Patent Publication Number: US-6708785-B1

Title: Fluid controlled adjustable down-hole tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This applications claims the benefit of application Ser. No. 9905050.2 filed in Great Britain on Mar. 5, 1999. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to adjustable down-hole tools employed in the oil and gas drilling industry. 
     2. Description of the Related Art 
     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activation when they are in a given position down hole to make them operative, and deactivation when they are to be withdrawn, or repositioned or indeed simply to go into a different operating condition.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), 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 However, if the pistons are withdrawn, then gravity can deflect the drill string so that it alters the inclination of the hole. 
     EP-A-0251543 describes a slabiliser that is activated by weight on the stabiliser from the drill string above it. Weight, or absence thereof, switches the stabiliser between activated and de-activated positions. The weight acts on a mandrel slidable in the bore of the stabiliser, which mandrel has ramps against which wedge-surfaces on the bases of the pistons slide. A mechanical detent is overcome by a compressive force on the stabiliser greater than a threshold value, so that unless substantial changes in weight act on the stabiliser, switching does not occur. This means that some variation in weight is permissable without changing the activation of the stabiliser. However, it is known that excessive changes in weight can occur unintentionally, possibly resulting in accidental activation and deactivation of the stabiliser. 
     It has been suggested to employ a rise in mud pump pressure to move the mandrel in the stabiliser. Changes in pressure switch the mandrel between different positions. Such a system is described in EP-A-0190529, in which a differential piston cooperates with a flow restrictor so that, if the fluid pressure rises beyond a low threshold, the piston (or flow restrictor) moves to rapidly and substantially increase the pressure differential across the piston which then drives the mandrel to activate the stabiliser. As a subsidiary feature the mandrel rotates on each stroke because the pads have pins which follow a barrel cam defined around the mandrel, which barrel cam has different steepness ramps so that the pads are extended different amounts. Unintentional variations in fluid pressure might also cause premature activation or deactivation. 
     GB-A-2263923 discloses a stabiliser control arrangement in which the object is to not be dependent on either fluid pressure or weight on the bit to maintain a stabiliser setting. This is achieved by lifting the drill string to positively disengage the locking mechanism, and then fluid pressure is employed to determine the stabiliser piston position. At the appropriate pressure the drill string is lowered to engage a lock, whereupon subsequent changes in fluid pressure have no effect on stabiliser position. 
     GB-A-2251444 has essentially the same aims as GB-A-2263923, except that, here, check valves prevent operation or deactivation of the stabiliser pads unless the pressure of the pump fluid exceeds or falls below upper and lower threshold values. 
     EP-A-0661412 has an arrangement similar to EP-A-190529. The position of a control piston determines the pressure drop across the mandrel which therefore controls the position of the mandrel. The control piston has a barrel cam in which a pin of the housing slides, so that the piston is constrained to follow a course determined by the track. A junction in the track is provided so that, at an intermediate pressure, if the pressure is reversed the pin does not return to its stating point but goes up a branch to a lesser (or greater) extent than its starting point. The stabiliser is activated between upper and lower pressures and that the pressure be taken from one level to an intermediate level whereupon the direction of pressure change is reversed. 
     GB-A-2314868 describes an arrangement in which the mandrel is hydraulically operated between operative and inoperative positions A first shoulder on the body of the stabiliser in which the mandrel slides has a serrated face. A facing shoulder on the body has a clutch face which is also set Between the two faces is a sleeve which is axially fixed but rotationally freely slidable on the mandrel. On the edge of the sleeve facing the serrated edge of the body is series of knobs to engage the serrations and rotate the sleeve through a small angle when the sleeve is axially pressed against the serrations. On its other edge, it has a series of fingers to engage the clutch face and either catch on ridges of the clutch face, which are provided with stops to prevent further rotation of the sleeve, or they miss the stops and hit a sloping serration of the lower shoulder causing further rotation of the sleeve until its fingers coincide with long slots in the shoulder whereupon the sleeve permits the mandrel to go to its operative position. 
     Consequently, as pressure is alternated and the mandrel moves back and forth, when it first moves down, for example, it may rest on the ridges of the clutch face and prevent the mandrel from going to its operative position When the pressure is released and the mandrel rises the knohs on the sleeve hit the serrations and turn the sleeve through a small angle; enough so that on the next stroke of the mandrel the fingers on the sleeve do not stay on the ridges. Instead, the fingers slide down the serrations of the clutch face and drop into slots therein. This movement takes the mandrel into its operative position. Finally on the return stroke, when the knobs again contact the serrated face the sleeve again rotates, repeating the cycle. 
