GB2373272A - A variable orientation downhole tool - Google Patents

Abstract

A variable orientation downhole tool such as a directional drill which comprises a first part 20 joined to a downhole tube and a second part (16, fig 1) attached by articulation to the first part whose orientation may be adjusted relative to the first part independently through the three 'Euler' angles. The articulation includes a first actuating member 65 which is axially moveable relative to the first part and a following member 62 against which the actuating member bears such that movement of the actuating member causes transverse displacement of the following member thus varying the angle between the first and second parts. The articulation includes a second actuating member 67 which may either be axially moveable or be eccentrically rotatable, acting to displace the following member in a transverse direction differing from that achieved by the first actuating member. In combination the two actuating members can adjust the orientation through two of the three 'Euler' angles. The third angle is adjusted by the motor shaft 20 being rotated. Also a variable orientation downhole tool is disclosed where the articulation is adjusted by means of rotating eccentric actuating means alone. Further a method of drilling borehole casings by means of a variable orientation down hole window forming tool is disclosed comprising two parts adjustable with respect to each other.

Description

Method and Apparatus for Directional Actuation The present invention relates to a method and apparatus for directional actuation, in particular for controllable pointing of an articulated member in downhole procedures and the like, such as borehole steerable drilling and power tool manipulation.

Drilling of earth formations for oil or mineral exploitation, river crossings and other purposes is generally performed using a drillpipe rotated from surface so as to rotate a cutter known as a drill bit at the downhole end.

Typically fluids are pumped through the drillpipe, through the drill bit and then back up to surface through the annulus between the outside of the drillpipe and the borehole wall. These fluids, which generally comprise solids laden liquid but may also be gaseous or a foam, are used to clean and lubricate the bit, to return cuttings to surface, and to stabilise the freshly cut earth formation. Such so-called rotary drilling has limited possibilities for controlling the direction of the borehole using distribution of weight and borehole contact, as is well-known, usually a few degrees in tens of metres drilled.

It is often necessary to effect significant changes in borehole direction in a relative short distance, as much as three degrees per metre or more. The predominant known means of achieving this is to use a downhole motor in the drillpipe, so that the bit may be rotated independently of the drillpipe by a motor shaft connecting motor to drill bit. Typically a bent housing (often called a bent sub) is fitted between motor and bit so that the bit and portion of the motor shaft and drill pipe below the bend are set at an angle to the motor axis. As the drillpipe is slid into the hole, the drill bit cuts a hole in a direction biased at an angle to the drillpipe above the bent sub, causing the direction of the hole to change, and a curved path to be followed. The azimuth angle in which the bent sub points (i. e. considered along the straight axis of the drill string just above the bent sub) is altered by rotating

the drillpipe, and hence the motor, bend and bit, to the desired azimuthal position. When the borehole direction is sufficiently changed the drillpipe may either be withdrawn from the borehole and the bent sub removed, or the drillpipe may be rotated slowly and continuously so that the drill bit orbits around the hole circumference, drilling a helical path which approximates a straight path. Since the bend is fixed, the overall desired hole trajectory is normally accomplished by a combination of sliding and rotating of the drillpipe and the judicious use of weight on the bit applied from surface to alter its drilling rate. These operations are time consuming and being effected from surface rather than near the drill bit are subject to error.

An alternative known method is to use a continuous tubing, wire-connected or autonomously powered unit to convey the motor and bit into the hole. In such cases there is no strong drillpipe with which to transmit rotation from surface, so that the fixed bent housing provides a means for steering but not for drilling straight ahead, and there is limited scope for applying weight on the bit compared to massive force which may be applied through a conventional drill pipe from the surface.

A further known apparatus for effecting direction drilling is shown in GB 2271795. The directional drilling tool comprises a drill bit support arranged upon a main support by means of a cam surface, such that rotation of the drill bit support with respect to the main support causes the drill bit support to be change its azimuthal orientation. Again, in order to drill a borehole at a different inclination, it is necessary to withdraw the drilling tool and fit an alternative tool (having a different cam profile) which is set at a different inclination. This limits the flexibility of the drilling process.

