CA2494229C

CA2494229C - Adjustable downhole tool

Info

Publication number: CA2494229C
Application number: CA2494229A
Authority: CA
Inventors: Mark Alexander Russell; Andrew Ollerenshaw
Original assignee: Toolbox Drilling Solutions Ltd
Current assignee: Toolbox Drilling Solutions Ltd
Priority date: 2004-02-02
Filing date: 2005-01-25
Publication date: 2011-01-11
Anticipated expiration: 2025-01-25
Also published as: CA2494229A1; GB2410510B; DE102005004606A1; GB0402220D0; GB2410510A

Abstract

An adjustable down-hole tool, for example a drill-string stabiliser (10), comprises a body (12) having a through bore (16). A mandrel (18) is rotationally fixed but axially movable in the body, the mandrel being movable by fluid pressure in the tool against the action of a first return spring (44) between a first, deactivated position and a second activated position. A sleeve (66) is between shoulders (68, 69) on the body and mandrel. Castellations (l8a,b, 69a,b) are on the body and facing edge or edges of the sleeve so that, when the castellations are in phase, the mandrel is prevented from travelling from said first to second position, and when they are out of phase they interdigitate and the mandrel is not prevented from travelling from said first to second position. A control piston (36) is slidable in the mandrel, being movable by fluid pressure in the tool against the action of a second return spring (50). The piston is axially slidable with respect to the mandrel. A circumferential barrel cam (56) is defined on the piston, a cam follower (58) being disposed in the mandrel but within the confines of the barrel cam so that axial movement of the piston with respect to the mandrel results in corresponding rotation of the piston with respect to the mandrel. Gears (61) in the barrel transmit rotation of the piston to the sleeve to move the sleeve between said activated and deactivated positions.

