Modelling suction caisson installation in sand using FLAC3D

Abstract

Suction caisson foundations have been very popular in oil and gas industry and the current trend is to extend their use to offshore wind farms. A suction caisson is an upturned ‘bucket’ of cylindrical shape made from steel. Seepage conditions play a pivotal role in suction caisson installation process in sand. Pressure gradients generated by the imposed suction inside the caisson cavity cause an overall reduction in the soil resistance around the caisson wall and tip. This transient soil loosening around the caisson wall helps caisson penetration into the seabed. However, these effects must be controlled to avoid soil failure due to critical conditions such as piping or loss of soil shear strength. In this paper, we endeavour to study the role of seepage on the suction caisson installation process in sand. We investigate the effect of seepage conditions on soil resistance to caisson penetration with a particular focus on how frictional and tip resistance are differently affected. For this purpose a series of finite difference simulations of suction caisson installation process are performed using FLAC3D models. The required suction is predicted using an explicit strategy which consists of updating current suction based on displacement history available after the previous prescribed displacement increment. The results of FLAC3D models show that this approach provides an insight on how soil resistance evolves under suction during the installation process and confirm the effect of seepage on total reduction of shear resistance around the caisson wall during installation in sand.