One or more forms of corrosion protection can be used to protect the substructure from corrosion to the extent that is required, such as a paint barrier with cathodic protection.
What it costs
About £22 million for a 450 MW floating offshore wind farm.
Who supplies them
Cathodic protection systems: Corrosion, Imenco, Impalloy and Metec Cathodic Protection.
Corrosion protection coatings: Hempel, Hutchinson Engineering, International Paint and Jotun.
In offshore wind, corrosion predominantly occurs when sea water interacts with metallic surfaces. This can lead to oxidation (or rusting) of metallic surfaces, which can compromise the strength and performance of metal structures such as substructures. Corrosion protection mitigates general and localised wall loss in steel substructures and is a prerequisite for attaining the fatigue of the structure. Corrosion can also occur from microbiological activity.
Methods for corrosion protection include corrosion protective coatings and cathodic protection. The in-built corrosion resistance of the material and allowing an additional material thickness for corrosion are also considered.
The external surfaces of the atmospheric and splash zones are normally coated with high performance marine coatings that reduce corrosion.
Parts of the substructure in the submerged zone use cathodic protection systems to provide corrosion protection. The application of a negative current to the steel structure reduces the voltage on the structure to a level at which oxidation, and hence corrosion, is suppressed. There are two types of cathodic protection systems:
- Galvanic anode cathodic protection systems (GACP) comprise several sacrificial anodes made of aluminium or zinc-based alloys that are fixed to the steel structure below the waterline. These can be designed to be replaced periodically to extend the lifetime of the corrosion protection.
- Impressed current cathodic protection systems (ICCP) use an external power source and rectifier to supply a negative current to the steel structure and a corresponding positive current to non-consumed anodes mounted adjacent to the structure. An ICCP is substantially lighter and causes less drag in the water than GACP but requires a reliable power supply and additional instrumentation.
The chemical reactions that cause corrosion can generate noxious gases which accumulate inside a floating substructure. The lower deck of the substructure is sealed for the safety of maintenance technicians working above, whilst gas detection and ventilation systems may be used to monitor and safely vent the concentrations of the gases.
In closed internal structural compartments of the floating substructure which are welded shut, corrosion may be mitigated by humidity control or oxygen depletion.