Questions about "effective RAOs"

Dear Jason,
I have gotten some calculation results using FAST for the 5MW-OC3-Hywind model, and from the OC3 report in Chapter 5, I found some analysis for the “effective RAOs”, but I don’t really get it.
The report mentioned that the definition for the “effetive RAOs” was the difference in response amplitudes between nonlinear time-domain simulations run with and without wave excitation.
But from what I’ve got, for constant wind and regular wave excitation of one specific frequency, the system might response in different frequencies, and therefore what defines the “amplitude of the motion” under this specific frequency in such case?

Thank you.

Dear Yu Ma,

I’ve attached a memo that I wrote to the OC3 group when we were working through the “effective RAO” load case (load case 5.4). This memo should answer all your questions about what is meant by “effective RAO”.

Note: after the memo was written, we decided in OC3 to run load case 5.4 at 8 m/s instead of at 11.4 m/s about which the memo was written.

Best regards,
5.4.pdf (324 KB)

Dear Jason,
Thanks for replying fast.
I’ve read the memo, and that really helps.
But I still have some confusions. Is it right that you designed the definition of “effective RAO” to evaluate the response induced merely by waves so that you used the quasi-steady response after a certain simulating time?
Here is a result of platform surge time series under the constant wind 8m/s and regular wave of 0.7rad/s. For what I can see, the motion of the platform reached stable at about 600s. And a significant low-frequency response could be found before 600s. Is this low-frequency response just neglected here?

Best regards.

Dear Yu Ma,

The short answer is “yes.” The large-amplitude low-frequency response you show at the start of the simulation is a start-up transient resulting from the choice of initial conditions that don’t coincide with periodic oscillations about the mean position characteristic of an RAO. The start-up transients could be reduced considerably by choosing initial conditions closer to the mean position of the structure for the given mean wind speed. Whatever start-up transients remaining should be ignored in the effective AO calculation.

Best regards,