Numerical Simulation Of Local Scour Around Pipeline In Cohesive Soil Seabed

The offshore pipelines that are exposed to the seabed are prone to scour,which will cause the pipeline safety problems.So far,almost all the research is aimed at the non-cohesive sandy seabed,and there is little literature about the local scour of pipeline in cohesive soil seabed.In this thesis,numerical simulation of local scour around pipeline in cohesive soil seabed are performed by the computational fluid dynamics(CFD)software Fluent.The VOF twophase model is used to calculate fluid phase and soil phase,the SST k-? turbulence model is used to close Navier-Stokes equation,PISO algorithm is used to solve incompressible flow field.In VOF model,the cohesive soil is modeled as Herschel-Bulkley fluid,with the parameter ? ?0.15 determined by the T-bar test with different penetration rates.This thesis combines the Herschel-Bulkley model with the undraned shear strength of cohesive soil achieving the simulation of cohesive soil in Fluent.For rigid fixed pipeline,first of all,to simulate the flow field analysis with the seabed setting to wall,to simulate the process of local scour of pipeline,then the important factors that may affect pipeline local scour such as soil strength,parameter k,flow velocity,pipeline diameter,initial suspension height are studied.It is concluded that the existence of exposed pipeline will increase the shear stress of the fluid to the seabed;When the pipeline suspension height reaches a certain height,there is obvious vortex shedding on the downstream of the pipeline,which can effectively simulate the wake scour;The larger the soil strength,the greater the critical shear stress,and the more difficult the local scour of the pipeline is to form;The smaller the parameter k,the lower the strength of the soil and the smaller the scour depth;The larger the velocity,the greater the shear stress,the deeper the scour depth.For elastic constraint pipeline,the external forces produced by soil constraints for pipeline are approximated as the linear forces for the X,Y direction in two-dimensions.The three phases coupling of soil-flow-pipe is come true with dynamic grid technology and user-defined function(UDF)in Fluent software.The simulation results of the local scour of the elastic constraint pipeline are compared with that of the rigid fixed pipeline.The study found that the scour depth and scour width of elastic constraint pipeline are significantly greater than that of the rigid fixed pipeline,because the pipeline vibration increases the turbulence intensity of flow around the pipeline,strengthens the erosion of fluid to the soil below the pipeline and behind the pipeline;In addition,the smaller the elastic stiffness,the larger the vibration amplitude of the pipeline,the turbulence intensity is higher,the stronger the effect on soil erosion,the greater the scour depth.