Methods and apparatus for reducing stick-slip

Patent metadata

Field	Value
Publication number	US-9885231-B2
Application number	US-201514589349-A
Country	US
Kind code	B2
Filing date	Jan 5, 2015
Priority date	Dec 2, 2008
Publication date	Feb 6, 2018
Grant date	Feb 6, 2018

Abstract

A method and apparatus for estimating the instantaneous rotational speed of a bottom hole assembly at the lower end of a drill string. In one embodiment, a method includes driving the drill string by a drilling mechanism at the upper end of the drill string. A fundamental frequency of stick-slip oscillations suffered by the drill string is estimated. Variations in a drive torque of the drilling mechanism are determined. Known torsional compliance of the drill string is combined with the variations in the drive torque. An output signal representing the instantaneous rotational speed is provided.

First claim

The invention claimed is:

1. A method of controlling a drilling mechanism for use in a drilling bore using an electronic controller, the method comprising:
rotating a drill string in the drilling bore;
damping stick-slip oscillations using a drilling mechanism at a top of said drill string;
controlling the speed of rotation of said drilling mechanism using a PI controller;
tuning said PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is at or near a fundamental frequency of said stick-slip oscillations; and
increasing a damping effect of said drilling mechanism for frequencies above said fundamental frequency by reducing an effective inertia of said drilling mechanism.

2. The method of claim 1 , wherein said stick-slip oscillations comprise torsional waves propagating along said drill string, and wherein the tuning comprises adjusting an I-term of said PI controller to be dependent on an approximate period of said fundamental frequency of said stick slip oscillations and on an effective inertia of said drilling mechanism, 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 fundamental frequency of stick-slip oscillations.

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

4. The method of claim 2 , wherein said effective inertia comprises total mechanical inertia of said drilling mechanism at an output shaft thereof.

5. The method of claim 1 , wherein reducing said effective inertia comprises tuning said PI controller with an additional torque term that is proportional to angular acceleration of said drilling mechanism.

6. The method of claim 5 , further comprising multiplying said angular acceleration by a compensation inertia (J c ), which compensation inertia (J c ) is adjustable so as to control an amount of the reduction of the effective inertia of said drilling mechanism.

7. The method of claim 6 , wherein said compensation inertia (J c ) reduces said effective inertia by between 0% and 80%.

8. The method of claim 5 , further comprising adjusting an I-term of said PI controller according to I=ω s 2 J, where ω s , is an approximate or estimated angular frequency of said stick-slip oscillations and J is the reduced effective inertia value of said drilling mechanism.

9. The method of claim 1 , wherein said drilling mechanism has a torsional energy absorption bandwidth for stick-slip oscillations, size of said bandwidth obtainable from its full width half maximum, whereby upon reducing the effective inertia of said drilling mechanism the size of said full width half maximum is greater.

10. The method of claim 1 , wherein said drilling mechanism has a frequency dependent damping curve having a point of maximum damping, the method further comprising shifting said point of maximum damping to higher frequencies whereby the damping effect of said drilling mechanism on at least some higher frequencies is increased and damping of said fundamental frequency is reduced.

11. The method of claim 10 , wherein said shifting comprises determining an I-term of said PI controller as I=ω s 2 J , in which a period value ω s , is greater than an approximate period of said fundamental frequency, whereby said frequency dependent damping curve is shifted toward higher frequencies and damping of at least one higher mode of oscillation is increased above the amount of damping obtainable when using said approximate period to determine said I-term.

12. The method of claim 11 , wherein said period value is greater by up to 40% from said approximate period.

13. The method of claim 11 , further comprising further reducing said effective inertia of said drilling mechanism when performing said shifting, whereby narrowing of an absorption bandwidth of said damping curve is inhibited.

14. The method according claim 13 , further comprising reducing said effective inertia and increasing said period value by a same factor.

15. The method of claim 1 , wherein the reducing said effective inertia comprises changing into a higher gear of said drilling mechanism.

16. The method of claim 1 , further comprising monitoring said drilling mechanism for occurrence of one or more higher mode of oscillation, and when detected, performing the method of claim 1 .

17. The method of claim 1 , further comprising:
monitoring a period of said fundamental frequency,
comparing said period against a period threshold and,
if said period exceeds said period threshold, reducing said effective inertia as said period increases.

18. The method of claim 1 , wherein said PI controller comprises a PID controller, and the reducing said effective inertia comprises using a negative D-term in said PID controller.

19. A method of controlling a drilling mechanism for use in a drilling bore using an electronic controller, 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 whereby the earth's surface is penetrated; and
in response to detection of stick-slip oscillations of said drill string:
damping said stick-slip oscillations using said drilling mechanism at a top of said drill string;
tuning a PI controller so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is at or near a fundamental frequency of said stick-slip oscillations; and
reducing an effective inertia of said drilling mechanism, whereby a damping effect of said drilling mechanism is increased for frequencies above said fundamental frequency.

20. A drilling control mechanism for use in drilling a borehole, the drilling control mechanism comprising:
an electronic controller comprising:
memory storing computer executable instructions that when executed: provide a PI controller, and cause the PI controller to:
control a speed of rotation of a drilling mechanism to dampen stick-slip oscillations in said drill string, said PI controller tuned so that said drilling mechanism absorbs most torsional energy from said drill string at a frequency that is at or near a fundamental frequency of said stick-slip oscillations, and
wherein said electronic controller is configured to reduce an effective inertia of said drilling mechanism, whereby a damping effect of said drilling mechanism is increased for frequencies above said fundamental frequency.

21. The drilling control mechanism of claim 20 , wherein said PI controller comprises a PID controller, and said PID controller is adapted to implement a term

J
c

⁢

d
⁢

⁢
Ω

dt

,

where J c is a user or computer adjustable inertia compensation value, and Ω is measured angular speed of the drilling mechanism.

22. The drilling control mechanis

Assignees

• Nat Oilwell Varco Lp

Inventors

• Nessjoen Pal Jacob
• Kyllingstad Age

Classifications

• G05B15/02

  electric · CPC title

• E21B44/00 Primary

  Automatic control systems specially adapted for drilling operations, i.e. self-operating systems which function to carry out or modify a drilling operation without intervention of a human operator, e.g. computer-controlled drilling systems; Systems specially adapted for monitoring a plurality of drilling variables or conditions · CPC title

• G05D19/02

  characterised by the use of electric means · CPC title

• E21B33/129

  with mechanical slips for hooking into the casing (E21B33/122, E21B33/124 take precedence) · CPC title

• E21B17/07 Primary

  Telescoping joints for varying drill string lengths; Shock absorbers · CPC title