Abstract:
A direction adjustment tool for a downhole drilling apparatus is disclosed. The tool has a tubular housing adapted to be incorporated into a downhole drilling apparatus, and steering blades mounted to the housing. A drive shaft transmits drive to a drill bit of a drilling apparatus, wherein the shaft defines a passage for transmitting drilling fluid to the drill bit. A first pressure chamber is defined between the housing and the shaft and communicates with the passage, wherein the steering blades are moved from retracted positions to extended positions thereof as a result of increase of fluid pressure the first pressure chamber. A pendulum member is pivotably mounted to extend in a vertical orientation when the shaft is not rotating relative to the housing, and pistons prevent movement of at least one steering blade to the extended position thereof as a result of the angle between a longitudinal axis of the shaft and the longitudinal axis of the pendulum member exceeding a predetermined amount. This causes at least one steering blade to adjust the direction of drilling of the drilling apparatus towards a vertical direction and/or to resist movement of the direction of drilling away from a vertical direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending PCT patent application serial number PCT/GB08/050473, filed on 20 Jun. 2008, which claims priority to GB patent application serial number 0712451.4, filed on 27 Jun. 2007, each of which is hereby incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a direction adjustment tool for a downhole drilling apparatus incorporating the tool, and relates particularly to a tool for correcting the direction of drilling of the drilling apparatus when it deviates from vertical. The invention relates particularly, but not exclusively, to such a tool for use in oil and gas well drilling apparatus. 
     Drilling direction adjustment tools are known for correcting the direction of drilling of a drilling apparatus incorporating the tool, and which include devices such as accelerometers or magnetometers, which provide an electrical signal representing the deviation of a longitudinal axis of the tool from the vertical. Electrical signals representing deviation of the longitudinal axis of the tool from the vertical are used to control the direction of drilling of the apparatus, for example by means of steering pushers which engage a wall of a borehole formed by the drilling apparatus, to cause the orientation of the tool to deviate, which in turn adjusts the direction of drilling back towards the vertical. 
     Existing tools of this type suffer from the drawback that the use of complicated electronic components increases the cost of production of the apparatus, and makes the apparatus more prone to failure. This can be particularly disadvantageous when the tool is located downhole, since drilling operations must be ceased while the tool is recovered. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art. 
     According to the present invention, there is provided a direction adjustment tool for a downhole drilling apparatus, the tool comprising: 
     a tubular housing adapted to be incorporated into a downhole drilling apparatus; 
     a plurality of steering pushers slidably mounted to said housing, wherein each said steering pusher is moveable between a respective extended position in which said steering pusher engages a wall of a borehole formed by the drilling apparatus, and a respective retracted position, in which the steering pusher does not engage the wall of the borehole; 
     a hollow rotary shaft adapted to be incorporated into the drilling apparatus for transmitting drive to a drill bit of the drilling apparatus, wherein the shaft defines a passage for transmitting drilling fluid to the drill bit; 
     at least one first pressure chamber defined between said housing and said shaft and communicating with said passage, wherein at least one said steering pusher is adapted to be moved from the retracted position to the extended position thereof as a result of increase of fluid pressure in at least one respective said first pressure chamber; 
     a pendulum member pivotably mounted relative to the housing such that a longitudinal axis of the pendulum member extends in a substantially vertical direction when said shaft is not rotating relative to the housing and when a longitudinal axis of the shaft extends in a substantially vertical direction; and 
     a control mechanism adapted to prevent movement of at least one said steering pusher to the extended position thereof as a result of the angle between the longitudinal axis of the shaft and the longitudinal axis of the pendulum member exceeding a predetermined amount, in order to cause at least one said steering pusher to adjust the direction of drilling of the drilling apparatus towards a substantially vertical direction and/or to resist movement of the direction of drilling away from a substantially vertical direction. 
