Abstract:
This invention relates to a torque control device for a downhole drilling assembly. The torque control device is adapted for connection to a drill bit, and has an outer sleeve and an inner shaft, the outer sleeve being movable longitudinally relative to the inner shaft. The torque control device also has a cylinder, a piston located within the cylinder, and a controller to control the volume of the cylinder. Changing the volume of the cylinder causes relative longitudinal movement between the outer sleeve and the inner shaft. The controller can receive inputs from a torque sensor and/or an accelerometer in order to determine if a threshold torque has been exceeded. Alternatively the controller can comprise a rotary valve which automatically responds to torque in the assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to United Kingdom Patent Application No. GB1203433.6 filed on Feb. 28, 2012, and United Kingdom Patent Application No. GB1211300.7 filed on Jun. 26, 2012, the contents of each one incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a torque control device for a downhole drilling assembly. 
       BACKGROUND OF THE INVENTION 
       [0003]    When drilling for oil and gas, the downhole drill bit is connected to surface equipment by way of a drill string. The drill string is hollow whereby drilling fluid or mud can be pumped down the borehole, the mud acting to lubricate the drill bit and to carry drill cuttings back to the surface. The mud and entrained drill cuttings return to the surface along the outside of the drill string, the drill string being smaller than the diameter of the borehole. 
         [0004]    In some drilling applications the drill string is rotated at the surface, with the rotation being communicated to the drill bit by the drill string. In other drilling applications a downhole motor such as a mud motor is provided, which uses the flowing mud to drive the drill bit to rotate. A downhole motor may be used with a rotating, or a non-rotating, drill string. 
         [0005]    The surface equipment applies a downhole force upon the drill string, which force is communicated to the drill bit. In addition to the torque seeking to rotate the drill bit there is also a force acting to advance the bit into the rock at the leading end of the borehole, the latter force typically being referred to as “weight on bit”. 
         [0006]    The drill operator will typically seek to maximise the weight on bit so that the drill advances as quickly as possible through the rock. However, there is a maximum limit for the weight on bit which depends upon the bit design and the drilling conditions. Exceeding the maximum weight on bit for the particular bit design and drilling conditions will increase the drag upon the drill bit and cause the drill bit to slow down or stall, i.e. the drill bit will rotate more slowly, or in extreme cases stop rotating altogether. 
         [0007]    If the drill bit does rotate more slowly than the drill string, or than the output of the downhole motor, then the drill string will be caused to twist as torque output from the surface equipment (or downhole motor) increases in response to maintain the original rate of rotation. Eventually, torque at the drill bit will exceed the resistance to rotation and the drill bit will start to rotate again. 
         [0008]    Such a phenomenon is known as “stick-slip” and is a major concern to drill operators. Firstly, the drill string may be damaged by the requirement to twist as the drill bit slows down or stops. Secondly, the drill bit will often rotate very rapidly, and uncontrolledly, as the torque in the twisted drill string is relieved. Periods of slow or non-rotation of the drill bit followed by rapid and uncontrolled rotation of the drill bit will often be repeated if they are not countered. 
         [0009]    Drill operators seek to avoid stick-slip by reacting to reductions in the rate of rotation of the drill bit by reducing the weight on bit, so that the drill bit resumes its desired rate of rotation quickly without undue twisting of the drill string. A reduction in the rate of rotation of the drill bit can be detected directly by measuring the rate of rotation of the drill bit, or (more typically) by measuring the torque being applied to the drill bit, the torque increasing as the rate of rotation reduces. 
         [0010]    The prior art includes torque control devices which can automatically reduce the weight on bit if the torque upon the drill bit exceeds a certain threshold. One prior art arrangement is described in WO 2004/090278 (Tomax). This document has an outer sleeve connected to the drill string and an inner shaft connected to the drill bit. The outer sleeve and the inner shaft are interconnected by a helical thread. A spring biases the inner shaft outwardly of the outer sleeve, into engagement with a fixed stop upon the outer sleeve. During normal drilling operations the inner shaft is driven to rotate by the sleeve, and in turn drives the drill bit to rotate at the same rate as the drill string. If the drill bit slows down or stops, however, the torque upon the drill bit increases sufficiently to drive the sleeve to rotate relative to the shaft, compressing the spring. The helical thread between the inner shaft and the outer sleeve means that rotation of the inner shaft relative to the outer sleeve causes the inner shaft to retract into the sleeve, thereby retracting the drill bit and reducing the weight on bit. As the weight on bit is reduced a point is reached where the drill bit can resume its rotation. The spring causes the inner sleeve to return to its extended position in engagement with the fixed stop, during which the drill bit rotates faster than the drill string. 
