You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND  
       [0001]     This invention relates to a steerable drilling system for use in the formation of subterranean boreholes, for example for subsequent use in the extraction of oil or natural gas.  
         [0002]     One form of steerable drilling system includes a bias unit having a plurality of bias pads. Each bias pad is moveable from a retracted position to an extended position by an associated piston operable by pressurised drilling fluid or mud, a suitable valve arrangement being provided to control the supply of drilling fluid or mud to each piston and thereby control the positions occupied by the bias pads at any given time.  
         [0003]     The drilling fluid or mud sometimes carries with it particles or debris from further up the borehole being formed. The presence of such particles or debris can cause wear or malfunction of the valve arrangement and/or the pistons. Clearly this is undesirable. In order to reduce wear and the risk of malfunction it is known to provide a junk basket or filter to collect particles carried by the flow of drilling fluid or mud before the fluid or mud reaches the valve arrangement and pistons. Such devices are intended to remove fairly large particles which, potentially, could cause malfunction or blockage. They are not intended for, or suitable for, removal of small, dense particles which cause erosion.  
         [0004]     It is an object of the invention to provide a steerable drilling system of reduced susceptibility to erosion.  
         [0005]     According to the present invention there is provided a steerable drilling system comprising a bias unit, the bias unit having at least one actuator operable by pressurised drilling fluid or mud, in use, and a separator arranged to remove particulate matter from the supply of pressurised drilling fluid or mud to the or each actuator, the separator comprising a hydrocyclone.  
         [0006]     The use of a hydrocyclone as the separator is advantageous in that it can be used to separate relatively dense particles from the supply of drilling fluid or mud, even when the dense particles are of fairly small dimensions and so may be capable of passing through a junk basket or filter. As it is the relatively small, dense particles that tend to cause erosion, the use of a hydrocyclone allows a reduction in erosion damage to be achieved.  
         [0007]     The separator may be integrally formed with the bias unit. Alternatively, the separator and bias unit may be separate components, the separator being located upstream of, preferably immediately upstream of, the bias unit so that the drilling fluid or mud from which matter has been separated by the separator can be supplied directly to the bias unit.  
         [0008]     The invention also relates to a hydrocyclone adapted for use in such a steerable drilling system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will further be described, by way of example, with reference to the accompanying drawing, in which:  
         [0010]      FIG. 1  is a diagrammatic illustration of part of a steerable drilling system in accordance with an embodiment of the invention;  
         [0011]      FIG. 2  is a schematic representation of part of the system of  FIG. 1 ; and  
         [0012]      FIG. 3  is a diagrammatic view illustrating the separator of the system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIGS. 1 and 2  illustrate, diagrammatically, part of a steerable drilling system for use in the formation of a borehole, for example for subsequent use in the extraction of hydrocarbons. The part of the drilling system illustrated in  FIG. 1  comprises a housing or drill collar  10  into which drilling fluid, referred to hereinafter as mud, is pumped. The drill collar  10  has mounted thereon a plurality of bias pads  12 , each bias pad being moveable between a radially inner position and an extended position in which it engages the wall of the formation being drilled to allow the application of a sideways acting load to the drill collar  10  which is used in steering of the drilling system. In one fairly simple form of drilling system, the drill collar  10  carries a motor  28  and a drill bit  30 , and it will be appreciated that the application of a sideways acting load to the drill collar  10  results in the application of a sideways acting load on the drill bit  30  which, in turn, urges the drill bit  30  away from the centreline of the adjacent part of the borehole, thereby promoting the formation of a curve. A steerable drilling system of this type is sometimes referred to as a push the bit drilling system. It will be appreciated, however, that a number of other forms of steerable drilling system are known, and the invention is equally applicable to such drilling systems.  
         [0014]     Each bias pad  12  is moveable between its radially inward and extended positions by means of an associated actuator  14 . The actuator takes the form of a piston  32  slidable within a cylinder  34 , mud under pressure from the interior of the drill collar  10  being supplied to one end of the piston  32  to cause movement of the piston  32  and hence movement of the associated bias pad  12  from its retracted, radially inner position to its extended position. The supply of mud under pressure to the actuators  14  is controlled by a suitable valve arrangement  16  operable under the control of a suitable control arrangement  18 . A range of suitable valve arrangements  16  and control arrangements  18  are known, and so these components will not be described in further detail.  
         [0015]     In accordance with the invention, a separator in the form of a hydrocyclone  20  is provided within the drill collar  10  to allow the separation of relatively dense particles from the mud supply to the actuators  14 . The hydrocyclone  20  includes an inlet  22  through which mud under pressure from the interior of the drill collar  10  enters the hydrocyclone  20 . In the interior of the hydrocyclone  20  is a substantially conical recess (see  FIG. 3 ), the mud being supplied through the inlet  22  generally tangentially into the large diameter part  36  of the conical recess. The centrifugal acceleration imparted into the mud forces the relatively dense particles carried by the mud towards the wall of the conical recess, the particles working their way towards the small diameter end  38  and apex of the conical recess from where they can exit the hydrocyclone  20  through a dirty mud outlet  24 , the mud exiting the hydrocyclone  20  through the dirty mud outlet  24  rejoining the flow of mud within the drill collar  10  towards the drill bit  30 . Mud from which the relatively dense particles have been separated by the hydrocyclone  20  exits the hydrocyclone  20  through a cleaned mud outlet  26  at the large diameter part  36  of the conical recess from where the cleaned mud is supplied to the valve arrangement  16  and actuators  14 .  
         [0016]     As hydrocyclones work by separating relatively dense matter from relatively low density matter, rather than by separating relatively large particles from the flow of mud, it will be appreciated that the use of the hydrocyclone  20  in accordance with the invention removes particles which would otherwise be likely to cause erosion within the valve arrangement  16  and actuators  14 .  
         [0017]     Although not illustrated, a conventional junk basket or filter may also be incorporated into the steerable drilling system in order to remove relatively large dimension particles from the supply of mud to the valve arrangement  16  and actuators  14 .  
         [0018]     It will be appreciated that a range of modifications and alterations may be made to the arrangement described hereinbefore without departing from the scope of the invention.

Summary:
A steerable drilling system comprises a bias unit  10,  the bias unit  10  having at least one actuator  14  operable by pressurised drilling fluid or mud, in use, and a separator  20  arranged to remove particulate matter from the supply of pressurised drilling fluid or mud to the or each actuator  14,  the separator  20  comprising a hydrocyclone  20.