Study On Mechanism Of Pipeline-soil Interaction In Deep Water Clay

To correctly evaluate the vertical penetration resistance and lateral resistance of pipeline,a large deformation approach was used to investigate the pipe-soil interaction,including the vertical shallow penetration of pipeline,the deep penetration of a pipeline and the lateral movement of a shallow embedded pipeline.Firstly,the Coupled Eulerian-Lagrangian approach(CEL)was used to simulate the vertical penetration of pipelines in cohesive soil,and the effects of strain rate behavior and strain softening on bearing capacity factor of pipelines were studied by the user subroutine VUSDFLD.The resistance of the heaved-soil on both sides of the pipeline during the penetration process was analyzed and the sensitivity analysis of the resistance was implemented,then a surface heave capacity factor was proposed.Results indicated that bearing capacity factor of the pipeline increases significantly with the increase of strain rate parameter.As the sensitivity of the soil increased,the rate of soil softening went up and the bearing capacity factor of the pipeline decreased.The surface heave capacity factor mainly depended on the submerged weight of the soil,and with embedment depth increasing,the effect of the submerged weight of the soil on the surface heave capacity became more obvious.Secondly,the CEL approach was used to analyze the penetration resistance and failure mechanism of clay in the process of deep penetration and cyclic penetration of pipeline,and an adhesive contact algorithm for the pipeline-clay contact was developed by the user subroutines VUSDFLD and VUFIELD.Accuracy of the proposed adhesive contact algorithm was verified with a simple rotational shear model,and then the adhesive contact algorithm was applied in the large deformation finite element analysis of a deep penetrated pipeline and cyclic penetrated pipeline in strain softening soil.Numerical results show the fluctuations of the penetration resistance can be explained by the intermittent formation of new shear bands in the soil around the pipeline.New formed shear bands would intersect the previous formed shear bands in the partially back-flow stage while the evolution of shear bands would form a similar fabric structure in the fully back-flow stage.The cyclic penetration resistance degradation model can be fitted accurately by a simple expression versus the accumulated plastic displacement in the cyclic damage zone for different rough conditions.The height of the velocity field was 1.79 D for the penetration process and 1.80 D for the extraction process as the soil was fully remolded,and the width of the velocity field decreased firstly and then increased to stable.Finally,for the lateral buckling of pipeline in deep water,an Arbitrary Lagrange Euler(ALE)method based on ABAQUS was used to investigate the influence of the pipe weight on the lateral resistance,soil failure mechanisms and pipe trajectory,and the adhesive contact between pipeline-clay was correctly modeled via the user subroutine VFRIC with consideration of the effect of strain rate and strain-softening.Results indicated that ‘‘light'' pipelines rose with the “sweep” failure mechanism,whereas “heave” pipelines dived downwards involved “plough” failure.The residual embedment and residual resistance were strongly affected by the pipe weight,and the yield envelopes can be used to well model the breakout resistance of ‘‘light'' and ‘‘heavy'' pipelines.