     By providing a pendulum member pivotably mounted relative to the housing such that a longitudinal axis of the pendulum member extends in a substantially vertical direction when the shaft is not rotating relative to the housing and when a longitudinal axis of the shaft extends in a substantially vertical direction, and a control mechanism adapted to prevent movement of at least one said steering pusher to the extended position thereof as a result of the angle between a longitudinal axis of the shaft and the longitudinal axis of the pendulum member exceeding a predetermined amount, this provides the advantage of providing a drilling direction correction tool which can be constructed using mechanical components only. This avoids the use of complicated electronic components, which in turn reduces the cost of manufacture and servicing time and costs and increases the robustness and reliability of the tool. 
     In a preferred embodiment, the tool further comprises a plurality of second pressure chambers, wherein at least one said steering pusher is adapted to move from the retracted position to the extended position thereof as a result of increase of fluid pressure in at least one respective said second pressure chamber, and said control mechanism comprises a plurality of pistons, wherein each said piston communicates with at least one said first pressure chamber and at least one said second pressure chamber and has a first condition in which the piston does not engage the pendulum member and increase of fluid pressure in a said first pressure chamber communicating with said piston causes said piston to move relative to the housing to increase pressure in the or each second pressure chamber communicating with said piston to move the corresponding steering pusher to the extended position thereof, and a second condition in which the piston engages the pendulum member to limit movement of said piston relative to the housing to prevent movement of the corresponding steering pusher to the extended position thereof. 
     By providing a plurality of pistons, wherein each said piston communicates with said first pressure chamber and a said second pressure chamber and has a first condition in which the piston does not engage the pendulum member, and a second condition in which the piston engages the pendulum member to limit movement of said piston relative to the housing, this provides the advantage of enabling the control mechanism to be constructed in a simple mechanical manner, which in turn increases the robustness and reliability and reduces the cost of manufacture of the tool. 
     The sealing means may further comprise a plurality of sealing plates, wherein each said sealing plate is adapted to be mounted to the housing to at least partially define a respective said second pressure chamber. 
     This provides the advantage of simplifying assembly of the tool and minimising the risk of the sealing plates being damaged during assembly of the tool by avoiding the necessity of sliding an assembly defining all of the second pressure chambers along the bore of the tool. Also, this provides the advantage of allowing the steering pushers to be sealed off from one another without having to stagger their location along the length of the housing. 
     Each said sealing plate may have a compressible seal adapted to be compressed between a body of said sealing plate and said housing. 
     The tool may further comprise at least one aperture formed in said housing and at least partially defining a respective said second pressure chamber. 
     By forming at least one aperture in the housing at least partially defining a respective second pressure chamber, this provides the advantage of reducing the risk of leaks occurring in the vicinity of the sealing means, and of simplifying manufacture of the tool by reducing the number of components needed. This is particularly advantageous when the sealing means is subjected to significant back pressure, for example after air, has been evacuated from the second pressure chambers and then when being filled with oil, which is driven by a pump into the second pressure chamber under pressure. 
     In a preferred embodiment, when the angle between a longitudinal axis of the shaft and the longitudinal axis of the pendulum member does not exceed said predetermined amount, none of said pistons engages the pendulum member in use, and all of said steering pushers are able to move to the extended positions thereof. 
     The tool may further comprise sealing means adapted to be mounted to the housing to at least partially define at least one said second sealing chamber. 
     The sealing means may comprise at least one closure member having a plurality of closure surfaces adapted to at least partially define a plurality of respective second pressure chambers. 
     The sealing means may further comprise alignment means for assisting alignment of said sealing means relative to the housing. 
     This provides the advantage of assisting assembly of the tool. 
     The tool may further comprise a piston chamber block arranged between said housing and said shaft and defining a plurality of curved piston chambers for slidably receiving a respective curved portion of each said piston. 
