M areike Collmann

Leibniz Universitaet Hannover, Hannover, Germany



Monopiles are still the predominant support structure for offshore wind farms in Europe with a market share of around 80 % in 2020. Due to greater water depths and increasing turbine dimensions, current monopiles have a diameter up to 10 m and maximum plate thicknesses up to 150 mm. Due to wind, wave and turbine operation, the monopile experiences up to 109 load cycles during its design service life of 20 years. The fatigue resistance is often decisive for the design. Circular and longitudinal welds are designed using the nominal stress approach based on experimentally determined SN curves. With increasing plate thickness, a thickness-dependent reduction of the fatigue strength becomes necessary. Due to the high test loads required, this reduction has hardly been experimentally proven so far. The influence of the sheet thickness and the medium stress sensitivity are difficult to differentiate in test series with small specimens. In this paper, existing test series from older and more recent literature as well as own test series are therefore statistically evaluated and assessed again. Even in the region of high cycles with N > 107, an experimental validation of the generally assumed fatigue limit is still insufficient due to the long duration of tests. Alternative high-frequency testing techniques are required to test specimens up into high cycle fatigue region. In this paper test results are shown and the sensitivity to medium stress and the dependency on frequency are evaluated. Finally, these results are placed in the context of data already evaluated.


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