Beside the requirements with respect to ultimate and fatigue limit state the lower eigenfrequencies of a wind turbine support structure are an important design consideration. The lower eigenfrequencies of the entire system of support structure and rotor-nacelle-assembly (RNA) are mainly driven by the stiffness of the support structure including its foundation and the mass of the RNA. A proper design has to prevent significant resonances between such eigenfrequencies and excitations from the waves or the rotor frequency and its higher harmonics. In this chapter it will be elaborated that for large offshore wind turbines in or beyond the 8 MW class the combination of a relatively stiff jacket support structure and the RNA can results in severe dynamic problems
At an early design stage the dynamics of wind turbines can be estimated by the Campbell diagram. For this purpose the excitation frequencies 1P, 2P, 3P, etc., i.e. multiple of the rotational speed, as well as the eigenfrequencies of the whole wind turbine system are plotted over the rotor speed. This way potential resonances are graphically indicated if an intersection of an excitation ray with an eigenfrequency takes place within the operational rotor speed range.
The Campbell diagram of the INNWIND reference turbine with the reference jacket is shown in Figure 1-1. At a rotor speed of 6 rpm a resonance between the blade passing frequency and the 1st combined eigenfrequency occurs. In order to reduce the dynamic excitation a rotor speed exclusion window between 5.5 and 6.8 rpm is considered.
Figure 1-1: Campbell diagram of reference turbine with focus on RNA eigenfrequency This resonance problem of the INNWIND reference turbine is considered to be the consequence of a direct up-scaling of both the support structure concept and the rotor design from the 5 MW class to larger turbines. In order to elaborate this Figure 1-2 illustrates two trends in the design of large turbines with rotor diameter between 100 and 250 m, i.e. the rotor rotational frequency and the first eigenfrequency of the entire support structure-RNA system.
Firstly, when the design tip speed of the rotor is maintained, the rated rotor speed decreases inversely proportional to the rotor diameter. In the diagram two excitation bands are related to the rotor speed, the operational range of the rotor speed and the corresponding range of the blade passing frequency with a three times larger magnitude of the frequencies at a three-bladed rotor. The lower frequency of these two ranges is given by the cut-in rotor speed which depends on the used generator-converter concept. A typical ratio between rated rotor speed and cut-in rotor speed is 1.6 and 2 for double fed-induction generator (DFIG) and full power converter with synchronous generator respectively. In the diagram the parameters of the INNWIND reference turbines with a design tip speed of 90 m/s, which is a quite typical value for offshore wind turbines above 3 MW, and a frequency ration of 1.92 is assumed.
Secondly, the increase of the overall height and tower top mass for growing rotor size results in a decrease of the first eigenfrequency of the entire support structure-RNA system. Even with the recently developed extra large (XXL) monopiles their applicability is limited up to the 6-8 MW class and by water depth as well. Typically the first eigenfrequency of these monopile structures ranges between 0.3 and 0.26 Hz as indicated by a transparent red band in Figure 1-2. This band has a negative slope with respect to the turbine diameter and is located close above the rated rotor speed range separated by an at least 10 % safety margin. For some monopile designs this lower limit of the design eigenfrequency rather than the fatigue strength is the design driver.
For large turbine size and deep water locations jacket type structures are considered the only economic alternative at present since floating structures are regarded not mature and competitive, yet. These jackets are providing inherently considerably higher stiffness even if combined with a slender tubular tower. Typical values for the first eigenfrequency range between 0.35 Hz for the 5MW class and 0.3 Hz for the 10 MW class. As a matter of fact the first eigenfrequency of the currently considered design concepts decreases only rather small when the size of the turbines increases. The reason for this trend is the reinforcement of the structural stiffness due to the larger footprint and member size required to provide sufficient strength against the ultimate limit state loads. Apparently the rather low slope of the band of the first eigenfrequency is penetrating more and more into the blade passing frequency range when the turbine size and diameter are rising. In the transparent red band entitled “jacket” in the diagram two dots are indicating the value of the first eigenfrequency of a typical 5 MW and the INNWIND reference turbine respectively. When the resonance frequency moves further towards the rated rotational frequency the resonating rotor speed is associated with a higher mean wind speed. This implies higher excitation energy as well as larger number of operational hours at this wind speed at exposed offshore sites. In order to limit the amplification of the fatigue loads a rather wide rotor speed exclusion zone (wider than ±10 %) would be required if such a mitigation attempt will be effective at all.
Figure 1-2: Design trends on first design eigenfrequency and the rotor and blade resonance ranges for three-bladed offshore turbines in the 5 to 20 MW class
In summary, a strong and severe 3P (blade passing) resonance is expected for very large offshore turbines with jacket structures. Therefore the next two chapters shall highlight and assess challenges and opportunities for an integrated design approach for RNA and support structure on the basis of the 10 MW INNWIND reference turbine.