Method and apparatus for reducing stick-slip

1. A method of damping stick-slip oscillations in a drill string, the method comprising:

damping said stick-slip oscillations, for every length of said drill string, using a drilling mechanism at a top of said drill string, wherein said stick-slip oscillations comprise torsional waves propagating along said drill string; and

controlling speed of rotation of said drilling mechanism using a PI controller;

characterised by

tuning said PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is approximately at a frequency of said stick-slip oscillations;

wherein the tuning comprises a step of calculating an I-term of said PI controller which uses a value of an estimated period of said stick slip oscillations and a value of an effective inertia of said drilling mechanism, but which step of calculating the I-term does not use a value of the length of said drill string and a value of a speed of the torsional waves, whereby said drilling mechanism has a frequency dependent reflection coefficient of said torsional waves, which reflection coefficient is substantially at a minimum at or near said frequency of stick-slip oscillations.

2. A method according to claim 1 , further comprising adjusting said I-term according to I=w s 2 J where w s is an estimated angular frequency of said stick-slip oscillations and J is the effective inertia of said drilling mechanism.

3. A method according to claim 1 , further comprising measuring said estimated period of stick-slip oscillations for use in adjusting said I-term.

4. A method according to claim 1 , further comprising adjusting a P-term of said PI controller to be a same order of magnitude as the characteristic impedance ζ of said drillstring.

5. A method according to claim 4 , further comprising adjusting said P-term such that said reflection coefficient does not vanish whereby a fundamental mode of said stick slip oscillations is inhibited from splitting into two new modes with different frequencies.

6. A method according to claim 4 , further comprising adjusting said P-term as P=ζ/a where a is a mobility factor that permits adjustment of said P-term during drilling, whereby energy absorption of said stick-slip oscillations by said drilling mechanism is increased or reduced.

7. A method according to claim 6 , further comprising increasing said mobility factor based on the magnitude of said stick-slip oscillations being not reduced.

8. A method according to claim 6 , further comprising reducing said mobility factor once the magnitude of said stick-slip oscillations has been reduced, whereby drilling efficiency is increased without re-appearance or increase in magnitude of said stick-slip oscillations.

9. A method according to claim 1 , wherein said PI controller is separate from a drilling mechanism speed controller, the method further comprising bypassing said drilling mechanism speed controller with said PI controller during damping of said stick-slip oscillations.

10. A method according to claim 1 , wherein said drilling mechanism comprises said PI controller, the method further comprising tuning said PI controller when said stick-slip oscillations occur, and leaving said PI controller untuned otherwise.

11. A method according to claim 1 , further comprising estimating an instantaneous rotational speed of a bottom hole assembly at the lower end of said drill string by combining a known torsional compliance of said drill string with variations in a drive torque of said drilling mechanism.

12. A method according to claim 11 , wherein said variations in drive torque are expressed only at a fundamental frequency of said stick-slip oscillations, whereby said estimating is implemented by a PLC and performed in real time.

13. A method according to claim 11 , wherein said estimating comprises band pass filtering a drive torque signal with a band pass filter centred on an estimated frequency of said stick-slip oscillations.

14. A method according to claim 13 , wherein said estimating of instantaneous rotational speed comprises determining a downhole speed using a total static drill string compliance and a phase parameter, and determining a sum of (i) a low pass filtered signal representing a speed of rotation of said drilling mechanism and (ii) said downhole speed.

15. A method according to claim 11 , further comprising determining said estimate periodically and outputting said estimate on a driller's console whereby a driller is provided with a real-time estimate of the instantaneous rotational speed of said bottom hole assembly.

16. A method according to claim 11 , further comprising determining a stick-slip severity as the ratio of dynamic downhole speed amplitude over the mean rotational speed of said drilling mechanism, which stick-slip severity is useable to provide an output signal indicating severity of stick-slip at that point in time.

17. A method according to claim 1 , wherein length of the drill string is up to 5000 meters.

18. A method of drilling a borehole, the method comprising:

rotating a drill string with a drilling mechanism so as to rotate a drill bit at a lower end of said drill string; and

in response to detection of stick-slip oscillations of said drill string, wherein said stick- slip oscillations comprise torsional waves propagating along said drill string, damping said stick- slip oscillations, for every length of said drill string, using a PI controller to control said drilling mechanism, which PI controller has been tuned so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is approximately at a frequency of said stick-slip oscillations; and

calculating an I-term of said PI controller using a value of estimated period of said stick slip oscillations and on a value of effective inertia of said drilling mechanism, wherein the calculating the I-term does not use a value of the length of said drill string and a value of a speed of the torsional waves, whereby said drilling mechanism has a frequency dependent reflection coefficient of said torsional waves, which reflection coefficient is substantially at a minimum at or near said frequency of stick-slip oscillations.

19. A drilling mechanism for use in drilling a borehole, the drilling mechanism comprising:

an electronic controller having:

a PI controller and

memory storing computer executable instructions that when executed cause said electronic controller to:

damp stick-slip oscillations, for every length of a drill string, using said drilling mechanism at a top of said drill string, wherein said stick- slip oscillations comprise torsional waves propagating along said drill string;

control speed of rotation of said drilling mechanism using said PI controller and to:

tune said PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is approximately at a frequency of said stick-slip oscillations; and

calculate an I-term of said PI controller using a value of an estimated period of said stick slip oscillations and a value of an effective inertia of said drilling mechanism, but not using a value of the length of said drill string and a value of a speed of the torsional waves in the calculation of the I-term, whereby said drilling mechanism has a frequency dependent reflection coefficient of said torsional waves, which reflection coefficient is substantially at a minimum at or near said frequency of stick-slip oscillations.

20. An electronic controller for use with a drilling mechanism for drilling a borehole, the electronic controller comprising:

a PI controller; and

memory storing computer executable instructions that when executed cause said electronic controller to:

damp stick-slip oscillations, for every length of a drill string, using said drilling mechanism at a top of said drill string, wherein said stick-slip oscillations comprise torsional waves propagating along said drill string;

control speed of rotation of said drilling mechanism using said PI controller; and to:

tune said PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is approximately a frequency of said stick-slip oscillations; and

calculate an I-term of said PI controller using a value of an estimated period of said stick slip oscillations and a value of an effective inertia of said drilling mechanism, but not using a value of the length of said drill string and a value of a speed of the torsional waves in the calculation of the I- term, whereby said drilling mechanism has a frequency dependent reflection coefficient of said torsional waves, which reflection coefficient is substantially at a minimum at or near said frequency of stick-slip oscillations.

21. A method of upgrading a drilling mechanism, the method comprising:

uploading computer executable instructions to an electronic controller to be used on a drilling rig, which electronic controller is for controlling operation of said drilling mechanism;

wherein said computer executable instructions comprise instructions for:

damping stick-slip oscillations, for every length of a drill string, using said drilling mechanism at a top of said drill string;

controlling speed of rotation of said drilling mechanism using a PI controller; and

tuning said PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is approximately a frequency of said stick-slip oscillations;

wherein said stick-slip oscillations comprise torsional waves propagating along said drill string, and wherein the tuning comprises a step of calculating an I-term of said PI controller which uses a value of an estimated period of said stick slip oscillations and a value of an effective inertia of said drilling mechanism, but which step of calculating the I-term does not used a value of the length of said drill string and a value of a speed of the torsional waves, whereby said drilling mechanism has a frequency dependent reflection coefficient of said torsional waves, which reflection coefficient is substantially at a minimum at or near said frequency of stick- slip oscillations.

22. The drilling mechanism of claim 19 , wherein length of the drill string is up to 5000 meters.