Effect of first eigenfrequency of the support structure – RNA system

2.5Effect of first eigenfrequency of the support structure – RNA system

Figure 2-16 and 2-17 illustrate the damage equivalent fatigue loads at the tower base in fore-aft and side-to-side direction respectively over the production wind speed range. The contour plots are normalized with the lower value in each diagram. The vertical red lines are indicating the wind speed at hub height when a rotor speed of 6 rpm is reached and the wind speed with the activation of the pitch control and the rated wind speed respectively. Furthermore the 3P excitation frequency and a ±10 % frequency band is shown. In the vicinity of the curve of the 3P excitation strongly amplified fatigue loads can be seen. Hence the first design eigenfrequency should not be positioned between 0.28 and 0.35 Hz. However, lowering it to a value of 0.275 Hz or below would result in significantly reduced fatigue loads in the entire production wind speed range.

Figure 216: Relative damage equivalent loads for N_ref=10⁷, SN-slope m = 4 over wind speeds and natural frequency for fore-aft tower base direction (Kuhnle 2015a)

Figure 217: Relative damage equivalent loads for N_ref=10⁷, SN-slope m = 4 over wind speeds and natural frequency for side-to-side tower base direction (Kuhnle 2015a)

3Cost-effective design methodologies

Table 3-1 compares five indicative 10 and 20 MW turbine designs.

The first and the second configuration include the 10 MW onshore INNWIND reference design and the 10 MW design with adjusted cut-in rotor speed and rotor speed exclusion zone combined with the 10 MW reference jacket. The latter configuration has been analysed in Chapter 2 and severe problems with the 3P blade passing resonance have been recognized. Therefore an adjusted 10 MW design is proposed here and described in more detail in the following. The fourth turbine parameter set corresponds to a direct up-scaling of the 10 MW INNWIND offshore reference design to 20 MW rated power. Since here an even more pronounced 3P resonance is expected also an adjusted 20 MW design is indicated. The dynamic configuration of the 10 and 20 MW up-scaled offshore design respectively (column 3 and 5 in Table 3-1) is shown in Figure 3-1. As expected form the discussion in Chapter 1 a 3P resonance takes place for the 10 MW design close to the cut-in rotor speed and the 20 MW machine would suffer from a likewise resonance more in the centre of the rotor speed range. Both machines feature a high power density of 400 W/m², an aerodynamic design with maximum tip speed of 90 m/s, classical high induction and solidity blade design, a relatively short tower with a soft-stiff dynamic characteristics and a rotor speed exclusion zone in order to mitigate in an insufficient manner the 3P resonance.

• 
  Increase of the hub height to approximately 130 m or taller in order to lower the first eigenfrequency from 0.30 to 0.275 Hz or below
  
• 
  Significant increase of the maximum tip speed to approximately 105 m/s. Such a value is relatively high for three-bladed design. Most likely the design tip speed ratio of the rotor should be increase as well which could provide additional benefits as described in Section 2.2.
  

• 
  Reduction of the rotor speed variability, i.e. ratio between rated rotor speed and cut-in rotor speed from 1.92 down to 1.61. This effect is indicated by dashed and dotted curves respectively in Figure 3-2.
  

• 
  Advanced controls like higher order individual pitch control (IPC) (see Task 1.4) aiming at the reduction of dynamic loads on both rotor and support structure.
  

In total it should be possible to realize a first design eigenfrequency which is at least 10 % below the cut-in rotor speed in order to facilitate sufficient distance to the strong 3P excitation without any rotor speed exclusion zone.

• 
  Aiming at a light-weight rotor-nacelle-assembly which is opposite to the square-cube law for such a giant turbine.
  
• 
  Maintaining a low or even a decrease of the hub height to approximately 170 m or lower in order to achieve a first eigenfrequency of at least 20 % above the rated (2P) blade passing frequency.
  
• 
  Choosing a two-bladed rotor with a moderate or low maximum tip speed to 100 m/s or lower with a low induction, large rotor diameter. Close to the rated wind speed a low maximum tip speed will result in an operational tip speed ratio which is significantly lower than the design tip speed. The reduced aerodynamics efficiency at the high wind speed offshore site will cause extra losses in energy yield. This should be compensated by the above-mentioned increase in rotor diameter.
  
• 
  A high rotor speed variability of 1.92 or even larger could be maintained.
  
• 
  Advanced controls like higher order individual pitch control (IPC) and/or smart blades with multiple flaps (see Task 1.4) aiming at the reduction of dynamic loads and especially the 2P excitation by the thrust force and the yaw moment on the support structure.