Torque control device for a downhole drilling assembly

1. A torque control device for a downhole drilling assembly, the downhole drilling assembly comprising a drill bit and connected componentry which are adapted to rotate during drilling, the torque control device being adapted for connection to the drill bit, the torque control device including an outer sleeve and an inner shaft, the outer sleeve and the inner shaft being respective parts of the connected componentry which are adapted to rotate during drilling, the outer sleeve being movable longitudinally relative to the inner shaft, the torque control device having a cylinder, a piston located within the cylinder, and a controller to adjust the volume of the cylinder, the volume of the cylinder being adjustable whilst the outer sleeve and the inner shaft are rotating.

2. A torque control device according to claim 1 in which the piston is connected to the inner shaft and the cylinder is provided in the outer sleeve.

3. A torque control device according to claim 1 in which changes in the volume of the cylinder cause the outer sleeve to move longitudinally relative to the inner shaft.

4. A torque control device according to claim 1 in which the controller has a memory in which is stored a threshold value, the controller causing the volume of the cylinder to increase when the threshold value is exceeded.

5. A torque control device according to claim 4 in which the threshold value can be adjusted during use.

6. A torque control device according to claim 4 in which the controller is connected to a sensor adapted to determine the torque in a part of the downhole drilling assembly.

7. A torque control device according to claim 4 in which the controller is connected to an accelerometer adapted to determine the rate of rotation of a part of the downhole drilling assembly.

8. A torque control device according to claim 1 in which the cylinder is filled with drilling fluid in use.

9. A torque control device according to claim 8 in which the torque control device has a through-bore for carrying drilling fluid to the drill bit.

10. A torque control device according to claim 9 in which the through-bore is located within the inner shaft.

11. A torque control device according to claim 9 having a fluid entry port between the through-bore and the cylinder, and an actuating valve controlling the flow of drilling fluid through the fluid entry port.

12. A torque control device according to claim 11 in which the cylinder has an exhaust port permitting the flow of drilling fluid out of the cylinder.

13. A torque control device according to claim 1 in which a return spring is provided which acts to reduce the volume of the cylinder.

14. A torque control device according to claim 1 in which the controller is a rotary valve.

15. A torque control device according to claim 14 in which the inner shaft comprises a first part and a second part, the first part of the inner shaft being rotatable relative to the second part of the inner shaft, the piston being connected to the first part of the inner shaft.

16. A torque control device according to claim 15 in which the rotary valve comprises a fluid entry port in the second part of inner shaft and a conduit through the piston, relative rotation of the piston and the second part of the inner shaft varying the overlap between the fluid entry port and the conduit.

17. A torque control device according to claim 16 including a torsion spring which acts to rotate the piston relative to the second part of the inner shaft whereby to reduce the overlap between the fluid entry port and the conduit.

18. A torque control device according to claim 17 in which the torsion spring also acts to reduce the volume of the cylinder.

19. A torque control device according to claim 15 having stops to limit the rotation of the piston relative to the second part of the inner shaft.

20. 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 outer sleeve being movable longitudinally relative to the inner shaft;

the torque control device having a cylinder, a piston located within the cylinder, and a controller to control the volume of the cylinder, the cylinder being filled with drilling fluid in use;

the torque control device having a through-bore for carrying drilling fluid to the drill bit, a fluid entry port between the through-bore and the cylinder, and an actuating valve controlling the flow of drilling fluid through the fluid entry port, the actuating valve being located within the piston.

21. 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 outer sleeve being movable longitudinally relative to the inner shaft;

the torque control device having a cylinder, a piston located within the cylinder, and a controller to control the volume of the cylinder, the cylinder being filled with drilling fluid in use;

the torque control device having a through-bore for carrying drilling fluid to the drill bit, a fluid entry port between the through-bore and the cylinder, an actuating valve controlling the flow of drilling fluid through the fluid entry port, and an exhaust port permitting the flow of drilling fluid out of the cylinder,

the exhaust port being permanently open, the exhaust port having a smaller cross-sectional area than the fluid entry port.