Low-Frequency RAOs from White Noise excitation in OpenFAST

[darkstar71]

Dear Dr. @jjonkman,
I'm reading this paper "Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines" [https://docs.nrel.gov/docs/fy13osti/58098.pdf] and following the procedure described in the paper but I was wondering

1. What was the white noise wave excitation frequency range used for this RAO generation?
2. Is there any valid range of frequencies to be used for WhiteNoise spectrum in HydroDyn/seaState in OpenFAST?
3. What is the meaning/use of RAO for low-frequency ranges?

My query is relevant for box part of this image.

Best regards,

[jjonkman]

Dear @darkstar71,

Here are my responses to your questions:

1. I don't recall what frequency range we used for white-noise excitation our ISOPE 2013 paper, but the data is plotted between 0 and 0.8 Hz, depending on the figure.
2. I would expect that you'd want to cover the first-order wave-energy range (e.g., 0.05 - 0.5 Hz), with extensions to lower and high frequencies to cover the floater natural frequencies.
3. The main reason to compute ROAs is to identify the natural frequencies and damping ratios of various modes of a floating offshore wind turbine.

Best regards,

[darkstar71]

Dear Dr. @jjonkman,
Thank you very much for your answers. I really appreciate it.

Regarding your response to Question 2, my understanding from the wave mechanics literature is that the practical lower-frequency limit for wind-generated waves is often taken to be around 0.03 Hz. What I am struggling to understand is that OpenFAST allows the generation of a white-noise wave spectrum at frequencies below this nominal physical limit. Could you please clarify how this should be interpreted?

My background is in shipbuilding and offshore engineering. In that context, I have frequently seen RAOs across the relevant wave-frequency range. My understanding is that, aside from the application you mentioned, RAOs may also be used as a linear scaling measure of system response for similar sea states with wave amplitude around unity.

If I follow that analogy, however, it seems that such an interpretation would break down below 0.03 Hz, because waves at those frequencies are generally not considered physically present in measured wind-wave spectra. Is this line of reasoning incorrect?

Figure 11 in this paper, https://docs.nrel.gov/docs/fy18osti/68050.pdf reported surge, heave and pitch RAOs within wave-frequency range [0.04 - 0.17 Hz) using white-noise spectrum without showing the low-frequency part which is very relevant for semi-submersibles. Is there any specific reason for that?

Best regards,

[jjonkman]

Dear @darkstar71,

I don't disagree that physical linear waves exist within a given frequency band. But normally, floater natural frequencies are placed outside of this band to avoid direct resonance of these modes. As such, we've found it useful at times (but not in all cases, such as the OC5 Phase II paper you cite) to compute RAOs beyond the physical limits of first-order waves, not because we care about the first-order wave response at those frequencies, but rather to identify the natural frequencies and damping of these modes, which can be excited by nonlinear hydrodynamic terms, e.g., 2nd-order terms.

Best regards,