     A problem with this arrangement, and with EP-A-0661412 is that the pressure which activates the stabiliser must be greater, of course, than the return force provided by springs, for example, which springs must themselves be very substantial in order to guarantee deactivation and overcome any jamming tendency which could occur through external pressure on the pistons. Consequently, there is wear on the components which are rotating, or causing the rotation, since they are simultaneously subject to substantial axial loads. Moreover, in the case of GB-A-2314868, because the fingers are the same components which result in rotation of the sleeve, they cannot be as substantial as their loading, particularly in an extended position, would ideally want them to be. Thus they may break. 
     GB-A-2314868 also discloses application of the mechanism described therein in relation to under reamers. 
     It is therefore an object of the present invention to provide a down-hole tool activation arrangement which does not suffer firm, or at least mitigates these or other problems. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is therefore provided 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, activated position and a second deactivated position; 
     a sleeve between a shoulder on the body and the mandrel; 
     at least two sets of castellations, one on one of said shoulder and said mandrel and the other on a facing edge of the sleeve so that, when the castellations are in phase the mandrel is prevented from travelling foam 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 mandrel between said in-phase and out-of-phase positions; 
     characterised in that 
     said means comprises a control piston slidable in the mandrel, being movable by fluid pressure in the tool against the action of a second return spring; and in that 
     one of said piston and mandrel is rotationally fixed with respect to the body. 
     Preferably, it is said mandrel which is rotationally fixed with respect to the body. Preferably, said control piston is axially slidable with respect to said sleeve and rotationally fixed with respect hereto Preferably, a circumferential barrel cam is defined in one of sad 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 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 angle. 
     When said mandrel is in said deactivated position, a rise in hydraulic pressure 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 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 biassed against a lip of the body or mandrel, respectively. Said lip may be of a circumferential groove around the body. 
     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. 
     The mandrel will usually have a through bore and be scaled to the body abut 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. 
     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 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. 
     Preferably, the piston extends from the mandrel and is sealed to the body. Indeed, the seal between the body and mandrel about said second circumference, and the seal between the piston and mandrel about said fourth circumference, may comprise an integrated seal between the piston and the body. 
     In said activated position, the bore of the piston preferably engages a plug 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 the body, whereby, when the mandrel moves from said deactivated to said activated position, the pistons extend from the body inn sing the working diameter of the stabiliser. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1 a, b  and  c  are side sections through the tool in accordance with the present invention, in different positions thereof; 
     FIG. 2 is a section on the line II—II in FIG. 1 c;    
     FIGS. 3 a, b, c  and d are, respectively, a side view of a control piston of the tool of FIG. 1, a section on the line X—X in FIG. 3 a , a section on the line Y—Y in FIG. 3 a  and a detailed view of the barrel cam in the direction of arrow A in FIG. 3 a;    
     FIGS. 4 a  and  b  are, respectively, an expanded side view of detail B in FIG. 1 a , and  a  side section on the line IV—IV in FIG. 1 a;    
     FIGS. 5 a  and  b  are, respectively, a view in the direction of arrow A in FIG. 5 b , and an expanded view of detail V in FIG. 1 a;    
     FIG. 6 is a view similar to FIG. 5 a , but of an alternative embodiment of the preset invention; 
     FIGS. 7 a  to  d  are enlargements of one end of the stabiliser according to the embodiment of the present invention shown in FIG. 6, and wherein development of a signalling constriction is shown; 
     FIGS. 8 a  and  b  are graphs showing changes in mud pump pressures with mandrel position and time, respectively; 
     FIGS. 9 a, b  and  c  are side sections through an alternative embodiment of a tool in accordance with the present invention; and 
     FIG. 10 is a detailed view of the inset marked X on FIG. 9 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings, a stabiliser  10  comprises a body  12  connectable to a drill string (not shown) 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  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 defied on the surface of the body  12  and bear against the 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 (not shown). 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 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 , (see FIG. 1 c ). 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. 
     A collar  25  is screwed onto the mandrel  18  at its upstream end  32  (see also FIG.  4 ). Above the collar  25  is a seal sleeve  34  which is sealed both to the mandrel  18  and the bore  16  of the body  12 . At its downstream 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  32  to its downstream end  33 . The control piston carries seals  46  which seal the piston with respect to the mandrel  18 . The piston  36  extends out of the end  33  of the mandrel  18  and is itself sealed at  48  to the bore  16  of the body  12 . 
