On the application of the beam model for linear dynamic analysis of pile and suction caisson foundations for offshore wind turbines

Abstract

Piles and suction caissons are the most common foundation solutions for fixed Offshore Wind Turbines at intermediate water depths. They are generally used as a single element, presenting large diameters and short aspect ratios. These specific dimensions drastically differ from the ones of classical applications (offshore platforms, bridges, tall buildings etc.). Thus, in this paper the validity of their modelling as beam elements for the particular problem of OWT is revised. The results of a soil-beam model, based on the integral Reciprocity Theorem in Elastodynamics and specific Green’s functions for the layered half-space for the soil behaviour coupled with Timoshenko’s beam Finite Elements, are benchmarked against the ones of a soil-shell model, based on Boundary Elements for the soil coupled with shell Finite Elements. The comparative study is conducted in terms of foundation characterization variables (impedance functions and kinematic interaction factors). Their influence on the OWT seismic response is also studied through a substructuring procedure. From the results, some expressions for determining the applicability range of the beam simplification are proposed as functions of the relative foundation-soil stiffness ratio. It is observed that this applicability range goes beyond that the one commonly considered.