With the offshore oil and gas industry shifting its attention to increasingly deeper waters, the request for accurate subsea positioning systems is also rising. Long Baseline (LBL) systems can provide an accuracy that is virtually independent of water depth.

LBL has already been used for over three decades, mainly in the offshore construction business, where ROV and structure positioning requirements are often too strict, for example, for vessel-mounted ultra-short baseline positioning. With LBL, the reference frame is placed on the seabed, thus providing a depth-independent positioning accuracy.

By measuring the ranges to at least four acoustic beacons on known locations on the seabed, a 3D position of a transducer can be calculated by means of the intersection principle. Such ranges are determined by measuring the time of flight of the acoustic signal. The network of known transponders is usually called an ‘array' and the baselines in such an array can be several kilometres long, depending on the frequency in use.

Technical development of LBL has been relatively slow, until recently, when spread-spectrum signalling was introduced in underwater acoustics. This significantly improved the ranging accuracy and increased the maximum baud rates for acoustic data telemetry. Most manufacturers now provide spread spectrum on their systems.

Hydro International is greatly indebted to all manufacturers who contributed to this product survey.