A Large Deformation Random Finite Element Study On Failure Mechanism And Bearing Capacity Of Spudcan

Jack-up rigs now have been extensively used in offshore oil and gas exploration and increasingly in temporary production and maintenance work,which are generally installed in shallow to moderate water depth up to 150 m.The particular type of spudcan footing is cone with an equivalent diameter ranging from about 10 m to 20 m.During the installation of a jack-up rig,a spudcan foundation would be preloaded vertically to about twice its working load,which can cause the spudcan to penetrate up to 2-3 diameters below the mudline,so as to ensure sufficient safety margin against extreme storms.The bulk of the published literature on the failure mechanism and bearing capacity of a spudcan is limited to homogenous soils,but the offshore seabed soils have been shown to exhibit strong spatial variability and autocorrelation due to many factors.In conventional deterministic geotechnical analysis,such uncertainties of soil properties used to be ignored,which may overestimate the bearing capacity of a spudcan footing.It is only recently that the influence of spatial variability of offshore soil properties on the bearing capacity of spudcan foundations has been explicitly explored using the random finite element method.However,due to technical and computational complexity,these are still limited to the small strain Lagrangian finite element calculation.In the framework of a Monte-Carlo simulation,this paper adopted the Coupled Eulerian-Lagrangian finite element calculation to model the continuous penetration of a spudcan,which combined the large deformation finite element calculation with random field theory.With the continuous penetration of the spudcan,the failure mechanism of soil progressively evolves from surface failure to localised flow-around mechanism at deeper depths.It is the onset of soil back-flow that marks the transition between these two failure mechanisms.Spatial variability was found to clearly affect the soil back-flow and thereby the limiting cavity depth.The range of cavity depths can be approximately modeled as a log-normal distribution,so as to determine the characteristic limiting cavity depths at various probability levels.An algebraic expression was proposed to explicitly predict the characteristic limiting cavity depths in random soils from the fractile.Particular attention was paid to the lower and upper 5% characteristic values,which are likely to be useful for reliability-based design.Results suggest that the randomness of soil strength can obviously alter the flow pattern and failure mechanism of soil,and in turn affects the bearing capacity of the spudcan.Based on the log-normal probability distribution of bearing capacity factor,a correlation between the failure probability and the safety factor was established,from which the level of reliability corresponding to different safety factors can be clearly evaluated.In addition,when considering the spatial variability of soils,the mobilized shear strength can also be employed to ensure that the bearing capacity of a spudcan meets the requirement of target confidence.Such a correlation may facilitate the further development of reliability based design.