Ultra Long Base Line Multi User Positioning System
Authors: Manson Robert , Protoolis Harry , Andronis Nick 
Published: October 2008

Acoustic Positioning has long played a critical role in the Offshore Oil and Gas Exploration Industry, being the main positioning system for sub-sea operations and providing a critical second reference for surface positioning.

Until now this function has been provided by a variety of different mutually incompatible acoustic positioning systems. They have tended to be single user, for short term deployment, designed for one specific task and then removed.

For the last 15 years Nautronix has been a pioneer in the field of acoustic spread spectrum (ADS2) signalling techniques. In the early 1990's this work was applied to a variety of applications for the Royal Australian Navy, to develop equipment which could be used in difficult noisy acoustic environments and be able to transmit information accurately, over very long ranges.

Nautronix is now bringing to the offshore oil and gas industry these same acoustic technologies. This includes the introduction of a wide area multi-user, highly accurate, positioning system, called NASNet?. The system is designed to allow one grid to be laid, potentially for the life of field and simultaneously used to position unlimited multiple sub-sea targets, and to provide a second reference for surface positioning, against a common baseline.

Nautronix has used its extensive experience in ultra long range positioning to solve some of the more difficult problems encountered, including resolving timing issues arising from multiple free running transmitters, dealing with the acute ray bending which commonly limits maximum range, multi-path and reverberation problems.

This technology allows Nautronix to offer a constellation of stations, which can be deployed for up two years (if batterypowered), providing a seamless positioning reference covering an arbitrarily large area. Throughout this area a common reference is available for an essentially unlimited number of surface and sub-sea users providing a accuracy of approximately 1 metre rms.

This technology promises to do for sub-sea positioningwhat DGPS has done for surface positioning; to provide a stable long-term common reference to multiple users, simultaneously. This technology promises significant costsavings and overall improvements in the operation of sub-sea Oil and Gas Fields.

Introduction

Underwater acoustics play a vital role in offshore applications including the positioning of underwater structures, ROV's,AUV's, and tracking of towed bodies for underwater surveying and mapping.

Current systems require an interrogation pulse to a transponder, or family of transponders, which then respond, and by measuring the time of return and knowing the speed of the sound in water range can be determined. By either the use of a number of hydrophones receiving one signal or a number of transponders responding to a single hydrophone a triangulation can be made which gives bearing. The main draw backs being encountered, particularly in an area where much activity is taking place, is that a number of independent systems with a limited amount of frequencies has to be used and this leads to interference and cross talk problems, which can seriously impair and delay work.