     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 sleeve seal  34  in the body  12  is much larger than the circumference of the seal  48  around the piston  36 . Consequently, hydraulic pressure of the mud in the tool  10  results in a larger downward force acting at the end  32  of the mandrel  18  via the seal sleeve  34 , than acting in the reverse direction on the piston  36  through its seals  48 . 
     Springs  44  act between a shoulder  42  in the body  12  (via compensation device  23  described further below) and the collar  25  on the mandrel  18 , 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 sprung  44 , the mandrel will move rightwardly in the drawing. 
     Likewise, hydraulic pressure acting on the control piston  36  across the circumference of its seals  46  to the mandrel result in a downward force on the piston  36  because the circumference of the seal  48  to the body  12  is smaller than seal circumference  46 . Again, springs  50  act 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 springs  50  are overcome, the piston  36  will move rightwardly in the drawings. 
     The piston has a barrel cam  56  defined in its surface (see FIG. 3 a ). 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 . 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 FIG. 1 a ). Suppose the pins  58  start at position  58   a , for example (see FIG. 3 d ), where they lie at the base of a first notch  56   a  of the barrel cam. They will thus move, relatively to the barrel cam  56 , until they contact the opposite wall thereof at  56   b . Further axial movement of the piston  36  then only occurs when the piston rotates through a small angle α 1 , so that the pin  58  effectively moves to position  58   b  in notch  56   c  on the opposite side of the barrel cam  56  from notch  56   a.    
     Should the hydraulic pressure be released, ream springs  50  force the piston  36  leftwardly in the drawings (FIG. 1 a-c ). The pin  58  is obliged to follow a course from position  58   b  in notch  56   c  of the barrel cam  56 , axially until the opposite wall of the barrel cam  56   d  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  56   e  on the same side of the barrel cam  56  as notch  56   a . In this movement, the piston has rotated through a further angle α 2 , which is not necessarily the same as α 1 . Nevertheless the sum (α 1 +α 2 ) is equal to α, 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 ″ 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 ′ land large pin  58 ′ is employed, then, in moving from notch  56   a  to contact surface  56   b , a rotational drift back in the direction of Arrow X in FIG. 3 d  of only 1° can be permitted. Any greater drift, which would generally be caused by the sprig having been “wound up” by previous movements, would cause contact of the pin  58 ′ with point  56   f  of the cam  56 ′, such that secure guidance of the pin to notch  56   c  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 FIG. 3 d ), 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 bearing 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 FIG. 3 a  and  c , the piston  36  has a longitudinal slot  60  in which is received a key  64  of a castellated sleeve  66  (see FIGS. 5 a  and  b  for more details). 
     The sleeve  66  is received between a shoulder  68  of the body  12  and end  33  of the mandrel  18 . The end  33  of the mandrel  18  is castellated having fingers  18   a  and slots  18   b . The end  69  of the sleeve  68  is likewise castellated having fingers  69   a  and slots  69   b . When the fingers  18   a , 69   a  of the mandrel and sleeve are in phase with one another, as shown in FIG. 5 a , then rightward movement of the mandrel  18  in the drawings, is limited, with the fingers  18   a , 69   a  abutting one another and the other end  70  of the sleeve  66  abutting shoulder  68  of the body  12 . 
     On the other hand, however, when the sleeve  66  is out of phase with respect to the mandrel  18 , fingers  18   a  face slots  69   b  and fingers  69   a  face slots  18   b  so that, when the mandrel  18  moves rightwardly in the drawings, the castellations on the mandrel 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 a (or multiples thereof), as described above. 
     Consequently, when the piston makes a complete return stroke serving to rotate the sleeve  66  through the angle 2α, the sleeve  66  moves from an in-phase position to an out-of-phase position, or vice versa. 
     Although FIGS. 5 a  and  b  show fingers  18   a , 69   a  and slots  18   b ,  69   b  extending across the thickness of both the mandrel  18  and sleeve  66  respectively, in FIG. 2, it can be seen that the respective fingers and slots extend only across a portion of the thickness of each element  18 , 66 . Both arrangements are functionally identical, the arrangement in FIG. 2 merely being mechanically more sound. 
