Testing the applicability of fiber optic gyroscopes for azimuth monitoring for measurement-while-drilling processes in the oil industry

Abstract

Measurement-while-drilling (MWD) surveying is currently performed to provide the drill bit attitude at certain survey stations using three-axis accelerometers to determine the pitch and the roll and three-axis magnetometers to determine the azimuth. The magnetometers used in this process suffer from the deviation of the Earth's magnetic field due to the presence of downhole ore deposits, and from drill string magnetic interferences. The aim of this study is to describe an experimental setup for replacing the magnetometers with a single fiber optic gyroscope (FOG). The experimental setup is designed to deliver rotations about three orthogonal axes and these rotations are performed using DC motors controlled by DC-servo amplifiers. Each amplifier compares the control signal from the operator with the feedback position signal from the motor and produces a command signal of certain angular change to the motor. In addition, optical encoders are installed on the shaft of each motor to monitor the speed and to send a feedback signal to the DC-servo amplifier. This feedback signal is used to control the motor speed. The FOG is installed in the horizontal plane of the setup together with three-axis accelerometers. The simulator provides precise angular changes about each axis as small as 0.5/spl deg/. The DC-servo amplifiers provide on-line tuning of the angular change corresponding to the variation in the command signal within /spl plusmn/10% error. The FOG can be oriented at near vertical and completely horizontal situations. Measurements from the FOG and the accelerometers are digitized using a data acquisition system and a laptop computer mounted inside the same setup. After integration, corresponding angular and velocity increments for processing by the developed navigation algorithms are provided. The experimental setup provides the flexibility to test the FOG at different orientations thus simulating various downhole situations.