provide a downhole drilling or reaming apparatus, either of the conventional, re-usable type or of the sacrificial type, which is more effective in use.

In sacrificial assemblies of the above mentioned type as well as in conventional, recoverable downhole drilling assemblies, the torque applied to the cutting head is reacted by the tubular string. The drillstring is typically of great slenderness relative to its length, and the resulting torsion leads to undesirable vibration and alternate slipping and sticking of the cutting head, causing potential damage to the drillstring and a loss of control at the cutting head which makes it more difficult to direct the drilling or reaming operation. In another aspect the invention sets out to provide a downhole drilling or reaming apparatus, either of the conventional, re-usable type or of the sacrificial type, in which this last mentioned problem is mitigated.

In yet another aspect the invention has as its object the provision of a bearing assembly and a corresponding method of manufacturing a bearing assembly which simplifies assembly of rotating cylindrical elements, inter alia in downhole drilling tools.

In accordance with its various aspects the present invention provides a drilling or reaming apparatus, a bearing assembly and a method of manufacturing the bearing assembly as defined in the claims. In a first embodiment, a drilling or reaming apparatus for use in a borehole includes a housing made substantially from at least one durable material such as steel, the housing being attachable, e.g. by means of a screwthread or the like to a tubular string of well casing. A sacrificial hydraulically powered drive element is arranged in the housing, the drive element being made substantially from at least one sacrificial material which is more readily drillable, fragmentable or soluble than the durable material, and is preferably easy to drill, for example, aluminium or a fibre reinforced phenolic resin composite. A cutting head, which is preferably also made from a readily drillable material, optionally the same material as the drive element, with hard cutting surfaces is driven in rotation by the drive element.

Advantageously, the drive element is hydraulically driven in reciprocation, preferably reciprocating along the longitudinal axis of the casing and the tubular string, and since it is preferably engaged with straight guideways in a fixed portion of the housing which prevent it from rotating about this axis, it can be drilled out without providing any additional means or taking any additional steps to lock it against rotation. Similarly, the helical guideways cause the cutting head and other rotating parts of the housing to lock in rotation relative to the tubular string after a partial turn in either direction, whereby they also may be readily drilled out. Preferably, the durable housing includes bearings supporting the rotating parts of the casing as well as transmission means such as helical guideways for converting the reciprocal motion of the drive element into rotary motion of the cutting head; since these elements are incorporated into the more durable housing, the useful life of the apparatus is prolonged. Preferably the drive element is arranged to impact against a seat so as to impart a percussive action to the cutting head, whereby the apparatus may remove debris more quickly. All of these measures enable the apparatus to perform more work during its lifespan, sufficient to enable the casing string to reach its target depth before the bearings and sacrificial parts fail. The housing is

advantageously very robust, since it includes the helical guideways and other bearing and transmission structures, and is unlikely to fail after the sacrificial parts are removed by drilling, fragmentation or dissolution by acids or the like. This avoids any danger of the housing becoming dislodged from the lower end of the cemented casing string on which it was deployed and blocking the freshly drilled portion of the borehole beneath it, which would seriously impede further operations. Preferably, the housing includes rotating parts but does not comprise any telescopic parts which might weaken it.

The novel apparatus may be used in various other applications, including for example to enable a subsea well conductor to be self drilled into the seabed from a platform support type vessel, with a umbilical from the vessel supplying the power fluid. This would save rig time and cost, enabling a rig subsequently positioned over the well to drill through the drillable core and deepen the well.

In another embodiment, a drilling or reaming apparatus includes a housing which is attachable to a tubular string, a hydraulically powered drive element arranged in the housing, and a cutting head driven in rotation by the drive element. The drive element comprises a mass which is hydraulically driven in reciprocation, and is arranged to impact against a seat so as to impart a percussive, i.e. jarring or impacting action to the cutting head. The combination of impact and rotation at the cutting head advantageously provides faster and hence more effective drilling, and may be desirable in drilling hydrocarbon wells and other boreholes as well as a wide variety of other applications, for example to provide a positive and fast method of penetrating a deep-set anchor into the seabed.

In embodiments, the apparatus may be made entirely from durable materials, and may be free from seals, particularly elastomeric seals, so that it can be used in high temperature geothermal drilling.

