Serviceability of suction caisson founded offshore structures

Suction caissons have recently been considered as a cost-effective alternative to conventional foundations for offshore met masts and wind turbines. Such foundation arrangements are suitable for applications within water up to 20 m deep. Most offshore structures have stringent serviceability limits imposed on their design dictating the allowable structural deflections and accumulated rotations throughout its operational life. This paper summarises the findings from a series of scale model tests and identifies key factors which influence the serviceability performance of an offshore structure founded on a single caisson. Tests were conducted using representative caisson models in loose sand under single-g conditions, replicating a fully drained prototype state. These experiments recorded the rotational foundation stiffness (soil–structure interaction), the evolution of foundation stiffness under cyclic loading and the accumulation of structural rotation with loading cycles. It was discovered that the caisson stiffness was dependent on the local soil strain, and under cyclic loading would increase in a logarithmic manner. Further it was found that a caisson system will retain and accumulate structural rotation under cyclic loading, following a power relationship. From these observations it was possible to produce an analytical model describing the changing serviceability state of a prototype structure with loading cycles.