US 5,386,724 A · Load cells for sensing weight and torque on a drill bit while drilling a well bore

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.

Claims36

Dependent claims

A load cell for sensing forces on a drill bit during the drilling of a well bore with a drill collar connected in a drill string above the bit, the drill collar having a radial pocket therein for receiving said load cell, said load cell comprising:

a) a substantially solid cylinder having a diameter such that said cylinder fits snugly in a radial pocket in a drill collar;
b) first, second, third, and fourth force sensors mounted on said solid cylinder at ninety degree intervals, said sensors each providing an electrical resistance which varies according to a magnitude of force applied, wherein
said first and third sensors are electrically coupled to each other to form a first sensor pair, and said second and fourth sensors are electrically coupled to each other to form a second sensor pair, each of said first and second sensor pairs having an electrical resistance which is the sum of the electrical resistance of each said sensor in said pair and said sensor pairs being electrically coupled to each other such that the electrical resistance of said second sensor pair is subtracted from the electrical resistance of said first sensor pair to provide a first output for said load cell.

An apparatus for sensing forces on a drill bit during the drilling of a well bore, said apparatus comprising:

a) a drill collar having a vertical axis and including means for connecting to a drill string above the bit, said drill collar having a pair of diametrically opposite radial pockets;
b) a pair of load cells, each load cell including,
i) a cylinder having a diameter such that said cylinder fits snugly in one of the radial pockets of the drill collar,
ii) first, second, third, and fourth force sensors mounted on said cylinder at ninety degree intervals, said sensors each providing an electrical resistance which varies according to a magnitude of force applied, wherein for each load cell said first and third sensors are electrically coupled to each other to form a first sensor pair, and said second and fourth sensors are electrically coupled to each other to form a second sensor pair, each of said first and second sensor pairs having an electrical resistance which is the sum of said electrical resistance of each said sensor in said pair;
and
c) electrical coupling means for coupling said first and second sensor pairs from both of said load cells to a source of voltage and to a voltage measuring means.

3 more elements

a) a substantially solid cylinder of high strength homogenous material having low residual stress, said substantially solid cylinder having a diameter such that said cylinder fits snugly in a radial pocket in a drill collar;
and
b) at least four force sensors mounted on and spaced around said cylinder, each force sensor providing an electrical resistance which varies according to a magnitude of force applied thereto.

Dependencies are inferred from the claim text.

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

Life of this patent

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

Life of this patent

Abstract

Claims36