         [0011]    The Tomax arrangement can include an oil damper, i.e. the spring and cooperating helical threaded components can lie within an oil reservoir which damps out the movement of the inner shaft relative to the outer sleeve, preventing uncontrolled rotation of the inner sleeve and therefore the drill bit. 
         [0012]    A similar arrangement is described in U.S. Pat. No. 7,044,240 (McNeilly), and also in Tomax&#39;s later U.S. Pat. No. 7,654,344, which uses a helical spring rather than a helical thread to interconnect the outer sleeve and the inner shaft. 
         [0013]    The prior art arrangements all rely upon compression springs, and it will be understood that the force provided by those springs must exceed the weight on bit. The design of the tools must therefore include a calculation for the maximum weight on bit which can be catered for, and once the spring rate has been determined it cannot be adjusted. When drilling for oil and gas, however, the rock type through which the drill must pass can vary significantly during a drilling operation, and if the spring force is set too low the tool may reduce the drilling torque even if the drill is not sticking, i.e. the drill operator cannot exceed the weight on bit determined by the spring force, even if the drilling conditions are more favourable than expected and the drill bit would not stick with a greater weight on bit. If, on the other hand, the spring force is set too high for the particular drilling conditions, the drill bit may undergo significant stick-slip without actuation of the torque control device. 
       SUMMARY OF THE INVENTION 
       [0014]    The inventor has realised that an improved device is required for reducing the weight on bit and thereby reducing the torque upon a drill bit whereby to reduce or avoid the likelihood of stick-slip. One object of the invention is to provide a device which enables the torque at which the weight on bit is reduced to be adjusted downhole to match the drilling conditions. 
         [0015]    According to the invention there is provided a torque control device for a downhole drilling assembly, the torque control device being adapted for connection to a drill bit, the torque control device including an outer sleeve and an inner shaft, the inner shaft being movable longitudinally relative to the outer sleeve, the inner shaft having a through-bore for carrying drilling fluid to the drill bit, the device having a piston and cylinder arrangement and a controller which controls the volume of the cylinder. 
         [0016]    It is arranged that the relative position of the inner shaft relative to the outer sleeve (in the direction of the longitudinal axis of the torque control device) is determined by the volume of the cylinder, so that the controller controls the (longitudinal) movement of the inner shaft relative to the outer sleeve. The controller preferably has a memory in which is stored a threshold value, the controller causing the inner shaft to move relative to the outer sleeve when the threshold value is reached or exceeded. The controller can desirably be adjusted (preferably downhole) whereby the threshold value can be adjusted to match the drilling conditions. 
         [0017]    The controller can be connected to a torque sensor adapted to measure the torque in a part of the downhole assembly, suitably the torque in a part of the downhole assembly connected to the drill bit. Alternatively, the controller can be connected to a sensor such as an accelerometer which measures the rate of rotation of the drill bit (or a part of the downhole assembly connected to the drill bit) whereby to detect reductions in the rate of rotation of the drill bit. The controller can in some embodiments receive and compare the inputs from two accelerometers, one accelerometer located close to the drill bit and the other accelerometer located remote from the drill bit. Sticking of the drill bit can be detected by changes in the relative outputs of the two accelerometers. 
         [0018]    Preferably the inner shaft is connected to the drill string and the outer sleeve is connected to the drill bit, but it will be understood that the orientation of these components can be reversed without departing from the invention. 
         [0019]    Desirably, the cylinder is connected to the through-bore whereby the cylinder will be filled with drilling fluid in use. The drilling fluid can therefore provide the hydraulic fluid for the piston and cylinder arrangement. In such an arrangement, the cylinder can also be open to the periphery of the downhole assembly, so that in use drilling fluid can flow out of the cylinder into the annulus surrounding the downhole assembly, and along which the drilling fluid returns to the surface. Such arrangements take advantage of the pressure differential which occurs between the drilling fluid within the through-bore (i.e. upstream of the drill bit) and in the annulus (i.e. downstream of the drill bit). 
