The dynamic response of fixed offshore structures and the loads on the foundation is strongly influenced by foundation stiffness and damping. Design optimization of offshore structures, e.g. wind turbines (OWT) and jack-up rigs, life extension and load increase for existing structures etc. often require detailed studies with due consideration of the cyclic soil behaviour for accurate calculation of foundation stiffness and damping. This is especially important for OWT structures, for which the lowest resonance frequency is designed to lie within a very narrow frequency band and foundation damping plays an important role on structural fatigue design. However, in the early stages of projects it is not always feasible to run full-fledged 3D finite element (FE) analysis. Therefore, a simple framework to estimate the rocking stiffness and hysteretic soil damping of skirted foundations is proposed. A comprehensive set of finite element analyses shows that there is a simple scaling relationship between foundation rocking stiffness and damping vs rotation and normalized soil element stiffness (modulus reduction) and damping vs shear strain for soil profiles with constant properties with depth. For layered soil profiles with large contrasts in stiffness and strength, further analyses are still needed to investigate the scaling law.
Jörgen Johansson, Nallathamby Sivasithamparam, Youhu Zhang and Harun Kursat Engin
Keywords: offshore structures, dynamics, nonlinear stiffness and damping, map shear strain to rotation
Publisher: DFI