Resonance enhanced rotary drilling

1. A method for controlling a resonance enhanced rotary drill comprising a rotary drill bit and an oscillator for applying axial oscillatory loading to the rotary drill bit, the method comprising:

controlling frequency (f) of the oscillator in the resonance enhanced rotary drill whereby the frequency (f) is maintained in the range

( D 2 U s /(8000 πAm)) 1/2 ≦f≦S f ( D 2 U s /(8000 πAm)) 1/2

where D is diameter of the rotary drill bit, U s is compressive strength of material being drilled, A is amplitude of vibration, m is vibrating mass, and S f is a scaling factor greater than 1; and

controlling dynamic force (F d ) of the oscillator in the resonance enhanced rotary drill whereby the dynamic force (F d ) is maintained in the range

[(π/4) D 2 eff U s ]≦F d ≦S Fd [(π/4) D 2 eff U s ]

where D eff is an effective diameter of the rotary drill bit, U s is a compressive strength of material being drilled, and S Fd is a scaling factor greater than 1,

wherein the frequency (f) and the dynamic force (F d ) of the oscillator are controlled by monitoring signals representing the compressive strength (U s ) of the material being drilled and adjusting the frequency (f) and the dynamic force (F d ) of the oscillator using a closed loop real-time feedback mechanism according to changes in the compressive strength (U s ) of the material being drilled.

2. A method according to claim 1 , wherein S f is less than 5.

3. A method according to claim 1 , wherein SF Fd is less than 5.

4. A method according to claim 1 , wherein S f is selected whereby

f≦f r

where f r is a frequency corresponding to peak resonance conditions for the material being drilled.

5. A method according to claim 4 , wherein S f is selected whereby

f ≦( f r −X )

where X is a safety factor ensuring that the frequency (f) does not exceed that of peak resonance conditions at a transition between two different materials being drilled.

6. A method according to claim 5 , wherein X>f r/ 100.

7. A method according to claim 5 , wherein one or both of X and Y are adjustable according to predicted variations in the compressive strength (U s ) of the material being drilled and speed with which the frequency (f) and dynamic force (F d ) can be changed when a change in the compressive strength (U s ) of the material being drilled is detected.

8. A method according to claim 5 , wherein X>f r/ 50.

9. A method according to claim 5 , wherein X>f r/ 10.

10. A method according to claim 1 , wherein

F d ≦S Fd [(π/4) D 2 eff U s −Y]

where Y is a safety factor ensuring that the dynamic force (F d ) does not exceed a limit causing catastrophic extension of cracks at a transition between two different materials being drilled.

11. A method according to claim 10 , wherein Y>S Fd [(π/4)D2 eff U s]/ 100.

12. A method according to claim 10 , wherein Y>S Fd [(π/4)D 2 eff U s]/ 50.

13. A method according to claim 10 , wherein Y>S Fd [(π/4)D 2 eff U s ]/ 10.

14. A method according to claim 1 , wherein the frequency (f) of the oscillator is controlled to be maintained in the range 100 to 500 Hz.

15. A method according to claim 1 , wherein the dynamic force (F d ) is controlled to be maintained within the range 20 to 1000 kN.

16. A method according to claim 1 , wherein the method further comprises controlling the amplitude of vibration of the oscillator to be maintained within the range 0.5 to 10 mm.

17. A method according to claim 1 , wherein power is supplied to the oscillator from a mechanism which drives rotary motion of the drill bit.

18. A method according to claim 1 , wherein the oscillator has a power consumption in the range 5 to 200 kW.

19. A method according to claim 1 , wherein S f is less than 2.

20. A method according to claim 1 , wherein S f is less than 1.5.

21. A method according to claim 1 , wherein S f is less than 1.2.

22. A method according to claim 1 , wherein SR Fd is less than 2.

23. A method according to claim 1 , wherein S Fd is less than 1.5.

24. A method according to claim 1 , wherein S Fd is less than 1.2.

25. A method according to claim 1 , wherein the dynamic force (F d ) is controlled to be maintained within the range 40 to 500 kN.

26. A method according to claim 1 , wherein the dynamic force (F d ) is controlled to be maintained within the range 50 to 300 kN.

27. A method according to claim 1 , wherein the method further comprises controlling the amplitude of vibration of the oscillator to be maintained within the range 1 to 5 mm.

28. A method according to claim 1 , wherein the oscillator has a power consumption in the range 5 to 150 kW.

29. A method according to claim 1 , wherein the oscillator has a power consumption in the range 5 to 100 kW.

30. A method according to claim 1 , wherein the oscillator has a power consumption in the range 5 to 50 kW.

31. An apparatus comprising a controller configured to perform the method of claim 1 .

32. An apparatus according to claim 31 , wherein the apparatus further comprises:

an oscillator for applying axial oscillatory loading to a rotary drill bit; and one or more sensors,

wherein the controller is configured to receive signals from the one or more sensors representing the compressive strength (U s ) of the material being drilled and adjust the frequency (f) and the dynamic force (F d ) of the oscillator using a closed loop real-time feedback mechanism according to changes in the compressive strength (U s ) of the material being drilled.

33. An apparatus according to claim 32 , wherein the oscillator comprises a piezoelectric actuator with mechanic amplification, a magnetostrictive actuator, a pneumatic actuator, or an electrically driven mechanical actuator.

34. An apparatus according to claim 32 , further comprising a vibration isolation unit which is couplable to a downhole end of a drill string whereby the apparatus is operable under downhole closed loop real-time control.