First- and second-order excitation loads

Dear sir,
I calculated the first-order and second-order excitation loads in the OC4’s surge direction in HydroDyn, and obtained the spectrogram through FFT, but it is very different from the spectrogram in your article. Your first-order excitation load curve is smooth, and the second-order sum frequency load lasts until 1Hz. I would like to ask if there is a problem with my settings. Is this related to the floating foundation (your article is the OC3 platform), and the output parameters of the HydroDyn wave excitation loads “B1WvsFxi” “B1WvsF1xi” "“B1WvsF2xi”.I have already modified the cutoff frequencies of the difference frequency and sum frequency, but the result still does not change
Best regards,



Dear @Hairui.Jiang,

I can’t really comment much without knowing more about your model set up. Just a few comments so far:

  • The second-order difference-requency and sum-frequency effects will depend on the floating platform and sea state.
  • The smoothness of the force curves will depend on the simulation settings and post-processing applied.
  • Which paper are you comparing your results to?
  • You shared an excerpt from your HydroDyn input file from the 2ND-ORDER WAVES section, but what are the other settings? Are you using the full QTFs provied by NREL for the OC4-DeepCwind semi model?

Best regards

Dear sir,
Thank you for your reply. My specific HydroD file settings have been uploaded. I am using the OC4 semi-submersible platform in the case. I want to know whether the natural frequency of the OC4 semi-submersible platform floating turbine is within the second-order wave frequency range, just as described in your article(“Effects of Second-Order Hydrodynamic Forces on Floating Offshore Wind
Turbines”), I set the same sea conditions as you, but the results are quite different. Is it because of the different floating platforms? I use QTFs to calculate the second-order wave force.
Best regards,



Dear @Hairui.Jiang,

Your input file settings look fine, but please note that it appears that you have enabled the second-order sum-frequency QTF (SumQTF = 12), but you have not enabled the second-order difference-frequency QTF (DiffQTF = 0). Are you using the first- and second-order WAMIT data provided by NREL or did you derive this data yourself?

The natural frequencies of the OC4-DeepCwind semisubmersible are summarized in Figure 3 of the following paper: https://www.nrel.gov/docs/fy14osti/61154.pdf.

Regardless, I would expect differences in results between the OC3-Hywind spar and the OC4-DeepCwind semisubmersible.

Best regards,