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1. FIELD OF THE INVENTION 
       [0001]    This invention relates in general to forming a subterranean bore using drilling rig with a top drive, and in particular measuring a torque from the top drive. More specifically, the present invention relates to estimating top drive torque by monitoring strain within linkage elements coupling the top drive to a support. 
       2. DESCRIPTION OF RELATED ART 
       [0002]    The most common way of drilling an oil or gas well involves attaching a drill bit to a string of drill pipe and rotating, the drill pipe to drill the well. A top drive can be used in a drilling rig for handling the string of drill pipe, also referred to as a pipe string, during drilling or casing a wellbore. In some well operations, an engaging apparatus, including an internal or external, pipe gripping mechanism, can be connected below the top drive to grip a joint of casing, so that the engaging apparatus and the joint of casing can be driven axially and/or rotationally by the top drive. In a drilling rig, the top drive can be hung in the mast with the engaging apparatus connected in drive communication and in substantial axial alignment therebelow. The top drive and engaging apparatus are hung in the mast above the well center and define a main axis of the drilling rig that is aligned with well center. The joints of casing, for connection into the casing or liner string, can be supported, for example in a V-door, adjacent the main axis of the drilling rig. For connection into the casing or liner string, the pipe joints can be engaged by an elevator and brought under the drive system for engagement and handling. Generally, the elevator is supported on link arms. 
         [0003]    It is important to know how much torque is being generated by top drive, particularly during make-up of the threaded connections. One method of estimating torque monitors the electrical current or hydraulic power being used by the top drive during pipe make-up. This method is not very accurate. Another method mounts a sub in the drill string between the quill and pipe gripper, the sub having means for measuring torque output from the quill. However, the sub lengthens the distance between the top drive and the lower end of the pipe gripper. 
       SUMMARY OF THE INVENTION 
       [0004]    Disclosed herein is a method and apparatus for estimating top drive torque generated during use. In one example embodiment of a method a top drive assembly is provided that has 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. Torque is delivered to the pipe string by operating the motor and a measurement is made of the strain in the linkage assembly imposed by reacting the torque to the rail position. The torque generated by the top drive assembly is estimated based on the strain measured in the linkage assembly. In an embodiment, the linkage assembly includes a clevis hinge mounted to a housing around the motor, a linkage member pivotingly coupled to the torque restraint, and a pin coupling the clevis hinge to the linkage member. In an example embodiment, the measured strain in the pin results from a bending moment created by the torque. In one example embodiment, the pin is oriented along a line substantially perpendicular to the pipe string. Optionally, the linkage assembly can include additional clevis hinges mounted to the housing along with additional linkage members and additional pins coupling the additional clevis hinges to the additional linkage members. The strain in one or more of the additional pins can also be measured. Alternatively, the pins being measured for strain are generally coaxial to one another and disposed on opposing lateral sides of the torque restraint; the pins being measured for strain may be set a distance apart from one another along a length of the top drive assembly. 
         [0005]    Also disclosed herein is a top drive assembly for use with a drilling rig. In an example embodiment the top drive assembly has 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, and a strain gauge 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 gauge can be used to measure the torque. In one example embodiment, the linkage assembly is made up of a clevis hinge mounted to a housing around the motor, a linkage member with an end pivotingly coupled to the restraint. A pin can be used for coupling the clevis hinge to the end of the linkage member distal from the end coupled to the torque restraint, wherein the gauge is disposed on the pin. Alternatively, the pin is oriented along a line substantially perpendicular to the drill string. In another optional embodiment, the linkage assembly further includes additional clevis hinges mounted to the housing along with additional linkage members 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. Additional gauges may be included on one or more of the additional pins. The pins being measured for strain may be generally coaxial to one another and disposed on opposing lateral sides of the torque restraint and can be set as distance apart from one another along a length of the top drive assembly. 
         [0006]    Yet further described herein is a drilling rig for forming a subterranean borehole. In an example embodiment the drilling rig includes a rail vertically disposed on a mast of the rig, and a top drive coupled to and selectively moveable along the length of the rail. The top drive is made up of 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. The linkage members can be linked to the rail by a torque restraint that is slideable along the length of the rail. In an example embodiment, the linkage members are elongate and define a linkage for articulating the top chive away from the rail. Additional linkage members may be included that are linked together by additional pins. Yet further optionally, additional strain gauges can be provided that are disposed on one of the additional pins. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic partial sectional view illustrating a top drive in a rig forming a borehole. 
