Underwater acoustic technology has been revolutionised over the last few years by the introduction of complex signalling techniques into subsea acoustic control, monitoring and positioning systems.
This technological innovation was made viable by the increasing availability of low power digital signal processing (DSP) devices resulting from the dynamic growth in the mobile telecommunications industry in recent years. Prior to this, such devices were too power hungry to be considered for use in commercial battery-powered equipment, such as subsea transponders, and through water data communication links. Consequently, acoustic system manufacturers were able to re-examine their approach to remote subsea control and monitoring as well as offshore positioning, resulting in a step change in the performance of these systems over recent years.
Additionally, Sonardyne has recently launched its second generation wideband systems, which build on these advantages by providing a more flexible modulation scheme that can be configured for the optimum trade-off between data rate and robustness - for even the most challenging environmental conditions. This process has also seen the introduction of new terminology: digital acoustics, wideband, spread spectrum, DSP and others into the field of subsea acoustics.
This paper is intended to provide guidance and clarification to existing and potential users, as well as those who specify these systems. It summarises the new technology, how it differs from conventional tone based acoustics and how it provides improved performance and cost-savings in existing subsea acoustic control, monitoring and positioning applications. Drawing upon Sonardyne's real-world experience, it provides examples of data from some recent offshore projects to illustrate the performance improvements achieved.
It also identifies the opportunities presented by the new acoustic technology, for replacement of umbilical cables by high integrity acoustic links in certain applications, offering the potential for substantial savings in both capital and operating costs.
Acoustic systems have been used for many years in the offshore industry for positioning and underwater navigation as well as certain control and monitoring applications, such as acoustic backup blow-out preventer (BOP) controllers.
As operations have moved to ever deeper waters and field development times have decreased, the concentration of rigs using acoustic positioning systems within relatively small field development areas has increased. This has led to increasing overcrowding of the acoustic spectrum in busy areas, such as the Campos Basin, and the potential for the acoustic system on one rig to interfere with that of its near neighbours, with potentially serious consequences.
With a very limited number of acoustic channels available, the management of channel allocation between neighbouring rigs has become increasingly difficult as the rig population have grown. This has placed increasing demands upon acoustic system suppliers to provide solutions that will enable many users to share the same acoustic spectrum without mutual interference.
The solution to this problem lies in the adoption of digital wideband signalling techniques, which allows many more channels to be accommodated within the same acoustic spectrum than is possible using conventional tone based acoustic signals.