Impedance functions for rigid skirted caissons supporting offshore wind turbines

Large diameter caissons are being considered as plausible foundations for supporting offshore wind turbines (OWTs) where reductions in overall cost and environmentally friendly installation methods are expected. The design calculations required for optimization of dimensions/sizing of such caissons are critically dependent on the foundation stiffness as it is necessary for SLS (Serviceability Limit State), FLS (Fatigue Limit State), and natural frequency predictions. This paper derives closed form expressions for the 3 stiffness terms (Lateral stiffness KL, Rotational Stiffness KR and Cross-Coupling term KLR) for suction caissons having aspect ratio between 0.5 and 2 (i.e. 0.5 < L/D < 2) which are based on extensive finite element analysis followed by non-linear regression. The derived stiffness terms are then validated and verified using studies available in literature. An example problem is taken to demonstrate the application of the methodology.