Linearization OC4Semi OpenFAST : unable to find steady-state solution

Hello everyone,

I try to line the wind turbine 5MW OC4deepcwind with OpenFast 2.5. For the moment, I just take into account 2 degrees: Gendof and Pitch Platform. With the SS-Fitting code, I used the pitch for the modelling of radiation efforts. However, I’m constantly getting “unable to find steady-state solution”. I changed the values of Trimtol and Trimgain but it does not give any convincing results. Linearization only works in the case where Calcstdeady = false.

I join with this post the files used for the case described

Best regards, (19.8 KB)

Dear Salic,

From my brief look at your input files, I see that you’ve disabled wave radiation altogether in HydroDyn (RdtnMod = 0). So, your state-space model is not being used.

Regarding convergence, your simulation is only 100-s long (TMax = 100), which is not long given the low natural frequency of the platform-pitch mode of the OC4-DeepCwind semisubmersible. I would first suggest plotting the time series (RotSpeed, PtfmPitch) to see if the solution is converging over time such that increasing TMax will lead to convergence; TMax may need to be over 1000 s.

Best regards,

Dear Dr. Jonkman,

Thanks for taking the time to respond. I have already done tests with a longer simulation time, but same conclusion, no steady-state solution has been found. I restarted the calculation taking into account the 6 degrees of freedom of the platform. And it works better than if we only allow the platform to pitch.

Best Regards,