     Turning now to FIG. 6, an alternative arrangement is shown to that described above with reference to FIG. 5 a . Here, the sleeve  66 ′ has alternate slots  69   b ′ which have different depths (shallow,  69   b ′, and deep,  69   b ′ 2 ). Similarly, the mandrel  18 ′ has alternate fingers  18   a ′ which are correspondingly short,  18   a ′, and long  18   a ′ 2 . Such an arrangement necessitates, of course, an even number of fingers and slots around the sleeve  66 ′ and mandrel  18 ′, which has a consequent effect ion the barrel cam  56 . In the previous embodiment, there were five fingers/slots around the periphery (as shown in FIG.  2 ), meaning that angle 2α was 72° of rotation Here, there are preferably six fingers/slots, so that angle 2α is 60°. 
     The result of varying depth of fingers  18   a ′ and slots  69   b ′ is that mandrel  18  can have three positions instead of just two, that is to say an intermediate position between deactivation and full activation. In FIG. 6 at its top, the mandrel is shown in its fully activated position  18 ′A, in which long fingers  18   a ′ 2  coincide with deep slots  69   b ′ 2 , so that this corresponds entirely with at activated position of the previous embodiment, At the bottom of FIG. 6, the fingers  18   a ′ 2  coincide with the fingers  69   a  of the sleeve  66 ′ (which fingers are all level, as in the embodiment described with reference to FIG. 5 a ), 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 FIG.  5 . However, in the middle of FIG. 6, there is shown the intermediate position  18 ′B in which long fingers  18   a ′ 2  coincide with shallow slots  69   b ′ 1 , with the result that the pistons  28  are only displaced radially outwardly to a lesser extent. 
     Returning to FIG. 1 a  and with reference also to FIG. 4 a , and  b , the mandrel has on the collar  25  a series of pockets  90  in which a plunger  92  is disposed. Springs  94  press the plunger radially outwards, the plungers being retained in the pockets  90  by threaded retainers  96 . The head  98  of each plunger  92  is received within a circumferential groove  100  in the body  12 . 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. For this purpose groove  100  is provided with an angled cam surface  102 . Thus when the mandrel is piss 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 pas over lip  104  of the groove  100 . In order to ensure that hydraulic effects do not influence the operation of this detent represented by the plungers  92 , each plunger has a through bore  106  connecting space  108  between the mandrel  18  and body  12  with space  110  behind the plunger  92  and within the pocket  90 . 
     While the detent plungers are shown spring loaded, the same result could be achieved with the plungers forming pistons as shown at  92 ′. Fluid behind the pistons here resists their radially inward displacement until the fluid leaked out around the sides thereof. Nevertheless, a return spring  94  is still required, and moreover a return flow path  106 ′ guarded by a check valve  95  is also required. The check valve comprises a ball  99  and spring  101  and it inhibits fluid leaving the space  97  behind the piston  92 ′, but permits inflow when the springs  94  push the piston  92 ′ out. 
     In operation of the stabiliser  10 , therefore, and beginning with the positions shown in FIG. 1 a , 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 sat on the components within the stabiliser tool. Because of the detent represented by the plungers  92 , he mandrel is at first prevented from moving. However, the piston  36  ha| no such detent and so commences to move rightwardly in FIG. 1 a  against the pressure of spring  50 . Rightward movement of the piston  36  is thus accompanied by rotation thereof through the angle α 1 , which, for the sake of argument, rotates the sleeve  66 , via the key  64  sliding in the slot  62  of the piston  36 , to the position shown in FIG. 5 a  where the fingers  69   a  of the sleeve  66  are in phase with the fingers  18   a  of the mandrel  18 . It must be borne in mind that the mandrel  18  is rotationally fixed in the body  12  by pin  20  received in slot  22 . 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 than shown in FIG. 1 a  by virtue of the fingers  18   a  at the end  33  of the mandrel contacting the fingers  69   a  of the sleeve  66 . Indeed, such movement as there is merely takes up the clearance between the fingers  18   a , 69   a , and between end  70  of the sleeve  66  and shoulder  68 . 
     However, should it be desired by the user that the stabiliser operate in its maximum working diameter, the operator reduces the pump press so that the spring  44  returns the mandrel (to the extent that this is necessary) to the position shown in FIG.  1   a . The springs  50  also return the piston from the position shown in FIG. 1 b  to that shown in FIG. 1 a . In doing so, the piston woes through the further angle α 2 . On the next occasion, therefore, that the hydraulic pressure is increased again so that the piston  36  moves once again towards the position shown in FIG. 1 b , and it rotates through a further angle α 1 , then, on this occasion, the castellations on the mandrel  18  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 FIG. 1 c , with the respective castellations on the mandrel and sleeve inter-digitating. 
