Large Deformation Penetration Mechanism And Bearing Capacity Of Spudcan Foundation Considering Spatial Variability Of Clay

Jack-up drilling platforms are widely used in offshore oil and gas exploration due to their advantages of easy installation and movement.However,the complex marine geological conditions and harsh marine environment lead the accidents to occur frequently during the spudcan penetration process and service phase.Therefore,it is particularly important to study the bearing capacity and failure mechanism of spudcan foundations under complex marine geological conditions and marine environmental loads.At present,whether in the installation stage of the spudcan foundations or in its service stage,most studies on the bearing capacity and failure mechanism of the spudcan foundation in the clay are based on the assumption that the soil is a single layer or a multilayer uniform clay.In fact,due to the effects of natural physical and chemical weathering,transportation,and deposition,the undrained shear strength of clay often has strong spatial variability.For this reason,this study focuses on the penetration mechanism and penetration resistance of the spudcan foundation in the preloading installation stage in the spatially variable clay,the failure mechanism and the bearing capacity failure envelope surface in the service stage.This study also aims to propose several effective methods which can quantify the installation risk and guide the engineering project.The main research contents and results of this paper are as follows:The principle model device of the spatially variable soil was developed,and the preparation method of the spatially variable saturated clay was proposed.First,through the quick shear test,the quantitative relationship between the compaction degree of saturated soil and its undrained shear strength is clarified.Subsequently,the circulant embedding of the covariance matrix method was used to generate undrained shear strength random fields with spatial variation characteristics.In the experiment,the compaction of the soil between the movable blades of the model device was controlled,and then the spatially variable soil can be reconstructed using the layered compaction method and vacuum saturation method.A high-precision micro static penetration probe was developed to measure the spat ial distribution of undrained shear strength of soil.The measurement results show that the preparation method of spatially variable saturated clay proposed in this paper is effective.Based on the preparation method of spatial variability clay,vertical loading test method and Geo PIV-RG soil displacement measurement technology,a model test method of spudcan penetration in spatially variable clay is proposed,which can measure the spatially variable soil deformation and spudcan penetration resistance and reveals the catastrophic mechanism.Spudcan penetration tests were carried out in three typical representative spatial variability soils(soil with large coefficient of variation and large horizontal scale of fluctuation,soil with large coefficient of variation and small horizontal scale of fluctuation,and soil with small coefficient of variation and large horizontal scale of fluctuation).The test results show that the spatial distribution of the undrained shear strength of the clay determines the penetration mechanism and penetration resistance of the spudcan foundation.Based on the results of the spudcan penetration test in the spatially variable clay,the existing method for predicting the penetration resistance of the spudcan is evaluated,and several suggestions that can guide actual engineering are put forward.Based on the random field theory,the principle of generating random fields by the EOLE method and the nonlinear finite element method,a random large deformation finite element analysis method is developed,which can simulate the large deformation interaction between geostructures and spatially variable soils.This method is implemented by the secondary development of ABAQUS using Python scripts.The entire random large deformation finite element analysis process can be fully automated and efficiently implemented,which greatly improves the efficiency of Monte Carlo simulation.The applicability,robustness and reliability of the method are verified by simulating the penetration of the T-bar penetrometer and the spudcan foundation.The random large deformation finite element analysis method is used to simulate the whole process of the spudcan penetration in the spatially variable clay,and the penetration mechanism and penetration resistance of the spudcan are further discussed.A Monte Carlo simulation was carried out,and the simulation results found that the depth of the soil,the penetration resistance of the spudcan foundation and the resistance ratio all follow the normal distribution.The mean and standard deviation of the resistance ratio have a certain functional relationship with the penetration depth.Finally,a percentile method that can predict the penetration resistance of the spudcan foundation is proposed,and the method is used to predict the piling cases in the seas of northeastern Brazil and the Gulf of Mexico.The prediction results show that the percentile method can give a certain de gree of confidence in the penetration resistance interval and penetration depth interval,which can be used to guide the on-site spudcan installation and reduce the risk.The penetration of the spudcan foundation in the normally consolidated clay will cause the foundation soil to be greatly disturbed,and the shallow weak soil with lower strength will flow back to the top of the spudcan.Based on the results of the random large deformation finite analysis of the spudcan penetration,this paper uses the probe test method to study the effect of the spudcan installation on the failure mechanism and failure envelope of the spudcan foundation in service phase.Subsequently,Monte Carlo simulation was carried out,and the failure envelopes of the spudcan foundation in 400 disturbed spatial variability strength fields were obtained.Finally,the probability statistical analysis was carried out.The failure envelopes are sorted and mathematically fitted,and the best fitting parameters of the failure envelopes with different confidence levels are proposed,which can be used to design the spudcan foundation.