Most mooring systems for FPSO's comprise of an upper chain section that may include some jewelry to control the catenary angle. This is usually connected to a wire rope taut catenary section that covers the majority of the water column. A further section of chain is often used to connect the taut catenary to the anchoring device. Fairleads or bending shoes are usually attached to the turret structures to control the attitude of the mooring lines.
The following section identifies some areas where we have been asked to investigate corrosion problems, and also contains recommendations to ensure that the moorings are not compromised through corrosion damage.
The fairleads are designed to allow movement, and have a number of mixed materials in the bearings and swivels. The main caution is to ensure that the fairleads are electrically continuous with the structure to which they are attached, if this is not checked, anodes provided on the cheek plates of the fairleads may prematurely consume due to current losses to the chain, after this localized corrosion may result in the bearing areas. If properly grounded with a flexible cable jumper, the turret or hull CP system can accommodate current losses to the chains while maintaining protection on the fairlead components.
Like the fairleads, the connectors between chains and rope sections have various mixed metallurgy that can result in localized galvanic attack (often on chain links adjacent to the connector plates). It is therefore important to ensure that an adequate weight of anode material is attached directly to the connector assembly, and that there are no electrically isolated components. Jumper wires should be used across all mechanical joints. A rule of thumb is to allow for current drain to approximately 30 m, of chain in each direction.
Chains are notoriously difficult to cathodically protect, and are therefore normally provided with a corrosion allowance. The biggest problem is that they drain some current from the CP systems on other components to which they are attached. It is important to allow for this drain by providing additional anode weight.
The large diameter spiral strand rope commonly used will normally have it's own corrosion protection scheme that may include blocking compounds, sheaths, sacrificial anode strands and galvanic coatings on the strands. It is preferable to use sheathed rope over non-sheathed and to use Zn/Al alloyed coatings on the strands rather than just galvanize. Always ensure that there is adequate anode material, and a good quality coating on the wire rope connectors.
See also: Fouling,FPSO, Ions in seawater,DO in seawater, Seawater scaling, Anti-fouling coatings