Need help to explain spectral density plots of motion response of a FOWT

From a coupled dynamic analysis of a semi-submersible FOWT, platform motion responses in the turbulent wind are obtained as one-hour time series. Later, these time series (for different values of Hs, Tp, and turbulent wind field) are transferred into the spectral density plots (w (rad/s) vs. S(w)). Wind and waves are aligned along the x-axis of the platform.

I know the 1P & 3P frequencies of the rotor, w_p (2*pi/T_p) from the wave spectrum, and natural frequencies of the platform from decay tests.

Now, using the spectral plots,

  1. Is it possible to comment/explain why the spectral peaks are at a certain frequency?

  2. When there are multiple peaks in a single plot, “peaks within the wave frequency range are due to the wave and outside wave frequency are due to non-linear effects” can I comment like that?


Dear @Touhidul.Islam,

Just a couple comments:

  • I would expect the FOWT response to be dominated by wind and wave excitation frequencies and system natural frequencies. So, identifying these frequencies on the plots is useful to determine what is driving what
  • It is often useful to run separate simulations with wind-only and wave-only excitation to help determine what is driving what.

Best regards,

Many thanks for your comments. It means a lot to me.
Kind regards,