Frequently Asked Questions (FAQ)

Questions

The internal_structure_2d_fem contains definitions that are not documented in the current version of windIO, what is offset_plane?

Correct. We anticipated that the used `offset_plane` section would be introduced sooner into windIO. During spring 2025 windIO will be updated to contain a method for intersecting the blade surface with a plane. Once solidified, the windIO defintion of the reference turbine will be updated. Until then, please use `start_nd_arc` and `end_nd_arc` defined for webs and spar caps.

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.

What is the diameter of the mooring chain?

The correct nominal diameter of the mooring chain is 225 mm. In hydrodynamic calculations, this value is multiplied by 1.8, resulting in a hydrodynamic equivalent diameter of 405 mm. This is why GitHub shows 225 mm in the YAML file and 405 mm in MoorPy—both are correct. However, Table 19 in the technical report is incorrect and should list 225 mm, not 400 mm. Some figures in the report may also be affected, as they were generated assuming 225 mm value in MoorPy. Although 225 mm is on the very high end for chain diameter and was initially considered controversial, the mooring experts confirmed it is still reasonable as a future-looking assumption, not an error.

Are there upper pontoons on the floating platform?

Yes, there are upper pontoons on the floating platform, similar to the VolturnUS-S design of the IEA 15-MW reference turbine. The pontoons were mistakenly omitted in the original report restored in later releases of the modeling files.