It is an objective of this invention to provide a convenient method of correcting or adjusting the direction of a down hole tool.

According to the present invention there is provided a variable orientation downhole actuation tool disposed near the end of a down hole tube, including a first part joined to the down hole tube whose orientation is fixed relative to the down hole tube, and a second part fixed by articulation to the first part, whose orientation may be adjusted relative to the first part, the orientation between being adjustable so as to vary a first angle, this being the included angle between the first part and the second part, a second angle through which the second part is rotated relative to the first part in the first part's axis, and a third angle through which the second part is rotated in its own axis, these three angles being independently adjustable, the articulation between the first part and the second part including an actuating member and capable of substantially axial movement relative to the first part, and a following member, which the actuating member bears against such movement of the actuating member causing a substantially transverse movement of the following member, and thereby varying the first angle between the first and the second part.

Preferably the articulation between the first part and the second part includes a second actuating member and capable of substantially axial movement relative to the first part, and a second following member, which the second actuating member bears against such movement of the second actuating member causing a substantially transverse movement of the second following member, such that the plane in which first transverse movement and the first part lie is non-parallel to the plane that the second transverse movement and the first part make.

According to another aspect of the present invention there is included a variable orientation downhole actuation tool disposed near the end of a down hole tube, including a first part joined to the down hole tube whose orientation is fixed relative to the down hole tube, and a second part fixed by articulation to the first part, whose orientation may be adjusted relative to the first part, the orientation between being adjustable so as to vary a first angle, this being the included angle between the first part and the second part, a second angle through which the second part is rotated relative to the first part in the first part's axis, and a third angle through which the second part is rotated in its own axis,

these three angles being independently adjustable, the articulation between the first part and the second part including an rotating member which may be rotated about an substantially axis parallel to the first part, the rotating including an eccentric surface not having rotational symmetry about the first parts axis, a steering member, which the eccentric surface bears against such that rotation of the rotating member causes displacement a substantially transverse movement of the following member, and thereby varying the first angle between the first and the second part.

According to a further aspect of the present invention there is provided a method of drilling borehole casings by means of a variable orientation downhole window forming tool comprising a first part which is fixed with respect to the end of a down hole tube and a second part which is adjustable with respect to the first part, characterised in that the forming tool is run in hole with a closely fitting straight orientation and then a bend is set to bring the window forming tool into contact with the casing upon further advancement of the window forming tool.

A joint is disclosed for coupling the first part to the second part, said joint permitting the aforementioned adjustments. Hydraulic means, electric means, or electro-hydraulic means may be used to control the aforementioned adjustments.

Efficient means of fast rotation and transmission of high torque of one of the first and second parts relative to the other is shown.

The actuation tool may be used for milling windows in casing that lines a previously drilled borehole. Diameter is a severe constraint and by means of the invention it is possible to run in hole with a closely fitting straight assembly and then to set a bend to bring the cutter into contact with the casing.

Furthermore, in order to make a rapid change in direction in a short drilled distance, it is advantageous that the bend must be close to the bit, while the bit must be supported by a robust thrust bearing to permit motor shaft rotation under heavy loads. It is also advantageous that a short and simple bottom hole assembly is likely to be much more reliable and much more easily integrated with instrumentation and surface handling equipment. The present invention further provides an adjustable bend, adjustable plane of bend, thrust bearing, rotating motor shaft and passage for drilling fluid in one integrated joint close to the bit.

In the preferred embodiments described below, the orientation of the tool is expressed in terms of three angles, in which the drill can be rotated in at least two of the angles freely. These three angles are (i) the angle between the drill pipe above the adjustable portion and below the adjustable portion (herein referred to as the included angle or inclination) (ii) the angle of the plane in which the included angle lies (for example considered from the axis of the drill string), herein referred to as (azimuthal) direction (iii) and the rotation of the second body about its reference axis.

These angles are known as Euler angles, and although different conventions for describing and defining them exist, it will be apparent that they described three dimensions with which any orientation of one body with respect to another can be described. Thus the tool of the specific

embodiments may orient itself within the envelopes of its movement with three degrees of freedom.