         [0020]    Preferably, the controller controls the position of an actuating valve whereby to control the flow of drilling fluid into the cylinder. It can be arranged that the port from the through-bore into the cylinder is of larger cross-section than the port in the periphery of the downhole assembly. This arrangement avoids the requirement for a separate actuating valve controlling the egress of drilling fluid from the cylinder, it being arranged that the larger entry port will act to increase the volume of the cylinder when the actuating valve is opened, and the (always open) exit port will allow the drilling fluid to drain out of the cylinder, so as to reduce the volume of the cylinder, when the actuating valve is closed. 
         [0021]    Desirably, a return spring is provided to bias the piston so as to reduce the volume of the cylinder. It is arranged that when the actuating valve is closed the biasing force of the return spring is sufficient to force drilling fluid out of the cylinder and into the surrounding annulus so as to reduce the volume of the cylinder and drive the inner shaft to move longitudinally relative to the outer sleeve. 
         [0022]    In certain embodiments the threshold value of the controller can be adjusted during use. It is known to communicate from the surface to a downhole tool, and it is also know to communicate by way of the drilling fluid. In the “RipTide” drilling reamer of Weatherford, Inc. radio frequency identification (RFID) units are injected into the drilling fluid and sent downhole with the fluid. As the RFID units pass a controller of the reamer they are read and used to adjust the status of the reamer. A similar system can be used with the present invention, with the controller being adapted to react to messages sent downhole, for example by way of RFID units, whereby the threshold value for actuation of the device can be adjusted during use. It is therefore not necessary to trip the downhole assembly in order to adjust the threshold value, and if the drilling conditions become more (or less) favourable and a greater (or lesser) weight on bit can be accommodated without incurring stick-slip, the threshold value can be increased (or decreased) accordingly. 
         [0023]    Certain embodiments of the present invention can avoid the requirement for sensors communicating torque and/or acceleration to the controller. In such embodiments the controller is in the form of a rotary valve, and admission of drilling fluid into the cylinder is controlled by the rotary valve which automatically moves to an open position (or to a more open position) when the torque within the downhole assembly exceeds a predetermined threshold. 
         [0024]    Whilst in the simplest embodiments of the present invention the drilling fluid is caused to flow into and out of the cylinder in order to determine the volume of the cylinder, in other embodiments a closed hydraulic system is used. In those other embodiments the volume of the cylinder, and therefore the position of the inner shaft relative to the outer sleeve, is determined by a hydraulic fluid which is isolated from, and independent of, the drilling fluid. Such alternative embodiments are more mechanically complex, but avoid the possible problems associated with the use of drilling fluid as the hydraulic fluid. The electrical and hydraulic power for a closed hydraulic system can be provided by a downhole pump in known fashion. 
         [0025]    The inventors have also realised that the device of the present invention can be used for other downhole applications where the torque transmitted to the drill bit requires adjustment. One such application is in drilling applications using an under-reamer for example. An under-reamer, such as the aforementioned “RipTide” drilling reamer of Weatherford, Inc., uses a reamer as well as a drill bit, the reamer following the drill bit and reaming out a larger diameter borehole along chosen lengths of the borehole. It is advantageous to balance the drilling torque provided by the drill string between the drill bit and the reamer so as to maximise the rate of advance of the downhole assembly. The present invention can be located in the downhole assembly between the reamer and the drill bit and can control the torque transmitted to the drill bit and thereby control the proportion of the drilling torque utilised by the drill bit and that used by the reamer. 
     
    
     
       BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
         [0026]    The invention will now be described in more detail, by way of example, with reference to the accompanying schematic drawings, in which: 
           [0027]      FIG. 1  shows a side view of a tool according to the present invention, in a normal, non-actuated, condition of use; 
           [0028]      FIG. 2  shows a side view of the tool of  FIG. 1 , in an actuated condition; 
           [0029]      FIG. 3  shows a representation of the tool of the present invention located in a downhole assembly between a reamer and a drill bit; and 
           [0030]      FIG. 4  shows a side view of a tool according to the present improvement. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The torque control device  10  of the present invention is part of a downhole assembly  12  which is adapted to drill a borehole  14  into the Earth  16 . The longitudinal axis A-A of the downhole assembly  12  (which corresponds to the longitudinal axis of the torque control device  10 ) is shown horizontal in  FIGS. 1 and 2 , but the orientation is unimportant and the present invention can be used with the longitudinal axis at any chosen angle. 
