Offshore wind turbines (OWTs) in relatively shallow waters are most often founded on monopile foundations, whose design is extremely relevant to the OWT dynamic performance under environmental loading.In this study, 3D finite element (FE) modelling is applied to the dynamic analysis of OWTs and proposed as a valuable support to current design practice. FE results are presented about the interplay of cyclic soil behaviour and hydro-mechanical coupling in determining the OWT natural frequency: in dilative sands, the natural frequency seems not to decrease monotonically at increasing loading amplitude, while slight influence of soil permeability is found.

Monopiles are at present the most widespread foundation type for offshore wind turbines (OWTs), due to their simplicity and economic convenience. The current trend towards increasingly powerful OWTs in deeper waters is challenging the existing procedures for geotechnical design, requiring accurate assessment of transient soil-monopile interaction and, specifically, of the associated modal frequencies. In this work, advanced 3D finite element (FE) modelling is applied to the dynamic analysis of soil-monopile-OWT systems under environmental service loads. Numerical results are presented to point out the interplay of soil non-linearity and cyclic hydro-mechanical (HM) coupling, and its impact on transient response of the system at increasing load magnitude. It is shown how the lesson learned from advanced modelling may directly inspire simplified, yet effective, spring models for the engineering dynamic analysis of OWTs.

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