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Frequency-Domain Hydrodynamic Analysis about OC4 #2578
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Dear @axxyyyyyyyy378, It looks like your OpenFAST model is physically or numerically unstable. To isolate the problem, I would first disable second-order hydrodynamics. Is your solution stable then? If not, I would check that your ANSYS AQWA-developed first-order solution (.1, .3, .hst) is consistent with the first-order solution provided in the OpenFAST r-test, which was derived from WAMIT. FYI: the body mass and inertia should not be needed to compute the first-order solution (.1, .3, .hst). But the values you say you are using for the second-order solution are not correct because they only represent the platform itself, not the mass/inertia of the full floating offshore wind turbine. Best regards, |
Dear @jjonkman, Merry Christmas! (maybe we have a time difference T.T) Thank you for your patient response. After reading your reply, I first disabled the second-order hydrodynamic calculations, but the results were still incorrect. I then manually compared the first-order solution generated by ANSYS AQWA with the reference solution provided by OpenFAST, and I found some mismatches in the hydrodynamic parameters. For instance, the hydrostatic stiffness coefficients for roll and pitch in the After correcting this, the **significant differences in the results of the Additionally, based on your comments, I did not account for the total mass/inertia of the full OC4 floating wind turbine when calculating the second-order hydrodynamic parameters ( Thank you in advance! |
Dear @axxyyyyyyyy378, I'm not sure why your first-order AQWA solution differs from the WAMIT solution provided by NREL. If it helps, the dimensional NREL solution is plotted in Figure 4-3 of the NREL/TP-5000-60601 report you reference. How do your results compare when plotted on top of each other? The full-system mass, center of mass, and inertias of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semisubmersible are documented in my post dated Feb 13, 2020 in the following topic on our forum: https://forums.nrel.gov/t/oc4-deepcwind-semisubmersible-wamit-files/2203. Best regards, |
Dear @jjonkman, Thank you for your response. As you mentioned, I compared the AQWA results with the NREL solution in Figure 4-3 of the NREL/TP-5000-60601 report. From the comparison, most results match well; however, there are significant differences in the surge-pitch elements ( Best regards, |
Dear @axxyyyyyyyy378, Good question. In addition to the sign flip on the 15 (surge-pitch) and 24 (sway-roll) coupling terms, I also see that the AQWA results are quite a bit smaller than WAMIT in magnitude for the 15, 24, 44, and 55 terms, while the results look nearly identical for the 11, 22, 33, and 66 terms. Are you using the same reference point in AQWA as used by the WAMIT results, which is the intersection of the vertical centerline of the platform and still water level? Best regards, |
Dear @jjonkman, Thank you for your patient response. You are absolutely correct—I mistakenly set the reference point at the actual center of gravity of the platform (13.46m below the waterline). When I adjusted the reference point to the intersection of the vertical centerline of the platform and the SWL, the results in Subsequently, I recalculated the second-order hydrodynamic coefficients ( However, as you previously pointed out, when calculating second-order hydrodynamic coefficients, I need to account for the mass and inertia parameters of full wind turbine , including tower, nacelle, and blades. Yet, based on the current results, it seems that considering only the floating platform has already yielded accurate outcomes, which leaves me somewhat puzzled. **the results in **comparison of the six-degree-of-freedom responses of the platform : In subsequent tests, I encountered another issue. I attempted to include all wave directions (-180° to 180°, in 10° intervals) in the second-order hydrodynamic parameter files ( Best regards, |
Dear @axxyyyyyyyy378, When calculating the second-order potential-flow solution for HydroDyn, the full system mass, center of mass, and inertia should be used and the reference point for the hydrodynamic outputs should be the same as the first-order solution. The reference point is user-specified in HydroDyn via If your results from AQWA match those from WAMIT without these settings, then perhaps the second-order effect is not playing a large role for the conditions you are simulating. Regarding the "sparse" error, does the direction and frequency data in second-order solution include all combinations of direction and frequencies pairs? Best regards, |
Dear @jjonkman, Thank you for your response. Your comments are very professional, and I now fully understand the issue with setting the reference point. As you mentioned, since the environmental loads I simulated were relatively mild, the second-order effects did not play a significant role. To address this, I made adjustments by using the full system mass, center of mass, and inertia in AQWA, and ensured that the hydrodynamic output reference point matched the first-order solution's reference point. I then "correctly" calculated the second-order hydrodynamic coefficients in HydroDyn. Subsequently, I compared the platform displacement responses obtained using AQWA and WAMIT results in OpenFAST under a set of extreme loads (turbulent wind Regarding the "sparse data" error, the second-order hydrodynamic coefficient file ( **platform displacement responses obtained using AQWA and WAMIT results in OpenFAST: Best regards, |