         [0032]    The downhole assembly  12  includes a female threaded connector  20  by which the assembly may be connected to a length of drill string (not shown) connected to the surface. Alternatively, the connector  20  can be connected to a downhole motor such as a mud motor, or to a downhole steering tool such as that of EP 1 024 245. It will be understood, however, that the tool can be located uphole of a steering tool if desired. 
         [0033]    The connector  20  is connected to an inner shaft  22 , which has a through-bore  24  through which drilling fluid can flow to the drill bit  26 , in known fashion. In common with prior art downhole assemblies, the drilling fluid passes out through ports (not shown) in the drill bit  26 , and then returns to the surface by way of the annulus  30  surrounding the downhole assembly  12  and the drill string. 
         [0034]    Though not shown in the drawings, it will be understood that the torque control device  10  will typically include a plurality of blades which engage the borehole  14  and serve to centralise the torque control device  10  within the borehole  14 . The downhole assembly may in practice also include a stabiliser located between the torque control device  10  and the drill bit  26 , and/or between the connector  20  and the drill string. 
         [0035]    The drill bit  26  is connected (in the embodiment of  FIGS. 1 and 2  directly, but in other embodiments indirectly) to an outer sleeve  32  which surrounds a part of the inner shaft  22 . At least one set of splines  34  interconnect the inner shaft  22  and the outer sleeve  32 , so that the inner shaft  22  can slide longitudinally relative to the outer sleeve  32 , but cannot rotate relative to the outer sleeve. The number and disposition of the splines will depend upon the torque which is to be transmitted from the inner shaft  22  to the outer sleeve  32 . 
         [0036]    During normal drilling operations, in the absence of stick-slip, the torque control device  10  is in the condition shown in  FIG. 1 . Rotation of the drill string (and/or downhole motor) is communicated to the connector  20  and, by way of the inner shaft  22  and splines  34 , to the outer sleeve  32  and the drill bit  26 . 
         [0037]    The through-bore  24  has a port  36  which opens into a valve chamber within the body of a piston  40 , the piston  40  comprising an enlargement of the inner shaft  22 . An actuating valve  42  is located within the valve chamber of the piston  44 , the actuating valve  42  being controlled by a controller  44 . The actuating valve  42  controls the passage of drilling fluid from the through-bore  24 , through the port  36  and into a cylinder  46 . The cylinder  46  has another port  50  which is open to the periphery of the device  10 , and therefore to the annulus  30  surrounding the downhole assembly  12 . 
         [0038]    It will be understood that the pressure of the drilling fluid within the through-bore  24  is substantially higher than the pressure of the drilling fluid within the annulus  30 , the difference in pressure being caused primarily by the pressure drop across the drill bit  26 . It is arranged that the entry port  36  is of significantly larger area than the exit port  50 , so that when the actuating valve  42  is opened drilling fluid flows into the cylinder  46  from the through-bore  24  at a faster rate than fluid can flow out of the cylinder  46  through the port  50 . 
         [0039]    If the weight on bit is too great for the particular drilling conditions, the rotation of the drill bit  26  will slow relative to the rotation of the connector  20 . In the present embodiment this is detected by a strain gauge  52  located upon the shaft  22 . It will be understood that the strain gauge  52  is sufficiently sensitive to detect very small angular twisting movements of the inner shaft  22 , as caused by small angular deviations of the drill bit  26  relative to the connector  20 , which are indicative of the drill bit slowing and the possible onset of stick-slip. The strain gauge  52  detects the strain in the inner shaft  22  and communicates this to the controller  44 . The communication is preferably by wires (not shown), but the form of data transmission is not critical to the invention. 
