Probabilistic capacity analysis of suction caissons in spatially variable clay

Suction caissons are increasingly used in offshore energy production to moor floating facilities in deep water. The holding capacity of a suction caisson is dependent on the angle of the mooring line and is often described in terms of a vertical-horizontal (VH) load interaction diagram, or failure envelope. These envelopes have commonly been defined by numerical methods using deterministic soil parameters, ignoring the natural spatial variability of seabed sediments. In this paper, spatial variability is modelled using a random field and coupled with finite element analysis to obtain a probabilistic characterisation of holding capacity. The increase of strength with depth that is characteristic of a marine clay is taken into account. A non-parametric approach using kernel density estimation is presented for constructing probabilistic VH failure envelopes that allow an appropriate envelope, associated with an acceptable level of risk, to be selected for design. A study of the autocorrelation distance, a quantity often difficult to obtain accurately in practice, has shown that the vertical autocorrelation distance has a much greater influence on the variability of holding capacity than the horizontal and should be carefully chosen in offshore applications.