INERTIAL NAVIGATION SYSTEMS
Inertial navigation is a very precise method of surveying used in aircraft and missile guidance systems. In the late 1970s this technique was adopted for borehole surveying in the North Sea. The FINDS tools (Ferranti Inertial Navigation Directional Surveyor) based on an inertial platform consisting of 3 accelerometers and 3 gyroscopes mounted on gimbals was the first IN system used in borehole surveying.
Although the FINDS tool is no longer used it is the most generic type of tool and will therefore be described On the surface the platform is automatically levelled and the N-S accelerometer aligned with true North. As the tool is run down the hole
on wireline any misalignment of the platform is detected by the gyroscopes which send signals to the gimbal mechanism to restore the platform to its original position. The running procedure is to stop the tool for 1 minute, then run for 1 minute and so on until it reaches bottom. During the 1 minute transit periods the accelerometer readings give the inertial velocity. Once back on surface this data can be integrated to give the incremental X, Y and Z displacements for each transit period. These distances can then be added to the previous co-ordinates to give the trajectory of the cased borehole (Note that the FINDS tool calculates the co-ordinates directly, not by measuring azimuth and inclination). The FINDS tool was generally considered to be the most accurate surveying device available. Its accuracy was about 0.2 ft. per 1000 ft. of hole length (i. e. it can locate a 13 5/8" casing shoe, set at 5000 ft, to within 1 foot, compared with 15 — 30 ft. using conventional gyro methods).
The FINDS tool does however have certain disadvantages :
• The tool diameter was 10 5/8", and so could only be used down to the 13 3/8" casing shoe.
• It is much more expensive to run than a gyro multi-shot.
• Only a limited number of tools were available.
Its major application was to provide a definitive trajectory of the hole from surface down to the 13 5/8" casing shoe. High accuracy is required here when drilling from multi-well platforms where the wells are very close to each other and there is a risk of intersection.
Since the FINDS tool a number of new surveying tools were introduced. In 1981 Schlumberger, introduced the GCT (Guidance Continuous Tool). This instrument is only 3 5/8" diameter and it can therefore be used to survey the entire well path down to TD (minimum casing size is 4 1/4"). The inertial platform in the GCT consists of a 2 axis accelerometer and a 2 axis gyroscope, mounted on gimbals. The spin axis of the gyroscope is parallel with one axis of the accelerometer and aligned with true North. Any drift of the gyro is detected by positional sensors and corrected by the gimbal mechanism. The inclination and azimuth are calculated from the accelerometer reading and the angle between the outer and inner gimbals. The inclination and azimuth are given on a surface display as the tool is being run. The survey depth is given by the wireline measurement. The accuracy of this tool is about 2.6 ft. per 1000 ft. per 1000 ft. of hole length, in the North Sea.