           [0008]      FIG. 2  is an overhead view of a top drive. 
           [0009]      FIG. 3  is a perspective view of a linkage for coupling a torque restraint to a top drive. 
           [0010]      FIG. 4  is a partial sectional view of a linkage for coupling a torque restraint to a top drive. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    An example embodiment of a top drive assembly  10  is shown in a side partial sectional view in  FIG. 1 . The top drive assembly  10  of  FIG. 1  is used for rotating a pipe string  12  shown having a hit  14  on its lower end. The pipe string  12  could be drill pipe, which is retrieved after drilling a well. Alternatively, the pipe string  12  could be casing for drilling and casing the well. Also, pipe string  12  could be casing being run into a previously drilled well. Rotating the bit  14  with a sufficient downward force forms a borehole  16  through the formation  18  below the top drive assembly  10 . In the example embodiment of  FIG. 1 , the top drive assembly  10  is mounted within a drilling rig  20  and the pipe string  12  is made of casing being used to drill the well, after which the casing can be cemented into place. As shown, the drilling rig  20  includes a frame  21  made up of support members and a generally vertically oriented rail  22  that is mounted within the frame  21 . A motor  23 , shown in a dashed outline within a housing  27 , mechanically couples to a quill  24  that drives the pipe string  12 . As is known, the rotation of the motor  23  and combined with torsional forces in the pipe string  12  during drilling, exert a resultant torque onto the top drive assembly  10  that is transferred to the drilling rig  20  via coupling between the top drive assembly  10  and the frame  21 . A casing gripper  29  is secured to quill  24  for gripping the pipe string  12 . 
         [0012]    In the example of  FIG. 1 , the top drive assembly  10  is coupled to the drilling rig  20  by a torque restraint  25  that slidingly mounts on the rail  22 ; thus allowing vertical movement of the top drive assembly  10  within the drilling rig  20 . A linkage  26  couples the housing  27  of the top drive assembly  10  to the torque restraint  25 . The linkage  26 , which is made up of elongate members connected with pivoting ends, may allow some articulated movement of the top drive assembly  10  away from die rail  22  for retrieving pipe segments to incorporate into the pipe string  12 . 
         [0013]    Further illustrated in the example of  FIG. 1  is a bail assembly  28  pivotingly mounted to casing gripper  29 . Alternatively, bail assembly  28  could be mounted to housing  27  and have an attached elevator  30 . The elevator  30 , in one embodiment, includes clamps that may be power driven for grappling and retaining pipe segments (not shown) for integration into the pipe string  12 . 
         [0014]    Provided in  FIG. 2  is an overhead embodiment of a top drive assembly  10  and illustrating the rectangular inner periphery of the torque restraint  25  that is configured for mounting around the rail  22  ( FIG. 1 ). Further illustrated in  FIG. 2  are components of the linkage assembly  26 , shown as clevis hinges  34  attached on the side of the torque restraint  25  facing the motor housing  27 . Each clevis hinge  34  is made up of a pair of generally planar vertically oriented members. Edges of the clevis hinges  34  attach to the side of the torque restraint  25  facing the motor housing  27 ; free ends of the clevis hinges  34  located opposite the attached edge have a lateral bore formed therethrough. The members of each clevis hinge  34  define an open space therebetween. Linkage members  36  are shown inserted between the open space, where the linkage members  36  have a corresponding bore registered with the bores through the clevis hinges  34 . 
         [0015]    Pins  38  insert through the registered bores of each clevis hinge  34  and elongate member  36  to pivotingly couple the elongate members  36  to the clevis hinges  34 . The elongate members  36  have, lower ends similarly pivotingly coupled with a portion of the top drive assembly  10  so that the top drive assembly  10  can selectively articulate away from and back towards the torque restraint  25 . Below the clevis hinge  34  is another clevis hinge assembly  40  shown set between the torque restraint  25  and lower housing  27 . Another elongate member  41  is pivotingly coupled on one end to the clevis hinge  40 , and connected on a lower end. (not shown) to the top drive assembly  10  for providing additional linkage connection between the top drive assembly  10  and torque restraint  25 . 
