Evaluation of post-storm soil stiffness degradation effects on the performance of monopile-supported offshore wind turbines in clay

Over their lifetime, offshore wind turbines (OWTs) will experience large storm surges that may negatively affect their structural performance. Current design codes do not provide sufficient guidance on tackling this problem. To address it, this study aims to provide a guideline on how shear stiffness degrades around monopiles in nor-mally consolidated clays after a storm surge. High-fidelity finite element models of monopile-supported OWTs are built and validated with 5.5e-05 mean square error against centrifuge tests. Parametric analysis of 108 scenarios is conducted, considering different pile diameters, lengths, soil strengths, and loading scenarios. Pile toe displacements are recorded and used to define a system that can classify the OWT monopiles as flexible or rigid. Shear stiffness degradation ratios at 1D (pile diameter) depth are utilized to predict the change of natural frequency, given the mudline loads. The data suggest that over 5% or 10% decrease in natural frequency in-dicates that pile head deformations exceed serviceability or ultimate limit state criterion, respectively. A data -base of soil strain -stiffness degradation ratio pairs is tabulated along each simulated pile as a reference for degraded p-y curve calibration. The proposed method and database are crucial for safer design and quicker assessment of OWTs.