     By providing curved piston chambers for receiving curved portions of the pistons, this provides the advantage of fully utilising the space occupied by the body of the piston chamber block between the housing and the shaft, which in turn reduces the axial depth of oil reservoir required. This in turn enables more robust pistons to be provided, while also reducing the difficulty of manufacturing and inspection thereafter, because the length over which the pistons and piston chambers must accurately match each other is reduced. 
     Each said piston chamber may communicate with a respective said second pressure chamber by means of a respective conduit formed in the housing. 
     This provides the advantage of further simplifying assembly of the tool. 
     The or each said second pressure chamber may contain oil. 
     Each said piston may include a respective first engaging portion for engaging a corresponding second engaging portion on said pendulum member. 
     Each said first engaging portion and/or said second engaging portion may define at least one respective inclined surface. 
     This provides the advantage of providing secure engagement between the pistons and the pendulum member, minimising the extent to which the pistons become inadvertently disengaged from the pendulum member even at elevated drilling fluid pressures, for example, due to drilling radial and axial vibrations. 
     Each said first engaging portion and/or said second engaging portion may define a plurality of respective inclined surfaces. 
     In a preferred embodiment, the pendulum member is pivotably mounted relative to the housing by means of a pivot adapted to move axially relative to said housing in response to increased fluid pressure in at least one said first pressure chamber. 
     This provides the advantage of maximising the length of overlap between the pistons and the pendulum member, which in turn maximises the sensitivity of the control mechanism to deviations of the tool from a substantially vertical orientation. This also provides the advantage of enabling the pendulum member to have a return stoke relative to the housing on removal of drilling fluid pressure in the first pressure chamber, to maximise reliability of engagement between the pistons and the pendulum member on subsequent increase of fluid pressure in the first pressure chamber. 
     The tool preferably further comprises first biasing means for urging said pendulum member axially relative to the housing towards said control mechanism. 
     This provides the advantage of maximising reliability of engagement between the pistons and the pendulum member on subsequent pressurisation of drilling fluid in the first pressure chamber. 
     The tool may further comprise at least one flow restrictor between the housing and the shaft and communicating with said first pressure chamber to restrict flow of drilling fluid therethrough to cause a pressure difference between the interior and the exterior of said first pressure chamber. 
     This provides the advantage of providing a more robust and lower cost alternative to a rotary seal for the first pressure chamber, and which can also act as a radial journal bearing. 
     The pendulum member may be pivotably mounted to at least one said flow restrictor. 
     This provides the advantage of minimising the number of components needed to construct the tool, which in turn improves the robustness and reliability of the tool. 
     Each said steering pusher may comprise a steering blade for contacting the wall of the borehole and mounted to the housing by means of at least one pusher piston communicating with a said second pressure chamber. 
     Each said pusher piston may be adapted to be removed from said housing from the exterior of the tool. 
     This provides the advantage of maximising the extent to which maintenance, adjustment and repair can be carried out at a drilling site. 
     The housing may be assembled from a plurality of parts. 
     This provides the advantage of reducing the cost of construction and maintenance of the tool by reducing the cost of the blade housing (or “housing”) by making it as short as possible, which may need to be replaced due to wear and tear in use downhole. 
     The tool may further comprise at least one nozzle arranged in said passage. 
     This provides the advantage of enabling the pressure inside the bore of the tool to be increased for a given flow rate if the nozzles fitted in the drill-bit have large orifice diameters. 
     According to another aspect of the present invention, there is provided a downhole drilling apparatus including a direction adjustment tool as defined above. 
