Damage Evolution Modeling And Finite Element Simulation For Corrosion Fatigue Of Submarine Pipeline

As the lifeline of offshore oil and gas transportation,the safety and reliability of the submarine pipeline are very important for offshore oil-gas exploitation.The risk of corrosion fatigue failure of submarine pipelines increases due to the corrosion induced by transport media and seawater,as well as the alternating stress caused by ocean wave-current action of pipelines.Therefore,it is of great importance to develop the fatigue and corrosion fatigue damage evolution modeling of submarine pipelines and to predict the corrosion fatigue life of pipelines for theoretical research and engineering application.In this paper,the corrosion fatigue damage evolution of submarine pipelines is investigated by the combined theoretical analysis with finite element simulation,based on existing theoretical and experimental research.The research contents can be summarized as the following parts:(1)Based on continuum damage mechanics,three kinds of damage evolution models,namely,high cycle and low cycle fatigue evolution model and stress corrosion damage evolution model of metal material,are established in this paper.Considering the interaction between stress corrosion and fatigue load on the material damage,the corrosion fatigue damage evolution model was developed by introducing a nonlinear accumulation factor,according to the nonlinear accumulation principle of damage.Taking API 5L X65 pipeline steel as an example,the parameters in the above damage evolution model were obtained by fitting the experimental data collected from the literature.Then,the low cycle fatigue damage evolution model and corrosion fatigue damage evolution model were programmed into UMAT subroutines using the FORTRAN programming language,which was embedded into the finite element model to simulate the crack initiation behavior under the testing conditions and verify the correctness of model parameters.(2)Different API 5L X65 pipeline steel samples with notches on both sides were designed.The influence of notch depth,notch root radius,and notch opening angle on the accumulation rate of fatigue damage of pipeline steel samples was studied by using the control variable method,and the stress and damage distribution of notch root were analyzed.The fatigue crack initiation process of the sample with different thicknesses was studied by combining theoretical analysis with finite element method,and the relationship between crack initiation position and sample thickness was discussed.The stress concentration factor was introduced to analyze the relationship between the stress concentration effect and the fatigue crack initiation life of the sample,and the expression of the relationship between them was obtained.(3)The shape of the corrosion pit is considered a semi-ellipsoid shape.By introducing the shape parameters,the pre-corrosion pit pipeline steel samples with different shape factors are designed.The corrosion fatigue damage evolution process of samples with corrosion pits was simulated by introducing the established corrosion fatigue damage evolution model into ABAQUS in the form of the subroutine.The influence of the shape factor of corrosion pits and the stress level on the corrosion fatigue crack initiation life of the samples was investigated.Besides,the influence of the shape factor of corrosion pit on the initiation position of corrosion fatigue crack was analyzed on the premise of comparing the samples with different corrosion pit depths.(4)The finite element models of corroded pipelines with different distribution patterns of corrosion pits were analyzed,and the influence of the corrosion pits on the crack initiation life of defective pipelines was summarized.Based on the above results,the influence of corrosion pit spacing coefficient on the stress and damage distribution near the corrosion pit of different defective pipelines was discussed.Finally,the relationship between the stress concentration factor of pipelines with different types of corrosion pits and corresponding corrosion fatigue crack initiation life was analyzed,and the mathematical formula between them was established.