Marine systems

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FPSO Sacrificial Anodes

New build

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

Sacrificial Anodes on the Hull (Courtesy Deepwater)

There are a number of reasons for this the main ones being:

  1. Ability to confidently design for 20 year + life
  2. No maintenance required
  3. Very high reliability
  4. No modifications on hull interior, and no hull penetrations
  5. Zero risk of interference
  6. Compatibility with other CP systems on subsea equipment
  7. Lower overall life cycle cost

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.

Retrofit Systems

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:

  1. Anodes (2) are at a remote location, and thus current distribution can be expected to be very uniform over all the protected areas of the vessel and its appurtenances.
  2. There is no risk of stray current interference with remote anodes, no locally high voltage gradients to deal with.
  3. Installation can be accomplished in one or two days, using only ROV support.
  4. Only two such sleds (400 Amperes each) are required. Installation is planned for 2003. A typical buoyant sled of the type to be used is shown below.

Buoyant Anode Sled (400 Ampere)

Buoyant Anode Sled (400 Ampere) Shown With Cable Dispensing Hopper (Courtesy Deepwater)

Recommended Strategy

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.

Adapted from Cathodic Protection Strategies for FPSO's Jim Britton

See also: Fouling,FPSO, Ions in seawater,DO in seawater, Seawater scaling, Anti-fouling coatings

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