         [0040]    The controller  44  has a memory in which is stored a high threshold strain value, and against which the strain measured by the strain gauge  52  is continuously or repeatedly compared. If the comparison is not continuous, it is sufficiently frequent so as quickly to identify unacceptable increases in the measured strain. The high threshold strain value may be determined by calculation or experiment. If the measured strain exceeds the high threshold strain value the controller opens the actuating valve  42  and permits drilling fluid to flow into the cylinder  46 . 
         [0041]    As shown in  FIG. 2 , when the actuating valve  42  is opened, drilling fluid flows into the cylinder  46  through the entry port  36 . Since the flow rate through the entry port  36  and past the valve  42  into the cylinder  46  is greater than the flow rate out of the cylinder through the exit port  50 , the volume of the cylinder  46  is thereby caused to increase. The piston  40  is fixed to the inner shaft  22  and does not move relative to the inner shaft  22 . Instead, as the volume of the cylinder  46  increases the outer sleeve  32  moves to the right as drawn. This rightwards movement is represented in  FIG. 2  by the drill bit  26  being lifted from the bottom of the borehole  14 ; in practice the actual movement may be very small, but the force with which the drill bit  26  engages the end of the borehole (i.e. the weight on bit) can be reduced significantly. 
         [0042]    During this retracting movement of the outer sleeve  32 , the connector  20  continues to rotate, and at some point the torque upon the drill bit  26  will exceed the frictional resistance to rotational movement and the drill bit will resume rotation (and will unwind any twist which has been imparted into the drill string). 
         [0043]    As the drill bit  26  resumes is rotation, the strain upon the inner shaft  22  will reduce, and will be detected by the strain gauge  52 . The memory of the controller  44  also stores a low threshold strain value, the low threshold strain value being a chosen amount lower than the high threshold strain value so as to avoid “hunting”. When the low threshold strain value is passed the controller  44  closes the actuating valve  42 . 
         [0044]    In other embodiments the controller  44  stores only a single threshold strain value, the controller opening the valve  42  when the measured strain rises above that value, and closing the valve  42  when the measured strain falls below that value. 
         [0045]    The controller  44  can if desired close the actuating valve  42  to an intermediate position at which the rate of drilling fluid flowing into the cylinder  46  closely matches the rate of fluid flowing out of the cylinder, and it may be arranged to maintain the intermediate position for a predetermined period of time, perhaps a few seconds, so that the device dwells in that operational position (with the volume of the cylinder  46  remaining substantially constant). 
         [0046]    When the actuating valve  42  is closed the compression spring  54  acts to drive the drilling fluid out of the cylinder  46 , through the exit port  50 , so that the tool returns to the condition of  FIG. 1 . Desirably, the exit port  50  is sufficiently small so that it takes several seconds (e.g. 2-3 seconds) for the device to move from the condition of  FIG. 2  to the condition of  FIG. 1 , it being preferred that the weight on bit be gradually increased back to its desired level rather than suddenly increased. 
         [0047]    The drill operator at the surface will be aware that the torque control device  10  has been actuated by virtue of the reduction in pressure of the drilling fluid caused by the opening of the actuating valve  42 . The drill operator will typically react by reducing the weight on bit at the surface so as to avoid the onset of stick-slip. The operator can check that the device  10  does not undergo repeated actuation, and if so can steadily increase the weight on bit back to the desired level. 
         [0048]    Since the actuation of the torque control device  10  is not dependent upon the force exerted by a spring, the drill operator can set the maximum weight on bit for the drilling conditions. The spring  54  can therefore be made sufficiently strong to exceed the maximum weight on bit which the surface equipment can impart (so that the spring  54  can drive the tool from the condition of  FIG. 2  to the condition of  FIG. 1  when the actuating valve  42  is closed, regardless of the actual weight on bit. It is not necessary to set the spring force dependent upon the likelihood of stick-slip as in the Tomax and other prior art arrangements. 
         [0049]    The drill operator can also adjust the high and low threshold strain values for the actuating valve downhole, without needing to trip the downhole assembly. Specifically, the drill operator at the surface can communicate with the tool  10 , and in particular with the controller  44 , whilst the tool  10  is downhole. Such communication may be effected by any of the known means for communicating with downhole tools, for example by wire, radio waves, mud pulsing, or RFID units injected into the drilling fluid. Thus, if it is determined that the threshold for actuating the valve  42  is set too low, so that the valve is actuated at strain levels which would not result in damaging stick-slip, the high threshold strain value may be increased without tripping the tool. The drill operator can also switch the torque control device  10  on and off remotely, it perhaps being desirable to switch the torque control device off in certain situations so as to save power. 
