Research Of Residual Strength Evaluation Methods Of The High-Strength Steel Gas Pipeline With Internal Corrosion Defects

In the long-distance transportation of natural gas, high-strength steel pipes are expected to replace ordinary steel pipes because of their excellent corrosion resistance, large diameter and compression resistance. And just because of the large diameter, high pressure transmission and thin thickness of the wall, the high-strength steel pipes will face more severe challenges compared with the ordinary steel pipes when the internal corrosion defects appeared during the operations, and the leakage and explosion caused by the defects can also spark catastrophic accidents. At present, studies are still not adequate of the corrosion model and residual strength assessment of the high-strength steel pipes for not considering the influence of the internal flow field of the transport fluid. The assessment of the key monitoring position and the estimation of the remaining operation life of the high-strength steel pipes have become increasingly crucial. Therefore, considering the flow field of the transport gas into the corrosion model and analyzing the residual strength of the high-strength steel pipe with internal corrosion defects under fluid-structure interaction (FSI) had the great significance on quick assessment of residual strength and operation status of the high-strength steel pipeline.Taking the existing corrosion model and residual strength assessment of ordinary steel pipes, the influence of the flow field of the natural gas in the pipes on the internal corrosion and its evaluation was investigated. And based on theoretical analysis, numerical simulation and test data, a more comprehensive corrosion model and residual strength assessment of the high-strength steel pipes was proposed.Firstly, the erosion test platform of the high-strength steel specimen was designed according to the similarity of the Reynolds, and the erosion experiments of X80samples were carried out. Then the erosion shape and weight of the X80samples under different erosion angle were obtained; and based on the two-phase particle dynamics models, the simulation correspond to the experiment was carried out by using finite element method. In the simulation, the parameters of the fluid which influenced the erosion of the X80sample of the experiment were analyzed, and the distribution of the velocity and turbulence kinetic energy in the simulation were similar to the erosion shape of the erosion defect in the experiment.Secondly, the components, yield strength and tensile strength of the X70and X80 samples were measured, and the tensile test of the X80samples containing a semi-elliptical defect was carried out. In the test, the change of the defect depth had the maximum impact on the tensile while the change of the defect length (15mm-55mm) had the minimum impact when the test samples reached the yield strength; the finite element simulation showed that there was a stress concentration around the X80defects. The stress concentration around the semi-ellipse defect after smooth transition can reflect the failure status of the tensile sample more precisely and efficiently.Thirdly, based on the turbulence compressible gas equation, parameters of the fluid in the pipe were analyzed in different inclination angles. Then pressure and temperature drop equations were deduced. The formula of the internal corrosion rate influenced by the flow field of the high-strength steel pipe was deduced based on the De waard corrosion model. Result shows that the fluctuation of the pipeline’s elevation causes the internal pressure of the pipeline. The CO2partial pressure has become the most important role on inducing the average corrosion rate along the pipeline. In the parameters of the flow, the velocity and turbulence kinetic energy of the transport gas, can replace the fluctuation of the pipeline’s elevation, and describe the influence of the corrosion along the high-strength steel pipeline. In the key position, the appearance of the corrosion distribution often appears in the form of an elliptical sheet.Fourthly, a finite-element pipe model was built based on an X70straight high-strength steel pipe with semi-ellipse defects, and the parameters of the fluid and equivalent stress was simulated with the pipe under FSI method. Results showed that the maximum stress under the influence of the flow field was higher than that under a simple pressure load at the semi-ellipse defect. Under the simulated pressure lOMPa, the depth of the defects had the strongest effect on the maximum stress of the pipe, while the length of the defects had little effect when it was more than100mm, whereas the width has the least effect. For the X70high-strength steel pipeline, the depth more than50%thickness would cause bad influence of the failure pressure. And for the pipe with double defects in the same axis, the maximum equivalent stress was inversely proportional to the distance between the double defects. At a particular length, the stress interaction of the double defects can be negligible.Finally, according to the stress distribution with different defects’ parameters in the stress distributions simulation, the failure pressure formula of the high-strength steel pipe with semi-ellipse defects under the influence of the flow field was fitted based on Marquardt method. Compared with the experiment data under a simple internal pressure load, the failure pressure prediction of the pipe with defects under the influence of the flow field was stability. Based on the internal corrosion rate equation under the influence of flow field of the high-strength steel gas pipeline and the failure pressure model of the high-strength steel gas pipe with internal defects under FSI, the prediction model of the internal corrosion remaining life of the high-strength steel gas pipe was established, which can focus on the important position of the pipeline.Based on the analysis of the corrosion rate with flow parameters in the high-strength steel gas pipe and the failure pressure of the high-strength steel pipe with internal corrosion defects under FSI, the evaluation method of the residual strength of the high-strength steel gas pipeline with internal corrosion defects was proposed, which can focus on the important detection position of the pipeline, calculates the residual strength of the pipeline with internal corrosion defects and predicts the re-evaluation intervals, which can be provided the theoretical basis for the reliable operation of the high-strength steel pipeline.An integrated evaluation system of the residual strength of the high-strength steel gas pipeline in operation was formed in the paper, which can be faster and more accurate. The technical support for safety operation of high-strength steel gas pipeline can be provided in the system.