Description

ADJUSTABLE DOWNHOLE TOOL
This invention relates to adjustable down-hole tools employed in the oil and gas drilling industry.
WO-A-0053886 discloses an adjustable down-hole tool comprising a body having a through bore; a mandrel axially movable in the body, the mandrel being movable by fluid pressure in the tool against the action of a first return spring between a first, deactivated position and a second activated position; a sleeve limiting movement of l0 said mandrel between said positions; at least two sets of castellations, one set on the sleeve and the other set on a facing edge so that, when the castellations are in phase, the mandrel is prevented from travelling from said first to second position and when they are out of phase they interdigitate and the mandrel is not prevented from travelling from said first to second position; and means to rotate the sleeve relative to the facing edge between 15 said in-phase and out-of phase positions; characterised in that said means comprises a control piston slidable in or on the mandrel and in the body and being movable by fluid pressure in the tool against the action of a second return spring.
In the specific embodiments disclosed in that application, the control piston 20 slides sealingly against the bore of the body, and also against the bore, or around the outside of, the mandrel, also sealingly. Both embodiments disclosed are satisfactory, but the present invention relates to a modification of the tool disclosed in that application.
The reason for the modification is that, in the previously described arrangement, the control piston seals against two components, the body and mandrel, that are not fixed 25 with respect to one another. Consequently, there could be leakage past the respective seals if the tolerance in manufacture of the body and mandrel is not very tight and there is excessive play between them. Also, since the piston must seal to both the body and mandrel, it inevitably extends beyond the end of the mandrel, increasing its overall length. It is an object of the invention to overcome these disadvantages.
In accordance with the present invention, there is provided an adjustable down-hole tool comprising a body having a through bore; a mandrel having a through bore axially movable in the body, the mandrel being movable by fluid pressure in the tool against the action of a first return spring between a first, deactivated position and a second activated position; a sleeve between the body and mandrel limiting movement of said mandrel between said positions; at least two sets of castellations, one set on the sleeve and the other set on a facing edge of the body or mandrel so that, when the castellations are in phase, the mandrel is prevented from travelling from said first to second position and, when they are out of phase, they interdigitate and the mandrel is not prevented from travelling from said first to second position; and a control piston to rotate the sleeve relative to said facing edge between said in-phase and out-of phase positions, the piston being movable by fluid pressure in the tool against the action of a second 1 o return spring; characterised in that said control piston is slidable in the mandrel, the mandrel carrying rotation transmitters that are in contact with both the piston and sleeve, whereby rotation of the piston relative to the mandrel rotates the sleeve relative to the mandrel.
Preferably, said rotation transmitters are carried by the mandrel intermediate its ends. Preferably, they are between axially spaced seals of the piston against the bore of the mandrel. Said transmitters may comprise a gear rotationally journalled in the mandrel about an axis parallel the throughbores. Both the piston and sleeve may have a rack engaged with the gear. A plurality of gears may be disposed around the 2o circumference of the mandrel.
Drill string stabilisers, under reamers and fishing tools are some of the down hole tools that require activation when they are in a given position down hole to make them operative, and deactivation when they are to be withdrawn, or repositioned or indeed simply to go into a different operating condition.
Taking stabilisers as an example, these tools centralise drill strings with respect to the hole drilled. They normally comprise a sub assembly in the drill string.
The stabiliser has a plurality of blades, (usually three and usually spirally arranged), 3o whose edges are adapted to bear against the bore-hole. The blades are not complete around a circumference of the drill string so that the return route for drilling mud pumped down the bore of the drill string is not blocked. In order to control the direction of drill bits, it is sometimes required that the stabiliser has variable diameter.
Pistons in the blades are extendable to give the stabiliser a maximum diameter, which ensures that the drill string is central in the bore-hole. The drill bit, assuming the stabiliser is close behind the drill bit, is thus kept straight. However, if the pistons are withdrawn, then gravity can deflect the drill string so that it alters the inclination of the hole.
Preferably, said mandrel is rotationally fixed with respect to the body.
Preferably, a circumferential barrel cam is defined in one of said piston and mandrel, a cam follower being disposed in the other thereof, the follower being within the barrel cam so that axial movement of the piston with respect to the mandrel results in 1 o corresponding rotation of the piston with respect to the mandrel. In this case, the barrel cam may be shaped so that movement of the piston in one axial stroke and return thereof results in rotation of the sleeve from a said in-phase position to a said out-of phase position or vice versa. Said castellations are preferably angularly spaced by a phase angle and said stroke and return of the piston results in rotation of the sleeve by said phase 15 angle. Instead of the piston rotating in its entirety in the mandrel, a separate component thereof may be rotationally freely, but axially fixedly, mounted in the piston, which component carries said barrel cam or follower. In this event, said separate component drives said rotation transmitters on rotation of said component in response to axial movement of the piston in said mandrel. This arrangement avoids the necessity to rotate 2o the piston with respect to the mandrel. Thus the barrel cam does not need to overcome the resistance of the piston's frictional contact with the mandrel through its seals thereto and other parts in contact therewith.
When said mandrel is in said deactivated position, a rise in hydraulic pressure 25 in the tool preferably results in movement of the piston before movement of the mandrel.
Said first return spring may be sufficiently stronger than said second return spring to ensure that, when said mandrel is in said deactivated position, a rise in hydraulic pressure in the tool results in movement of the piston before movement of the mandrel.
Alternatively, or in addition, a spring loaded detent may be provided between said 3o mandrel and body to retain the mandrel in said deactivated position until a threshold hydraulic pressure has been exceeded, which pressure is greater than that required to move said piston. Said detent may comprise a plunger in a radial bore of the mandrel or body, spring biased against a lip of the body or mandrel, respectively. Said lip may be of a circumferential groove around the body or mandrel.
Preferably, the plunger has a through bore connecting the space between the mandrel and body with a space behind the plunger so that hydraulic effects are substantially eliminated. Moreover, there are preferably a plurality of said detents arranged around the circumference of the mandrel. This reduces any moment on the mandrel relative to the body.
1 o The mandrel is preferably sealed to the body about first and second circumferences, the first being a larger circumference upstream, in terms of fluid flow through the tool, of the second, smaller circumference. Thus hydraulic forces act on the mandrel relative to the body urging the mandrel in a downstream direction.
References to upstream and downstream are purely for convenience, of course. The direction of 15 movement of the components in question is dependent only on hydraulic pressure, not on direction of flow.
The piston preferably also has a through bore and is sealed to the mandrel about third and fourth circumferences, the third being a larger circumference upstream, in 2o terms of fluid flow through the tool, of the fourth, smaller circumference.
Thus hydraulic forces likewise act on the piston relative to the mandrel, also urging the piston in a downstream direction.
In said activated position, the bore of the mandrel preferably engages a throat 25 in the bore of the body to create a flow restriction and consequent back pressure detectable to indicate the position of the tool.
Said tool can be a drill-string stabiliser, in which case said mandrel has wedge surfaces to engage corresponding surfaces on radially disposed pistons slidable in 3o the body, whereby, when the mandrel moves from said deactivated to said activated position, the pistons extend from the body increasing the working diameter of the stabiliser.

