Numerical Calculation of Damping for Monopile Foundations under Cyclic Load During Steady-State Vibration

Dynamic loading and fatigue has significant effects on offshore wind turbine foundations. To obtain a proper design of a turbine and its foundation it is therefore important to understand how the structure and soil interact regarding damping. Especially, this paper demonstrates the damping and stiffness of the saturated soil within the seabed due to pore water flow generated by the cyclic motion of a monopile. The concept of a Kelvin-Voigt model is employed and combined with a Finite Element Method (FEM) model of the pile. The two-dimensional analysis of each individual soil layer is performed in ABAQUS by coding input files, utilizing Python and MATLAB. A parametric study is performed to illustrate the effects of model size, soil properties such as permeability, void ratio, Young’s modulus, bulk modulus, and load parameters such as amplitude and frequency. It is found that by increasing the bulk modulus, the damping and stiffness in the soil increase; however, the increase is small. Further, it is observed thatYoung’s modulus has a significant effect on the dynamic response of the soil. Finally, the permeability of the seabed has a strong influence on the damping of wind-turbine-tower vibrations.