Stress Calculation Model And Fatigue Analysis Of Tensile Armors On Unbonded Flexible Pipes

With the oil and gas industry gradually moving to the deep sea,flexible pipes which transport oil and gas from the seabed to the platform or FPSO have been used more and more widely.Compared with traditional rigid pipes,flexible pipes have good corrosion resistance and flexural compliance,and can adapt to deeper water and higher pressure.However,due to the complexity of its own structure,the fatigue analysis of flexible pipes is extremely complex.It has been shown that the fatigue of flexible pipes is mainly manifested in the tensile armor.This paper studies the stress calculation model of deep sea flexible riser tensile armor strips and conducts fatigue analysis research,which mainly involves the following aspects:1)A brief overview of flexible pipes and a summary of the current research status are given.The structure of flexible pipe,the function of each layer,the layout and the failure modes are introduced.Then the stress calculation and fatigue analysis of flexible pipes are summarized.2)Finite element numerical simulation of flexible pipes under tension,torsion and bending is carried out by using ABAQUS,and compared with the experimental and theoretical results.3)During the bending process of flexible pipes,the tension armor layers have different slip conditions: stick,partial slip and full slip.According to different slip conditions,the calculation model of the axial stress of the tension strip of flexible pipes is deduced by using the return mapping algorithm,the poisson effect and the bending induced tension are considered,too.It is verified by ANSYS numerical model.4)Based on static equilibrium,deformation compatibility and constitutive relation,a general calculation model for local bending stress of flexible pipe strips is deduced,and the effect of axial strain is considered.Different simplified models can be deduced by different simplified conditions,and can verified by ANSYS numerical model.5)Calculate the fatigue of flexible pipes.The calculation process of fatigue life of flexible pipe is introduced.Based on the numerical software OrcaFlex,the overall response of a FPSO flexible pipe in the South China Sea is calculated.Then,the strip stress is calculated by the model derived in this paper.The stress time history is counted by rainflow counting method.Finally,the fatigue life of flexible pipe is calculated by S-N curve.