In each of its embodiments the apparatus is powered by a hydraulic fluid preferably pumped from the surface down the tubing string or via a separate conduit within the tubing string, which preferably passes through a valve mechanism which causes the drive element to mechanically reciprocate. The exhaust power fluid can be discharged below the tool to assist the cutting and debris removal or alternatively may be exhausted into a contained flow path back to surface. The entire drive assembly of the tool including the drive element and valve mechanism could be made of a durable material or alternatively of a sacrificial, preferably readily drillable material, enabling full bore access below the tool in the event that the tool is left permanently in a well.

In a further embodiment a drilling or reaming apparatus includes a housing which is attachable to a tubular string, e.g. by means of a conventional screwthread, a hydraulically powered drive element arranged in the housing, and a cutting head driven in rotation by the drive element. The drive element is hydraulically driven in reciprocation, and the apparatus includes first and second rotary portions which are driven in rotation in opposite directions by the drive element, whereby the output drive is preferably concentric and counter rotating, and may be engineered in accordance with well known principles in the art so as to be torque balanced. One or both of the rotary portions may be arranged to form an outer surface of the housing in contact with the ground, and may include cutting edges and/or other surfaces for engaging the ground. The novel apparatus advantageously avoids reacting the torque from the (or each) cutting head against the drillstring or other tubular string to which the housing is attached, thus avoiding the problems associated with torsion of the drillstring.

In a yet further embodiment, a bearing assembly comprising a cylindrical sleeve and a plurality of spherical balls, each ball being located in a respective hole extending radially through the sleeve, each hole having an inner edge portion insersecting an inner wall of the sleeve and an outer edge portion intersecting an outer wall of the sleeve; wherein each of the inner and outer edge portions defines an aperture of smaller diameter than the ball, whereby the ball is retained in the hole.

In a related embodiment, a method of manufacturing a bearing assembly comprises the steps of forming a plurality of holes in a cylindrical sleeve, each hole extending radially through the sleeve from an inner edge portion intersecting an inner wall of the sleeve to an outer edge portion intersecting an outer wall of the sleeve; inserting a plurality of spherical balls, each into a respective one of the holes; and then locally deforming at least one of the inner and outer edge portions of each hole to retain the respective ball in the hole.

The novel bearing assembly may be used in the apparatus of the other embodiments so as to support the concentric cylindrical bearing surfaces of the respective, fixed and rotary portions of the housing, and is found to make assembly much easier than similar prior art assemblies in which the balls must be retained in the holes during assembly, for example by means of grease. It should be understood however that the novel bearing assembly and method of manufacture may be used in any application where two cylindrical surfaces are separated by a plurality of balls arranged in a foraminous sleeve, and are not limited to use in drilling or reaming tools.

Although the novel apparatus is described in various embodiments, it will be understood that the features of the various embodiments may be combined as required for the intended application. Thus the apparatus may provide rotational and if necessary impacting force, may be recoverable or may remain in the hole, and its reciprocating core can be made of a drillable, fragmental, or soluble material to enable full bore access below it once it is no longer required. It may be used high temperature drilling in geothermal type wells or the like, in which case it may be completely seal-less, and it may also be used to install a pile. Further features and advantages will be evident from the following illustrative embodiments which are described, purely by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a section side view of one embodiment of the tool

Figure 2 is a section side view of a further embodiment of the tool Figure 3 is a section side view of another further embodiment of the tool Figure 4 is a section side view of the tool with the central reciprocating mass at its lowest (start) position.

Figure 5 is a section side view of the tool with the central reciprocating mass almost at its highest (end) position.

Figure 6 is a section side view of the tool with the central reciprocating mass at its highest (end) position. Figure 7 is a section side view of the tool with the central reciprocating mass at its lowest (start) position.

Figure 8 is a section side view of the tool with its central reciprocating portion of the tool removed (by drilling out or another process) in situ to enable full bore access below it.

Figure 9 is a side section view of another embodiment of the tool, with a more robust construction, with no seals and for use in a very high temperature environment.

Figure 10 is a side section view of a further embodiment of the tool, where the power fluid is not discharged below the tool, but circulated back in the direction it came via a annular flow area.