An embodiment of the invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which:-Figures 1 a, b and c are part side sections through different parts (downstream, middle and upstream sections) of a stabiliser in accordance with the present 5 invention;
Figure 2 is a section on the line II-II in Figure lb;
Figure 3 is a side view of a control piston of the tool of Figure 1;
Figure 4 is a view in the direction of arrow A in Figure 3;
Figure 5 is a side view of the facing castellations of the sleeve and mandrel of I o the stabiliser of Figures 1 a to c;
Figure 6 is a view similar to Figure 5, but of an alternative embodiment of the present invention; and Figure 7 is a partial side section of a variation of the embodiment shown in Figures 1 a, b and c.
IS
In the drawings, a stabiliser 10 comprises a body 12 connectable to a drill string 13 by means of male and female connectors 14 at either end thereof. A
bore 16 extends from one end of the body 12 to the other, to permit flow of mud to lubricate the drill bit (not shown) at the end of the string. Slidable in the bore 16 is a mandrel 18 2o which is rotationally fixed therein by virtue of a stud 20 in the body 12 which extends into a slot 22 in the mandrel 18. The slot 22 extends axially of the mandrel 18 permitting axial movement thereof within the body 12.
Spiral blades 24 are defined on the surface of the body 12 and bear against the 25 surface of the bore hole (not shown) being drilled to guide the drill bit.
The blades permit the return passage of drilling mud by being spaced around the body 12. The blades 24 have radial bores 26 defined in spaced relation along each blade 24. Within each bore 26 is a piston 28 urged radially inwards by springs 29. The base of each piston is formed with a wedge surface 30 against which a wedge 31 of the mandrel 18 acts. Thus, if the 3o mandrel moves rightwardly in the drawings, the pistons 28 are thrust radially outwardly projecting beyond the circumference of the stabiliser 10 defined by the blades 24. In this way, the working diameter of the stabiliser increases with the faces of the pistons 28 bearing against the wall of the bore hole.

