Acoustic Control System

This paper describes some features of an acoustic control system. It is designed primarily for acoustic control of blow out preventers and subsea production units. The system is combined with a super short baseline acoustic navigation system. The communication between the central unit and the subsea unit to be controlled is 2-way. The communication from the subsea unit is used for readback of both status and acknowledgement of received command. This is utilized in a message handshake to obtain an improved security. The messages are coded by using multifrequency keying (MFSK). Each frequency is transmitted with high power, giving a favourably signal to noise ratio. Therefore the system works well in noise environments. Each frequency is transmitted only once in a message. That gives the system good immunity against reflections. All received frequencies are stored in memory for some time by the algorithm which decodes the frequencies to messages. By using this information the algorithm detects, if however possible, a message received together with noise pulses and pulses from either reflections or other acoustic activity in the area. The information also enables the algorithm to detect an ambiguity in the received message and thereby rejecting the message.