Numerical model for dynamic installation of large diameter monopiles
This paper presents the numerical code VibPile for simulation of the nonlinear dynamic response of large monopiles under harmonic loading and installation by vibration or impact driving. VibPile is based on a nonlinear FE model of the pile-soil interaction along the shaft and the tip by elasto-plastic springs representing the near field and elasto-dynamic elements (spring and dashpot) for the far field. While for the shaft friction and tip resistance the classical engineering solutions, such as those by standards or guidelines (e.g. API) and literature, are used in the numerical simulations, new computational models are developed for the dynamic response of the soil inside the pile and the far-field elasto-dynamic springs. What primarily differentiates the present model from the existing ones is the treatment of the soil inside the monopile together with the stiffness at the pile tip. Most existing models are based on the solutions for piles with solid sections or pipe piles in which the soil inside the pile follows the pile vibrations like a solid section. For large diameter monopiles commonly used for offshore wind turbines, both the soil response inside the pile and the stiffness of the pile tip are different. The developed model is verified against the vibro-pile test data collected at Altenwalde, Germany. The results include axial pile strains, accelerations, and shear stresses along the pile shaft and at the pile tip. Both the strains and accelerations as well as the rate of pile driving measured at Altenwalde are reproduced by VibPile with reasonable accuracy. Sensitivity analyses are presented to highlight the effects of the shear wave velocity and shear strength at the pile-soil interface on the pile driving rate. Moreover, the effect of the natural period of the inner soil on the pile driving is investigated.