Figure 11 is a cross section at AA of figure 9;

Figure 12 is a cross section at BB of figure 9 showing the bearing assembly;

Figure 13 is an enlarged view of detail C of figure 12;

Figs. 13A and 13B are still further enlarged views of the bearing assembly shown in Fig. 13, respectively before and after deformation of the sleeve; Figure 14 is a longitudinal section through the bearing assembly of Figs. 12 - 13 showing how it is retained relative to the upper and lower housings;

Figure 15 is a cross section side view of another embodiment of the tool with the reciprocating central mass in its lower most position.

Figure 16 is a cross section side view of the embodiment of the tool shown in figure 15 with the reciprocating central mass in its upper most position. Referring to figure 1 -3 there is shown a longitudinal section of an embodiment of the tool which comprises a drilling or reaming apparatus for use in a borehole 405. The tool includes a housing 403, the housing including a rotary portion 13 and an upper tubular fixed housing portion 1 which is attachable by means of a screwthread 2 in fixed relation to a tubular string 400. A hydraulically powered drive element 59 is arranged in the housing, and a cutting head 401 is arranged to be driven in rotation by the drive element. The cutting head is mounted on the screwthread 402 on the distal end of the rotary portion 13 of the housing so that it is not moveable relative to the housing in an axial length direction D3 of the housing. The drive element 59 is hydraulically driven in reciprocation by a hydraulic fluid 404 which is pumped down the tubular string from the upper end of the borehole at the ground surface.

The rotary portion 13 forms an outer suface 13' of the housing, and the drive element slidingly engages the rotary portion via a helical guideway 9 so as to drive the rotary portion in rotation.

A resilient bias element 52, which in this embodiment comprises a helical spring, is arranged to urge the drive element from a second position (as shown in Fig.5, in which the valve is about to open) to a first position (shown in Figs. 1 - 4). The drive element operates in the manner of a piston and includes a wall 405 of an expandable chamber 30 having an outlet 406 sealed by a valve 26 which comprises a ball, and a supply of pressurised hydraulic fluid 404 is connected to the chamber so as to expand the chamber by movement of the drive element from the first position to the second position. The drive element is arranged to open the valve 26 when it is in the second position (best shown in Fig. 6, showing the valve immediately after opening); and the valve is arranged to close when the drive element is in the first position (best shown in Fig. 7), and to remain open when the drive element is between the second position and the first position, the valve being held open in the position shown in Fig. 6 by a resilient valve control element 407 (comprising in this embodiment a coil spring) when the drive element is between the second position (Fig.6) and the first position (Fig. 7) and closing only when the drive element reaches the first position (Fig. 7). In this manner a self sustaining reciprocal action is achieved whereby the hydraulic fluid alternately expands the chamber and is exhausted via the outlet.

The foregoing features are common to each of the drilling or reaming tools described in the further embodiments below although some features which are functionally common to all embodiments are indicated by different reference numerals and referred to by other terminology.

