Effect of Sea State Realizations on the Geotechnical Design of OWT Foundations-Paper

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The study investigates how different realizations of the same sea state affect cyclic soil behavior and the design response of offshore wind turbine monopile foundations. Soil strength and stiffness depend on recent loading: severe cyclic loads can weaken soil temporarily, while quieter periods may allow recovery. Consequently, a storm’s load history—not just its peak loads—can influence foundation performance.<br /><br />The authors modelled a monopile supporting a generic 10 MW turbine at a North Sea site. They compared three 35-hour storm profiles (BSH, DNV and WAS-XL), each represented using three random seed files. Cyclic load histories were assessed using laboratory-based soil contour diagrams, then incorporated into finite-element analyses of foundation response under ultimate limit state loading.<br /><br />Predicted peak rotations varied by up to 41%, while horizontal deflections varied by up to 51% across the examined cases. Even different random-seed realizations of the same WAS-XL storm produced differences of 18–20%. These results show that averaging loads across simulations may not accurately represent foundation response: soil behavior is nonlinear, and cases causing greater soil utilization may need more weight in the design assessment.<br /><br />The authors conclude that current structural design load cases and guidelines do not adequately address geotechnical effects or uncertainty from sea-state modelling. The BSH storm, widely used as a default, may not always be the most critical. They recommend developing geotechnical-specific design load cases and modelling uncertainty factors that account for soil nonlinearity, storm history, and the effects of averaging across random-seed analyses.

Keywords

offshore wind turbine

monopile foundation

cyclic soil behavior

sea state realization

storm load history

soil strength recovery

finite-element analysis

ultimate limit state

random seed variability

geotechnical design load cases