A finite element analysis of the pullout capacity of suction caissons in clay

Abstract

Suction caissons are increasingly becoming the foundation of choice for offshore structures in deep water. They are used extensively in Tension Leg Platforms and provide the most efficient foundations for many offshore wind turbine structures. One of their major advantages is the ability to withstand large uplift forces by mobilizing shear on their external and internal surface and by the suction forces induced in the enclosed soil plug. These suction forces can be relied upon for short-term loading, while the behaviour of the soil remains undrained, but are more questionable for the sustained loading induced by storms and loop currents. This study uses finite element analysis to investigate the uplift capacity of suction caissons under three loading conditions: a) short-term undrained loading, b) long-term drained loading and c) sustained loading for short and long periods of time. The study compares the capacity from 5 different geometries with length to diameter ratios, L/d = 0.5,0.65,1,2 and 3 under these three loading conditions. For the sustained loading case, a minimum time under which the load can be sustained is established for different load levels. The commercial finite element program Plaxis is used and a Mohr-Coulomb model is assumed for the soil. Comparisons are presented between the results of this study, the theoretical Mohr-Coulomb model predictions and other finite element analysis found in the research for undrained and drained loading.