The dynamic soil structure interaction imposes both kinematic and inertial interactions on monopile-supported wind turbines. The two effects can be calculated individually, but there is no clear guidance in the current design standard for combining them. A theoretical study using the finite element method to model a simplified single degree of freedom system has been conducted to analyse the phase relationship between the two effects and obtain the combination laws through parametric studies using the enhanced hysteresis control soil constitutive model. The results show that when the fundamental frequency of the soil is smaller than the superstructure, an in-phase relationship is observed. Monopile’s bending moments from the two loadings have different signums. Considering the inertial loading alone with an additional 5% modification is sufficient to capture the maximum bending moment and the profile. When the natural frequency of the soil is larger than the superstructure, an out-of-phase relationship is observed. The kinematic loading has a significant domination on the lower part of the pile. The worst-case scenario can be obtained when both loadings are combined using the simple addition rule. These findings enable engineers to combine two loadings in a simple manner, avoiding performing the lengthy dynamic time-history numerical analysis and facilitating an efficient design.
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