Gyroscopic survey instrument and method

A gyrocompass platform for surveying subsurface wells, comprising:

• a frame rotatably mounted in a first gimbal, the first gimbal disposed in a housing adapted to traverse a subsurface well, an axis of rotation of the frame being parallel to a longitudinal axis of the housing;
• a motor for rotating the frame with respect to the housing;
• at least a first rocker gimbal rotatably mounted in the frame;
• a motor for rotating the at least a first rocker gimbal with respect to the frame;
• a second rocker gimbal rotatably mounted in the frame;
• a motor for rotating the second rocker gimbal with respect to the frame;
• gyroscopic sensors (gyros) mounted to the frame, to the at least a first rocker gimbal and to the second rocker gimbal, the gyros arranged to have input axes along each of three orthogonal directions, the frame being rotatable and the at least a first rocker gimbal being rotatable with reference to the frame to reorient the gyro input axes along opposed directions to the three orthogonal directions wherein a different gyro input axis is oriented along each opposed direction than a one of the gyros oriented along each orthogonal direction prior to rotation of the frame, the at least a first rocker gimbal and the second rocker gimbal;
• wherein the gyros comprise three at least single input axis gyros, a first of the at least single input axis gyros mounted to the frame, a second of the at least single input axis gyros mounted to the at least a first rocker gimbal and a third of the at least single input axis gyros mounted to a second rocker gimbal mounted in the frame and having a motor for rotating the second rocker gimbal, the at least a first and the second rocker gimbals each having a rotation axis in a plane of the frame and at an angle of 45 degrees with respect to the axis of rotation of the frame;
• and
• wherein the at least a first rocker gimbal and the second rocker gimbal are rotatable between a first position in a plane of the frame and a second position 180 degrees apart from the first position.

A gyrocompass platform for surveying subsurface wells, comprising:

• a frame rotatably mounted in a first gimbal, the first gimbal disposed in a housing adapted to traverse a subsurface well, an axis of rotation of the frame being parallel to a longitudinal axis of the housing;
• a motor for rotating the frame with respect to the housing;
• at least a first rocker gimbal rotatably mounted in the frame;
• a motor for rotating the at least a first rocker gimbal with respect to the frame;
• a second rocker gimbal rotatably mounted in the frame;
• a motor for rotating the second rocker gimbal with respect to the frame;
• gyroscopic sensors (gyros) mounted to any two of the frame, the at least a first rocker gimbal and the second rocker gimbal, the gyros arranged to have input axes along each of three orthogonal directions, the frame being rotatable and the at least a first rocker gimbal being rotatable with reference to the frame to reorient the gyro input axes along opposed directions to the three orthogonal directions wherein a different gyro input axis is oriented along each opposed direction than a one of the gyros oriented along each orthogonal direction prior to rotation of the frame, the at least a first rocker gimbal and the second rocker gimbal;
• and
• wherein an axis of rotation each of the at least a first rocker gimbal and the second rocker gimbal are oriented at an angle of 45 degrees to the axis of rotation of the frame and wherein the at least a first rocker gimbal and the second rocker gimbal are rotatable between a first position in a plane of the frame and a second position 180 degrees apart from the first position.

A method for gyrocompass surveying a subsurface well, comprising:

• moving a gyrocompass sensor to a survey position along the well, the gyrocompass sensor comprising gyroscopic sensors (gyros) having input axes along three mutually orthogonal directions and accelerometers having input axes along the three mutually orthogonal directions;
• measuring output of the gyros and output of the accelerometers;
• rotating the gyros such that the input axes of the gyros are oriented along opposed directions to the three mutually orthogonal directions wherein a different gyro input axis is oriented along each opposed direction than the gyro input axis oriented along each of the three mutually orthogonal directions prior to rotation;
• repeating measuring output of the gyros;
• determining at least a geodetic azimuth of the gyrocompass sensor using the measured gyro output, the repeated measured gyro output and the measured output of the accelerometers, the geodetic azimuth corrected for gyro bias;
• and
• wherein the gyros comprise three at least single input axis gyros, a first of the at least single input axis gyros mounted to a frame rotatably supported in a housing, a second of the at least single input axis gyros mounted to at least a first rocker gimbal mounted in the frame and having a motor for rotating the at least a first rocker gimbal at most 90 degrees, and a third of the at least single input axis gyros mounted to a second rocker gimbal mounted in the frame and having a motor for rotating the second rocker gimbal at most 90 degrees, the first and second rocker gimbals each having a rotation axis in a plane of the frame and perpendicular to an axis of rotation of the frame, and wherein the rotating the gyros comprises rotating each of the frame, the at least a first rocker gimbal and the second rocker gimbal by 90 degrees from the plane of the frame to perpendicular to the frame.