At its upstream end 33, the mandrel receives a control piston 36. The control piston is slidable in a bore 38 of the mandrel which extends from its upstream end 33 to its downstream end 32. The control piston carries seals 46,48a which seal the piston with respect to the mandrel 18. The mandrel is sealed to the body at its upstream end 33 by seal 34, and at its downstream end 32 by seal 48b.
As far as the body 12 is concerned, the mandrel and piston are a single unit, and it can be seen that the circumference of the seal 34 in the body 12 is much larger than 1 o the circumference of the seal 48b. Consequently, hydraulic pressure of the mud in the tool 10 results in a larger downward force acting at the end 33 of the mandrel 18, via the seal sleeve 34, than acting in the reverse direction through the seal 48b.
A spring 44 acts between a shoulder 42 in the body 12 and the mandrel 18, 15 urging the mandrel in the upstream direction. Should the pressure differential be such that the force acting on the mandrel exceeds the return force of the spring 44, the mandrel will move rightwardly in the drawing.
Likewise, hydraulic pressure acting on the control piston 36 across the 20 circumference of its seals 46,48a to the mandrel result in a downward force on the piston 36 because the circumference of the seal 48a is smaller than seal circumference 46.
Again, a spring 50 (not shown) acts between shoulder 52 in the mandrel 18 and shoulder 54 on the piston 36 to urge the piston in an upstream direction. Again, should the hydraulic pressure be such that the force of the spring 50 is overcome, the piston 36 will 25 move rightwardly in the drawings.
The piston has a barrel cam 56 defined in its surface (see Figure 3). Pins 58 in the mandrel are received within the confines of the barrel cam 56 so that movement of one relative to the other forces the piston to follow a course defined by the barrel cam 56.
30 If the mandrel is considered, for the moment, to be stationary, then, as hydraulic pressure increases in the bore 38 of the mandrel 18, the piston 36 begins movement from left to right (with reference to Figure 1). Suppose the pins 58 start at position 58a, for example (see Figure 4), where they lie at the base of a first notch 56a of the barrel cam. They will thus move, relatively to the barrel cam 56, until they contact the opposite wall thereof at 56b. Further axial movement of the piston 36 then only occurs when the piston rotates through a small angle al, so that the pin 58 effectively moves to position 58b in notch 56c on the opposite side of the barrel cam 56 from notch 56a.
Should the hydraulic pressure be released, return spring 50 forces the piston 36 leftwardly in the drawing (Figure 1). The pin 58 is obliged to follow a course from position 58b in notch 56c of the barrel cam 56, axially until the opposite wall of the barrel cam 56d is contacted. Thereafter, further axial movement of the piston can only occur on further rotation of the piston. In this event, the pin moves to the base of notch 56e on the same side of the barrel cam 56 as notch 56a. In this movement, the piston has rotated through a further angle a2, which is not necessarily the same as a~ .
Nevertheless the sum (al + a2) is equal to 2a, the angle of rotation of the piston 36 on one complete return stroke thereof in relation to the mandrel 18.
A subsidiary feature of the barrel cam 56 and pins 58 is that the pins 58 have a large diameter section 58' and a small diameter end 58". The barrel cam has a correspondingly wide slot 56' and a deeper, narrow slot 56", so that the wide slot 56' accommodates the large diameter section 58' of the pin 58, while the narrow slot 56"
2o accommodates the thin pin end 58". The purpose of this is that a wide slot is inevitably somewhat coarse compared with a narrow slot, which can be precise. On the other hand, a wide slot with a large diameter pin significantly reduces point loads, both on the pin and cam surface it is following. Given that the control piston is spring loaded, it inevitably resists rotation due to frictional forces, although these can be alleviated, for example, by employing a thrust bearing between the spring 50 and piston 36.
However, even with this measure, if only a coarse cam surface 56' and large pin 58' is employed, then, in moving from notch 56a to contact surface 56b, a rotational drift back in the direction of Arrow X in Figure 4 of only 1 ° can be permitted. Any greater drift, which would generally be caused by the spring having been "wound up" by previous 3o movements, would cause contact of the pin 58' with point 56f of the cam 56', such that secure guidance of the pin to notch 56c could not be guaranteed. Because slot 56" can be more precise, however, the permitted angle of drift can be much greater, such as 15° (see Arrow Y in Figure 4), while still ensuring that the pin is guided correctly and rotation of the piston 36 in the correct direction is guaranteed. At the same time, however, it is only during these extreme situations that loading only occurs through the narrow slot 56" and thin pin end 58". Most of the time, and indeed mostly all the time when thrust bearings are employed, both surfaces 56' and 56" are contacted by both pin parts 58' and 58", so that wear on the pin 58 and slot 56 is minimised, even though accurate guidance is ensured.
As shown in Figure 3, the piston 36 has a splined section 60a which is engaged with a plurality of gears 61 disposed in pockets formed in the mandrel 18. The 1o gears 61 axe journalled for rotation in the mandrel pockets about axes 63 that are parallel the length of the stabiliser 10. The gears 61 mesh with a splined rack 64a disposed internally of a castellated sleeve 66. Consequently, since the mandrel is held rotationally fixed by stud 20 engaged with slot 22, when the piston rotates through angle 2a on one complete return stroke thereof, the sleeve likewise rotates relative to the mandrel about 15 the same angle 2a, albeit in the opposite direction.
The sleeve 66 is received between a shoulder 68 of the body 12 (in fact, on an end of compensation collar 25) and shoulder 69 of the mandrel 18. The sleeve is axially fixed on the mandrel 18 by a retention ring 67, but it is freely rotatable on the mandrel.
2o The shoulder 68 is castellated having fingers 18a and slots 18b. The end 569 of the sleeve 66 is likewise castellated having fingers 69a and slots 69b. When the fingers 18a,69a of the body and sleeve are in phase with one another (as shown in Figure 5), then rightward movement of the mandrel 18 (in Figure lb of the drawings -leftwardly in Figure 5), is limited, with the fingers 18a,69a abutting one another and the other end 70 25 of the sleeve 66 abutting shoulder 69 of the mandrel 18.
On the other hand, however, when the sleeve 66 is out of phase with respect to the body 12, (as shown in Figure 2) fingers 18a face slots 69b and fingers 69a face slots 18b so that, when the mandrel 18 moves rightwardly in the drawings, the 3o castellations on the body and sleeve interdigitate, so that further rightward movement of the mandrel 18 is possible than when the castellations are in phase. The angular separation of the fingers and slots in the mandrel and sleeve is arranged to be the same angle 2a (or multiples thereof), as described above.

