Good mooring,

I directly downloaded the latest version of OpenFAST 2.4.0, and used it to do a free decay test of a 5WM semi-submersible wind turbine. I found that its surge natural period was 120s, but I read other papers about 107s. What is the reason? If I want to use OpenFAST to compare with previous studies, how can I modify it?

Best wishes.

Jingyu.Bian

Dear Jingyu.Bian,

Is this OpenFAST model of the semisubmersible that you are using supposed to be the same as the one from the paper you are referencing? Is this an OpenFAST model provided by NREL (e.g., the NREL 5-MW turbine atop the OC4-DeepCwind semi) or a model you made yourself?

The surge natural frequency is dictated by the full system mass and added mass in surge and the mooring stiffness in the surge direction. So, changing the surge natural frequency would requiring changing the structural mass, added mass, or mooring stiffness.

Best regards,

Dear Jason,

Thank you for your quick reply. I used the NREL5WM semi-submersible floating wind turbines and did not modify the parameters of the model. Is the surge natural period correct? If I want to modify the model, where is the additional mass modified? Is it to modify the value of AddF0 in HydroDy.dat?

Best wishes,

Jingyu.Bian

Dear Jingyu.Bian.

I expect the natural frequency of the NREL 5-MW turbine atop the OC4-DeepCwind semi to have a surge natural frequency closer to 107 s than 120 s. And I would expect the OpenFAST model provided by NREL to predict a frequency closer to 107 s.

Can you clarify how you set-up the free-decay simulation and show the results you’ve obtained?

Best regards,

Dear Jason,

(1) My settings are: Disable CompInflow, CompAero; Ptfmsurge=5, (Ptfmsway, Ptfmheave, Ptfmroll, ptfmpitch, ptfmyaw=0), Wavemod=0; Using Jason_PSD to analyze the surge time-series, the spike corresponds to the natural frequency.

(2) Because I also passed the irregular wave verification and got the surge PSD. It is found that the natural period is consistent with the result of the decay test. So, I don’t know what to do.

Best wishes,

Jingyu.Bian

Dear Jingyu.Bian,

How long of a surge free-decay simulation did you run (are you capturing many oscillations)? Can you share your plots of the resulting surge time series and PSD?

Best regards,

Dear Jason,

The free decay simulation time is 600s, and the results are shown in the attachment.

Timeseries_PSD.docx (52.7 KB)

Dear Jingyu.Bian,

Your time series shows a period less than 120 s, otherwise the last peak would be at 5*120 = 600 s. My guess is the PSD is too coarse at low frequency due to the limited simulation length of 600 s, so, the frequency identified in your PSD has been rounded. To get adequate resolution of the PSD at low frequency, you should include at least 10 periods of oscillation, which would require increasing your simulation length to about 1200 s.

Best regards,

Dear Jason,

Thank you for your reply. I set the decay-test simulation time to 1200s, and the surge natural period is really about 107s.Is the simulation time bigger the better?

Best wishes,

JIngyu.Bian

Dear Jingyu.Bian,

I’m glad you now get the result you expect. A good rule of thumb when selecting a simulation length is to ensure that there are at least 10 oscillations of the lowest period.

Best regards,