     The apparatus may further comprise at least one nozzle adapted to increase fluid pressure in said passage. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross sectional view of part of a first part of a vertical drilling tool of a first embodiment of the present invention; 
         FIG. 2  is a cross sectional view of a second part of the tool of  FIG. 1 ; 
         FIG. 3  is a cross sectional view of a third part of the tool of  FIG. 1 ; 
         FIG. 4  is a cross sectional view along the line IV-IV in  FIG. 3 ; 
         FIG. 5  is a cross sectional view of a fourth part of the tool of  FIG. 1 ; 
         FIG. 6  is a perspective view of a sealing plate of the tool of  FIGS. 1 to 5 ; 
         FIG. 7A  is a perspective view of a piston of the tool of  FIGS. 1 to 5  from one side; 
         FIG. 7B  is a perspective view of the piston of  FIG. 7A  from the other side; 
         FIG. 8A  is a perspective view of a piston oil chamber block of the tool of  FIGS. 1 to 5  from one side; 
         FIG. 8B  is a perspective view of the piston oil chamber block of  FIG. 8A  from the other side; 
         FIG. 9  is a cross sectional view of part of a first part of a vertical drilling tool of a second embodiment of the present invention; 
         FIG. 10  is a cross sectional view of a second part of the tool of  FIG. 9 ; 
         FIG. 11  is a cross sectional view of a third part of the tool of  FIG. 9 ; 
         FIG. 12  is a cross sectional view of a fourth part of the tool of  FIG. 9 ; 
         FIG. 13  is a cross sectional view of a fifth part of the tool of  FIG. 9 ; 
         FIG. 14  is a cross sectional view of a sixth part of the tool of  FIG. 9 ; 
         FIG. 15  is a perspective view of a sealing member of a third embodiment of the present invention; and 
         FIG. 16  is a cross sectional view of part of a tool of a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A vertical drilling tool  2  for incorporation into a drilling apparatus for drilling an oil or gas well has an upper end  4  ( FIG. 1 ) for attachment to an upper part (not shown) of a drill string and a lower end  6  ( FIG. 5 ) for attachment to a lower of the drill string. The tool  2  has a tubular housing  8  in which three steering blades  10  are slidably mounted by means of pusher pistons  12  such that the steering blades  10  are slideable relative to the housing  8  between a retracted position and an extended position in which the respective blade  10  engages the wall (not shown) of a borehole being formed by the drilling apparatus. Each of the blades  10  is adapted to be moved outwardly to its extended position by means of increased drilling fluid pressure, in a manner which will be described in greater detail below, and is urged inwardly relative to the housing  8  by means of a respective leaf spring  14 . 
     A drive shaft  16  for transferring rotary drive from the surface to the drilling bit is rotatably mounted in the housing  8  by means of bearings  18 ,  20  and flow restrictor assemblies  22 ,  24 ,  26  and  66  (which also functions as a piston oil chamber block and radial bearing which can also have axial slots along the bore of the oil chamber at the right-hand end and so is not then a flow restrictor) and defines a hollow passage  28  for conveying drilling fluid along the bore of the tool to the drill bit (not shown). The drive shaft  16  is provided with a series of apertures  30  in its wall which communicate with a first pressure chamber  32  defined between the shaft  16  and the housing  8 . Flow restrictor assembly  24  includes a flow restrictor member  34  slidably mounted to the shaft  16  and defining a flow restriction channel  36  between the flow restrictor member  34  and the drive shaft  16 , such that when drilling fluid passes through apertures  30  into the first pressure chamber  32 , the flow restrictor member  34  is urged to the left in  FIG. 2  against the action of compression spring  38  abutting spring retainer  40 , and controlled by engagement of pins  42  in corresponding axial slots  44  in the external surface of flow restrictor member  34 . The spring retainer  40  is held in position by means of a circlip  46 . 
     The flow restrictor assembly  24  operates such that when high pressure drilling fluid is located in the first pressure chamber  32 , it flows through flow restriction channel  36  between the flow restrictor member  34  and the shaft  16 , regardless of the axial position of the flow restrictor member  34  on the shaft  16 , such that a pressure drop occurs between the interior of the first pressure chamber  32  and annular space  45  defined between the housing  8  and the shaft  16 . 