The upper tubular portion of the tool 1 has an internal threaded portion 2 which enables it to be connected to an upper tubular member 400. This tubular portion has a maximum diameter 3 and a minimum diameter 4 which is not less than the ID of the upper tubular member not show. All parts within these two diameters 3 and 4 are made from structural material. Tubular member 1 has an annular internal recess 5. A second tubular member 6 is located at the lower end of this recess, and is connected to member 1 by a set of straight key ways 7 and pins 8. Member 6 can telescope into the recess 5 the length of the keyway 7. At the lower end of member 6 is a set of key ways 9 which form a helical path. A pin 10 connects this keyway to member 1 1. As member 6 moves up within recess 5 the annular member 11 is turned by the distance of the helical keyway 9, this in turn transmits the torque through a transmission mechanism which in this embodiment comprises a ratchet mechanism 12 to the lower annular part of the tool 13. The transmission mechanism 12 is arranged to transmit unidirectional rotary motion to the cutting head. On the downward movement of the member 6, the ratchet mechanism 12 allows member 1 1 to rotate under the direction of the helical keyway, but disconnects the drive to the member 13, so as member 6 moves up and down the lower annular part of the tool 13, only turns in one direction. This would be preferably clockwise, as it would tighten any connections 2 in the assembly. Referring to figures 4-8, within the diameter 4 of the tool is a mechanism which achieves reciprocation movement from the fluid pumped internally through it. This is achieved as follows; Fluid flows through port 20 passing a spring 21 mounted in an annular chamber 22, through the centre port 24 of a drive element comprising a reciprocating mass 25 which is attached to annular member 6. The fluid passes a valve 28, which has a ball at each end connected by an X cross sectioned stem 29 so fluid is in communication with chamber 30 via passage 31. The fluid flow pushes a ball 26 on a seat 27, this both generates a differential pressure across the seals 40, 41 and the fluid pump displaces the reciprocating mass 25 in an upward direction. Between the faces 42 of the valve 29 and 43 of the reciprocating mass 25, a spring 44 is compressed, when fully compressed, its lift the valve stem upward, lifting the ball 26 of the seat 27, causing pressure to vent below the tool. At the same time the spring 44 extends to its fully relaxed position 50. The upper surface 51 of the reciprocating mass 25 is compressing a spring 52 at this position, so this spring pushes the reciprocating mass downward causing the faces 55 and 56 to contact, providing a hammer or jarring effect. Instead of a return coil spring, a rubber mass 57 operated in shear, could be used which is bonded to annular member 58 and the drive element 59 comprising a reciprocating mass; holes 60 would be necessary to equalise pressure across it.

A pressure compensating piston 60 could be provided to equalise the pressure in the chamber 61 , if it was desirable to keep clean oil in this chamber.

The drive element 59 is arranged to impact against a seat 408 (Fig. 3) in the first position so as to impart a percussive action to the cutting head, whereby the resilient bias element 52 imparts the percussive action. Alternatively the mechanism could be arranged so that the hydraulic fluid drives the drive element in the downhole direction against the seat 408 to impart the percussive action, and the resilient bias element 52 returns the drive element in the reverse (uphole) direction to a rest position.

The resilient bias element and/or the resilient valve control element may comprise coil springs as in the Figs. 1 and 3 embodiments. Alternatively the resilient bias element 57 and/or the resilient valve control element 409 may be made from an elastomer as shown in the Fig. 2 embodiment, in which case they are easy to remove, e.g. by drilling.

Finally, referring to figure 8, it be desirable to use this device in a consumable way, such that the reciprocating internal components are made from a drillable or fragment able or soluble material so that full bore access can be achieved. Examples of this requirement, could this device is used to install a casing conductor, prior to rig arriving on location, so as the rig lowers its drilling assembly into the conductor, the internal components are penetrated by the drill bit in a rapid and efficient manor. Similarly, for extended reach wells or horizontal wells, it may help to rotate a reamer shoe to assist in getting a casing or liner to total depth, again this tool would assist and allow the hole to be deepened afterwards.

The housing 403, 1, 13 is made substantially from at least one durable material such as steel or steel alloy, whereas the drive element 59 is made substantially from at least one sacrificial material, which is to say, one or more materials which may be destroyed relatively more rapidly or easily than the durable material by a selected destruction means, which may comprise one of drilling, fragmentation, or dissolution, being respectively relatively more readily drillable, fragmentable or soluble than the durable material. In a preferred embodiment the sacrificial material is more readily drillable than the durable material, which is to say, a material which compared with the durable material is relatively easy to drill. It may be relatively soft and may be selected from the list consisting of aluminium, zinc, lead, copper, tin, and alloys thereof (including for example suitable brasses and bronzes);

plastics materials and fibre reinforced composite plastics materials; and cast iron. In

embodiments, the sacrificial material may be aluminium, aluminium alloy, or phenolic resin reinforced by glass fibre or the like.

Alternatively the sacrificial material may be a material which compared with the durable material is more readily dissolved by a fluid, for example, an acid. Alternatively the sacrificial material may comprise a frangible material, e.g. a glass, ceramic or hard plastics material, which compared with the durable material is more brittle and more readily fragmented. In this embodiment the sacrificial material is preferably not subjected to impact in use.

The apparatus may provide a discharge path 410 for discharging hydraulic power fluid into the borehole. Alternatively the apparatus may provide a return path 205 for returning hydraulic power fluid to surface via the tubular string, as exemplified in the Fig. 10 embodiment.

