WAMIT .ss file

Dear all,

I am using FASTv8 and in the HydroDyn.dat file it is requiered WAMIT output files. With .hst, .1 and .3 is not problem but how is compound .ss file?

I was reviewing WAMIT documentation but it is not there.

Thanks in advance,

Dear Joe,

I’m assuming you are referring to the linear state-space model input file (*.ss) of HydroDyn. The *.ss file is not generated by WAMIT, but by the SS_Fitting preprocessor of HydroDyn, which itself makes use of WAMIT output. SS_Fitting is available from: nwtc.nrel.gov/SS_Fitting. The *.ss file is only needed by HydroDyn when the radiation memory effect is calculated using a linear state-space model (RdtnMod=2) in place of the convolution method. If you use the convolution method (RdtnMod=1), only the WAMIT *.hst, *.1, and *.3 files are needed (plus files related to the second-order effects, if enabled).

Best regards,

Dear Jonkman,

Was that. Thank you so much.

Best regards,

Dear Jonkman,

I return to the subject of *.ss files.

I execute ‘ss_fitting.m’ to my hydrodynamic and hydrostatic results (*.1, *.3 and .hst). But the obtained *.ss file is not good enough. I analyzed the resulting plots and are very different to original ones. There are some oscillations where there should not be. Why can be this?

I attached *.ss results and my hydrodynamic and hydrostatic results.

Thank you in advance,
K.zip (49.5 KB)
Hydro.zip (27.8 KB)

Dear Joannes,

I’m not an expert on the application of the SS_Fitting software. I would send detailed questions to Tiago Duarte of EDP (who developed SS_Fitting) (and I don’t believe Tiago checks this forum regularly).

Which identification method did you use and what fit accuracy did you select? Have you followed the guidance described in Duarte et al’s OMAE 2013 paper: nrel.gov/docs/fy13osti/58099.pdf?

Best regards,

Dear Jonkman,

Thanks for your quick reply.

Yes, I followed the guidance. I try with fourth identification methods and the one that fits best was Frequency Domain Identification (FDI) with Fit Accuracy of 0.97. But the results are not very accurate as previously attached results shows.

If I introduce this *.ss results in FAST simulations, an error is displayed saying that the system is unstable. And without this *.ss file FAST works well.

And, do you think that FAST simulations without this wave radiation memory-effect activated would be valid? I mean, the system behaviour changes slightly. I test it with the ITIEnergy Barge and the difference is very small.

Thank you for getting in touch with Tiago Duarte.

Kind regards,

Dear Joannes,

Hopefully Tiago can help regarding the SS_Fitting software.

Can you run the simulation in FAST/HydroDyn with the convolution method (RdtnMod = 1) used in place of the state-space method?

The importance of the radiation memory effect likely depends on the platform. The radiation memory effect will influence the level of platform damping so it could be important especially for resonance excitation.

Best regards,

Dear Jonkman,

Ok, thank you.

I can not run the simulation in FAST/HydroDyn with the convolution method (RdtnMod = 1). Error displayed says that it is not allowed with the current version of HydroDyn. I used the version 2.03.

Did you sent to Tiago the files attached by me?

Thank you again,

Dear Joannes,

I’m not familiar with this error. The convolution method should be available in all versions of HydroDyn. Can you paste a copy of the error shown in the command window?

No, I have not reached out to Tiago. I suggested that you reach out to him yourself. I don’t need to be the mediator.

Best regards,

Dear Jason,

This is the error displayed:

Error using Run_Simu (line 21)
Error reported by S-function ‘FAST_SFunc’ in ‘FOWT_Simulink_Jonkman_detuned/FAST Nonlinear FOWT 5MW NREL/S-Function’:
FAST_InitializeAll:SrvD_Init:ValidatePrimaryData:Yaw angle and rate are not commanded from Simulink model.
ValidatePrimaryData:HSS brake is not commanded from Simulink model.
FAST_InitializeAll:HydroDyn_Init:The value of Conv_Rdtn is not equal to the glue code timestep. This is not allowed in the current version of HydroDyn.

Ok I will get in touch with Tiago.

Best regards,

Dear JOE,

You can avoid that error by setting RdtnDT equal to the DT specified in the FAST primary input file.

Best regards,