         [0050]    An alternative embodiment of torque control device  110  is shown in  FIG. 4 . Though not shown in  FIG. 4 , the downhole assembly  112  will also include a drill bit (perhaps similar to the drill bit  26  of the embodiment of  FIGS. 1 and 2 ) which is secured by way of a male threaded connector  56 . Alternatively, a mud motor for example may be located between the drill bit and the torque control device  110 . 
         [0051]    The connector  120  is connected to an upper shaft  60 , which has a through-bore  124  by which drilling fluid can flow to the drill bit (not shown), in known fashion. 
         [0052]    The connector  56  is connected to an outer sleeve  132  which surrounds a lower shaft  122  and part of the upper shaft  60 . At least one set of splines  134  interconnects the lower shaft  122  and the outer sleeve  132 , so that the lower shaft  122  can slide longitudinally relative to the outer sleeve  132 , but cannot rotate relative to the outer sleeve. As with the embodiment of  FIGS. 1 and 2 , the number and disposition of the splines will depend upon the torque which is to be transmitted from the lower shaft  122  to the outer sleeve  132 . 
         [0053]    The upper shaft  60  is separate from the lower shaft  122 ,  FIG. 4  showing an exaggerated gap  62  between the facing ends of these shafts. The upper shaft  60  has an enlarged end which forms a piston  140  as described below. A part of the piston  140  surrounds the end of the lower shaft  122 , and a set of axial bearings  64  interconnect the piston  140  and the lower shaft  122 . The axial bearings  64  permit relative rotation between the piston  140  and the lower shaft  122 , but resist relative longitudinal movement. It is therefore arranged that the piston  140  is fixed upon the upper shaft  60 , and can rotate relative to the lower shaft  122 . 
         [0054]    The through-bore  124  within the lower shaft  122  has a port  136  which lies within the region of the lower shaft  122  which is surrounded by the piston  140 . The piston has a conduit  66  which can be aligned with the port  136  whereby drilling fluid can pass from the through-bore  124  into a cylinder  146 . 
         [0055]    The cylinder  146  has an exhaust conduit  150  which in this embodiment passes through the piston  140 , and opens into a spring chamber  68 . An exhaust port  70  is provided for the spring chamber  68 , the exhaust port  70  being open to the periphery of the downhole assembly  112 . 
         [0056]    It is arranged that the port  136  and conduit  66  are of larger cross-sectional area than the exhaust conduit  150 , so that when the conduit  66  is fully aligned with the port  136  drilling fluid flows into the cylinder  146  from the through-bore  124  at a faster rate than fluid can flow out of the cylinder  146  through the conduit  150 . 
         [0057]    A spring  72  is located within the spring chamber  68 . One end of the spring  72  is located in a piston spring pocket  74  and the other end of the spring is located in a sleeve spring pocket  76 . The spring  72  acts primarily as a torsion spring, and seeks to rotate the piston  140  relative to the sleeve  132 . Since the sleeve  132  is non-rotatably connected to the lower shaft  122  by way of the splines  134 , the spring  72  also acts to rotate the piston  140  relative to the lower shaft  122 . It is arranged that the spring  72  is biased to move the conduit  66  out of alignment with the port  136 . 
         [0058]    Thus, in normal operation the conduit  66  is out of alignment (or at least out of full alignment) with the port  136 , whereby drilling fluid either cannot flow into the cylinder  146  at all, or at most flows into the cylinder  146  at a rate below that at which it flows out along the conduit  150 . The volume of the cylinder  146  is therefore minimised, and the sleeve  132  is extended (to the left as drawn) to its farthest extent relative to the upper shaft  60  and piston  140 . 
