A freak-wave event-curation workflow for predicting mooring-line tensions and platform response of floating offshore wind turbines

Floating offshore wind turbines (FOWTs) in extreme sea states may be governed by rare freak-wave groups that control mooring-line tension and platform motions. This study develops a freak-wave event-curation workflow for early-stage floating-wind assessment, focusing on three linked questions: the simulation duration required to capture at least one freak wave, the effect of documented freak-wave morphologies on response, and the extent to which short event-based simulations can reduce computational effort while preserving useful accuracy in extreme-response prediction. Long-crested irregular sea-surface elevations consistent with a standards-based extreme sea state were generated and grouped into six duration bins from 150 h to 20 min under constant total exposure. Statistical consistency was verified using zero-up-crossing analysis, Rayleigh-Haring-Tayfun statistics, and the Kolmogorov-Smirnov test. Freak-wave candidates were classified into four morphology types and extracted as standardized 700 s clips for OpenFAST simulations of the IEA 15 MW turbine on the UMaine VolturnUS-S platform, using the second-order Sharma and Dean method for nonlinear wave kinematics and second-order sum- and difference-frequency QTFs for nonlinear platform forces. Results show that longer records improve the likelihood of capturing freak waves, that curated morphologies concentrate elevated mooring-line tension demands, with peak changes of ?6.0% to +35.0% relative to the baseline, and CPU-runtime reductions of up to 50%. © 2026 Published by Elsevier Ltd.