Experimental results of a multimode monopile offshore wind turbine support structure subjected to steep and breaking irregular waves

We present experimental data from MARIN on a bottom-fixed offshore wind turbine mounted on a monopile in intermediate water depth subjected to severe irregular wave conditions. Two models are analysed: the first model is fully flexible and its 1st and 2nd eigenfrequencies and 1st mode shape are representative of those of a full-scale turbine. This model is used to study the structural response with special focus on ringing and response to breaking wave events. The second model is stiff and is used to analyse the hydrodynamic excitation loads, in particular the so-called secondary load cycle. The largest responses are registered when the second mode of the structure is triggered by a breaking wave on top of a ringing response. In such events, the quasi-static response accounts for between 40 and 50% of the total load, the 1st mode response between 30 and 40%, and the 2nd mode response up to 20%. A statistical analysis on the occurrences and characteristics of the secondary load cycle shows that this phenomenon is not directly linked to ringing.