Dear OpenFAST community,
- I am using the 5MW_TLP_DLL_WTurb_WavesIrr_WavesMulti file for simulation in OpenFAST.
- I am trying to comparison of motion RAO (Figure 26-RAO WAMIT 27m) calculated by Denis Matha in “Model Development and Loads Analyis of an Offshore Wind Turbine on a Tension Leg Platform, with a Comparison to other Floating Concepts”.
- The way I do it is calculating motion RAO using white noise spectrum, with reference to the paper("Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines) and forum posts
[Method]
I used 8000 seconds out of a 10000second simulation.
wave time series → Sxx (spectral density ) using Blackman-Tukey
ouput response(surge, pitch) → Sxy (spectral density) using Blackman-Tukey
motion RAO = Sxy/Sxx
- For surge motion RAO, I think the peak values match and y(RAO)-values for each frequency appear to be influenced by smoothing.
However, For pitch motion RAO, the magnitude of the peak values varies significantly.
Is there something I am forgetting or doing wrong?
Please give suggestions on how to analyze.
Thank you for your help in advance.
Dear @Wonjun.Choi,
It looks like your natural frequencies are lining up well between your results and Matha’s results for both the surge and pitch modes, but the level of damping is quite different, with your results showing significantly less damping. This was done many years ago now, and I don’t recall the details, but I see in section 3.5 of Matha’s thesis-turned NREL report that it says his results include aerodynamics enabled with the rotor spinning at a constant 12.1 rpm at 11 m/s. I don’t see that your results include aerodynamics, but the lack of aerodynamic damping in your model could explain the difference.
Best regards,
Hello, dear Dr.Jonkman
First of all, I appreciate your compassionate response.
- Modified content: Aerodynamic effects were considered when the rotor rotates at a constant speed of 12.1 rpm at 11 m/s.
- Result: Aerodynamic damping effects have been confirmed. It was effective in Pitch RAO, but the peak value(y) decreased in Surge RAO.
I was comparing the pitch WAMIT 27m in Figure 26 with WJ(OpenFAST).
I think there could be a difference between the value transformed from time domain to frequency domain and the WAMIT(spoke 27m).
I am reviewing whether there are any errors in value settings and trying various approaches.
Best regards
WonjunCHOI
1 Like
Hello, dear Dr.Jonkman
First of all, I resolved the error based on your advice. Thank you.
- I have a question regarding the values in Denis Matha’s “Model Development and Load Analysis of Offshore Wind Turbines Installed on Tension Support Platforms, and Comparison with Other Floating Concepts.”
- First, looking at the previous results, the peak frequency of the pitch motion RAO matches at 1.8 [rad/s] (Figure 26).
- However, the natural frequency in Table 6 is 0.2211[Hz], which corresponds to 1.3892[rad/s].
- When the full system(platform+turbine) is considered as rigid and a static offset test (free decay) is performed, the results match the natural frequencies in Table 6.
- The values I calculated match the natural frequency values in Table 6, but they differ from the RAO(5) Pitch WAMIT 27m in Figure 26.
- The natural frequency of RAO(5) Pitch WAMIT 27m in Figure 26 should also be the same, as it is calculated as a rigid body.
- Are the analysis conditions for Table 6 and wamit (Figure 26) different?
I will attach the values I calculated along with the relevant data (Table 6 & Figure 26).
Best regards
WonjunCHOI
Table 6
Figure 26
Dear @Wonjun.Choi,
This work was done many years ago by Denis Matha and I don’t recall the details now. But my understanding is that the pure WAMIT analysis assumes a fully rigid system whereas the FAST model has structural flexibilities, like the tower, which shifts the platform-pitch frequency lower. The FAST 27m result in Figure 26–with a peak around 1.4 rad/s–seems quite consistent with the pitch natural frequency in Table 6 of 1.39 rad/s.
Best regards,
Hello, dear Dr.Jonkman
Your reply makes perfect sense. Thanks again for sharing your time.
- Next, I will proceed with secondary wave load analysis(QTF, sum frequency) using NREL TLP model.
- I plan to create input file for OpenFAST using AQWA and compare the results with OrcaFlex.
- I will use the information in forum for reference.
I will share the results to forum after calculation is complete.
Best regards,
WonjunCHOI
