The importance of foundation modelling for the support-structure fatigue damage estimation of a 10 MW monopile based offshore wind turbine is investigated in different operational states and wind-wave misalignment conditions. Three different models are used: (1) a non-linear elasto-plastic model including hysteretic behaviour effects, (2) a linear elastic model and (3) a non-linear elastic model, using numerical simulations with an aero-hydro-servo-elastic computational tool. Depending on the environmental condition, different dynamic processes dominate the responses. For parked states, deviations between models up to 160% were found. For wind wave-misalignment over 30° in operational cases, differences up to 180% were found for low sea states and 119% for high sea sates. Both nonlinear foundation damping and stiffness formulation have considerable effect on the responses, with hysteretic effects becoming crucial when aerodynamic damping is negligible in the direction of the response. Attention is required when comparing the fatigue damage only at the mudline, as larger variations between the models may occur in the embedded part of the monopile, where the absolute maximum is found.

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As a part of the assessment of foundation resistance for monopiles, several offshore wind standards prescribe symmetric 35-hour (or 42-hour) storm sequences in terms of wind speed and significant wave height. The temporal evolution of the peak period is not specified explicitly in the standards, despite the fact that large monopile wind turbines are sensitive to the wave period. In the present work, the storm sequences according to the standards are first compared to hindcast data for intermediate water depth locations in the North Sea. An alternative storm sequence is proposed based on the hindcast data, and possible values of the peak period evolution are proposed for the standard models. The responses of a 10 MW monopile wind turbine are then computed for both the standard and proposed sequences using a time domain aero-hydro-servo-elastic code coupled to a macro element model for the soil-structure interaction. The resulting mudline load cycles are then compared for the different storm sequences.

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