Frequently Asked Questions (FAQ)

Questions

Are the values of the nacelle inertia correct?

Unfortunately we have a typo in Table 11 of the technical report. The values of Ixx, Iyy, and Izz for the line Nacelle no hub/yaw were reported wrong

Wrong 1.25E+07 1.62E+07 1.68E+07

Correct 1.12E+07 1.45E+07 1.46E+07

Also, x TT should be negative, so -5.85m (the center of mass is upwind, toward the rotor)

The other values in Table 11, in the aeroelastic models, and in the windIO yaml are correct.

Are the values of tower/monopile diameters realistic?

We have received feedback that the state of the art of the maximum diameter at the top of the monopile is between 8 and 9 meters. The limit is imposed by limitations in the drilling equipment. Monopiles can be wider at the base, but at the top today should not exceed 9m. That also translates to maximum outer diameters for the tower of 9m (unless an unusual conical transition is used). The IEA22 adopts a constant outer diameter on the monopile and at tower base of 10m, see Figure 32 in the technical report, and it's therefore outside of what is possible today by 1 m.

Is the floating tower too light?

Yes, we have been told so and we are planning a redesign. Currently, floating and fixed-bottom towers are identical. However, loads in floating are higher and a new tower is needed.

What is the basis of the airfoil polar 360 degree extrapolation?

The report describes using the tool AirfoilPreppy for 360 degree extrapolation, but not what the maximum drag coefficients are based on. AirfoilPreppy can compute $C_{d-max}$ based on the blade aspect ratio (Cdmax = 1.11 + 0.018*AR), but for the IEA 22 MW RWT airfoil data $C_{d-max}$ is prescribed for each polar, with values $C_{d-max}$=1.5 for the 21% and 24% airfoils and $C_{d-max}$=1.3 for the airfoils further inboard. The choice of values are based on Zahle et al (https://doi.org/10.2514/6.2014-0359) and Bak et al (Risoe-R No. 1434(DA)). However, since aspect ratios of blades have generally increased since these studies, the airfoil characteristics in deep stall are quite uncertain.

What are the natural periods of the floating sytem?

Surge: 107.9 s
Sway: 108.4 s
Heave: 17.4 s
Roll: 27.3 s
Pitch: 27.3 s
Yaw: 83.9 s

What is the fundamental frequency of the floating tower? Is the report accurate?

The original report stated in Section 5.5 that the floating tower frequency is 0.22Hz. Simulation shows a value closer to 0.32Hz.