Referring to figure 9 there is shown a cross section through another embodiment of the tool, this embodiment has no seals, and is intended for operation in very hot wells such as geothermal type wells. The apparatus includes a hydraulic flow path 411 defined by (which is to say, confined by a wall or internal surface comprising) mutually sliding surfaces 100, 100', 101, 10 which are provided with a hard surface coating whereby a pressure differential is maintained across the surfaces without the use of elastomeric seals.

The moving surfaces 100, 100' and 101, 101 ' would be a close fit and coated in a material such as hardide. This is a technology of the chemical deposition of a coating containing tungsten carbides. This process is used to hard face surfaces to provide high resistance to wear, erosion and chemicals. A completed description of this process is described in US patent US 6,800,383 Bl Referring to Figs. 12 - 14, a bearing assembly comprises a cylindrical sleeve 102 and a plurality of spherical balls 103, each ball being located in a respective hole 104 extending radially through the sleeve, each hole 104 having an inner edge portion 104' insersecting an inner wall 102' of the sleeve and an outer edge portion 104" intersecting an outer wall 102" of the sleeve; wherein each of the inner and outer edge portions defines an aperture 412 of smaller diameter Dl than the diameter D2 of the ball, whereby the ball is retained in the hole. A method of manufacturing a bearing assembly, comprises the steps of forming a plurality of holes 104 in a cylindrical sleeve 102, each hole extending radially through the sleeve from an inner edge portion intersecting an inner wall of the sleeve to an outer edge portion intersecting an outer wall of the sleeve; inserting a plurality of spherical balls 103, each into a respective one of the holes; and then locally deforming at least one of the inner and outer edge portions of each hole to form a local deformation 413 which retains the respective ball in the hole. The deformation is created by an annular press tool 414 in direction D4 radially inwardly or outwardly towards the respective hole containing the ball.

Similarly the bearings could be ceramic roller bearings 103 mounted in a carrier 102, and retained in their respective holes 104 by a small deformation 105, 106 generated by an annular tool pushed together by a hydraulic press. At each end of the carrier, the balls would be clear of the carrier, so that they may contact the end rings 140, and 141. All the balls are retained in the carrier for ease of handling, but the balls have point contact for transmitting forces axially, and radially 1 10, 111 , 112, 113, 114. The carrier locates on a male part 131 of an upper outer housing 130. It is retained onto this male part by a ring 140 pinned using dowel pins 132. The female bore 133 of the lower out housing slides over the bearings 103 and carrier 102, and similarly retained using an upper ring 141 which is pinned using dowel pins 134, thus the upper and lower housing are free to rotate while also transmitting compression and tension forces. A plurality of large holes 150 and 151 allows wellbore fluids to equalise around moving reciprocating mass 160. The tool works identically as earlier described.

Figure 11 shows the one way sprag clutch which is a one-way freewheel clutch, operating as a transmission element to transmit unidirectional rotary motion to the cutting head. It resembles a roller bearing, but instead of cylindrical rollers, non-revolving asymmetric shaped sprags are used. When the unit rotates in one direction the rollers slip or free-wheel, but when a torque is applied in the opposite direction, the rollers tilt slightly, producing a wedging action and binding because of friction. The sprags are spring-loaded so that they can lock without backlash. Figure 10 shows an embodiment of the tool where it is desirable for the power fluid not to be discharged, but circulated back in the direction it came. This is achieved by having a central flow passage 200, the power fluid cycles the reciprocating part 201 and discharges through ports 202, around reciprocating part 201 via a plurality of passages 203, through a plurality of passages 204 into the annular space 205. This would be particularly important in piling a foundation support where it would be undesirable to have excessive fluid around the outer surface of the pile.

Referring to figure 15 and 16 there is shown a further embodiment of the tool, in this example, the tool has a concentric counter rotating output.

The apparatus includes first 308, 310 and second 314, 317 rotary portions which are driven in rotation in opposite directions by the drive element 300. The drive element 300 slidingly engages each of the first and second rotary portions via a respective helical guideway 305, 306, referred to below as a keyway.

