Question regarding OpenFAST capabilities for extreme loads and superelements

[YuhengShey]

Dear Dr. Jonkman,

I am a PhD student researching jacket-type offshore wind turbines. I am writing to ask about the reliability of OpenFAST in simulating extreme environmental conditions, such as 50-year return period wind and waves. And the degradation or damaged scenarios.

Specifically, I would like to know:

1. Is OpenFAST considered accurate for capturing nonlinear hydrodynamic or wind loads in these extreme limit states?

2. Can the SubDyn superelement method effectively handle the nonlinearities introduced by extreme waves or damage (fatigue crack or corrosion)?

I am looking forward your answers or suggestions.
Thank you for your time and guidance.

[jjonkman]

Dear @YuhengShey,

If you are asking whether OpenFAST can support the IEC-style loads-analysis requirements for the storm load cases involving 1- and 50-year wind and wave conditions (e.g., DLC 6.X and 7.X), the answer is "yes". For example, the Extreme Wind Model is supported by TurbSim (whose output is used as in input to OpenFAST) and the SeaState module of OpenFAST includes constrained NewWave theory, e.g., for embedding the 50-year extreme height into an otherwise irregular seastate.

If you are asking whether OpenFAST does not have limitations when modeling extreme events, the answer is "no". For example, SeaState only includes wave kinematics up to second-order and SubDyn is inherently linear and so does not consider plasticity or other types of structural damage, but these are not requirements for the loads analysis dictated by IEC design standards.

Best regards,

[YuhengShey]

May I ask which documents validate OpenFAST's performance under extreme wave and wind conditions (e.g., 50-year return period)? Additionally, how can the accuracy of OpenFAST results be proven for such scenarios?

[jjonkman]

Dear @YuhengShey,

I'm not aware of publicly available documentation on the validation of OpenFAST under full-scale 50-year conditions.

There certainly have been wave-basin experiments where 50-year-equivalent wave conditions have been tested, which OpenFAST has been validated against. The same could likely be done for wind conditions in a wind tunnel, but I don't recall specific publications.

Regarding the constrained NewWave theory, I suggest reviewing our IOWTC 2022 paper: https://docs.nrel.gov/docs/fy23osti/83679.pdf.

Best regards,