It will be evident that the invention may be used for pointing without necessarily rotating, such as without limitation for directing a fluid or plasma or other cutting or welding jet, arc or implement. It may also without limitation be used for orbiting a rotating abrasive or cleaning head for cleaning, cutting or dressing casing and casing joints or for expanding casing by orbiting side force.

The following is a more detailed description of an embodiment of the invention by way of example only, reference being made to the accompanying drawings, in which: Figure la shows a longitudinal section of a first embodiment of the downhole actuation tool with an integrated steering joint in a bent position; Figure Ib shows a longitudinal section of the tool in a straight position; Figures 2a and 2b show a second embodiment of the downhole actuation tool with, deployed at a maximum offset position; Figures 2c and 2e show a transverse sections of figures 2a and 2b respectively, whilst figure 2d shows an intermediate position.

Figures 3a and 3b show a longitudinal sections of a third embodiment in straight and maximally offset positions respectively; Figures 3c to 3f show transverse sections of the third embodiment in various positions

Figure 4 shows a longitudinal section of a fourth embodiment of the downhole actuation tool with a hollow bore hydraulic actuator driven from a through motor shaft.

The invention will now be described with reference to the representative embodiments in the figures. It will be understood that many features of engineering practice such as seals, fastenings, and bearings may be changed according to preference and physical size of the apparatus without altering the invention.

Referring to figure 1, a housing 10 is a continuation of the drillpipe and motor assembly and may conveniently be considered as a fixed reference for the purpose of the description, without affecting the generality of application of the invention. Drill bit 12 is assembled into bit box crossover 14 and thence to hollow steering shaft 16, by tapered threaded joints for stiffness and strength. Shaft 16 may be keyed on its internal bore to receive a tool whereby it may be tightened to crossover 14 without adding length to the shaft for external gripping tongs.

A limited-angle split spherical plain bearing, is set into housing 10, retained by the shoulder 11 of the housing 10, a spacer 19 and a nut 40. The inner rings 29 of this spherical bearing capture the steering shaft 14 on a flange 15 and provide massive and stable support for thrust loads, side loads and extraction loads that may be reacted into the shaft from the drill bit. The steering shaft is thus permitted to pivot in any direction about the centre 27 of the spherical bearing 18. The steering shaft may advantageously be made from a strong bearing material such as Beryllium Copper alloy, and is

free to rotate about its longitudinal axis, joumalled in the internal mounting faces of the spherical bearing ring 29'. This free rotation avoids premature wear of bearing 28 that would otherwise be caused if ring 29 was required to rotate within it at fast motor speed, and permits the addition of a peg or similar means between the parts of the bearing to prevent such rotation.

Motor shaft 20 is rotated by the drilling motor (not here shown) and terminates in a crown spline 28. These splines engages in corresponding straight splines cut in the bore of the steering shaft. The crown spline coupling provides a means of transmitting torque through the between two shafts capable of being articulated through small angles to each other.

Motor shaft 20 may be assembled to steering shaft 16 by various methods depending on the required strength and space availability. In the embodiment shown, the spline is too large to insert from one end of the steering shaft, and may be conveniently assembled as two parts screwed together to act as a single stiff whole.

Preferably as shown the steering shaft 10 rotates directly within the bearings 28 to minimise the size and number of parts in the assembly. If space permits, separate bushings and even needle bearing or other bearing means may be used to support the rotating steering shaft in its spherical bearing.

It will be appreciated that by tilting the steering shaft 16, the drill bit 12 can be pointed in any direction within the limits of the spherical bearing 28 and crown spline 28, and further that at any such direction the motor shaft 20 will rotationally drive the drill bit, the torque being transmitted through the crown spline.

In this embodiment, the motor shaft runs centred and concentric to the housing centreline, making it balanced and easy to support in bearings. The crown spline coupling inherently provides lateral support for the lower end of motor shaft and this may be sufficient in many applications. The motor end of the shaft and its sealing arrangements are not here shown, and may be implemented by any convenient well known means.

