Steerable drilling system

1. A steerable drilling system comprising a fluid powered motor having a rotor and a stator within a housing, the stator being mounted on a drillstring so as to be rotatable therewith, and a bias arrangement having plurality of bias pads connected to the stator so as to be rotatable therewith, the bias pads being moveable by a pressure differential between drilling fluid upstream of the fluid powered motor and drilling fluid in an annulus outside of the housing and downstream of the fluid powered motor to allow the application of a side load to the steerable system.

2. A system according to claim 1 , wherein each bias pad is moveable by an actuator.

3. A system according to claim 2 , wherein each actuator comprises a piston to which fluid can be supplied to move the associated bias pad from its retracted position towards its extended position.

4. A system according to claim 3 , further comprising a control arrangement for controlling the operation of the actuators.

5. A system according to claim 4 , wherein the control arrangement includes a plurality of solenoid actuated valves.

6. A system according to claim 5 , wherein each solenoid actuated valve includes a bistable actuator.

7. A system according to claim 4 , wherein the control arrangement includes a control unit, the motor is used to drive a drill bit, and the motor is located between the drill bit and at least part of the control unit.

8. A system according to claim 7 , wherein the control unit includes at least one sensor arranged to sense a drilling parameter.

9. A system according to claim 1 , wherein the pads of the bias arrangement are mounted upon a separate housing rotatable with the stator.

10. A system according to claim 9 , wherein the separate housing is connected to the stator by a flexible drive connection to transmit rotary motion of the stator to the separate housing, but to allow the separate housing to be angularly displaced relative to the axis of the stator.