The drive element is arranged to impact against a respective seat 320', 321 ' on each of the rotary portions 310, 314 so as to impart a percussive action to both of the rotary portions. The reciprocating drive element comprising a central mass 300, as described above in figures 1 to 7, moves up and down relative to the upper housing 301 as a function of the fluid flow rate being displaced down the bore 302. and it is prevented from rotating by a pin 303 held in the housing 301 and keyway 304 in the reciprocating mass 300. The reciprocating mass 300 has two further keyways, a clockwise keyway 305 and a counter clockwise keyway 306, equal to the same length as keyway 304 and each generating an angle of rotating for each stroke of the reciprocating mass 300. In keyway 305 is a pin 307 which is held firmly in a hole in outer housing 308. Housing 308 can rotate relative to housing 301 via a bearing assembly 309, a further lower outer housing 310 is connected to outer housing 308 via a one way clutch 311 and further bearing assembly 312. As previously described, for each upward reciprocation of 300, the housing 308 turns clockwise, which turns outer housing 310, and for each downward movement of 300, the housing 308 turns anti clockwise, but the clutch 31 1 slips and housing 306 stays stationary.

In key way 306, is a pin 313 which is held firmly in a central cylindrical member 314. Member 314 is mounted in bearings 315 and 316 and can rotate relative to reciprocating mass 300, a further lower central cylindrical member 317 is connected to 314 via a one way clutch 318 and further bearing assembly 319. for each upward reciprocating of 300, the cylindrical member 314 turns anti clockwise, which turns the lower cylindrical member 317. and for each downward movement of 300, the cylindrical member 314 turns clockwise, but the clutch 318 slips and lower cylindrical member 317 stays stationary.

Therefore it can be seen for each upward and downward movement of cylindrical member 300, the lower outer housing 310 and central cylindrical member 317 counter rotate. It will also be appreciated, that at the end of each downward movement the faces 320, 321 of the central cylindrical member 300 provide an impact force on faces 320' and 321 '.

In some applications, such as when installing a pile foundation, where it may be undesirable to have a power fluid being exhausted to the exterior of the pile, a return path for the power fluid may be provided either coaxially or parallel with the high pressure power fluid supply path, so that the exterior of the pile remains in intimate contact with the formation it was being installed in.

In summary, a downhole drilling or reaming tool comprises a drive assembly including a reciprocating hydraulically powered drive element arranged in a housing which is fixed to the lower end of a tubing string, for example, a drillstring or a casing string. The drive element and other parts of the drive assembly may be made from a durable material or alternatively from a sacrificial material which is drilled or otherwise removed after installation of the casing string, leaving the housing in position. The drive element may drive two counter-rotating elements whereby the tool is torque balanced and avoids applying torque to the tubing string. The drive element may comprise a reciprocating mass which imparts percussive action to the or each rotating element and cutting head. In a further aspect the invention provides a more convenient bearing assembly and method of manufacture wherein a plurality of bearing balls are retained in holes in a cylindrical sleeve by deformation of marginal portions of the holes.

Claims

1. A drilling or reaming apparatus for use in a borehole, comprising a housing made substantially from at least one durable material, the housing being attachable to a tubular string; a sacrificial hydraulically powered drive element arranged in the housing, the drive element being made substantially from at least one sacrificial material which is more readily drillable, fragmentable or soluble than the durable material; and a cutting head driven in rotation by the drive element; wherein the drive element is hydraulically driven in reciprocation.

2. A drilling or reaming apparatus according to claim 1 , wherein the housing includes a rotary portion which is driven in rotation by the drive element.

3. A drilling or reaming apparatus according to claim 2, wherein the drive element slidingly engages the rotary portion via a helical guideway.

4. A drilling or reaming apparatus according to claim 1 , wherein the apparatus includes first and second rotary portions which are driven in rotation in opposite directions by the drive element.

5. A drilling or reaming apparatus according to claim 4, wherein the drive element slidingly engages each of the first and second rotary portions via a respective helical guideway.

6. A drilling or reaming apparatus according to claim 1, wherein the drive element is arranged to impact against a seat so as to impart a percussive action to the cutting head.

7. A drilling or reaming apparatus according to claim 1, wherein the cutting head is not moveable relative to the housing in an axial length direction of the housing.

