Top drive output torque measurement method

1. A method of estimating torque generated by a top drive assembly while forming a wellbore, comprising:

a. providing a top drive assembly having a motor for rotating a pipe, a torque restraint engageable with a rail portion of a drilling rig, and a linkage assembly coupled between the torque restraint and motor, wherein the linkage assembly comprises:

i. a clevis hinge mounted to a housing around the motor;

ii. a linkage member pivotingly coupled to the torque restraint; and

iii. a pin coupling the clevis hinge to the linkage member;

b. delivering torque to the pipe string by operating the motor;

c. measuring strain in the pin imposed by reacting the torque to the rail portion, and wherein the strain in the pin is due to a bending, moment created by the torque; and

d. estimating the torque generated by the top drive assembly based on the strain measured in the pin.

2. The method of claim 1 , wherein the pin is oriented along a line substantially perpendicular to the pipe string.

3. The method of claim 1 , wherein the linkage assembly further comprises additional clevis hinges mounted to the housing, additional linkage members, each with an end pivotingly coupled to the torque restraint, and additional pins coupling the additional clevis hinges to the additional linkage members.

4. The method of claim 3 , further comprising measuring strain in at least one of the additional pins.

5. The method of claim 4 , wherein the pins being measured for strain are generally coaxial to one another and disposed on opposing lateral sides of the torque restraint.

6. The method of claim 4 , wherein the pins being measured for strain are set a distance apart from one another along a length of the top drive assembly.

7. A top drive assembly for use with a drilling rig comprising:

a motor selectively connectable to a pipe string;

a restraint slideable along a rail extending vertically along a mast of the drilling rig;

a linkage assembly coupled between the restraint and motor, the linkage comprising a clevis hinge mounted to a housing around the motor, a linkage member with an end pivotingly coupled to the restraint, and a pin coupling the clevis hinge to the end the linkage member distal from the end coupled to the torque restraint; and

a strain gauze disposed on the pin for measuring strain in the linkage assembly, so that when the torque generated by the motor is transferred to the restraint and the rail through the linkage assembly, the strain gauge can be used to measure the torque.

8. The top drive of claim 7 , wherein the pin is oriented along a line substantially perpendicular to the drill string.

9. The top drive of claim 7 , wherein the linkage assembly further comprises additional clevis hinges mounted to the housing, additional linkage members, each with an end pivotingly coupled to the torque restraint, and additional pins coupling the additional clevis hinges to the ends of the additional linkage members distal from the ends coupled to the torque restraint.

10. The top drive of claim 9 , further comprising additional gauges on at least one of the additional pins.

11. The top drive of claim 10 , wherein the pins being measured for strain are generally coaxial to one another and disposed on opposing lateral sides of the torque restraint.

12. The top drive of claim 10 , wherein the pins being measured for strain are set a distance apart from one another along a length of the top drive assembly.

13. A drilling rig for forming a subterranean borehole comprising:

a rail vertically disposed on a mast of the rig;

a top drive coupled to and selectively moveable along the length of the rail and comprising:

a motor for rotating a pipe string,

linkage members linked between the top drive and rail,

pins in opposing ends of the linkage members, so that torque generated by the motor is transferred through the linkage members and the pins to the rail; and

a strain gauge set on at least one of the pins for measuring strain in the pin for estimating a torque generated by the motor.

14. The drilling rig of claim 13 , wherein the linkage members are linked to the rail by a torque restraint that is slideable along the length of the rail, and the linkage members are elongate and define a linkage for articulating the top drive away from the rail.

15. The drilling rig of claim 13 , further comprising additional linkage members linked together by additional pins.

16. The drilling rig of claim 15 , further comprising another strain gauge on one of the additional pins.