At a time when there are major challenges facing the oil and gas industry, particularly with respect to the lead times on umbilicals, remote locations and increasing water depths, alternative thinking and approaches are playing an important role in finding solutions to these issues. To address this, this paper reviews the case for secure, high reliability acoustic signalling through water, with applicability to communication requirements for the subsea industry.
NASCoM is the name given to the family of products discussed in this paper. Starting with Nautronix Acoustic Subsea Blow-Out Preventer (NASBOP) control system, this is a proven safety critical system that is currently used for subsea drilling operations by various operators worldwide. NASBOP is an eight-channel acoustic system that was developed in 2002 to meet Shell's requirements. Shell had recognised the benefits to its drilling operations - particularly development drilling - from the surface BOP drilling technique, but required a reliable acoustic control and monitoring system to make the concept a reality. This is because with a surface blow-out preventer (BOP) there is no control umbilical running to the seabed.
Shell identified Nautronix' Acoustic Digital Spread Spectrum (ADS2) signalling as the most advanced and robust available in the market for this critical application. Subsequent to this, Nautronix worked closely with Shell and Cameron, the provider of the subsea isolation device, to develop NASBOP as the primary control and monitoring system and to provide an emergency disconnect capability. The system was first used in Brazil in 2926m of water.
NASBOP provides a number of benefits to the customer. The most significant is that it is an enabling technology that allows the use of surface BOP operations with a subsea isolation device. NASBOP provides reliable control and monitoring of the subsea isolation device, as well as the capability to expand the depth rating on the majority of rigs through the use of surface BOP drilling. The use of a reliable acoustic control and monitoring system provides the possibility for reduced capex on umbilicals in other applications, and hence will reduce maintenance costs. The concept can also be expanded to full BOP control called Nautronix Acoustic Subsea Multiplex (NASMUX, discussed in Section 3).
There were a number of design criteria which the NASBOP system was required to meet. Obviously, the reliability of the system and, in particular, the acoustic system was critically important to a successful outcome. As with all control systems, the probability of failure and demand had to be low and there had to be no possibility of an accidental triggering of the system.
Due to the increasing issues of frequency management and conflicts in field developments, the system required to be immune to interference from other acoustic systems. Redundancy was always a key consideration in all aspects of the system, and good system operability was also a requirement. It is thought that the success of NASBOP has changed the perception of the drilling companies using this acoustic system
