Hello NREL Forum Users,
I have conducted an analysis of a floating offshore wind system built with the IEA 15MW Turbine and the VolturnUS-S platform.
The analysis was performed for DLC 6.2 using OpenFAST and OrcaFlex respectively.
The modeling has been fully completed, and upon reviewing the analysis results, the positions of mooring lines with high tension under certain environmental conditions, as well as the motions of the floater, showed similar behavior between the two tools.
However, the maximum mooring tension observed in OpenFAST is significantly lower than that observed in OrcaFlex (approximately 1.5 times lower in OpenFAST).
Could it be that something is wrong with the OpenFAST analysis?
Has anyone else performed an analysis of the IEA 15MW model using other tools and observed similar results to mine?
Or could there be an issue in the analysis process, such as the steps shown in the figure below?
I would appreciate any thoughts or insights.
Sincerely,
Jisu Lim.
Dear @Jisu.Lim,
Certainly we’ve compared OpenFAST against OrcaFlex many times, generally with good agreement, but I’m not aware of the specific case were you are comparing.
You mention DLC 6.2, which presumably refers to the storm case of an idling turbine under 50-year wind and wave conditions with large yaw error (due to grid loss). Normally this case is run without BEM and without an unsteady airfoil aerodynamics model enabled; however, I see these mentioned in your screen capture. Can you clarify?
Best regards,
Dear Jason Jonkman,
Thank you for your prompt response.
First of all, I would like to share with you the environmental conditions and .dat files used in my OpenFAST analysis, as shown below.
The environmental conditions are listed in the table below.
Additionally, I have captured and shown the key parts of each .dat file (Hydrodyn, Elastodyn, etc.) that may influence the analysis results, as shown below.
Hydrodyn:
Hydrodyn member joint:
Elastodyn:
FST:
Please let me know if you need any additional information.
For your reference, the tension magnitude difference compared to Orcaflex was about twice as much.
Sincerely,
Jisu Lim.
Dear @Jisu.Lim,
From this information, I see that you are simulating the rotor parked with blades feathered under severe wind and wave conditions. In my prior post, I expressed concern with your AeroDyn settings, but you didn’t share those for me to check.
FYI: Normally a rotor of this size would be idle (GenDOF = TRUE with torque control disabled) rather than parked (GenDOF = FALSE) in severe winds.
Best regards,
Dear Jason Jonkman,
Thank you for your prompt response!
Apologies for not providing the information about AeroDyn earlier.
I’ve attached the details regarding the AeroDyn settings below.
Additionally, I will incorporate the (GenDOF = TRUE) setting you mentioned and try running the analysis again.
Do the settings for the other .dat files (such as HydroDyn, ElastoDyn, etc.) seem okay to you?
Sincerely,
Jisu Lim.
Dear @Jisu.Lim,
I have seen your AeroDyn input file. I think the issue is that you set WakeMod=1.
I know that when simulating idling wind turbine, WakeMod=0
Best Regards,
Riad
Dear @Jisu.Lim,
I agree with @Riad.Elhamoud that WakeMod should be 0 when modeling an idling rotor. I would also recommend setting AFAeroMod = 1 when modeling an idling rotor because the unsteady airfoil aerodynamics models are likely not valid in deep stall.
Your other input file settings look fine to me.
Best regards,
Dear Jason Jonkman, Riad.Elhamoud
Thank you sincerely for always providing prompt responses.
Since you confirmed that there are no issues with the input files I sent, I plan to proceed with the simulation using the options WakeMod = 0 and AFAeroMod = 1, along with reviewing the additional .dat files.
Thank you for answering my questions.
If I have any further inquiries, I will ask again through the forum.
Thank you. 
Jisu Lim.