A method for gyrocompass surveying a subsurface well, comprising:

• moving a gyrocompass sensor to a survey position along the well, the gyrocompass sensor comprising gyroscopic sensors (gyros) having input axes along three mutually orthogonal directions and accelerometers having input axes along the three mutually orthogonal directions;
• measuring output of the gyros and output of the accelerometers;
• rotating the gyros such that the input axes of the gyros are oriented along opposed directions to the three mutually orthogonal directions wherein a different gyro input axis is oriented along each opposed direction than the gyro input axis oriented along each of the three mutually orthogonal directions prior to rotation;
• repeating measuring output of the gyros;
• determining at least a geodetic azimuth of the gyrocompass sensor using the measured gyro output, the repeated measured gyro output and the measured output of the accelerometers, the geodetic azimuth corrected for gyro bias;
• and
• wherein the gyros comprise either (i) three at least single input axis gyros, a first of the at least single input axis gyros mounted to a frame rotatably supported in a housing, a second of the at least single input axis gyros mounted to at least a first rocker gimbal rotatably mounted in the frame and having a motor for rotating the at least a first rocker gimbal, and a third of the at least single input axis gyros mounted to a second rocker gimbal rotatably mounted in the frame and having a motor for rotating the second rocker gimbal, the at least a first and the second rocker gimbals each having a rotation axis in a plane of the frame, and at an angle of 45 degrees to an axis of rotation of the frame and wherein the rotating comprises rotating each of the at least a first rocker gimbal and the second rocker gimbal by 180 degrees, (ii) one at least single input axis gyro and one at least dual input axis gyro, the at least single input axis gyro mounted to the frame and the at least dual input axis gyro mounted to the at least a first rocker gimbal;
• or (iii) the at least single input axis gyro is mounted in the at least a first rocker gimbal and the at least dual input axis gyro is mounted in a second rocker gimbal rotatably mounted in the frame and having a motor for rotating the second rocker gimbal.

A method for drift tuning a gyroscopic survey instrument, comprising:

• measuring output of gyroscopic sensors and accelerometers in the instrument as the instrument is moved along a wellbore;
• determining an attitude of the instrument as the instrument is moved along the wellbore using the outputs;
• stopping the instrument in the wellbore;
• determining that a platform comprising the gyroscopic sensors and accelerometers in the instrument has stopped moving;
• measuring output of the gyroscopic sensors to determine a first bias and drift therein;
• reorienting the gyroscopic sensors;
• again measuring output of the gyroscopic sensors to determine a second bias and drift therein;
• determining a geodetic orientation of the survey instrument using the gyroscopic sensor outputs before and after reorienting the gyroscopic sensors;
• and
• using the geodetic orientation to refine the first and second determined biases and drifts.

A method for gyrocompass surveying a subsurface well, comprising:

• moving a gyrocompass sensor to a survey position along the well, the gyrocompass sensor comprising gyroscopic sensors (gyros) having input axes along three mutually orthogonal directions and accelerometers having input axes along the three mutually orthogonal directions;
• measuring output of the gyros and output of the accelerometers;
• rotating the gyros such that the input axes of the gyros are oriented along opposed directions to the three mutually orthogonal directions wherein a different gyro input axis is oriented along each opposed direction than the gyro input axis oriented along each of the three mutually orthogonal directions prior to rotation;
• repeating measuring output of the gyros;
• determining at least a geodetic azimuth of the gyrocompass sensor using the measured gyro output, the repeated measured gyro output and the measured output of the accelerometers, the geodetic azimuth corrected for gyro bias;
• and
• measuring acceleration in an accelerometer frame mounted to an instrument housing containing the gyrocompass sensor, the measuring acceleration performed in in left-right-up-down configuration, wherein measurements of acceleration in the accelerometer frame are used to correct measurements from the gyros for angular acceleration in the platform gyrocompass sensor.