Numerical Simulation Of Mechanical Properties Of Marine Flexible Riser

The marine flexible riser is affected by the movement of the pipe-laying vessel or the marine environment during the installation operation,and is subject to the combined effects of tension,bending moment and water pressure.As the water depth increases,the buckling failure of the tensile armor layer has become the main failure mode of the flexible riser.Then,in order to prevent the occurrence of buckling failure and to ensure that the mechanical response of the global pipeline is within the safe range during the actual installation operation,it is necessary to study in detail the mechanical properties of the flexible riser under axially symmetric load and bending load and the buckling failure mechanism of the tensile armor layer.The research has important theoretical value and engineering practical value.In this paper,the mechanical properties of the flexible riser under axially symmetric load and bending load are first explained.According to the mechanical properties and stiffness equation of each layer,a method of replacing the carcass layer,the internal pressure protection layer,the pressure armor layer and the inner anti-wear layer with an equivalent layer is proposed.A partially simplified numerical model for simplifying the cross section of a complex pipe is establelished,and verify the calibration with theoretical models and test results.Secondly,based on the simplified numerical model,the ultimate strength of the flexible riser under axially symmetric load and bending load is analyzed.The buckling failure mechanism of the tensile armor layer and the influence of external pressure and friction coefficient are studied.Finally,based on the research results of the mechanical properties and ultimate strength of the local flexible riser,the numerical model of the global flexible riser with nonlinear bending stiffness is es Tablelished by the lumped mass method,the bending moment,effective tension and curvature distribution along the pipeline under the condition of pipe laying operation are studied.Based on the ultimate strength analysis,the safety of pipe-laying operation was explained,and the influence of parameters such as suspension angle,seabed stiffness and current velocity on the mechanical properties of the global flexible riser was analyzed.The research results show that the simplified numerical model can simulate the mechanical properties of nonlinear contact between flexible riser layers,and the solution results are accurate,the calculation efficiency is high,and it is easy to converge.When the axial compression failure occurs,when the anti-wear layer and the outer protective layer reach the yield strength of the material,the inner and outer tensile armor layer has a separation gap;when the anti-wear layer and the outer protective layer reach the ultimate strength of the material,the inner and outer tensile armor of the layer separation gap is the largest and the riser loses axial compression resistance.When the axial tensile failure occurs,the inner tensile armor layer first reaches the yield strength of the material,the tensile strength of the riser is reduced,and the outer tensile armor layer reaches the yield strength of the material.This results in a three-stage change in the tensile stiffness of the pipe.When bending occurs,the helical strip at the beginning is a viscous contact,and the bending stiffness is large;then it becomes a sliding contact,and the bending stiffness becomes smaller;eventually,the local bending failure of the riser caused by the excessive slip deformation of the strip.At the same time,the external pressure and friction coefficient have an important influence on the buckling failure of the strip and the ultimate strength of the riser.The global analysis shows that the flexible riser with nonlinear bending stiffness can better conform to the marine environment,however,the tension of the suspension point and the bending moment of the touchdown point is large,which is easy to occur the buckling damage;the suspension angle,the seabed stiffness and the current velocity have an important impact on the force of the global pipeline,and the pipe-laying plan should be reasonably selected during pipe laying operations.