Preferably the distance between bit 12 and centre 27 is short, so only a modest angle of tilt, typically 2 degrees, suffices to achieve a high radius of

curvature of the borehole path. Moreover this short distance and small angle mean that reactance forces experienced at the end of the steering shaft furthest from the drill but are kept to manageable magnitudes.

A second spherical bearing 30 is located on the upper end (that is, the end furthest from the drill bit) of the steering shaft 10, for example a non-split bearing. The steering shaft 10 turns freely within the inner bearing ring 31 of the spherical bearing 30. Continuous rotation of between the steering shaft 10 and the bearing 30 is unnecessary and may be prevented by a key to prevent premature wear. Tilting of the steering shaft may thus be accomplished by displacing bearing 30 transversely from the centre line in any direction.

Housing 49 holds bearing 30 and suitable means of displacement will be disclosed below.

When bearing 30 is moved upwards (that is, radially outwards from the centre axis of the housing 10, but within the plane of the paper in the figure) results in the steering shaft 10 pivoting about the pivot point 27 so that the axis of the steering shaft is inclined relative to the axis of the housing as shown in Figure la. The bit box crossover 14 and drill bit therefore are similarly inclined to the housing and drillpipe immediately above the housing. Movement of the bearing 30 in the opposite direction (in a downwards direction radially way from the housing's 10 centre axis) results in a similar inclination angle but in a direction 180 degrees about the housing axis to the former position, when considered looking downwards along the drill pipe axis. Moving to set the bearing centred on the axis of the housing leaves the bend straight (that is, the axis of the steering shaft 16 and the housing 10 are coincident), as shown in Figure lb. It will be appreciated that moving the bearing out of the plane of the paper and up and down allows all azimuthal directions from 0-360 degrees and all bends (that is, magnitude of angle between the housing axis and the steering shaft axis) from zero to the aforementioned practical limit to be reached.

Moving the centre of bearing 30 along a fixed radius circle centred upon housing 10 centreline causes the bit to be describe a cone, since the bend will be fixed as the bit changes directions through 360 . Thus, importantly, a complete circular traverse of the bearing centre causes a complete circle of bit direction, but not a rotation of the drill bit about its axis. Bit rotation is effected by turning the motor shaft 20, the rotation being transmitted through the torsional keying of the crown spline to the splines on the steering shaft.. That is, if the motor shaft did not rotate then the bit would be seen to deflect in different directions but a point marked on it would not rotate. Conversely an actuator acting on housing 49 may rotate in a complete circle to cause the centre of the bearing 30 centre to traverse a circular path. Actuator rotation is decoupled from the steering shaft by bearing 30, so does not cause the steering shaft or bit thereon to rotate.

It will be seen that this embodiment of the present invention, with the single centre 27 of deflection and torque transmission through that centre, have an additional degree of freedom, within the maximum bend angle, as compared to prior art movement systems having a bent sub with a fixed bend which is rotated distant from the bend centre, which can be rotated so that the drill bit describes a cone, but the angle of the cone cannot be varied.

It will also be seen if a motor shaft were used with a bent sub arrangement, with the motor shaft being passed through the bend direct to the bit then the bit would require its own bearings and a lower housing. The motor shaft would need to be deflected through the bend axis, or split and coupled. If it was split and coupled at the centre of the steering joint then the joint would be weakened by the need to house a non-integral coupling. If the shaft was split into one or more lengths so as to pass through the constriction of the steering joint centre then additional couplings, possible eccentric location of the housing would be required and this is difficult to accomplish reliably.

When bearing 30 is moved transversely to the housing 10, its relative axial position along the steering shaft 20 will vary slightly, as the upper steering

shaft end 16 describes an arc. This slight movement is easily accommodated by allowing the bearing ring 31 to slide along the shaft 16 and/or the entire bearing 30 to slide within its housing 49.

A particular benefit of the spherical bearing 30 is that only side force can transmitted between the housing 49 and steering shaft 16, and not bending moment. This greatly reduces the strength needed for reliable operation and reduces flexure.

