Free decay test for 15MW RWT UMaineSemi

Dear all,

I am using OpenFAST-v3.0.0, and trying to reproduce the free decay in “Definition of the UMaine
VolturnUS-S Reference Platform Developed for the IEA Wind 15-Megawatt Offshore Reference Wind Turbine”. The case is downloaded from GitHub. When I give an initial displacement, time series show that the magnitude and frequency of the displacement are fine. The only problem is that the pitch is not centered at zero. Then I removed the initial displacement, i.e., zero initial displacement, without wind, wave and current. The plot below shows the pitch center is at around -1.3 deg, surge is off as well. What could cause this center shifting? Any suggestions are greatly appreciated!

Best regards,
Xiaoqin

Dear Xiaoqin,

I’m not too familiar with this specific model, but I would guess the nonzero offset in surge and pitch in the absence of wind and wave loading is caused by the overhanging weight of the rotor-nacelle assembly, offset from the tower centerline.

Best regards,

Dear Jason,

I compared the mass of RNA and related parameters, they are indeed different from the paper. I suppose there is offset of CG of RNA from tower axis for the setting in the GitHub package. Thank you for pointing in the right direction.

Regards,

Xiaoqin