         [0016]    Referring now to  FIG. 3 , a rear perspective view of an example of a top drive assembly  10  is shown in a perspective view. In this example embodiment, the motor housing  27  is disposed proximate to the torque restraint  25 . Also shown in  FIG. 3  is the lower pivoting connection of the elongate member  41 . A pair of clevis hinges  42  are shown mounted on a side surface of the motor housing  27  and set a lateral distance apart on opposing lateral sides of the elongate member  41 . In the example embodiment of  FIG. 3 , the clevis hinges  42  each have an upper portion with an outer side wall that slopes away from the motor housing  27 , defines a peak, then slopes back towards the motor housing  27  and terminates at a location between the upper and lower ends of the clevis hinges  42 . An inner side wall on the clevis hinges  42 , disposed adjacent the elongate member  41 , projects outward and parallel with the outer sidewall, but extends substantially the entire length of each of the clevis hinges  42 . Bores are formed through the inner and outer side walls of the clevis hinges  42  at their respective upper and lower ends. Bores are also provided in the lower ends of the elongate members  36  that register with the bores in the upper ends of the clevis hinges  42 . Pins  44  are inserted through the registered bores in the lower end of linkage member  36  and upper ends of the clevis hinges  42  thereby pivotingly coupling the linkage member  36  with the clevis hinge  42 . 
         [0017]    Because the outer side wall terminates above the lower end of the clevis hinges  42 , the bores in the lower end of the clevis hinges pass only through the inner side wall. The bores in the lower ends of the clevis hinges  42  register with bores formed laterally through lower depending legs  48  shown on the elongate member  41 . Pins  46  project through the registered bores in the respective lower ends of the clevis hinges  42  and the lower depending legs  48  from the elongate member  41 . Thus, strategically providing the bores for insertion of the pins  44 ,  46  enables articulated movement of the main body of the top drive assembly  10  from the torque restraint  25  by pivoting of the linkage members  36 ,  41 . 
         [0018]    Schematically illustrated in  FIG. 4  is a partial sectional view of the coupling between the torque restraint  25  and motor housing  27  by the clevis hinges  42 . In optional embodiments, the coupling of  FIG. 4  represents the connection between the clevis hinges  42  and the members  36  or member  41  respectively by pins  44 ,  46 . Further illustrated in  FIG. 4  are strain gauges  52  for measuring strain within the pins  44 ,  46 . Optional bores  54  are shown through the pins  44 ,  46  through which a counter pin or other lynch type pin (not shown) is inserted to retain the pins  44 ,  46  within the pivoting coupling. The strain gauges  52  measure strain through bending moment in the pins  44 ,  46  and can thereby provide a measurement of torque, represented by the curved arrow T, from the motor  23  ( FIG. 1 ). The use of a strain gauge  52 , rather than the known ways of measuring amperage and/or a torque sub, not only increases accuracy and repeatability, but provides a quicker response so that adjustments in motor controls can be more quickly made during drilling operations. 
         [0019]    To facilitate control of the systems, a processor  56  is shown coupled with the strain gauge  52  by a communication link  58 , such as a hard wire or telemetry communication. In an example embodiment, the processor  56  receives a signal from the strain gauge  52  via the communication link  58  and converts the signal into a correlative torque value. The signal typically is a voltage that changes in response to the strain imposed on the strain gauges  52 . Optionally, the processor  56  sends a command to adjust operation of the motor  23  based on the signal received from the communication link  58  and/or the converted torque value. The command can be transmitted directly to the motor  23  or to an optional motor controller (not shown). 
         [0020]    While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited thus susceptible to various changes without departing from the scope of the invention.

Summary:
A top drive assembly that includes a gauge for measuring strain in a linkage coupling the top drive to a drilling rig frame. The strain measuring gauge, which can be a strain gauge, is disposed on a pin that pivotingly links members of the linkage coupling. When a motor in the top drive assembly operates to rotate an associated pipe string, the torque generated by the motor can be estimated by monitoring strain measured in the pin.