Fatcor: corrosion fatigue for offshore applications

Thanks to our extensive metrology equipment and the combined knowledge of surface characterisation and corrosion, we were able to develop a new methodology to describe the statistical distribution of corrosion pits for unprotected surfaces exposed in seawater

Philippe Thibaux

The Fatcor project is a fundamental research project funded by Belgium’s Federal government. Coordinated by OCAS, Fatcor is a collaboration with the companies Iemants, Parkwind, and Rentel, Ghent University, and the Belgian Welding Institute. The objective is to define the fatigue life of a component that was originally protected against corrosion, but for which the corrosion protection system is defective. In this condition, the design fatigue life is theoretically decreased dramatically.

In the first year of the project, OCAS made a summary of the corrosion protection systems applied in the Rentel and Parkwind offshore wind parks. The most sensitive part is in the splash zone, which is protected by a coating, but which can be damaged. A new quantitative method was developed to characterise the dimensions of the individual pits present on a corroded surface. For this investigation, samples that were exposed in a monopile for 18 months were characterised. The analysis makes it possible to describe the statistical distribution of the surface defect present in an unprotected offshore structure. As these defects are expected to be stress raisers and locations in which fatigue cracks are expected to be initiated, their quantification brings information to reduce unnecessary conservatism. Now that the statistical distribution of the pits on a surface exposed to sea water has been characterised, the next steps are to reproduce these surfaces in the laboratory at an increased pace, and to measure the fatigue performance of samples in either dry or wet conditions. OCAS has engineered and put into service a specific device to measure fatigue performance in wet conditions. The first results look promising.