Can the second tower mode become significant for fatigue loads of offshore wind turbine monopiles?

Abstract The fatigue loads of the first offshore wind turbines were assumed to be primarily dominated by wind loads. With growing turbine sizes and dropping natural frequencies wave loading became the dominant fatigue loading, because the first tower mode frequency is near resonance to the wave frequency. As a result there is a strong focus in the design and monitoring of offshore wind turbines on the first tower mode and its properties. With the trend of the increasing size of offshore wind turbines and thus decreasing natural frequencies, the concern might be raised, if the second tower mode can have a significant contribution the fatigue loads of larger offshore wind turbines. In this contribution, a preliminary investigation on the contribution of the first and second tower mode on the fatigue loads of three reference wind turbines, IEA 10 MW, IEA 15 MW, and IEA 22 MW, at rated wind speed and two different wind-wave misalignment conditions is performed. Results show an increase of the significance of the second tower mode at rated wind speed for larger offshore wind turbines as well as in Side-Side direction of the investigated reference wind turbines. The significance of the second tower mode in Side-Side direction persisted even with wind-wave misalignment.