Abstract

Offshore wind farms are typically designed with an anticipated service life of 25 years. Earlier it was assumed that the internal side of foundations below the lower working platform was airtight. If airtight, corrosion was assumed to cease when the oxygen present inside the foundation was consumed. This assumption has been shown not to be fully valid as both sea water and oxygen have access to the inside of the monopile under certain conditions, not the least on sites where large tidal variations exist (up to approx. 10m). This may result in active corrosion which can compromise the durability of the wind farm and reduce the service life if not prevented.

We undertake condition assessment of our wind farm foundations (+850 foundations altogether) on a regular basis. The monitoring comprises visual inspections, corrosion measurements and evaluation of the corrosion environment (water quality). The use of cathodic protection is one of the mitigation measures where premature corrosion is seen.

A number of foundations have been installed with sacrificial anodes inside the foundations. However, due to partial close compartments and no established measures to control the current output from the anodes, this has in a number of cases resulted in acidification and health issues which compromise the structure integrity and safety.

This paper presents details of the design and installation of an internal cathodic protection system where individual anodes' current output can be controlled. The anode service life is increased, the protective level can be adjusted and not the least, this provides very good options for limiting the gas development to an acceptable level and hence health and safety can be considered.

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