Research On Prediction Methods Of Corrosion Fatigue Life Of Submarine Pipelines

Corrosion fatigue failure is a common cause of accidents in submarine pipelines.In order to study the method of predicting the corrosion fatigue life of submarine pipeline,based on the investigation of the corrosion fatigue expansion mechanism,the influencing factors and the fatigue life prediction method at home and abroad,a research route of predicting corrosion fatigue life of submarine pipeline by Paris formula in fracture mechanics theory is established.In order to obtain the corrosion fatigue properties and parameters of the submarine pipeline materials,the material performance test for two kinds of submarine pipeline materials,16Mn and X56,are designed,and both of them from the actual submarine pipeline are made into standard compact tensile(CT)tests.The specimens are subjected to the corrosion fatigue crack propagation test under seawater conditions and the fatigue crack propagation test under the air environment.In the test measurement method,crack lengths are measured by a combination of finite element method and flexibility method,and the differences in the fatigue life and fatigue crack growth rates of 16Mn and X56 steels in seawater and air environments were recorded.The results show that:The fatigue expansion characteristics have a great influence.The corrosion fatigue crack growth rate of the two types of submarine pipeline materials in seawater environment is far greater than the fatigue crack growth rate in air.Based on the measured data of fatigue crack growth of CT specimens,the Paris constants of 16Mn steel and X56 steel in air and seawater environment were fitted.From the fitting results,the Paris constant is more sensitive to the material and the environment.In different experimental environments,the Paris constants of the two materials are approximately similar in magnitude,but are numerically identical.The Paris constant obtained from the corrosion fatigue crack propagation test of the CT specimen provides the necessary parameters for the corrosion fatigue life prediction of the submarine pipelineOn the basis of corrosion fatigue test of submarine pipeline material,a corrosion fatigue test of a full-size X56 submarine pipeline specimen is carried out.The finite element model of submarine pipeline with cracks is further established by the combination of ABAQUS finite element simulation software and FORTRAN programming software.The value of stress intensity factor at the fatigue crack tip of X56 submarine pipeline is analyzed by using this model.The pipeline crack propagation process is divided into several stages by using approximate product method.The stress intensity factor at the end point of each stage is taken as the average value of the stress intensity factor in this stage.Finally,the fitted Paris constant value is brought into the Paris formula and the total corrosion fatigue life of the submarine pipeline can be obtained.Comparing with the corrosion fatigue test results of the submarine pipeline specimens,the relative error between the FEM prediction results and the test results is only 6.4%.Therefore,it can be considered that the above-mentioned corrosion fatigue prediction model and prediction method can effectively predict the corrosion fatigue life of the submarine pipeline.