     In this position, as shown in FIG. 1 c , an end  37  of the piston  36  moves into close proximity with a plug  19  in the body  12 , with the result that a substantial constriction  110  is created in the fluid flow. The operator at ground level is then advised that the mandrel has moved to its activated position by a sudden rise in pump working pressure. 
     Here, as shown in FIG. 1 c , 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 . 
     It will be apparent to the skilled reader that, m moving within the body  12 , the mandrel  18  and piston  36  compress the space between the body and mandrel/piston and defined by the seats  34 , 48  and primarily occupied by the space containing springs  44  and  50  and sleeve  66 . This space is filled with hydraulic oil and is isolated both from fluid pressure external of the stabiliser  12 , as well as hydraulic pressure internally of the bore  38 . Thus firstly there is a requirement to provide for relief of the oil in that space as the mandrel moves and compresses that space. Secondly, since the hydraulic pressures both internally and externally are intense, a means to match pressure in that space is desirable in order to avoid disruption of the seals. 
     For this purpose, pressure relief chamber  23  is provided This chamber is of known construction per se and consequently only brief description is required here. Chamber  23  comprises an annular bellows  23 ′ which, internally, is in fluid communication with the space and springs  44  and  50  and sleeve  66 , and externally is in communication with the outside environment through port  27 . Thus the pressure in the space referred to must correspond with the outside pressure. The chamber  23  is itself sealed to the bore  16  of the body  12 , but not to the mandrel  18 . The movement of the mandrel and compression of the space around spring  44  is also, indeed primarily, taken up by radially outward movement of the pistons  28 . 
     Referring again to FIG. 4 c , on rightward movement of the mandrel  18 , the detent plungers  92  move into over lip  104  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 . 
     By this arrangement, two connected effects are experience. The first is that the piston  36  moves with very little extraneous loading upon it. Thus the mandrel  18  is held in position by the detent plungers  92  so that sleeve  66  is freely rotatable between the end  33  of the mandrel  18  and the shoulder  68  on the body  12  by movement of the piston  36 . Consequently there is little wear on the barrel cam  56  or the pins  58  received therein. Secondly, because the fingers  18   a , 69   a  have no function beyond meeting one another and resisting the heavy fortes imposed by the hydraulic pressure, or inter-digitating when out of phase, they can be substantial components with little need to provide mutually sliding surfaces, for example. Thus they are able to be made as structurally strong components less liable to fail, without adversely affecting operation of the stabiliser. 
     It is intended that the present invention operates (that is to say toggles between positions) at pressures well below normal operating pressures of the drill string, which may be in the region of 500 psi or more. At these pressures, the control piston is designed to remain in the position shown in FIG. 1 b  or  1   c  relative to the mandrel, the latter being in either of its activated or deactivated positions (the fingers and slots on the mandrel and sleeve being entirely in-phase or entirely out-of-phase). On sing from zero pressure, both the mandrel and control piston would begin to move together but, due to the strength of the springs and their design the piston can be arranged to have completed its stroke before the mandrel has substantially begun to move. In any event, as mentioned above, the dent mechanism actively prevents the mandrel moving until the forces on it exceed a predefined limit. Indeed, that limit is arranged so that once the detent has been released, the mandrel moves from its stat position to its final position without further increase in pressure. In other words it is a clean switching action. 
     This is illustrated in FIG. 8 a  which is a grape of mud pump pressure (P) versus position (M) of the end  37  of the piston  36  with respect to its position shown in FIG. 1 a . As pressure increases from some value x above zero (there will be a preset loading of the spring  50 ) to P 1 , the control piston moves gradually from CP 1  to CP 2 , ie to the position shown in FIG. 1 b . Thereafter there is no movement until the pressure reaches P 2 , whereupon the detent mechanism is overcome and the mandrel moves from position M 1  to M 2 , being the position shown in FIG. 1 c  without further change in pressure P. Of course, should the fingers  18   a , 69   a  be in phase, then the mandrel will stay at M 1  and further increase in pressure will follow the phantom line in FIG. 8 a . If the stabiliser is as the alternative embodiment described with reference to FIG. 6, then the mandrel may move instead to position M i , being the intermediate position, and further increase in pressure will follow the dashed line in FIG. 1 a . In any event, all lines will reach working pressure WP, except that it will be less when the mandrel is in position M 1  than M 2 , because of the constriction  110  caused by plug  19 . 
     Turning to FIGS. 7 a  to  7   d , there is shown an arrangement of the piston  36 ′ and plug  19 ′ which assists in signalling to the user the position that the mandrel is in, and thus the state of activation of the stabiliser  10 , when the stabiliser is modified as described with reference to FIG.  6 . 
