Method and apparatus for automated drilling rig sheave friction calibration

1. A method for monitoring hookload in a well drilling system, the method comprising:

measuring raw hookload;

determining a velocity of a travelling block;

holding the travelling block in a non-lowering state for a first period of time;

lowering the travelling block for a second period of time;

calculating contribution to hookload of a component linear in the block velocity;

calculating contribution to hookload of a component linear in a direction (sign) in the block velocity; and

correcting measured raw hookload by the calculated contributions.

2. A method for monitoring hookload in a well drilling system as claimed in claim 1 , wherein the first period of time is between about 1 second and about 120 seconds in length, and wherein the second period of time is between about 1 second and about 20 seconds in length.

3. A method for monitoring hookload in a well drilling system as claimed in claim 1 , wherein the calculating contribution to hookload of a component linear in the block velocity and the calculating contribution to hookload of a component linear in a direction in the block velocity comprise using linear regression.

4. A method for monitoring hookload in a well drilling system as claimed in claim 1 , wherein the correcting measured raw hookload by the calculated contributions comprises subtracting the calculated contributions from the measured raw hookload.

5. A method for monitoring hookload in a well drilling system as claimed in claim 1 , further comprises preventing the weigh-on-bit from exceeding a preset limit during said lowering the travelling block for a second period of time.

6. A method for monitoring hookload in a well drilling system as claimed in claim 1 , further comprises preventing the weigh-on-bit from decreasing below a preset limit during said holding the travelling block in a steady position for a first period of time.

7. A method for monitoring hookload in a well drilling system as claimed in claim 1 , further comprising repeating the steps of holding the travelling block and lowering the travelling block during a calibration period.

8. A method for monitoring hookload in a well drilling system as claimed in claim 7 , wherein the first and second periods of time are different when the steps of holding the travelling block and lowering the travelling block are repeated.

9. A method for monitoring hookload in a well drilling system as claimed in claim 7 , wherein the repeated first and second periods of time each comprise a mixture of full amplitude fast and slow block movements, together with some shorter movements, whereby a variety of travelling block velocity profiles are used when the steps of holding the travelling block and lowering the travelling block are repeated.

10. A method for monitoring hookload in a well drilling system as claimed in claim 7 , wherein the calibration period is between about 5 minutes and about 45 minutes in length.

11. An autodriller for monitoring hookload in a well drilling system, the autodriller comprising:

a hookload receptor that receives a signal corresponding to raw hookload;

a travelling block receptor that receives a signal corresponding to travelling block velocity;

a processor;

a non-transitory storage medium; and

a set of computer readable instructions stored in the non-transitory storage medium, wherein when the instructions are executed by the processor allow the autodriller to:

command the travelling block to hold not lower for a first period of time;

command the travelling block to lower for a second period of time;

calculate contribution to hookload of a component linear in the block velocity;

calculate contribution to hookload of a component linear in a direction in the block velocity; and

correct measured raw hookload by the calculated contributions.

12. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein the first period of time is between about 1 second and about 120 seconds in length, and wherein the second period of time is between about 1 second and about 20 seconds in length.

13. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein the calculating contribution to hookload of a component linear in the block velocity and the calculating contribution to hookload of a component linear in a direction (sign) in the block velocity comprise using linear regression.

14. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein the correcting measured raw hookload by the calculated contributions comprises subtracting the calculated contributions from the measured raw hookload.

15. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein when the instructions are executed by the processor the instructions further allow the autodriller to prevent the weigh-on-bit from exceeding a preset limit while the travelling block is commanded to lower for the second period of time.

16. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein when the instructions are executed by the processor the instructions further allow the autodriller to prevent the weigh-on-bit from decreasing below a preset limit while the travelling block is commanded to hold a steady position for the first period of time.

17. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 11 , wherein when the instructions are executed by the processor the instructions further allow the autodriller to repeatedly command the travelling block to hold in a steady position and repeatedly command the travelling block to lower during a calibration period.

18. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 17 , wherein the first and second periods of time are different when the command of the travelling block to hold in a steady position for a first period of time is repeated and the command of the travelling block to lower for a second period of time is repeated.

19. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 17 , wherein the repeated first and second periods of time each comprise a mixture of full amplitude fast and slow block movements, together with some shorter movements, whereby a variety of travelling block velocity profiles are used when the steps of holding the travelling block and lowering the travelling block are repeated.

20. An autodriller for monitoring hookload in a well drilling system, as claimed in claim 17 , wherein the calibration period is between about 5 minutes and about 45 minutes in length.