Anodes for hull
Sacrificial anodes in zinc or aluminium designed for preventing corrosion on underwater hull, ballast tanks, drill water tanks and cargo tanks.
Cathodic protection is designed to ensure full corrosion control throughout the entire operation time of offshore floaters, and between SPS surveys for rigs and mobile offshore units.
Sacrificial anode design for external hull
Anodes can be customised to any design or compostion to suit most application and requirements. See the full product range of sacrificial anodes.
Below illustrations show the most common anode models applied for corrosion protection of external submerged areas of rigs, MOUs and offshore floaters.
Planning for special periodical survey (SPS)
For MOUs (semisubs and jack-ups), survey and replacement of anodes is normally done during the 5 yearly SPS (Special Periodical Survey). Good planning of design, procurement and installation of anodes is crucial for a good result.
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For semi-submersibles, the new system of anodes to be installed during the SPS should be designed with basis in design criteria from DNVGL-RP-B401, however optimised with basis in observed anode consumption and other collected data. A normal practice is to order a partial delivery of relevant anode types before the rig arrive quayside, and add the remaining number of anodes after a rig survey and detailed design has been made.
Sea chests, thrusters and fairleads will need a separate set of sacrificial anodes. A typical design can be summed up as follows:
- SEA CHEST: 6 pcs each 10 kg net aluminium flush anode per compartment. The anode type 114-AL is a good choice. The number of anodes can be reduced from 6 to 4 pcs if the sea chest is equipped with a well operated MGPS/ICAF antifouling system.
- THRUSTERS: Anode number and types usually according to design by thruster manufacturer. Equivalent anodes can be proposed upone request.
- FAIRLEADS: 1 pc 10 kg net aluminium flush anode on each side of the fairlead. The anode type 114-AL is a good choice.
A proper anode design for a semi-submersible moored with anchor chains should include current drain to minimum 30 m of chains from the fairlead and to the chain lenght up from fairlead to operational water level. The resulting number of anodes should be installed on the outboard side of the corner columns.
Ship-shape offshore floaters
For offshore floaters, such as FSO, FSUs and FPSO’s a corrosion strategy should be established early to avoid problems later on in the vessels lifetime. Cathodic protection must be designed to ensure full corrosion control throughout the entire time the floater shall be offshore.
The design considerations would usually include environmental parameters, detrimental effects, fastening devices, coating breakdown factor (coating cathegory) and anode life time requirements.