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.

The massive development of the offshore wind industry is motivating substantial research efforts worldwide, where offshore wind turbines (OWTs) of increasing size are being installed in deeper water depths. Foundation design is a major factor affecting the structural performance of OWTs, with most installations founded to date on large-diameter monopiles.This work promotes advanced 3D finite element (FE) modelling for the dynamic analysis of OWT-monopile-soil systems. A detailed FE model of a state-of-the-art 8 MW OWT is analysed by accounting for dynamic soil-monopile interaction in presence of pore pressure effects. For this purpose, the critical-state, bounding surface SANISAND model is adopted to reproduce the hydro-mechanical cyclic response of the sand deposit. The response to realistic environmental loading histories (10 minutes duration) are simulated, then followed by numerical rotor-stop tests for global damping estimation.While linking to existing literature, all FE results are critically inspected to gain insight relevant to geotechnical design. The modelling tools adopted (i) support the robustness of 'soft-stiff' foundation design with respect to natural frequency shifts, even during severe storm events; (ii) provide values of foundation damping in line with field measurements; (iii) suggest that pore pressure effects may more likely affect soil-monopile interaction under weak-tomoderate environmental loading.

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.

Cyclic loading caused by earthquake in sandy soils often leads to the development of positive pore pressures. In extreme conditions, the pore pressure may increase until reaching a state of zero effective stress associated with a dramatic reduction of the soil shear stiffness and strength. Even when liquefiable soils are found at depth, and capped by a non-liquefiable crust, once liquefied they can act as a seismic isolator, significantly modifying both amplitude and frequency content of the vertically propagating shear waves. As a result, seabed seismic motions at offshore sites characterized by the presence of liquefiable layers within the soil stratigraphy may change considerably with respect to a non-liquefied scenario. Non-linear Seismic Site Response Analyses (SSRA) are often used for site-specific evaluation of seismic input for offshore projects. Severe earthquake motions induce non-linear soil response associated with significant stiffness reduction for soft soils. The hysteretic non-linear soil behavior leads to modifications in terms of magnitude and frequency content compared to the postulated stiff soil input. This is even more important where pore pressure build-up and liquefaction may occur, leading to further modification of the seismic accelerations at mudline. However, standard industry practice consists in performing total stress SSRA that are not able to model the softening response in presence of liquefiable soil layers. This paper compares the seabed seismic motion assessed by means of total and effective stress SSRA in order to evaluate the effect of the soil stiffness degradation in liquefied layers within the soil profile, building upon the findings of Ardoino et al. (2015), who observed in-profile liquefaction to have a limited effect on mudline response spectrum. In particular, it is shown how modelling methodologies able to replicate the transient nature of excess pore pressure build-up during earthquake excitation (i.e. time-domain analyses), are better suited to capture seismic motion modifications in presence of in-profile liquefaction, with respect to response spectrum analyses. The effects of deep foundations embedded across the liquefied layers, on the propagation of the seismic motion, is also investigated and discussed.