Dear @axxyyyyyyyy378, Regarding the second-order solution calculated by AQWA, have you plotted the QTFs against the equivalent results from WAMIT at 0 degrees wave direction (like you did with the added mass and damping matrices)? Regarding the "sparse" error, presumably you mean 49 unique frequencies (rather than 47. Regardless, it seems like you are missing a dimension in your QTF. I would expect there to be 6 * nd^2 * nf^2 rows in the QTF, where nd = number of wave directions and nf = number of frequencies. With nd = 37 and nf = 49, this would be 19,721,814 rows; you are missing the combinations of wave direction pairs. Best regards, |
Dear @jjonkman , Thank you for your response, but I’m not sure how to post-process the second-order hydrodynamic data or what kind of plots to create for comparison and validation. Do you have any suggestions? Best regards, |
Dear @axxyyyyyyyy378, For the second-order data at a given wave direction (zero degrees), I would plot the QTF as a contour / surface plot as a function of the two frequencies. The QTF is complex valued; so, you could plot the magnitude and phase or the real and imaginary components. You could scale the data to physical units or keep the data nondimensional. Best regards, |
Dear @jjonkman, Thank you for your prompt response. I compared the QTF magnitudes for three degrees of freedom (surge, heave, and pitch). However, the results show a significant discrepancy between the reference values from WAMIT and the values I obtained using AQWA. Considering that the first-order hydrodynamic coefficients matched very well previously, what do you think could be the reason for this difference? Best regards, |
Dear @axxyyyyyyyy378, I'm not sure. The second-order difference-frequency QTF depends both on the quadratic interaction of first-order quantities as well the solving of the second-order potential. Perhaps you could isolate the contribution of each, although I don't believe I have access to a WAMIT solution for the OC4 DeepCwind semisubmersible that separates the two contributions. The diagonal of the difference-frequency QTF (where f1-f2 = 0, representing the mean drift term) depends only on the first-order solution, so, hopefully that compares well (it is hard to tell from your contour plots). If that differs, I would first identify what is causing that. Best regards, |
Dear @jjonkman, Happy New Year! Thanks for your professional advice. I compared the QTF magnitudes for the three degrees of freedom (surge, heave, and pitch) as quickly as possible when f1 = f2. From the comparison, the overall trend of the curves appears similar, but the results from AQWA exhibit more significant fluctuations and pronounced spiky noise. Could you provide some other suggestions? Best regards, |
Dear @axxyyyyyyyy378, I'm not sure why AQWA has more "noise" than WAMIT for the slow-drift terms. FYI: You could generate similar plots at other difference frequencies by plotting the results for f1-f2 = constant, where the constant is the specific difference frequency of interest. Best regards, |
Dear @jjonkman, Thanks for your advice. As far as I know, the wave periods worth attention in certain sea areas are roughly between 3s and 16s, so the wave frequencies fall within the range of 1/16 Hz to 1/3 Hz. I have redrawn the QTF magnitudes for the three degrees of freedom (surge, heave, and pitch) for f1-f2 = constant. From the results, the surge shows good agreement, the heave matches less well in the high-frequency range, and the pitch exhibits the poorest agreement. However, it seems that this is the best we can achieve for now. It looks very challenging to completely resolve this issue. Best regards, |
Dear @jjonkman ,
Today, I attempted to use
ANSYS AQWA
as an alternative toWAMIT
(since I do not have access to the latter) to calculate the frequency-domain hydrodynamic parameters for the OC4 wind turbine. Below, I have attached some of the geometric parameters and wave conditions used during the calculations. I have modeled the geometry of the floating platform and defined the baseline parameters following the reference (Robertson, A., Jonkman, J., Masciola, M., Song, H., Goupee, A., Coulling, A., Luan, C., 2014. Definition of the Semisubmersible Floating System for Phase II of OC4 (No. NREL/TP-5000-60601, 1155123).However, it seems there were some errors in my settings.Beacuse when I imported the hydrodynamic parameter files generated by AQWA (
.1
,.3
,.hst
,.12s
,.12d
) into OpenFAST, the simulation produced incorrect results. Under a steady unidirectional wind ofU = 9 m/s
and unidirectional irregular waves withHs = 1.2646 m
andTp = 10 s
, the wind turbine exhibited unreasonable platform displacements.I have also included the erroneous simulation results below. Could you kindly help identify where my settings might be incorrect?
**AQWA settings below:
**Unreasonable results:
Thanks in advance.
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