Motor shaft 20 is preferably tubular to permit the flow of drilling fluid or other substances or artefacts. Such a tubular bore is continued by bore 17 in the steering shaft and it is desirable to seal these elements together to prevent ingress into the mechanism of the joint. Importantly, as the steering shaft 16 and motor shaft 20 are coupled by splines 22 they cannot rotate relative to each other. Therefore seal 41, here shown as an elastic tube sealed statically to the motor shaft 16 at region 42 and the and to the steering shaft 16 at region 43, and merely deflects as the bit is steered, as may be seen by comparing Figures la and lb. In the normal case where the drilling fluid pressure greatly exceeds the internal pressure of the joint, the seal is supported by the surrounding steering shaft bore. In the case that the drilling fluid pressure is less than the internal pressure of the joint, the seal may be prevented from collapse by reinforcement such as wire hoops or a loose steel liner tube.

The joint mechanism is protected by being sealed to the outside of the housing. Rotary seal 44 and ring gutter seal 45 allow the steering shaft to rotate relative to housing 10 when driven by the motor shaft and combine the performance of a pure rotary seal with the static deflection capability of the gutter seal, but it will be appreciated that many sealing arrangements are possible. The gutter seal accommodates deflection in a similar manner to seal 41, and again its action is evident by comparing Figures la and Ib, but is also shaped for the different deflection range and space available, and

works best when the internal pressure of the joint is reasonably balanced to the external pressure. Referring now to Figure 2, a first means of controlling the steering shaft 16 using a dual parallel rotary actuator for adjusting a lever is disclosed. Bearing housing 49 is circular in cross-section and integrally includes an elongated inner cylindrical sleeve 52, which is free to rotate within an outer cylindrical sleeve 50. This outer sleeve 50 is free to rotate within the housing 10, and has centre of rotation indicated in Figures 2a and 2b by a line of dashes 54. The inner sleeve has a centre shown by a solid line 58, and shown by a cross having horizontal and vertical bars in the cross sectional views. Figures 2c and 2e respectively show the configurations depicted in Figures 2a and 2b through the line X-X, and are depicted so that the lines 54 and 58 carry over into these figures to indicate the level. The housing 49 has centre 58, indicated by a cross having diagonal bars. The centres 54 and 58 are displaced by one half the maximum desired offset of housing 49.

Referring to Figure 2a and Figure 2c the inner and outer sleeves are shown rotated such that the offset is a maximum upwards in the plane of the paper (that is, as shown in the figure). Accordingly the distances separating 54 from 58, and 58 from 56 are equal, and a hypothetical reference mark 60 fixed on housing 49 is at its maximum distance from the outer sleeve centre 54..

Rotating inner sleeve 52 within outer sleeve 50, causes the inner sleeve centre 56 will spiral towards centre 2-4. Figure 2D shows the inner sleeve rotated through one-quarter turn. The reference mark 60 has moved in direction but also is closer to centre 54. Referring to figure 2e, after the inner sleeve has been rotated through one-half turn, centre 56 is brought to coincide with centre 54. Figures 2b and 2e show that housing 49, and hence bearing 31 now coincides with the centre line of the apparatus, 54.

The inner sleeve rotation may be continued onwards or reversed to bring housing 49 back to its starting position.

It will now be appreciated in conjunction with Figures la and lb that rotation of inner sleeve 52 provides a means to operate the steering joint continuously from maximum bend to straight.

At any intermediate rotation the offset of the steering shaft has been adjusted, but also its direction. The direction may now be brought to a desired position while keeping the offset fixed, by rotating outer sleeve 50 in housing 10 but keeping the relative rotation of inner sleeve 52 fixed relative to the outer sleeve. It will also be appreciated that by co-ordinating the rotations of the outer sleeve 50 relative to the housing 10 and the inner sleeve 52 relative to the outer sleeve 50, any desired bend and direction of the steering joint may be obtained.

Various methods can be employed by those skilled in the art to cause the two sleeves to rotate, and measure the amount of rotation. Two are now briefly mentioned. Rotation can be from the output of a gearmotor, in which case, coupled with reversing of the motor, continuous adjustment will be obtained. Motor brakes can be used to lock the rotations. Alternatively a motor and screw, or hydraulics, can be used to reversibly longitudinally thrust a key, constrained not to rotate, along a screw thread cut in the sleeve to be rotated. The key's axial position thereby controls sleeve rotation. Motors can be individual units or, using clutches, the drilling motor shaft can be used as a source of power. Bend and direction angles can readily be calculated from the measured rotations and basic trigonometry.

