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 depths of 20-30m. Most offshore structures have stringent serviceability limit states 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 a caisson founded offshore structure. These tests were conducted using representative caisson models in loose sand under single-g conditions, replicating a fully drained prototype condition. 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 foundation stiffness was dependent on the local soil strain, and under cyclic loading would increase in a logarithmic manner. In addition it was found that under cyclic loading, a caisson system will retain and accumulate structural rotation following with a power relationship. From these observations it was possible to produce an analytical model and describe the changing serviceability state of a prototype structure with loading cycles.