Page | 64
10.6 Survey Tool Calibration
The purpose of calibration is to evaluate the coefficients for the various ‘ o ’ msdscbd bo .H ng s blshd h p fomnc fgu so ‘ch c sd’ ch s nso , ny sys m c o s m y b compensated thus enhancing its accuracy. To achieve this, the survey tool is placed on a calibration stand which allows the tool to be rotated between a series of known fixed orientations with respect to the local geographic axis set defined by the directions of true north and the local gravity vector. At each position, the turn rates and accelerations to which the sensors will be subjected in each controlled position are very accurately known. The series of positions are selected to excite each error contribution and so allow each error term to be identified separately and evaluated.
The highly accurate, custom made calibration stands used for this purpose were designed and developed by Gyrodata . Each calibration stand comprises a stabilised gimbal system with precisely controlled and instrumented gimbal angles. The stands are mounted on a plinth of granite that has its own foundations separate from, and vibrationally isolated from, the laboratory. The stands are aligned to true north within 0.001 degrees and are able to rotate the tool to any angle of inclination, azimuth or tool-face to an accuracy of 5 arc seconds (5/3600 degrees). Figure 55: Typical calibration stand.
10.7 Survey Tool Operating Modes
Gyrodata gyroscopic survey tools contain up to three accelerometers and up to two dual-axis gyroscopes installed in various configurations within the survey tool. Systems designed to operate at all attitudes generally require a full complement of gyroscopes and accelerometers in order to provide measurements of both angular rate and acceleration about three orthogonal axes; essential for all attitude operation. Some systems operate by taking sensor measurements at discrete intervals of depth along the well path trajectory when the survey tool is stationary, in what is described as gyro-compassing mode. Such systems provide estimates of inclination, high-side tool-face and azimuth angle as described above. Other systems can be operated in a continuous measurement mode. Given knowledge of the survey tool orientation at the start of a period of continuous surveying, changes in attitude that occur thereafter can be tracked by effectively integrating the subsequent gyro measurements of turn rate as the survey tool traverses the well path. 10.7.1 Gyro-compassing mode Gyro biases, which have an unpredictable behaviour, are measured and corrected for directly at each gyro-compassing station though a process of indexing the gyro. This involves mounting the gyro on a rotatable platform and driving it between two positions that are 180 0 apart. Measurements of turn rate are taken when the gyro is stationary at each index position. Whilst the turn rate to which the gyro is subjected is reversed between the two index positions, any bias which is present in the measurements remains fixed. Hence, an estimate of the measurement bias can be obtained by summing the two measurements and dividing the result by two. Any residual bias which remains can still be significant and must therefore be estimated in the field. Gyro mass unbalance is stable when the gyro is at rest. However, as discussed earlier, it may change significantly if the gyro is exposed to a mechanical impact, as can occur during transportation or surface handing. The average mass unbalance for the entire survey should therefore me estimated and corrected in the field. Accelerometer calibrations are usually very stable, but they can change over time or as a result of temperature exposure. It is therefore important that the performance of the accelerometer pack is always verified for every recorded measurement and for the survey as a whole.
Made with FlippingBook flipbook maker