Study On Pore Pressure Field Inside Soil And Consolidated Settlement Of Spudcan

Jack-up rigs have been extensively used for drilling operation and production of oil and gas fields.A modern jack-up rig typically consists of a buoyant triangular platform supported by three or four independent truss-work or cylindrical leg system with individual footings called “spudcan”.The rapid jack-in process of spudcan footing tends to generate substantial build-up of excess pore pressure below the spudcan underside.At any stage in which the preloading is stopped,partial consolidation of the soil around the spudcan can occur due to the dissipation of excess pore pressures.This results in a zone of increased strength of the soil and an improvement in the bearing capacity of the spudcan at that depth.The spudcan will produce settlement during the consolidation stage.When the consolidated settlement of spudcans at different locations have large differences at the same time,the stability of the platform will be affected.An intimate knowledge of the excess pore pressure concentration is thus of significance for understanding both the short-term and long-term mechanical behavior of spudcan,and the analysis of the consolidated settlement of spudcan in the consolidation stage is useful for to effectively predicting the future settlement development based on the existing data.As yet there is a dearth of information in these areas.This paper used the finite element method to simulate the penetration and consolidation process of pile shoes in soft soil.Main work and achievements of the research are presented as below:The coupled Eulerian-Lagrangian technique(CEL)based on effective stress was used to simulate the process of penetration of spudcan into soft soil,and the feasibility of CEL method was verified by comparing with the centrifuge results.The comparison results shows that the strength of the normally consolidated Malaysian kaolin clay used in experiment can be well predicted by Modified Cam-clay model.The penetration resistance obtained from the numerical simulation agrees well with the centrifuge results,but the numerical results are slightly higher.This discrepancy may be attributed to the soil strength in the numerical simulation is slightly higher,and the soil constitutive model fails to consider the strain-softening and remoulding effects.Numerical simulation can reasonably predict the pore pressure changes underneath the spudcan and inside soil in a satisfactory manner,in terms of the trend as well as the magnitude.The bulb-shaped excess pore pressure concentration zone was identified below the spudcan underside.Sizes of pressure bulbs as well as the magnitudes of excess pore pressures inside them bore close relationship to the penetration depths and soil's properties.The results of parametric study led to the development of a simple yet useful approach to estimate the spudcan-penetration-induced excess pore pressure in soil without carrying out the complicated computation.Necessary charts and tables were provided to facilitate its usage.This paper presents a dual-stage Eulerian-Lagrangian analysis for modelling the entire process of spudcan installation in soft clay,followed by consolidation and working load operation.The feasibility of numerical simulation was verified by comparing with the centrifuge results.The comparison results show that numerical simulation can predict the dissipation of excess pore pressure underneath the spudcan and inside soil and consolidated settlement of spudcan in a satisfactory manner,in terms of the trend as well as the magnitude.Three methods were adopted to predict the spudcan settlement during consolidation stage based on numerical and experimental results.The comparison results show that the three-point method is greatly influenced by the time interval.When using this method,a large time interval is needed to ensure the prediction accuracy.Besides,the three-point method will underestimate the final settlement.The Asaoka method is less affected by the time interval,but it will also underestimate the final settlement.The hyperbolic method can gives good predictions of the consolidated settlement of spudcan in experiments and simulations.To generalize these findings,a comprehensive parametric study was conducted.It was found that the time history of the consolidated settlement of spudcan in each case can be well represented by the hyperbolic formula.