A simplified procedure for seismic response spectrum analysis of offshore caisson foundations

Analyzing the seismic performance of caisson foundations that support wind turbines or subsea structures usually relies on oversimplified static or pseudo-static methods that completely ignore structural dynamics and soil-structure interaction (SSI) or using complex and expensive time-domain analyses that are sensitive to assumptions about key input parameters. The proposed paper will present a simplified response spectrum analysis (RSA) procedure that estimates the peak seismic response directly from an earthquake design spectrum and captures the structural dynamics and SSI in a simplified way. To appropriately account for the nonlinear soil response - which complicates what would otherwise be a straightforward use of RSA - an iterative procedure is used. The procedure considers all the main factors important for the earthquake response of caisson foundations, including the mass and inertia of the caisson, attached subsea structure, soil, and hydrodynamic mass; and determines the corresponding foundation stiffnesses based on a set of nonlinear pushover analyses. The paper will present the theory underlying the method, outline the main steps involved, and demonstrate its accuracy with a numerical example based on a recent offshore project.