8. A drilling or reaming apparatus according to claim 1 , wherein a transmission mechanism is arranged to transmit unidirectional rotary motion to the cutting head.

9. A drilling or reaming apparatus according to claim 1 , wherein the sacrificial material is more readily drillable than the durable material.

10. A drilling or reaming apparatus according to claim 9, wherein the sacrificial material is selected from the list consisting of aluminium, zinc, lead, copper, tin, and alloys thereof; plastics materials and fibre reinforced composite plastics materials; and cast iron.

11. A drilling or reaming apparatus according to claim 1 , wherein a resilient bias element is arranged to urge the drive element from a second position to a first position; and the drive element includes a wall of an expandable chamber having an outlet sealed by a valve, and a supply of pressurised hydraulic fluid is connected to the chamber so as to expand the chamber by movement of the drive element from the first position to the second position; and the drive element is arranged to open the valve in the second position; and the valve is arranged to close when the drive element is in the first position, and to remain open when the drive element is between the second position and the first position.

12. A drilling or reaming apparatus according to claim 11 , wherein the drive element is arranged to impact against a seat in the first position so as to impart a percussive action to the cutting head.

13. A drilling or reaming apparatus according to claim 11 , wherein the resilient bias element is made from an elastomer.

14. A drilling or reaming apparatus according to claim 1 1 , wherein a resilient valve control element is arranged to hold the valve open when the drive element is between the second position and the first position.

15. A drilling or reaming apparatus according to claim 14, wherein the resilient valve control element is made from an elastomer.

16. A drilling or reaming apparatus according to claim 1 , wherein the apparatus provides a discharge path for discharging hydraulic power fluid into the borehole.

17. A drilling or reaming apparatus according to claim 1 , wherein the apparatus provides a return path for returning hydraulic power fluid to surface via the tubular string.

18. A drilling or reaming apparatus for use in a borehole, comprising a housing, the housing being attachable to a tubular string; a hydraulically powered drive element arranged in the housing; and a cutting head driven in rotation by the drive element; wherein the drive element is hydraulically driven in reciprocation, and the drive element is arranged to impact against a seat so as to impart a percussive action to the cutting head.

19. A drilling or reaming apparatus according to claim 18, wherein a resilient bias element is arranged to urge the drive element from a second position to a first position; and the drive element includes a wall of an expandable chamber having an outlet sealed by a valve, and a supply of pressurised hydraulic fluid is connected to the chamber so as to expand the chamber by movement of the drive element from the first position to the second position; and the drive element is arranged to open the valve in the second position; and the valve is arranged to close when the drive element is in the first position, and to remain open when the drive element is between the second position and the first position.

20. A drilling or reaming apparatus according to claim 19, wherein the drive element is arranged to impact against the seat in the first position.

21. A drilling or reaming apparatus according to claim 19, wherein the resilient bias element is made from an elastomer.

22. A drilling or reaming apparatus according to claim 19, wherein a resilient valve control element is arranged to hold the valve open when the drive element is between the second position and the first position.

23. A drilling or reaming apparatus according to claim 22, wherein the resilient valve control element is made from an elastomer.

24. A drilling or reaming apparatus according to claim 18, wherein the cutting head is not moveable relative to the housing in an axial length direction of the housing.

25. A drilling or reaming apparatus according to claim 18, wherein the housing includes a rotary portion which is driven in rotation by the drive element.

26. A drilling or reaming apparatus according to claim 25, wherein the drive element slidingly engages the rotary portion via a helical guideway.

27. A drilling or reaming apparatus according to claim 25, wherein the rotary portion comprises an outer surface of the housing.

28. A drilling or reaming apparatus according to claim 18, wherein the apparatus includes first and second rotary portions which are driven in rotation in opposite directions by the drive element.

29. A drilling or reaming apparatus according to claim 28, wherein the drive element slidingly engages each of the first and second rotary portions via a respective helical guideway.

30. A drilling or reaming apparatus according to claim 18, wherein a transmission mechanism is arranged to transmit unidirectional rotary motion to the cutting head.

31. A drilling or reaming apparatus according to claim 18, wherein the apparatus provides a discharge path for discharging hydraulic power fluid into the borehole.

