Internal cathodic protection of offshore wind turbine monopoile foundations: Fid-Bau Portal

Internal cathodic protection of offshore wind turbine monopoile foundations

Tavares, Isaac / Ernst, Petra / John, Gareth / Jacob, Robon / Wyatt, Brian

The following deductions from the laboratory study can be made in respect of optimising CP designs for offshore wind farm monopiles: If no seawater replenishment takes place and aluminium anodes are used, the increase of acidity [lowering of pH] will increase the cathode currents required for protection of the steel, increase the self-consumption rate of anodes and increase the rate of hydrogen evolution. It is possible that protection of the steel may not be maintained. Zinc anodes do not significantly alter the pH and will, therefore, not have the adverse consequences described for aluminium anodes in non-replenished conditions. They will also evolve less hydrogen than aluminium anodes. The use of a gas impermeable membrane to reduce oxygen exchange at the seawater surface does not change the pH variations due to CP effects in a non-replenished situation but is effective in reducing oxygen level in the water but its merits should be considered on a project by project basis. If tidal replenishment can be achieved, the pH may be maintained at levels close to those in natural seawater, depending on the percentage seawater exchange on each tide, and aluminium anodes may be used. However, care in the design of the flushing system is important to mitigate against the stratification of the water column; this could present problems of decreasing pH even if tidal replenishment is employed, in particular for those monopiles that were drilled and have deeper water columns below the sea bed.