     The flow restrictor member  34  cooperates with a flow restrictor nut  48  to define a part spherical internal surface  50  on both the flow restrictor member  34  and the flow restrictor nut  48  which engages a part spherical upper end  52  of a pendulum member  54  to enable pivoting of the pendulum member  54  through a small angle in any direction relative to the housing  8 . The opposite end of pendulum member  54  defines a circumferential flange  56  defining a rearwardly inclined surface having a negative rake angle for engagement with corresponding engagement portions  58  on three pistons  60 , the engagement portions  58  being located in the first pressure chamber  32 . 
     As shown in greater detail in  FIGS. 7A and 7B , each of the pistons  60  has a head  60   a  defining slightly less than 120 degrees of circular arc and is slidably mounted by means of a respective O-ring  62  or more preferably an elastomer seal with an external profile which is more resistant to the seal rolling in its groove when the piston travels axially, in a respective piston chamber  64  of a cylindrical piston chamber block/flow restrictor/radial bearing  66  shown in greater detail in  FIGS. 8A and 8B  and located between the housing  8  and the drive shaft  16 . The piston chamber block/flow restrictor  66  defines three piston chambers  64 , each of which is filled with oil and slidably receives a piston  60  and communicates via conduits  68 ,  70  with a respective second pressure chamber  72  defined between pusher pistons  12  and a respective sealing plate in the form of a seal pad  74  located between the housing  8  and the shaft  16 . 
     Three seal pads  74  are mounted to the housing  8  by means of screws  76 ,  78  such that each seal pad  74  and O-rings  80  define a respective oil-filled second pressure chamber  72  between the seal pad  74  and the pusher pistons  12 , as shown more clearly in  FIG. 6 . Additional screws could also be added which are not shown to the centre of the seal pad to enhance the ability of the gasket seal around the periphery of the seal pad to seal properly in all instances of internal and external pressures applied to the oil-filled second chamber. Each second pressure chamber  72  communicates via conduits  68 ,  70  with a respective piston chamber  64  such that increase of pressure of drilling fluid in the first pressure chamber  32  is communicated by pistons  60  to the second pressure chambers  72  to enable the corresponding steering blades  10  to be pushed outwards against the action of two leaf springs  14  which are located along both sides of the steering blade  10 . 
     Because three separate second pressure chambers  72  are provided, each steering blade  10  is only able to extend outwards of the housing  8  to its extended position to engage the wall of the borehole if the corresponding piston  60  is able to slide to a sufficient extent in the corresponding piston chamber  64 . However, if the engaging portion  58  of any of the pistons  60  engages the corresponding engaging portion  56  of the pendulum member  54  as the pendulum member  54  moves to the left as shown in  FIG. 2  and the pistons  60  move to the right as shown in  FIG. 2 , the piston  60  is prevented from moving to the right to a sufficient extent to cause the corresponding steering blade  10  to move outwards relative to the housing  8  into engagement with the borehole wall. Engagement of one or two of the pistons  60  with the pendulum member  54  occurs as a result of the angle between the longitudinal axis of the pendulum member  54  and the longitudinal axis of the piston chamber block/flow restrictor  66  being more than a threshold very small amount. 
     In the design variant shown in the Figures only 0.45 deg of offset from vertical is required for the pendulum member  54  to catch the piston  60  on the low side of the hole. 
     The operation of the tool  2  will now be described. 
     In order to correct any deviation of the drilling direction of the drilling apparatus incorporating the tool  2  from the vertical direction, or to maintain a substantially vertical drilling direction, pumping of drilling fluid along the bore of the shaft  16  is first ceased in order to reduce the pressure of drilling fluid in the shaft  16  and the first pressure chamber  32 , down to the ambient static environment pressure. As a result, the flow restrictor member  34  and therefore the pendulum member  54  are urged to the right in  FIG. 2  (i.e. downwards in the borehole) by means of the compression spring  38 . Removal of drilling fluid pressure also causes the steering blades  10  to be urged inwardly relative to the housing  8  under the action of leaf springs  14 , as a result of which pressure in the second pressure chambers  72  urges the pistons  60  to the left as shown in  FIG. 2  so that the upper parts  58  of the pistons  60  overlap the lower part  56  of the pendulum member  54 . At the same time, the pendulum member  54  pivots freely about part spherical surface  52  so that its longitudinal axis is aligned generally towards the vertical by gravity. 
