Study On The Interaction Between Steel Catenary Risers At Touchdown Zone And The Seabed

In the exploitation of deep-sea oil and gas resources,steel catenary riser is a very important equipment for connecting the offshore floating platform with the subsea wellhead for oil and gas transportation.Steel catenary riser has the advantages of simple structure,easy installation,low cost and so on,so it has a very broad application space.Steel catenary risers are subjected to cyclic loading during installation and service.Two points of steel catenary riser are prone to fatigue failure.One is the hang point where steel catenary risers are connected to the offshore floating platform,and the other is the touchdown point on the touchdown zone.The topic of this paper is the interaction between steel catenary riser touchdown zone and the seabed.First,the p-y curve of pipe-soil interaction is established by n-segment elastic,and a soilsoil interaction model is established based on the model of soil with non-linear stiffness.According to the depth,the soil can be divided into different soil layers,and each soil layer corresponding to different stiffness.When the pipeline comes into contact with the seabed and penetrates into the seabed to a certain depth,the penetration depth of the pipeline along the length of the pipeline will be different,and the pipeline will be in different soil layers.According to the different soil layers,the pipeline is divided into several sections and a governing differential equation is established for multi-section pipeline penetration response in the touchdown section.In addition,the analytic expression of pipe penetration response in static state is obtained with the boundary conditions by transfer matrix method.Through the finite element software ANSYS,a reasonable finite element model is established to verify the analytical solution.The nonlinear soil model and the linear elastic soil model are compared.The effects of undrained shear strength,soil strength gradient and pipeline external diameter on penetration depth,moment and shear of pipeline were studied.The results show that the soil resistance obtained by the nonlinear seafloor model is closer to the p-y curve than the single-stiffness seafloor model,and the results obtained by the linear elastic soil model are more conservative.The penetration depth of pipeline will decrease with the increase of undrained soil shear strength and soil strength gradient.In comparison,the mudline shear strength has a greater influence on the pipeline than soil strength gradient.The penetration depth of the pipeline will decrease as the outer diameter of the pipeline increases.Then,the penetration response of the pipeline in the touchdown section under cyclic loading is analyzed.Through the secondary development of the finite element software OpenSees,a model of soil cyclic degradation was established and verified with previous experimental results.The displacement control loading method was used to study the penetration response of pipeline under cyclic load.The results show,considering the cyclic degradation of the seabed,the penetration depth of the pipeline increases with the increase of the number of cycles,and the horizontal distance from the touch point to the hang point decreases with the increase of the number of cycles;the maximum bending moment of the pipeline will increase with the increase of the number of cycles.The increase of pipeline bending moment leads to the increase of cyclic stress of pipeline.