Study On Evaluation Method Of Failure Pressure And Reliability Of Corroded Pipeline

Pipeline is widely used in the transportation of oil,natural gas and other liquids in petroleum,chemical and other energy industries due to its large transportation capacity,strong resistance to external loads,high reliability and little impact on the environment.In order to transport materials,pipelines are usually under internal pressure during operation and internal pressure must be considered in the design and bearing capacity evaluation of pipeline.Due to the electrochemical characteristics of the external environment and the corrosiveness of the transported materials,the corrosion defects appear on the internal and external surfaces of pipelines.The ultimate bearing capacity of corroded pipeline will be weakened due to the thinning of wall thickness.If the pipeline leaks under internal pressure,it will not only cause huge economic losses,but also result in great damage to the surrounding ecological environment.Therefore,the evaluation of internal pressure bearing capacity and reliability research of corroded pipeline are of great economic and social value for operation and management,maintenance and replacement of pipeline.Due to the randomness of corrosion,different defects will interact with each other,which will affect the internal pressure bearing capacity.The existing evaluation methods are based on the unified calculation model to evaluate the internal pressure bearing capacity of adjacent defects,and the differences of failure modes of pipeline with different distribution modes of adjacent defects under internal pressure are not been taken into account.Moreover,the existing reliability research is based on the internal pressure bearing capacity evaluation model of pipeline with single defect,and the interaction between defects is not considered,which overestimates the bearing capacity of pipeline.Based on full-scale burst tests and numerical simulations,the failure modes of pipeline with adjacent defects composed of different shape defects and arrangement modes under internal pressure are revealed in this study.Then the evaluation methods of failure pressure of pipelines with single defect,irregular-shaped defect and interacting defects are proposed.On the basis of the above research results,the reliability calculation method is developed by considering the interaction of defects,and the temporal variation of failure probability of corroded pipeline is analyzed according to the actual engineering example.The main work of the thesis is summarized as follows:(1)In view of the poor adaptability of the bulging factor in the failure pressure evaluation of pipeline with single volumetric defect,a new calculation method of bulging factor is determined based on the finite element analysis.Then,a more accurate calculation formula for the failure pressure of corroded pipeline is obtained by combining the failure pressure evaluation method of defect-free pipeline and the newly proposed bulging factor.(2)A three-dimensional nonlinear finite element model of pipeline with irregular-shaped defect is established.The effects of depth,width,length and axial spacing of deep defects on the failure pressure of corroded pipeline are analyzed.Based on the equivalent shape and evaluation length of irregular-shaped defect with different irregularity,a new calculation formula for the failure pressure of corroded pipeline is proposed,and the calculation formula is verified by comparing with the burst test data of actual pipeline.(3)In view of the heavy workload of the existing evaluation methods for the failure pressure of pipelines with interacting defects,the modified calculation method for the effective depth of interacting defects is developed based on the finite element results.Then,new calculation formulas for the failure pressure of pipelines with circumferentially aligned defects,axially aligned defects and defects with complex distribution are proposed,and the accuracy of these method is confirmed by the actual burst test.(4)The existing evaluation method of failure pressure is difficult to be applied to the time series analysis of failure pressure reliability for corroded pipeline due to its complicated steps.Therefore,a "central failure position" assessment method for the failure pressure evaluation of corroded pipeline is proposed based on the failure position of pipeline with corrosion clusters.Compared with the existing evaluation methods,the newly-developed method is more convenient and widely applicable with less evaluation steps and identical prediction error.(5)Based on the new calculation formulas and evaluation method proposed in the above,a reliability calculation method for the corroded pipeline is developed by considering the interaction between defects and the distribution law of tensile strength of service pipeline.Compared with the failure probability of pipeline with single defect,for the long-distance pipeline with general corrosion,the failure probability increases faster by considering the interaction of defects,and the service time corresponding to the same failure probability is shorter;for pipeline with local corrosion,the failure probability of the same service time is greater by incorporating the interaction of defects.