Result of Fast Track Project: Corrosion Protection of Monopile Structures

Unexpected internal corrosion in monopile foundations calls for a need for revised future corrosion protection strategies in the offshore wind energy business. A literature study was carried out and recommendations for future corrosion protection of monopiles were discussed with interested parties and attempts to obtain consensus on future corrosion protection strategies were initiated.

Visible corrosion of ladder, image courtesey of FORCE Technology

Background

Historically, internal corrosion in monopile foundations has led to unexpected costs in relation to retrofits and repairs. Years ago, it was a common belief internal corrosion would never be an issue to consider as the monopile was assumed to be a sealed compartment. However, several monopiles have been inspected since and unexpected corrosion rates have occasionally been observed.

Coating the inside of an existing monopiles is not realistic and the second-best solution is to retrofit CP, either GACP or ICCP. However, offshore repair is always very expensive and should be avoided on future offshore structures.

Potential

By understanding the malfunctions of previous internal corrosion protection of monopile foundations we believe discussions with contractors in the wind energy business may lead to a consensus in selecting a strategy with a high potential for industrialized mass production and a low risk for failure. Hence cost will decrease and we shall be able to maintain Danish competitiveness and secure jobs in offshore wind energy.

Objective

The aim of the fast-track was therefore; a) to review and extract learning’s on monopile corrosion protection in order to avoid any mistakes in the future and; b) to give more recommendations for future corrosion protection of monopiles and; c) to discuss the recommen-dations with other interested parties in the industry – also outside the societal partnership.

Main findings

The literature available today suggests that it is difficult to maintain an oxygen starved compartment of the size of a monopile. Recent guidelines even advise caution against using this approach.

Recommendations on new corrosion protection strategies were therefore given.

Conclusion

It was found that it is difficult to come up with a universally applicable solution for all monopile foundations. However, this investigation opened for discussions with contractors in the wind energy business attempting to obtain consensus on future corrosion protection strategies thus eliminating repetition of previous mistakes.

Outcome

It It is our view that overall recommendations for future foundations should be focused on either an optimized fully closed structure or fully open structures. A redesigned closed foundation includes removal of problematic components such as inspection hatches, grout connections and seals. The recommendations on open monopiles include coating and CP system internally as well as externally.