If the vessel is a new build, a life cycle economic comparison will almost certainly favor the use of galvanic anodes over impressed current.
Sacrificial Anodes on the Hull (Courtesy Deepwater)
There are a number of reasons for this the main ones being:
Anodes selected are normally platform-sized anodes 160 - 230 kg modified for flush mounting. Aluminum alloys are preferred based on higher efficiency coupled with lighter weight.
With approximately 70 FPSO's currently in operation or under construction worldwide, there are inevitably some vessels that require CP retrofits. One such FSU vessel operating offshore West Africa is being studied for application of a new type of impressed current retrofit system. The original sacrificial anodes are virtually depleted, and offshore replacement of the depleted system would be cost prohibitive due to the extended amount of diving activity required. Economic studies of various types of retrofit system have shown the remote buoyant anode to be the most favorable. The anode sleds will sit on the seabed and the feed cables will be deployed in a "Lazy S" configuration. The FSU in question is spread moored and is thus particularly suited to this strategy. The advantages of this system are very obvious:
Buoyant Anode Sled (400 Ampere) Shown With Cable Dispensing Hopper (Courtesy Deepwater)
For any vessel whether new-build or conversion, the recommendation would have to be for a sacrificial anode system if the expected on station life is greater than 10 years. This recommendation will be supported by both economic and reliability studies. For depleted systems requiring offshore retrofit the recommendation will usually be to deploy impressed current, if the system is allowed to "weather vane" 360 degrees around a turret, a deep suspended system would almost certainly be the most cost effective. If full 360-degree rotation is limited, or the vessel has a spread mooring, then the seabed-deployed system will usually be the best option. We can see little if any justification for keeping a close fitted impressed current system on what is essentially a floating production system.
See also: Fouling,FPSO, Ions in seawater,DO in seawater, Seawater scaling, Anti-fouling coatings