Consequently, when the piston makes a complete return stroke serving to rotate the sleeve 66 through the angle 2a, the sleeve 66 moves from an in-phase position to an out-of phase position, or vice versa.
Turning now to Figure 6, an alternative arrangement is shown to that described above with reference to Figure 5. Here, the sleeve 66' has alternate slots 69b' which have different depths (shallow, 69b'~ and deep, 69b'2). Similarly, the body 12' has alternate fingers 18a' which are correspondingly short, 18a'z and long 18a'2.
Such an to arrangement necessitates, of course, an even number of fingers and slots around the sleeve 66' and body 12', which has a consequent effect on the barrel cam 56.
In the previous embodiment, there were three fingers/slots around the periphery (as shown in Figure 2), meaning that angle 2a was 120° of rotation. Here, there are preferably six fingers/slots, so that angle 2a is 60°.
The result of varying depth of fingers 18a' and slots 69b' is that mandrel 18 can have three positions instead of just two, that is to say an intermediate position between deactivation and full activation. 1n Figure 6 at its top, the mandrel is shown in its fully activated position 18'A, in which long fingers 18a'2 coincide with deep slots 69b'2, so that this corresponds entirely with at activated position of the previous embodiment. At the bottom of Figure 6, the fingers 18a'Z coincide with the fingers 69a of the sleeve 66' (which fingers are all level, as in the embodiment described with reference to Figure 5), so that the mandrel is in its deactivated position 18'C, again corresponding with the deactivated position of the previous embodiment and as shown in Figure 5.
However, in the middle of Figure 6, there is shown the intermediate position 18'B in which long fingers 18a'2 coincide with shallow slots 69b'~, with the result that the pistons 28 are only displaced radially outwardly to a lesser extent.
Returning to Figure 1 b, the body has on a collar 25 a series of pockets 90 in 3o which a plunger 92 is disposed. Springs 94 press the plunger radially outwards, the plungers being retained in the pockets 90 by threaded retainers(not shown).
The plunger 92 is adapted to be received within a circumferential groove 100 in the mandrel 18. It is therefore apparent that rightward movement of the mandrel in the body 12 is only possible if the plungers 92 are first pressed radially inwardly and released from the groove 100. For this purpose groove 100 is provided with an angled cam surface 102.
Thus when the mandrel is pressed sufficiently strongly in the rightward direction in the drawings, the returning force of the springs 94 may be overcome and the plungers 92 are pressed radially inwardly so that they pass over the lip of the groove 100.
In operation of the stabiliser 10, therefore, a user at ground level who wishes to increase the working diameter of the stabiliser 10 increases the flow and pressure of drilling mud down the bore of the drill string so that hydraulic pressure begins to act on 1 o the components within the stabiliser tool. Because of the detent represented by the plungers 92, the mandrel is at first prevented from moving. However, the piston 36 has no such detent and so commences to move rightwardly in Figure 1 aagainst the pressure of spring 50. Rightward movement of the piston 36 is thus accompanied by rotation thereof through the angle al which, for the sake of argument, rotates the sleeve 66, via the gears 61, to the position shown in Figure 5 where the fingers 69a of the sleeve 66 are in phase with the fingers 18a of the body 12. Thus, even if the pressure in the tool 10 should continue to rise sufficient to release the detent plungers 92 from the slot 100, the mandrel 18 cannot move much further rightwardly by virtue of the fingers 18a contacting the fingers 69a of the sleeve 66. Indeed, such movement as there is merely takes up the 2o clearance between the fingers 18a,69a, and between end 70 of the sleeve 66 and shoulder 69.
However, should it be desired by the user that the stabiliser operate in its maximum working diameter, the operator reduces the pump pressure so that the spring 44 returns the mandrel (to the extent that this is necessary) to its home position. The spring 50 also returns the piston. In doing so, the piston rotates through the further angle a2.
On the next occasion, therefore, that the hydraulic pressure is increased again so that the piston 36 moves once again, and it rotates the sleeve through a further angle al, then, on this occasion, the castellations on the body 12 and sleeve 66 will be out of phase.
Consequently, once the hydraulic pressure rises sufficiently to force the mandrel past the detent plungers 92, the mandrel will move fully rightwards as shown in Figure lb, with the respective castellations on the mandrel and sleeve inter-digitating.