     If the longitudinal axis of the housing  8  is also arranged at a vertical orientation of less than 0.45 deg inclination, in this design example, as the pressure of drilling fluid in the bore of the shaft  16 , and therefore in the first pressure chamber  32 , is increased, the flow restrictor member  34  and pendulum member  54  are urged to the left as shown in  FIG. 2  and the pistons  60  are urged to the right, and none of the pistons  60  engage the flange  56  of the pendulum member  54 . The pistons  60  can therefore slide to their full extent, as a result of which all of the steering blades  10  are urged outwards of the housing  8  to engage the borehole wall to maintain the vertical orientation of the tool  2 . 
     If, on the other hand, the longitudinal axis of the housing  8  of the tool  2  is aligned a small angle clockwise as shown in  FIG. 2  of the vertical direction, as the pendulum member  54  aligns itself in a generally vertical direction, the longitudinal axis of the pendulum member  54  will be arranged at a small angle (0.45 deg or so is possible) anticlockwise of the longitudinal axis of the housing  8 . As the pressure of drilling fluid in the shaft  16  is gradually increased, the pressure increase is communicated via apertures  30  to the first pressure chamber  32 , as a result of which the flow restrictor member  34  and pendulum member  54  are urged to the left  2  against the action of compression spring  38 , and the pistons  60  are urged to the right. 
     Because of the orientation of the pendulum member  54 , the engaging portion  58  of one or two of the pistons  60  engages the flange  56  on the pendulum member  54  as a result of which further movement of the engaged piston  60  to the right as shown in  FIG. 2  is prevented. This prevents the corresponding steering blades  10  from being urged outwardly of the housing  8  to engage the wall of the borehole. In the arrangement shown in  FIGS. 2 and 3 , the upper piston  60  shown in  FIG. 2  engages the pendulum member  54  because the housing is oriented slightly clockwise of its intended position. The steering pusher  10  shown at the top of  FIG. 3  is prevented from engaging the wall of the borehole, as a result of which the other two steering pushers blades (not shown) engage the borehole wall and urge the housing  8  in an anti-clockwise orientation, which therefore urges the tool  2  back towards a generally vertical orientation to correct deviation of the drilling direction away from the vertical. 
     Referring to  FIGS. 9 to 14 , in which parts common to the embodiment of  FIGS. 1 to 8  are denoted by like reference numerals but increased by 100, second pressure chambers  172  are defined by respective gun drilled holes  173  formed directly in the housing  108  of the tool, as shown in detail in  FIG. 13 . The gun drilled holes  173  are connected to the respective piston chambers  164 , by means of hollow tubes  165  having longitudinal apertures in the ends of piston chamber block  166 . As a result, the angled conduits  70  in the housing  8  of the embodiment of  FIGS. 1 to 8  are no longer necessary. Similarly, the formation of the gun drilled holes  173  directly in the housing removes the need for seal pads, bolts, bolt gaskets seals and seal pad gasket O-rings of the embodiment of  FIGS. 1 to 8 , thus simplifying construction of the tool and reducing its cost and making the assembly less prone to leakage. 
     The lower part of the second pressure chambers is defined by a flow restrictor  175  located by means of three screws  177  in the housing  108 , and which is mounted to the corresponding gun drilled holes  173  by means of three blanking plugs  179 , in order to prevent the flow restrictor  175  from rotating with the central shaft  116  while the tool is in use in a drilling apparatus, and the screws  177  also prevent the flow restrictor  175  from sliding downwards as a result of gravity and/or vibration. Two grooves  181 ,  183  are provided on the internal surface of the flow restrictor  175 . The first groove  181  is provided to locate an O-ring seal for pressure testing on assembly of the tool, and the second groove  183  enables the flow restrictor  175  to be pulled out of the blade housing  108  to enable the tool to be dismantled with an expandable wire puller service tool (not shown). 
