Abstract
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 normally 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 indicates that pile head deformations exceed serviceability or ultimate limit state criterion, respectively. A database 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.
•This study established a new pile behaviour categorization system for intermediate piles (between rigid and flexible) typical of OWTs.•A set of curves are given in this paper that estimate the post-storm shear modulus degradation around an OWT monopile according to the maximal mudline forces that occur during a storm.•A set of formulae are given that can predict the change in natural frequency of an OWT according to the level of shear modulus degradation in the foundation soil.•An extensive database of the shear strain - shear modulus degradation data extracted from the 108 transient pile-soil interaction simulations is attached to this paper.