The shipping industry is comfortable with ICCP (impressed current cathodic protection) systems for the hull. This reasoning is well justified when one considers the operation and function of a tanker, for example. Impressed current systems offer a number of advantages for these vessels:
1. The low profile of impressed current hull anodes reduces drag and saves fuel, ultimately reducing operating costs.
2. The small number of anode sites required for a hull ICCP system makes dry dock maintenance relatively straightforward. This also reduces the amount of welding required on the hull exterior.
3. ICCP hull systems can be controlled according to cathodic protection potential, optimizing system performance in various geographical areas, particularly when the ship moves from seawater to brackish or river water conditions.
4. The systems are fairly standard, and ship builders are quite familiar with them. This does lower initial installed cost.
5. Classing agencies are very familiar with systems, and they have pre-approved many designs.
The operation and function of an FPSO varies significantly from an ocean-going tanker in the following areas:
1. It is moored in one offshore location for protracted periods of time.
2. Regular dry-docking is not possible.
3. There are a number of appurtenances, risers, umbilical, mooring lines and turret structures that may be in close proximity to the hull.
These differences should be considered when developing a long term cathodic protection strategy. Certain implications of these differences might prove to make the use of sacrificial anode systems more attractive to many cathodic protection designers.
Comparison of Different Hull Systems
Overview of Corrosion Control
Coatings - As with many other offshore structures, the primary corrosion control method is the coating applied to the hull. This "front-line" is backed up by a cathodic protection system that takes care of exposed steel at coating defects. When designing the cathodic protection system, it is necessary to predict how efficient the coating system will be initially, and also at the end of its life. Given that regular ocean-going vessels are required to be dry-docked at intervals not exceeding 5 years, there is always an opportunity to repair or re-coat the hull during the ships life, thus the coating degradation has only to be considered over this time span. An FPSO may be offshore for 15+ years. This will require a re-evaluation of the coating performance at the end of this period, and the cathodic protection system will have to be beefed up by some degree to deal with the increased bare steel area that will exist over the long life of the coating system.
Special attention must be paid to the compatibility of the coating system with cathodic protection, this is particularly true of impressed current systems that will generate higher negative potential values at the edges of the dielectric shields associated with hull mounted anodes.
Impressed Current Cathodic Protection - Many vessels that are candidates for conversion to FPSO service are fitted with hull mounted impressed current systems. For this reason most operators want to use the same system to protect the vessel in it's new role. If this is the proposed strategy the following areas must be carefully considered to avoid problems later on in the life.
Control Electrode Location - The control reference electrode(s) monitor the potential of the hull at the area where they are located. This signal is monitored by the controller built into the transformer rectifier power supply. The output current supplied from the anode systems is perpetually adjusted to maintain the potential of the hull (within an acceptable range). On a regular ship system, electrodes will usually be somewhat close to the anode locations (Fig. 3 below). This site selection is based on the desire to minimize the risk of coating damage at areas where the potential is expected to be more negative. When other subsea structures are introduced, particularly turret structures and risers, two problems can (and frequently do) arise;
1. Due to the intricate nature of the turret structure (Fig. 4 below) and the risers that pass through it, there may be problems of shielding that make it difficult for the impressed current system to adequately polarize the annular spaces. This is usually addressed by locating sacrificial anodes on some areas of the turret structure, to protect locally. Because the turret and the hull are electrically bonded, this can cause the sacrificial anodes to also provide protective current to the hull. The control reference electrode may then sense protected potentials and will not allow the impressed current system to activate. This can lead to early consumption of the small sacrificial anodes, accompanied by subsequent under protection of critical areas.
2. The addition of dynamic riser components and the creation of more highly stressed joints associated with their support structures may shift the priority of where high potentials or under protected potentials are unacceptable. The "standard" electrode locations may not lend this degree of flexibility.
It is recommended that additional reference electrodes are located at these critical areas with a facility to have one of them control the system if required. Existing electrodes can be used for monitoring rather than control. It is also a good idea to use dual element reference electrodes (Fig. 5 below) the standard electrodes provided are normally not expected to last 15 - 20 years. The dual element electrodes combine accuracy of silver/silver chloride sw (Ag/AgCl) with the long-term reliability of Zinc sw (Zn).