| Pipeline transportation is one of the main forms of long distance oil and gas transportation at home and abroad.At present,China has nearly 136,000 kilometers of long-distance oil and gas pipelines.With the accelerated development of the national economy,the demand for energy in all industries is gradually increasing.With the vigorous promotion of west-to-east gas transmission project,the pipeline mileage will further increase and present a rapid development.The safety and economy of pipeline operation has become an important issue in its operation.As a common defect in oil and gas pipelines,crack detection becomes an important part of pipeline safety evaluation.The lifetime of crack seriously affects the overall life of pipeline,so the study on the lifetime of crack can effectively improve the economics of pipeline operation.A large number of studies have been conducted on the evaluation of crack life in pipeline steels,but the existing studies are limited to the known crack size and the stress intensity factor at the crack tip.In practical engineering applications,how to obtain the crack size and stress intensity around the crack has become the focus of attention of inspectors.In recent years,the weak magnetic stress internal detecting technology has become a new non-destructive testing technology for long-distance oil and gas pipelines.Compared with ultrasound,ray detection methods,it inherits the advantages of the magnetic flux leakage testing technology such as dynamic online,without human contact and so on.Compared with the magnetic flux leakage detection technology,weak magnetic stress detecting technology could detect tiny cracks and stress concentration area,which make up for the inadequacy of the technique of magnetic flux leakage and meet the needs of inspectors about the detection data of cracks.Therefore,the study of weak magnetic signal characteristics of cracks under different stresses with different sizes can provide theoretical basis and data support for the identification and evaluation of pipe cracks,which is of great significance for the life evaluation and analysis of cracks.This subject is based on the weak magnetic stress internal detection technology.The stress-permeability relation and the calculation model of magnetic charge are established.The signal characteristics of stress concentration area are studied according to the relationship between magnetic permeability and stress.Based on the stress of pipe crack under load,the weak magnetic signal characteristics of pipe cracks with different length and depth or with different stress intensity are calculated and analyzed.The theoretical analysis of the stress concentration area and the weak magnetic signal of the crack is verified by the pipe pressing experiment.The prediction model of crack life of X80 pipe was established based on the crack fatigue test of pipe.The pipe crack life was evaluated according to the actual operation condition of the pipe.The results show that the relationship between tube stress and permeability under geomagnetic field is not monotone.The permeability increases linearly and then decreases linearly with the increase of stress.The characteristics of weak magnetic signals in stress concentration area are different under different stress intensities.The characteristics of the weak magnetic signal in the absence of stress in the crack are different from those in the presence of stress.The stress of the crack can be distinguished according to the characteristics of weak magnetic signal.The relationship between crack size and signal strength is linear.The influence of the change of crack size on the signal above the crack is much less than that above the stress concentration area at the crack tip.The crack life of pipe can be evaluated according to the life prediction model of X80 pipe and the operation condition of pipe. |
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