9.5.1 Surface Reference Orientation

Prior to the use of North Seeking Gyros the gyro had to be ‘Referenced’ and set to a known bearing at surface (On the rig floor) before entering the borehole. On land, this was a relatively simple process involving the establishment of a Sight Mark or Back Sight (Reverse Sight), from a distant mark clearly visible from the rig floor through the Sighting Scope oriented and attached to the Survey Tool in line with the internal Gyro Compass Reference. This Land Sight Mark was on occasion provided by the client, using the land survey company responsible for locating the rig but was often determined by the well survey company, using conventional land survey techniques and a Magnetic Brunton Compass. For low angle wells the gyro reference was then set to the Grid or True North orientation (dependent on the client requirements).

For offshore locations this observed and required Sight Reference had either to be pre-established (for each Slot Position) relative to Platform Centre or calculated using distant installations or features visible from the rig floor rotary. If/when no feature was visible from the rig floor the Sight Orientation Reference would be transferred from a position external to the rig floor (e.g. Helideck).

These techniques were also applicable to semi-submersible (floating) installations, with the added complication that the sight reference value might be subject to small but constant change. As a last resort the rigs Ships Compass Heading value would be used. This mobile condition had implications for both the start and end reference for the survey and its effect on Drift Curve closure and survey reference accuracy.

When setting the Gyro Orientation Reference for higher inclination surveys the gyro would be set with the spin axis perpendicular (oriented across) to the wellbore path, with due account taken of the expected gyro drift rate and any change in the wellbore azimuth trajectory. Setting the spin axis in this attitude, provides the most stable orientation for gyro operation as the gimbals tilt with inclination and reduces the potential for Gimbal Lock (System Bearing Jam) and subsequent gyro spin out and survey miss-run. The offset reference orientation value (relative to True or Grid North) applied in this technique was accounted and corrected for by applying a baseline shift to the calculated Drift Curve.