Figure 3 discloses a second means of controlling the steering shaft using dual parallel translating actuators for adjusting the steering shaft via bearing 31. The bearing housing comprises an inner sleeve 62 within an intermediate sleeve 65 and an outer sleeve 67. These sleeves are non

rotating with respect to the housing 10. A radial groove in housing 62 engages in a fixed flange or ring 63 on the inner surface of housing 10, axially constraining the housing 62. Housing 62 features two pairs of diametrically opposed keys 70,72, which are slanted with respect to the housing's 62 axis. These keys engage in elongate slots or grooves 69,71 cut in sleeve 65 at the same angle to the housing's 62 axis. The assembly may be made by pressing the keys into 62 via slots in 65. If sleeve 65 is forced forwards or backwards along the axis of the apparatus, then since inner sleeve 62 cannot move axially, the keys 70,72 of inner sleeve 62 lift or lower (that is, displaces upwards or downwards in the plane of the paper) the inner sleeve to which they are attached.

Figures 3b and 3d show the inner sleeve 62 lifted (as compared to figures 3a and 3c) as 65 and 67 the are together pulled away from ring 63. Figure 3e shows the inner sleeve 62 in a lowered position as 65 and 67 are together pushed towards the ring 63.

Intermediate sleeve 65 also carries two pairs of diametrically opposed angled keys on it, 86,88, positioned one quarter turn from keys 70,72, i. e. lying in perpendicular planes. These keys engage in angled slots or grooves 85,87 cut in outer sleeve 67. The principle is identical to that of intermediate sleeve 65 and inner sleeve 62 already described except now that it will be apparent axial movement of the intermediate sleeve relative to the outer sleeve will cause transverse motion (that is, into and out of the plane of the paper). Figure 3f shows leftwards movement of the inner sleeve 62 when starting from Figures 3a and 3c, the outer sleeve is moved to a first axial extreme relative to the intermediate sleeve. Conversely Figure 3g shows rightwards movement of the inner sleeve when the outer sleeve is moved to the opposite axial extreme relative to the intermediate sleeve.

It will now be readily appreciated that by co-ordinating the axial positions of the outer sleeve 67 and intermediate sleeve 65 with respect to the

housing 10, the inner sleeve 62 and hence (via the bearing 31) the steering shaft 16 may be made to move in any direction and offset from the apparatus centre line. Figure 3h shows such an intermediate position.

Axial movement of the sleeves may be achieved in many ways, such as hydraulic cylinders 91 and 94. Bi-directional pistons 90 and 93 carry thrust/pull rods 92 and 95 connected to sleeves 67 and 65 respectively.

These connections must allow for the small transverse movements of the sleeves. As an example, if the keys are set at a rate of one in eight, and the stroke length of cylinder 94 is plus or minus one centimetre, then it will cause a lift of inner sleeve 92 by plus or minus one-eighth centimetre.

Since this axial motion is relative to outer sleeve 97, then cylinder 91 must have a stroke length of plus or minus two centimetre, to allow for its plus or minus one centimetre stroke to move inner sleeve sideways plus or minus one-eighth of a centimetre when cylinder 94 is at either end of its own stroke.

Hydraulic pistons may be operated via flow lines and remote pumps, or preferably using a local pump and valve assembly as representatively shown in Figure 4. Here the drill motor rotation of motor shaft 20 is used via coupling 81 to turn an axial swash plate piston pump, comprising swash plate 82, pump pistons 83 and pump valves 84. Cylinder operating valves may be fitted into annular valve block 80.