         [0059]    If the weight on bit exceeds the maximum for the drilling conditions, the rate of rotation of the drill bit will reduce. The drill bit is connected to the sleeve  132  so that the rate of rotation of the sleeve, and thereby the lower shaft  122 , also reduce. The drill string and therefore the upper shaft  60 , however, continue to rotate, so that there is relative rotation between the piston  140  and the lower shaft  122 . The conduit  66  and the port  136  will thereby be forced into greater alignment, against the torsional bias of the spring  72 , and perhaps into full alignment as shown in  FIG. 4 . When so aligned, the flow rate of drilling fluid into the cylinder  146  will exceed the flow rate of fluid out of the cylinder  146 , so that the volume of the cylinder  146  increases and the sleeve  132  is forced towards the right as viewed, automatically reducing the weight on bit. 
         [0060]    As the weight on bit is reduced the rate of rotation of the drill bit increases and the torque within the downhole assembly  110  is reduced. The spring  72  can then rotate the conduit  66  and port  136  out of alignment and the drilling fluid bleeds out of the cylinder  146 . 
         [0061]    It will therefore be understood that the port  136  and conduit  66  act as a rotary valve to automatically control the volume of the cylinder  146  by allowing drilling fluid (or more drilling fluid) into the cylinder when the rate of rotation of the drill bit drops below that of the drill string. 
         [0062]    The spring  72  can determine a threshold value for the torque which will be required to open the rotary valve. It will be understood that the piston  140  needs to rotate through only a few tens of degrees in order to move a totally misaligned conduit  66  and port  136  into full alignment, and the range of relative rotation may be limited by stops (not shown). The torque control device  110  can be assembled with the spring  72  under a chosen pretension, i.e. the spring  72  can in normal conditions bias the piston  140  against a rotational stop. 
         [0063]    Whilst the primary function of the spring  72  is to control the rotary valve  66 ,  136 , it also acts as a compression spring and assists the movement of the sleeve  132  (and therefore the drill bit) to the left as drawn as the drilling fluid drains from the cylinder  146 . However, unlike the prior art arrangements, the compression force of the spring  72  does not provide the upper limit for the weight on bit. 
         [0064]    In the embodiment shown in  FIG. 4  the relative rotation of the piston  140  and the lower shaft  122  is directly dependent upon the torque applied to the drill bit by the drill string. In a further modification, a detent mechanism can be provided between the piston  140  and the lower shaft  122 , the detent mechanism allowing relative rotation only when a predetermined threshold torque has been exceeded. With such a modification, the opening movement of the rotary valve would be less progressive than the embodiment of  FIG. 4 . 
         [0065]    It will be understood that a small gap is shown between the inner shaft  22  and the outer sleeve  32  in  FIGS. 1 and 2 , and similarly between the inner shafts  60  and  122  and the outer sleeve  132  in  FIG. 4 , for the purposes of clarity. In practice, these components would be in sliding engagement, with suitable seals for the cylinder  46 ,  146  etc. 
         [0066]      FIG. 3  represents schematically another useful application of the torque control device  10 ,  110 . In this application, the torque control device  10 ,  110  is located between the drill bit  26  and a reaming tool  60 . In known fashion, the reaming tool  60  includes cutting blades  62  which can be refracted into the body of the tool  60  when not required (during passage through a borehole casing for example) and then actuated to their extended condition as shown at a chosen location downhole. When the cutting blades  62  are extended, the drill bit  26  and the reaming tool  60  are both engaging respective sections of rock. To maximise the rate of advance of the downhole assembly it is desirable to impart a proportion of the torque provided by the drill string to the drill bit  26  and another proportion of the torque to the reaming tool  60 , the actual proportions depending on the drilling conditions and the cross-sectional area of rock being removed by the respective components. The tool  10 , 110  can be used to reduce the torque being imparted to the drill bit  26 , and thereby to increase the torque being imparted to the reaming tool  60 , the respective proportions being determined by the threshold strain value set for the actuating valve  42  in the embodiment of  FIGS. 1 and 2 , or that set for the rotary valve  66 ,  136  in the embodiment of  FIG. 4 . If the threshold strain value is set correctly, the efficiency of the downhole assembly will be increased, i.e. both the drill bit  26  and the reamer blades  62  will be driven against the respective rock faces with an appropriate force and the advance of the downhole assembly will be maximised. 
         [0067]    The torque control device  10 , 110  is expected to have its greatest utility when used with PDC drill bits, but the invention can be used with other types of drill bit if desired.