Gyro Drift Values

Nature and Source of Drift

The Drift is the rotation of the outer gimbal because of the Earth's rotation sensed and imperfections of the system. Drift is expressed in (degrees per hour) and can range from 10 to 100 (°/hr).

Generally, drift values may range as follows:

• 0.5° to 1° per minute for cheap gyros
• a few degrees per hour for directional gyros
• 1/100th degree per hour for inertial gyros using gimbal flotation
• 1/1000th degree per hour for some inertial gyros with spherical spinning rotors supported by electrical fields

The rotation of the Earth causes apparent drift. The torque which may cause drift may be separated into two main categories.

Error Torque, such as gimbal error, for which adjustment or compensation may be applied.

Uncertainty Torque is the random component of drift that bears no correlation with any inputs. The drift due to error torques is of three types: Nonacceleration sensitive drift, Generally caused by elastic or magnetic torques.

Acceleration sensitive drift: Generally caused by mass unbalance.

Acceleration sensitive drift

Other sources of systematic drift rate errors may be temperature-sensitive torques due to differential expansion – torques due to the nonorthogonality of the principal axes and wheel speed change.

The random drift is due to small uncertainty torques such as those caused by bearing noise, friction, and temperature gradients, all of which are time variables. Random drift is generally determined by statistical analysis of many drift tests.

Essential Considerations To Be In Mind While Using Gyro Wireline Tool In Oil & Gas Wells

Gyroscopes are very sensitive to vibration, so the gyro single shot must be run and retrieved on a wireline.

The gyroscope may drift away from its set direction while running in the hole. When the instrument is recovered, its alignment must be checked, and a correction applied to the readings obtained from the survey. Gyro single shots are often used to orient deflecting tools near the casing.

Gyro Multi-Shot is used in cased holes to obtain a series of surveys along the wellbore length. The magnetic multi-shot cannot be used because of the interference with the Earth's magnetic field caused by the magnetization of the casing.

The directional surveyor must keep track of the depth at which the pre-set timer takes a picture. Only those shots taken at a known depth when the pipe is stationary will be recorded.

When the multi-shot is recovered, the film is developed, and the survey results read. In the case of both single-shot and multi-shot instruments, adequate centralization must be provided so that the device is correctly aligned with the wellbore.