Research On Pipe-soil Interaction Mechanism Of Steel Catenary Riser In The Touchdown Zone

During the exploitation of ocean gas and oil resources,the steel catenary riser(SCR)has been widely used due to its economical efficiency and good adaption to the movement of the upper platform.An SCR goes downward from its top end attached to the upper platform,experiencing the suspension section in the water and the touchdown zone where it contacts the seabed,and finally extends to the dragged section on the seafloor and the following long shallow-buried section.The pipe-soil interaction in the touchdown zone(TDZ)of the SCR is pivotal to predict the fatigue life of the pipeline,and the scour around the pipe section dragged on the seafloor can affect the stability of the pipeline during the service period.Now there are still many problems about the mechanisms of pipe-soil interaction and of incipient motion of particle during the local scour around the pipe.So this thesis aims to carry out studies about the pipe-soil interaction mechanism of steel catenary riser in the touchdown zone by indoor experiments,theoretical analysis and numerical simulation,specifically including the following works:1.This thesis analyzes and compares three exsiting typical pipe-soil interaction models:RQ model,AB model and ABY model,and presents an modified pipe-soil interaction model for improvement,which can consider the cyclic softening and the strain rate effect of the soil strength,as well as the variation of the seabed profile.Furthermore,with the use of the improved model,a numerical model of SCR-fluid-seafloor interaction is established.The embedment developments of a heaving pipe in the touchdown zone are deeply investigated and the variations of the seabed profile are analyzed in detail,revealing the stress response pattern of the SCR considering the softening of soil strength and the variation of seabed profile.Then the fatigue damage of pipe in the touchdown is estimated.2.Lateral pipe-soil cyclic interaction is simulated through an indoor large scale experiment.The developments of pipe embedment,stress responses and pore pressures at different locations along the pipe are monitored and recorded.The lateral movement locus of pipe at different cross sections is observed.The trenching process and the evolution of the profile and range of the touchdown zone the postion of the touchdown point(TDP)are analyzed and the mechanism of the pipeline penetrating into the soil and the softening and displacement of soil around the pipe during the lateral cycles are investigated.Furthermore,the entire responses of the pipe during the following vertical cycles are observed.Finally,the soil strengths underneath the pipe are tested after the cyclic experiments to assess the effect of pipe movement on the soil strength around.3.The mechanism of incipient motion of seabed sediments below a pipe is analyzed through a numerical method coupled with model tests.The traditional Shields number is modified by considering seepage effect.Based on the results of two kinds of existing scouring experiments in which the pipe is either fixed or sagging,a fluid-pipe-seabed interaction model is established.The evolution of fluid field at different moments during scouring is analyzed by S ST model and the seepage field in the seabed is obtained by solving a porous medium seabed model.Then the effect of seepage force on the incipient motion of particle is studied.The deficiency of the traditional judgement method on the incipient motion of particle is indicated and the importance of seepage in evaluating the incipient moiton of particle is revealed.It is suggested to use the Shields number modified by seepage to judge whether the incipient motion of partical occurs.