Hello,
I am trying to model the 5MW DeepWind OC4 semisubmersible in ANSYS based on the information provided (nrel.gov/docs/fy14osti/60601.pdf). My question regards the file available in CertTest regarding this same model. Does that FAST model in accordance with this document or is it only for tests purposes? I’ma asking this becausa as I ws checking the data I noticed the platform mass defined in the ElastoDyn file to be 3.85218E+06 kg, while in the document it is said to be 1.3473E+7 kg. Is there any reason for this or am I misunderstanding something?
Thank you in advance
Francisco Pimenta
Dear Francisco,
Yes, the FAST / OpenFAST model of the OC4-DeepCwind semi as provided in the CertTest / r-test are fully consistent with the specifications report 60601. The total mass of 1.3473E7 kg includes the mass of the water ballast. However, in this FAST / OpenFAST model, only the steel mass is considered in ElastoDyn; the water ballast is included in the strip-theory solution of HydroDyn.
Best regards,
Thank you very much. I have also noticed that the model includes an additional hydrostatic stiffness defined. As I have not found this topic in the report, I was wondering I there was a need to introduce this in the FAST model?
Thank you
Francisco Pimenta
Thank you very much. I have also noticed that the model includes an additional hydrostatic stiffness defined. As I have not found this topic in the report, I was wondering I there was a need to introduce this in the FAST model?
Thank you
Francisco Pimenta
Dear Francisco,
The hydrostatic stiffness specified in the AddCLin matrix is associated with the water ballasting in HydroDyn. See section 6.8.3 of the draft HydroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.
Best regards,
Thank you very much for your time and help. I assumed it had something to do with the ballast and as I hadn’t find a reference to that in the report I should have searched in the theory manual as well. Thank you for your time.
One final question, if I may. The added mass and damping files based on the WAMIT model don’t seem to be the values plotted in Figure 4.3 of NREL/TP-5000-60601 report.
In my ANSYS model I am able to get values near the ones in the report, but the response is still off when I try to compare it with the one I’m getting through the FAST model, even for free motion of the structure when displaced from its equilibrium position (the damping of my model and the FAST one just don’t seem to be the same). As I noticed I this went to the files to try to compare the added mass and damping coefficients at the frequency of interest and I just don’t seem to be getting the plots in Figure 4.3 of NREL/TP-5000-60601 report when using the WAMIT output files.
Is any reason why they don’t match that I’m just not taking into consideration?
Thanks in advance
Dear Francisco,
The WAMIT data should match that which is plotted in Figure 4.3. Are you using the WAMIT data stored in the marin_semi.1 file? Please note that the WAMIT data is stored nondimensionally. The added mass is normalized by the density (1025 kg/m^3) and the damping is normalized by the density times angular frequency. The normalization additionally includes a length scale (L) raised to some power, but for the OC4-DeepCwind semisumbersible, a length scale of unity (L = 1 m) was used. See Chapter 4 of the WAMIT User Manual for more information–available online at: wamit.com/manual6.4/Chap4.pdf.
I can’t really comment on why the results from your ANSYS model are not matching that of the FAST model.
Best regards,
Thanks for your time. I was already aware of that adimensionalization procedure used in WAMIT. I was misreading the files and they do match the ones presented in the report. I am still checking everything trying to understand the differences between the 2 models. If I find anything that I think could be helpful for anyone else doing the same kind of tests I will post it here.
Thank you for your time.
Best regards
Francisco Pimenta