The Macro-Element Method for Offshore Wind Turbines Exposed to Extreme Loads in the Asia-Pacific Region-Paper

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The paper evaluates a nonlinear hysteretic macro-element model for representing soil–structure interaction in monopile offshore wind turbines exposed to extreme Asia-Pacific conditions, including typhoon-driven waves. Such loads can govern support-structure design, making accurate foundation stiffness and damping important.<br /><br />The model was tested against three-dimensional finite-element analyses (FEA) at extreme load levels approximately 150% higher than those used in earlier verification. It reproduced the mudline response reasonably well, although some discrepancies arose at peak loading and during unloading and reloading, partly because the macro-element model does not represent soil gapping. Its estimated hysteretic damping (11.3% of critical damping) was close to the FEA result (12.0%).<br /><br />The study also examined damping estimates based on the logarithmic decrement method. Because nonlinear soil stiffness and damping can create amplitude, phase, and frequency-interference effects, this method may give inaccurate system-damping estimates under extreme, multi-frequency loading. The authors recommend further investigation and comparison with energy-based damping calculations.<br /><br />In a time-domain case study of an idling turbine under extreme wave loading, the macro-element model reduced structural response compared with API p-y foundation modelling. A macro-element model calibrated to 3D FEA produced further reductions, particularly at tower-interface level, reflecting a more realistic and stiffer foundation response.<br /><br />Overall, the results support using the macro-element model in ultimate-limit-state analyses of offshore wind turbines, as it can capture FEA-like foundation behaviour while reducing computational demands. Its improved representation of nonlinear stiffness and damping can lower predicted support-structure response, but damping assessment under extreme loads requires care.

Keywords

offshore wind turbines

macro-element model

soil-structure interaction

monopile foundations

hysteretic damping

finite-element analysis

typhoon-driven waves

extreme loading

mudline response

API p-y method