Here, as shown in Figure lb, the pistons are pressed radially outwardly so that they stand proud of the surface of the blades 24 and increase the working diameter of the stabiliser 10.
Refernng again to Figure 4c, on rightward movement of the mandrel 18, the detent plungers 92 move into a shallow groove 112 in the body 12, which has a much less steep return face 114. Consequently, springs 44, once hydraulic pressure has been released, have no problem in compressing plungers 92 to return them over lip 104.
t o Finally, Figure 7 shows an arrangement in which the barrel cam 56 and splines 60a of the piston 36' are mounted on a separate component 361' carned by the piston 36'. The component 361' is mounted on the piston between bearings 362',364' to axially fix the component with respect to the piston, but permit it to rotate freely. This has the effect of reducing the amount of work that the barrel cam arrangement 56,58 has 15 to do to rotate the sleeve 66. It removes the necessity to rotate the whole piston, so removing the resistance of frictional force between the seals 46,48a and the mandrel 18, as well as other contacts between the piston and mandrel.

Claims

1. An adjustable down-hole tool comprising:
a body having a through bore;
a mandrel having a through bore axially movable in the body, the mandrel being movable by fluid pressure in the tool against the action of a first return spring between two positions, one being a first, deactivated position and the other a second activated position;
a sleeve between the body and mandrel limiting movement of said mandrel between said positions;
at least two sets of castellations, one set on the sleeve and the other set on a facing edge of the body or mandrel so that, when the castellations are in phase, the mandrel is prevented from travelling from said first to second position and, when they are out of phase, they interdigitate and the mandrel is not prevented from travelling from said first to second position; and a control piston to rotate the sleeve relative to said facing edge between said in-phase and out-of-phase positions, the piston being movable by fluid pressure in the tool against the action of a second return spring;
wherein said control piston is slidable in the mandrel, the mandrel carrying rotation transmitters that are in contact with both the piston and sleeve, whereby rotation of the piston relative to the mandrel rotates the sleeve relative to the mandrel.

2. A tool as claimed in claim 1, in which said rotation transmitters are carried by the mandrel internaediate its ends.

3. A tool as claimed in claim 2, in which said rotation transmitters are between axially spaced seals of the piston against the bore of the mandrel.

4. A tool as claimed in claim 1, 2 or 3, in which said rotation transmitters comprise a gear rotationally journalled in the mandrel about an axis parallel the throughbores, both the piston and sleeve having a rack engaged with the gear.

5. A tool as claimed in claim 4, in which a plurality of said gears are disposed around the circumference of the mandrel.

6. A tool as claimed in any one of claims 1-5, in which said tool is a drill-string stabiliser and said mandrel has wedge surfaces to engage corresponding surfaces on radially disposed pistons slidable in the body, whereby, when the mandrel moves from said deactivated to said activated position, the pistons extend from the body increasing the working diameter of the stabiliser.

7. A tool as claimed in any one of claims 1-6, in which a circumferential barrel cam is defined in one of said piston and mandrel, a cam follower being disposed in the other of said piston and mandrel, the follower being within the barrel cam so that axial movement of the piston with respect to the mandrel results in corresponding rotation of the piston with respect to the mandrel.

8. A tool as claimed in claim 7, in which a separate component of the piston is rotationally freely, but axially fixedly, mounted in the piston, which component carries said barrel cam or follower, said separate component driving said rotation transmitters on rotation of said component in response to axial movement of the piston in said mandrel.

9. A tool as claimed in claim 7 or 8, in which the barrel cam is shaped so that movement of the piston in one axial stroke and return thereof results in relative rotation of the sleeve and said facing edge from a said in-phase position to a said out-of phase position or vice versa.

10. A tool as claimed in any one of claims 1-9, in which said first return spring is sufficiently stronger than said second return spring to ensure that, when said mandrel is in said deactivated position, a rise in hydraulic pressure in the tool results in movement of the piston before movement of the mandrel.

11. A tool as claimed in claim 7, 8 or 9, in which a spring loaded detent between said mandrel and body retains the mandrel in said deactivated position until a threshold hydraulic pressure has been exceeded, which pressure is greater than that required to move said piston.

12. A tool as claimed in claim 11, in which said detent comprises a plunger in a radial bore of the mandrel or body, spring biased against a lip of the body or mandrel respectively.

13. A tool as claimed in claim 12, in which said lip is of a circumferential groove around the mandrel.

14. A tool as claimed in any one of claims 1-13, in which said facing edge is in the body.

15. A tool as claimed in any one of claims 1-14, in which the mandrel is sealed to the body about first and second circumferences, the first being a larger circumference upstream, in terms of fluid flow through the tool, of the second, smaller circumference.

16. A tool as claimed in claim 15, in which the piston has a through bore and is sealed to the mandrel about third and fourth circumferences, the third being a larger circumference upstream, in terms of fluid flow through the tool, of the fourth, smaller circumference.