     Compared with the embodiment of  FIGS. 1 to 8 , the piston chambers  164  and pistons  160  are wider and the chamber walls are provided with greater thickness, as a result of which the components become more robust and can withstand greater negative pressure which may occur in the piston chambers  164  as a result of the pistons  160  being caught by the pendulum  154  and pulled upwards as a result of movement of the flow restrictor  134 . The piston chamber block  166  is also less expensive to manufacture and to replace when worn than the corresponding component of the embodiment of  FIGS. 1 to 8 . 
     As can be seen from  FIGS. 12 and 13 , the blade housing  108  is formed from two components (a lower component  108  and an upper component  108   a ), and has been made shorter, as a result of which it is of lower cost to replace when worn or damaged. Seals  185  on the pusher pistons  112  are located into the housing  108  and are not provided on the pistons  112 , which enables the sliding surfaces of the pistons  112  to be coated with a hard corrosion resistant coating, such as HVOF tungsten carbide. This is easier to apply to the pusher pistons  112  than to the corresponding recesses on the housing  108  in which the pusher pistons  112  slide. 
     As shown in greater detail in  FIG. 14 , a nozzle  187  is provided in the output shaft  116 . This enables the back pressure on the pusher pistons  112  to be increased if there is insufficient pressure drop across the drill bit during drilling. In addition, multiple thin longitudinal strips of hard facing (e.g. tungsten carbide) are provided on the outer surfaces of the blades  110 , as a result of which the blades  110  are less likely to allow the blade housing to rotate as a result of rotation of the main drive shaft  116  assembly. 
     Referring to  FIG. 15 , in which parts common to the embodiment of  FIGS. 1 to 8  are denoted by like reference numerals but increased by 200, the seal pads  74  of the embodiment of  FIGS. 1 to 8  are replaced by a single tubular sealing member  274 , which provides greater stiffness than the case of three separate seal pads  74  of the embodiment of  FIGS. 1 to 8 . The tubular sealing member  274  is provided with three bolting points  275  at each of its ends in order to enable the sealing member  274  to be correctly located relative to the housing  108  of the tool. The sealing member  274  is located in position, and the second pressure chambers are defined by suitable recesses  277  in the sealing member  274 , The second pressure chambers are sealed by means of suitable gasket O-rings (not shown) between the sealing member  274  and the internal surface of the housing  108  of the tool. 
     Referring to  FIG. 16 , in which parts common to the embodiment of  FIGS. 1 to 8  were denoted by like reference numerals but increased by 300, upper flow restrictor assembly  324  differs from the upper flow restrictor  24  of the embodiment of  FIGS. 1 to 8  in that it is restrained by means of screws  325  from sliding axially in the housing  308 . This provides the advantage that the walls of the piston chambers  364  are not exposed to significant negative pressures, since the flow restrictor member  334  is unable to forcibly pull the compensating pistons  360  against the direction in which they are pushed by the high internal pressure. The lower part of the pendulum member  354  is provided with a larger number of serrations  356  than the corresponding components of the embodiment of  FIGS. 1 to 8 , and the compensating pistons  360  are provided with corresponding serrations  357 . As a result, this component has significantly greater strength than the corresponding component of the embodiment shown in  FIGS. 1 to 8  as a result of the enlarged engagement area. In addition, the axial force passing through the mutually engaging parts of the pendulum  365  and the pistons  360  is lower because the seal area on the compensating pistons is lower than the seal area on the moving flow restrictor  34  of the embodiment of  FIGS. 1 to 8 . 
     It will be appreciated by the person skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.