A research and development project named Subsea Wells Acoustic Control System (SWACS), was carried out by Tecnomare (Italy) and Kongsberg Vaapenfabrikk (Norway), acting in joint venture as main contractors under assignment of Norsk AGIP (Norway), with the technical supervision of AGIP (Italy). The system can control and monitor up to 15 wells in water depths up to 1000 m. No umbilicals to the surface are required, either for electric lines or for hydraulic ones The system relies on an autonomous energy source for power supply and on acoustic link for signal transmission. It also features low-power-consumption electronics with intrinsic well safety procedures and closed-loop type hydraulics The control module can be installed without guidelines, either with a drill string or a simpler wire line, by means of a dedicated positioning, soft landing and orientation systems The interface with the subsea Christmas tree is assured by remotely operated subsea electrical and hydraulic connectors.
Following exhaustive dry tests, a control system prototype was installed in the Agip Luna Field (Ionian Sea, Mediterranean) for long-term sea trials The project demonstrated that an autonomous control system, with no need of umbilical connection to surface, is technically feasible, this will allow a more reliable and economic solution to the critical issue of subsea control
This paper describes the most outstanding characteristics and components of the SWACS and the results of the field trials Particular care is paid to the description of the control module, which features many innovative solutions, and to the most important technical problems faced during the design, dry test and installation phases
The control systems used for the subsea production of hydrocarbons have always had the surface units connected to the subsea control modules by means of one or more umbilicals, composed of different combinations of hydraulic and electrical lines depending on the type of control system.
This approach has some economic and technical implications In fact, the umbilicals are quite expensive to purchase and require costly marine operations for laying and connection Moreover, during the operative life, they are subject to the risk of being damaged by anchor or trawl gears; this risk cannot be eliminated even if the lines are buried. In addition the connectors have been shown to be critical components, in particular 'the electrical ones, with insufficient reliability; this is made more serious by the difficulty and the cost of recovery and replacement operations. These problems can be even more complicated in very deep waters, where the connections cannot be made by divers nor can they be helped by the use of guidelines On the basis of previous Tecnomare research (1974–1979) on acoustic underwater transmission, sponsored by AGIP and other Italian Companies and with financial support from IMI and EEC, autonomous power generation and acoustic link appeared to be the correct solutions in order to overcome such problems.