Effects of soil-structure interaction on real time dynamic response of offshore wind turbines on monopiles

Offshore wind turbines are highly dynamically loaded structures, their response being dominated by the interrelation effects between the turbine and the support structure. Since the dynamic response of wind turbine structures occurs in a frequency range close to the excitation frequencies related to environmental and parametric harmonic loads, the effects of the support structure and subsoil on the natural vibration characteristics of the turbine have to be taken into account during the dynamic simulation of the structural response in order to ensure reliable and cost-effective designs. In this paper, a computationally efficient modelling approach of including the dynamic soil–structure interaction into aeroelastic codes is presented with focus on monopile foundations. Semi-analytical frequency-domain solutions are applied to evaluate the dynamic impedance functions of the soil–pile system at a number of discrete frequencies. Based on a general and very stable fitting algorithm, a consistent lumped-parameter model of optimal order is calibrated to the impedance functions and implemented into the aeroelastic nonlinear multi-body code HAWC2 to facilitate the time domain analysis of a wind turbine under normal operating mode. The aeroelastic response is evaluated for three different foundation conditions, i.e. apparent fixity length, the consistent lumped-parameter model and fixed support at the seabed. The effect of soil–structure interaction is shown to be critical for the design, estimated in terms of the fatigue damage 1Hz equivalent moment at the seabed. In addition, simplified foundation modelling approaches are only able to capture the dynamic response reasonably well after tuning of the first natural frequency and damping within the first mode to those of the integrated model. Nevertheless, significant loss of accuracy of the modal parameters related to the second tower modes is observed.