Load cells for sensing weight and torque on a drill bit while drilling a

Abstract

A load cell for sensing weight and torque on a drill bit during a drilling operation includes a cylindrical load cell made from a material with minimum residual stress. A plurality of strain gages are arranged symmetrically on each load cell and electrically coupled to form two Wheatstone bridges. The load cells are mounted 180 degrees apart in radial pockets drilled in a drill collar wall. A half Wheatstone bridge from one load cell is electrically coupled with a half Wheatstone bridge from the other load cell to form a Wheatstone bridge for measuring WOB and a similar coupling is made to form another Wheatstone bridge for measuring TOB. In another embodiment, additional strain gages in another Wheatstone bridge measure differential pressure. The output of the pressure and weight bridges are electrically combined to provide a weight signal unaffected by differential pressure fluctuations. The load cells are force fit in the radial pockets in the drill collar and a cap is installed on each pocket housing a load cell to protect the load cell from drilling fluid. The strain gages in each load cell are placed to minimize the effects of bending, hydrostatic pressure, temperature, differential pressure and cross talk between WOB and TOB measurements. The load cells can be replaced easily and drill collar gauging is not required. Since the strain gages are not directly attached to the drill collar, the high residual stress in the drill collar does not cause the strain gages to drift.