These embodiments are readily varied according to requirements. For example, if the loads are relatively small then the fore and aft key pairs 70, 72 may be conveniently substituted for a single elongate pair, as may the fore and aft key pairs 86,88. The cylinder 78 and piston 79 may be made to have the same stroke as cylinder 93 and piston 94, but cylinder 91 being made slidable and locked to the motor shaft 20.. The cylinders may be annular, or as shown divided into one or more small cylindrical units. Instead of keys and axial sleeve movement, the sleeves may be directly

lifted relative to each other and housing 10 using transverse hydraulic cylinders. It will also be appreciated that hybrid methods of control based on combining elements from the different separate embodiments disclosed herein could be used. For example, intermediate sleeve 65 of figure 3 may be used to impart an offset to sleeve 62 as already disclosed, but without keys 70,72, but may instead be rotated like the outer sleeve 50 shown in figure 2 to choose direction.

Claims (1)

1. Claims 1. A variable orientation downhole actuation tool disposed near the end of a down hole tube, including a first part joined to the down hole tube whose orientation is fixed relative to the down hole tube, and a second part fixed by articulation to the first part, whose orientation may be adjusted relative to the first part, the orientation between being adjustable so as to vary a first angle, this being the included angle between the first part and the second part, a second angle through which the second part is rotated relative to the first part in the first part's axis, and a third angle through which the second part is rotated in its own axis, these three angles being independently adjustable, the articulation between the first part and the second part including an actuating member and capable of substantially axial movement relative to the first part, and a following member, which the actuating member bears against such movement of the actuating member causing a substantially transverse movement of the following member, and thereby varying the first angle between the first and the second part.

   2. A tool according to claim I wherein the articulation between the first part and the second part includes a second actuating member and capable of substantially axial movement relative to the first part, and a second following member, which the second actuating member bears against such movement of the second actuating member causing a substantially transverse movement of the second following member, such that the plane in which first transverse movement and the first part lie is non-parallel to the plane that the second transverse movement and the first part make.

   3. tool according to claim 1 wherein the articulation between the first part and the second part includes an rotating member which may be rotated about an substantially axis parallel to the first part, the rotating including an eccentric surface not having rotational symmetry about the first parts axis, a steering member, which the eccentric surface bears against such that rotation of the rotating member causes displacement a substantially transverse movement of the following member, and thereby varying the first angle between the first and the second part, such that the plane in which first transverse movement and the first part lie is nonparallel to the plane that the second transverse movement and the first part make.

   4. A tool according to any previous claim wherein the first part and the second part are torsionally coupled such that rotation of the first part about its axis causes rotation of the second part about the axis of the second part.

   5. A tool according to any previous claim wherein the following member includes a substantially radially extending protrusion and the actuating member includes an accepting portion with which the protrusion engages, the accepting portion being inclined with respect to the first part's axis.

   6. A tool according to any previous claim wherein the first part and the second part include a through bore for the passage of fluids.

   7. A tool according to any preceding claim, characterised in that the tool is a steerable drilling tool.

   8. A tool according to any preceding claim, characterised in that a rotary power means is provided for of supplying mechanical rotary power to the tool.

   9. A variable orientation downhole actuation tool disposed near the end of a down hole tube, including a first part joined to the down hole tube whose orientation is fixed relative to the down hole tube, and a second part fixed by articulation to the first part, whose orientation may be adjusted relative to the first part, the orientation between being adjustable so as to vary a first angle, this being the included angle between the first part and the second part,

   a second angle through which the second part is rotated relative to the first part in the first part's axis, and a third angle through which the second part is rotated in its own axis, these three angles being independently adjustable, the articulation between the first part and the second part including an rotating member which may be rotated about an substantially axis parallel to the first part, the rotating including an eccentric surface not having rotational symmetry about the first parts axis, a steering member, which the eccentric surface bears against such that rotation of the rotating member causes displacement a substantially transverse movement of the following member, and thereby varying the first angle between the first and the second part.

   12. A tool substantially as herein described and illustrated.

   13. A method of drilling borehole casings by means of a variable orientation downhole window forming tool comprising a first part which is fixed with respect to the end of a down hole tube and a second part which is adjustable with respect to the first part, characterised in that the forming tool is run in hole with a closely fitting straight orientation and then a bend is set to bring the window forming tool into contact with the casing upon further advancement of the window forming tool.