Modelling of soil-structure-interaction for flexible caissons for offshore wind turbines

Published foundation models and procedures for calculation of caisson foundation response typically assume a rigid caisson in deformable soil. However, recent published measurement data from a prototype suction bucket jacket has revealed that the lid flexibility of the caisson foundations significantly influences the dynamic foundation stiffness felt by the jacket legs. This paper investigate this soil-structure-interaction problem and presents a modelling approach for including the effect of caisson flexibility in a macro-element. The macro-element, originally developed by assuming a rigid foundation, was modified by included a stiffness correction and a procedure to account for changes in the elastic coupling between horizontal load and moment due to the caisson flexibility. The modified macro-element successfully re-produced the response to general load paths computed by geotechnical finite element analyses, where the foundation was modelled in detail with structural elements and the surrounding soil was represented by continuum elements. The general principles behind the modification is generic in the sense that it can be implemented in other foundation models.