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Paper Number 2815-MS
TitleUNDERWATER SURVEY USING AN INERTIAL NAVIGATION SYSTEM
AuthorsAlain Stankoff, Intersub Developpement, and R. A. R. Tait, Ferranti, Ltd.
Source

Offshore Technology Conference, 2-5 May , Houston, Texas
Copyright1977. Offshore Technology Conference
LanguageEnglish
PreviewABSTRACT

Recent trials of an Inertial Navigation System onboard a manned submersible have proved that underwater distances over several hundred metres can be measured with an accuracy of about ±15 cm.

The system is completely self-contained, independent of transponders or beacons and free of underwater environment disturbances. It permits, in addition, to navigate over very long distances with accuracies comparable to those obtained with long or short base acoustic systems.

This means that a new tool is now available to the offshore industry to simplify and/or improve the efficiency and the accuracy of underwater jobs such as tie-ins, measurement of flow-line position and length, pipe-line surveys, installation of platforms, sea bottom profiling, Navigation in acoustically disturbed environment, etc?

INTRODUCTION

Inertial navigation systems have, in the past, found application in, particularly, fighter aircrafts and space vehicles. Such aircrafts required the system to be light enough, self contained and largely immune from external influences. Navigational data is presented on a plane perpendicularly to the local vertical and oriented with respect to the True North. The system basically measures the components of the acceleration along the reference axes and compute the position by integrating them twice. Due to various imperfections, the resulting integrals are drifting as a function of time, and, for aircraft systems, the order of magnitude of the drift is I Nm/hr which is unacceptable for underwater usage. To offer convenient navigation performances required the system to be completed by an additional information relative to the INS velocity drift. Unlike aircraft, submarines can easily stop during their survey, and stay motionless on the seabed. Then, during such a station, the inertial system is informed that the speed is null, can measure its own drift and takes it into account to correct the navigational data until another stop occurs, where the drift data will be updated. The accuracy of the correction depends on the residual motion of the system during the drift check and of the time interval between two stations. Preliminary laboratory results had shown that such an inertial system, corrected every minute, could give the position within a few centimeters, which was at least one order of magnitude better than conventional underwater positioning systems. Furthermore, in the underwater environment, being self contained and immune from external influences offered immediate advantages over transponder systems. As it was necessary to demonstrate the performances of such an accurate system over significant distances, but impossible to verify the absolute accuracy over pre-determined distances underwater, the present evaluation was performed in two steps :
  • First, the absolute performances were accurately checked on a well-known circuit onshore, with the system mounted on a vehicle and driven at speeds simulating the submarine motion.
Number of Pages8
File Size 496 KB
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