This paper investigates the sensitivity of the natural frequency of a support structure of an offshore wind turbine to parameter variation and variation in models for pile foundations. The predicted natural frequencies are compared with measurements for several wind turbines in two wind farms in the Dutch ‘inland sea’ IJsselmeer.
Offshore wind energy is at the threshold of large-scale application throughout Europe, followed by the US and Canada [Henderson (2001), Zaaijer (2004)]. Offshore wind farms are also planned for Japan shortly (2004–2005). After a series of demonstration projects up to 40 MW installed capacity, mainly at benign sites, the first large offshore wind farms of 160 MW have been built at ‘Horns Rev’ and ‘Rødsand’, both in Denmark. Right from the beginning of the exploration of offshore wind energy, the importance of dynamic behaviour for the design of the support structure was well recognised. Both fatigue and extreme loading depend on the dynamic response characteristics of the system and the application of simple dynamic amplification factors had already proven the hard way to be insufficient for onshore wind turbines. The predictability of dynamic response of offshore wind turbines differs in some important aspects from that of platforms for the offshore oil and gas industry on the one hand and onshore wind turbines on the other. Loading of the rotating wind turbine blades causes at least two significant extra excitation frequencies: one at the rotation frequency and one at the blade passing frequency. Therefore, the first natural frequency of an offshore wind turbine is wedged between wave and rotor excitation frequencies, whereas the natural frequencies of a fixed platform for the offshore oil industry are usually designed to be well above the main wave frequencies [van der Tempel (2002)].