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degs of inclination. Multiple surveys were performed with both Gyro and Magnetic tools, with surveys taken at 25 ft intervals. In the final analysis the accuracy of both Survey Systems were proven and the East Texas Claims Issues finally settled. 1.75” Gy o ools d lop d us ng 26,000 pm A o o s. p c f c lly d s gn d fo c on l ll ng ool Orientation and surveying of Production wells in Tubing. 1971: Atomic Energy Commission Test Bore intersected at 6000 ft TD with Bottom Hole location land surveyed in at < 5 ft variance/error. 1974: Multi-well Platform drilling is prevalent. Level Rotor Gyro System developed with glass file Mercury Switch used to control the inner gimbal horizontal position to +/- 1-2degs but system is sensitive to gimbal/switch attitude, resulting in azimuth error propagation. 1977: n oduc on of u f c co d ng Gy os ( G’s - 3” nd 1.75” s ons), nsm ng d o su f c Wireline. Inner gimbal position now monitored and controlled by electronic resolver to +/- 0.01 degs resulting in significantly reduced error propagation. 1978: Ferranti Full Inertial System introduced into the North Sea as reconfigured Harrier Jump Jet IN System, developed for Shell, Mobil, BNOC (BP) and marketed by Eastman Christiansen (Inteq). Proven Accuracy < 1/1000ft potentially at all attitudes.
9.4 Improving Performance and Service Capability
As outlined above, Gyro Survey Tools were initially and primarily introduced into the industry, to provide a means of obtaining or checking borehole attitude, when the wellbore was already cased, negating the application or repeated use of magnetic based tools (Lease Scandals). Similar conditions were to foster the requirement for reduced diameter tools when later legal argument ensued over deeper smaller diameter wells. However, the primary motivation for technology advancement has been the requirement to survey ever deeper, higher angle boreholes (beyond horizontal), with significant azimuthal change at greater latitudes. The first gyros used by the industry had no means of inner gimbal/spin axis control but the surveyor could determine its approximate position from the film record at each survey station. These gyros were originally intended for use up to 20 – 30 degs inclination. However, as borehole inclinations increased, hardware improvements were made and operational techniques were developed which enabled these tools to be used successfully beyond 60 degs inclination. As noted above, Gimbal Tilt control and Drift Curve correction methods were refined to better account for the increasingly difficult operating conditions encountered and its interactive effects on gyro stability and data quality at a relatively early stage. kngGyos h gyoh d ob ‘ f nc d’ nds o knownb ng suf c ( n h 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). 9.5 The Gyro Survey Process 9.5.1 Surface Reference Orientation o o h us of o h
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
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