     In FIG. 7 a , the piston  36 ′ is in position CP 1 , ie no pump pressure. In FIG. 7 b , it has moved to position CP 2 /M 1 , where constriction  110  is negligible and not yet having any significant effect. A graph of pressure P versus time T is shown in FIG. 8 b , where it can be seen that reaching position M 1  has no precise impact on the shape of the developing pressure. However, if the mandrel stays in the position M 1 , then the pressure continues to develop to working pressure WP 1  along the solid line in FIG. 8 b.    
     If, on the other hand, the mandrel moves to the intermediate position M i , then the piston moves to the position shown in FIG. 7 c  where an internal lip  39 , which is formed by a circumferential groove  41  formed in the bore of the piston  36 , passe over a lip  43  on the plug  19 ′. Here, not only has the constriction  110  formed, but also, in moving to this position a very tight constriction was temporarily formed while the lips  39 , 43  overlapped. This results in a strong pressure pulse (at M i  in FIG. 8 b ) before the pressure continues rise to WP 2 , which is higher than WP 1  in view of the constriction  110 . 
     Finally, as the piston moves to the position shown in FIG. 7 d , where the mandrel is in its fully activated position M 2 , lip  43  moves over groove  41  and causes an even tighter constriction within the bore of the piston  36 ′. This further increases the pressure at M 2  in FIG. 8 b , before the pressure continues to rise to WP 3  which is again higher than WP 2 . 
     Thus by this mechanism not only are the final working pressures different for the different working positions of the mandrel  18 , but also a pressure pulse is experienced at each change of position. Indeed, with sensitive detection equipment at the surface and connected to the drilling mud pressure line, it may even be possible to dispense with the constriction  110  per se, and simply rely on the pulses to detect position rather than final working pressures. 
     Finally, FIGS. 9 and 10 illustrate a different embodiment of the present invention in which the control arrangement for movement of the mandrel is moved to the upstream end of the stabiliser. In these figures, parts with equivalent function to the embodiment described with reference to FIG. 1 are given the same references numeral, except for the addition of an apostrophe (′) or double apostrophe (″) if the element in question differs in any way from previous embodiments. 
     In this embodiment, the mandrel  18 ″ is a sliding fit inside the piston  36 ″, which is itself a sliding fit in the bore of the body  12 ′. Instead of the piston rotating, here a component  361  of the piston rotates on it through beans  362 ,  364 . The component  361  is rotationally mounted through bearings  362 ,  364  on the piston  36 ″ and rotates relative thereto as the piston moves up and down the body  12 ′. The cam track  56 ″ is formed on the surface of the component  361 , whereas the cam follower pins  581  are mounted on sleeve  66 ′, which is now effectively just a pat of the body  12 ′. The sleeve  66 ′ is prevented from rotating relative to the body by a bolt  64 ′ or by similar means. A mandrel drive ring  366  is carried by the piston  36 ″ and rides in an annular groove  182  in the mandrel  18 ″. The ring  366  is in two part and is returned by collar  54 ′ screwed onto the end of piston  36 ″. 
     When mud pressure increases, the piston  36 ″ moves rightwardly in the drawing and, depending on the rotational position of the Sleeve  66 ′, fingers  69   a ″/ 18   a ″ on the sleeve  66 ′ and component  361  either oppose one another or interdigitate with each other falling into slots  18   b ″/ 69   b ″. If they interdigitate, then drive ring  366  hits the end of slot  182  and the piston  36 ″ drives the mandrel rightwardly in the drawing to set it in its full gauge, activated position. If, however, the fingers  69   a ″/ 18   a ″ face one another then even if mandrel  18 ″ slides rightwardly relative to piston bore  36 ′ under the influence of mud pressure (which is minimised by substantial equality of diameter of the mandrel upstream, to the piston, (seal  46 ′) on the one hand, and downstream, to the body, (seal  34 ′) on the other hand), drive ring  366  prevents rightward movement of the mandrel  18 ″ and the mandrel remains in its under gauge or deactivated position of the stabiliser. 
     It is to be noted that here, the cam track  56 ″ and the component  361  move with the mandrel and therefore cam extensions  56 ″ a  in an axial direction are needed, at least in positions where the fingers  69   a ″/ 18   a ″ interdigitate and the axial movement of the piston  36 ′ and component  361  is extensive relative to the sleeve  66 ′.