32. A drilling or reaming apparatus according to claim 18, wherein the apparatus provides a return path for returning hydraulic power fluid to surface via the tubular string.

33. A drilling or reaming apparatus according to claim 18, wherein the apparatus includes a hydraulic flow path defined by mutually sliding surfaces which are provided with a hard surface coating whereby a pressure differential is maintained across the surfaces without the use of elastomeric seals.

34. A drilling or reaming apparatus for use in a borehole, comprising a housing, the housing being attachable to a tubular string; a hydraulically powered drive element arranged in the housing; and a cutting head driven in rotation by the drive element; wherein the drive element is hydraulically driven in reciprocation, and the apparatus includes first and second rotary portions which are driven in rotation in opposite directions by the drive element.

35. A drilling or reaming apparatus according to claim 34, wherein the first rotary portion comprises an outer surface of the housing.

36. A drilling or reaming apparatus according to claim 34, wherein the drive element slidingly engages each of the first and second rotary portions via a respective helical guideway.

37. A drilling or reaming apparatus according to claim 34, wherein the drive element is arranged to impact against a seat so as to impart a percussive action to the cutting head.

38. A drilling or reaming apparatus according to claim 34, wherein the drive element is arranged to impact against a respective seat on each of the rotary portions so as to impart a percussive action to both of the rotary portions.

39. A drilling or reaming apparatus according to claim 34, wherein the cutting head is not moveable relative to the housing in an axial length direction of the casing portion.

40. A drilling or reaming apparatus according to claim 34, wherein a transmission mechanism is arranged to transmit unidirectional rotary motion to the cutting head.

41. A drilling or reaming apparatus according to claim 34, wherein a resilient bias element is arranged to urge the drive element from a second position to a first position; and the drive element includes a wall of an expandable chamber having an outlet sealed by a valve, and a supply of pressurised hydraulic fluid is connected to the chamber so as to expand the chamber by movement of the drive element from the first position to the second position; and the drive element is arranged to open the valve in the second position; and the valve is arranged to close when the drive element is in the first position, and to remain open when the drive element is between the second position and the first position.

42. A drilling or reaming apparatus according to claim 41 , wherein the drive element is arranged to impact against a seat in the first position so as to impart a percussive action to the cutting head.

43. A drilling or reaming apparatus according to claim 41 , wherein the drive element is arranged to impact against a respective seat on each of the rotary portions in the first position so as to impart a percussive action to both of the rotary portions.

44. A drilling or reaming apparatus according to claim 41 , wherein the resilient bias element is made from an elastomer.

45. A drilling or reaming apparatus according to claim 41 , wherein a resilient valve control element is arranged to hold the valve open when the drive element is between the second position and the first position.

46. A drilling or reaming apparatus according to claim 45, wherein the resilient valve control element is made from an elastomer.

47. A drilling or reaming apparatus according to claim 34, wherein the apparatus provides a discharge path for discharging hydraulic power fluid into the borehole.

48. A drilling or reaming apparatus according to claim 34, wherein the apparatus provides a return path for returning hydraulic power fluid to surface via the tubular string.

49. A drilling or reaming apparatus according to claim 34, wherein the apparatus includes a hydraulic flow path defined by mutually sliding surfaces which are provided with a hard surface coating whereby a pressure differential is maintained across the surfaces without the use of elastomeric seals.

50. A bearing assembly comprising a cylindrical sleeve and a plurality of spherical balls, each ball being located in a respective hole extending radially through the sleeve, each hole having an inner edge portion insersecting an inner wall of the sleeve and an outer edge portion intersecting an outer wall of the sleeve; wherein each of the inner and outer edge portions defines an aperture of smaller diameter than the ball, whereby the ball is retained in the hole.

51. A method of manufacturing a bearing assembly, comprising:

forming a plurality of holes in a cylindrical sleeve, each hole extending radially through the sleeve from an inner edge portion intersecting an inner wall of the sleeve to an outer edge portion intersecting an outer wall of the sleeve;

inserting a plurality of spherical balls, each into a respective one of the holes;

and then locally deforming at least one of the inner and outer edge portions of each hole to retain the respective ball in the hole.