Random finite element method for spudcan foundations in spatially variable soils

• A random finite element method is presented for assessing the failure of a spudcan embedded in spatially varied seabed soils.
• The mean bearing capacity factor of the spudcan in spatially varied soil is smaller than that in uniform soil.
• The mean bearing capacity decreases with decreasing vertical scale of fluctuation.
• The COV of the bearing capacity factor increases with increasing scales of fluctuation.

Spudcans are large diameter (~ 20 m) conical foundations holding up mobile jack-up platforms for offshore oil and gas developments in shallow to moderate water depths. During self-installation the spudcan often penetrates several tens of meters into seabed soils, where large variations often occur in the shear strength of the soils. Although the failure mechanisms and bearing capacity of spudcan foundations have been studied in homogeneous soil and layered uniform soil, the effect of the spatial variability in soil strength on the spudcan response is still unclear. This study presents a random finite element method (RFEM) for assessing the failure mechanisms and the bearing capacity of a spudcan foundation embedded in spatially varied seabed soils. A comprehensive review of seabed soils discusses typical ranges of the mean, the coefficient of variation and the scale of fluctuation of the undrained shear strength. Random fields are generated and mapped into a non-linear finite element analysis to reveal the failure mechanisms of the spudcan in spatially varied soils. The influence of the scale of fluctuation in different directions is investigated. The results indicate that ignoring the spatial variability of soil strength leads to an overestimation of the bearing capacity. It is unconservative for the foundation design to assume an isotropic random field model in the RFEM analysis.