Research On Pipe Crack Detection Technology Under External Load

The stress concentration area is formed in long-distance oil and gas pipelines under the external loading for a long time when the pipeline structure is unreasonably arranged or the material has defects,and stress cracks will occur at the stress concentration area under long-term internal pressure.The crack will grow rapidly under fluctuating pressure,which directly affect the safe operation of engineering equipment.Therefore,the detection to the stress fatigue damage before the generation of macro-defects in pipeline is crucial to reduce the accident rate.Pipeline internal inspection equipment is used to determine the degree of deformation,corrosion,and defects of the pipeline.Pipeline internal inspection equipment provides a scientific basis for pipeline safety assessment,and pipeline internal inspection technology is the most effective method of pipeline safety inspection.Conventional NDT methods,such as magnetic flux leakage,magnetic particle,eddy current,permeating,ultrasonic,radiographic,etc.,have played an important role in pipeline defect detection and accident prevention.However,pipeline internal inspection technology has non-contact,high-speed detection(the operating speed of the internal detector is 1–5 m/s),dynamic continuous and other technical requirements.At present,magnetic flux leakage detection technology is the most widely applied and most mature pipeline internal inspection technology because it meets the above requirements.Magnetic flux leakage detection technology is adopted by more than 90% of the pipeline internal detection equipment in the world.But there are limitations to the safety assessment of microcracks in pipeline magnetic leakage detection technology.The main reason is that the size of the microcracks is small,resulting in a weak magnetic flux leakage signal generated by the microcracks.And the magnetic flux leakage signal is easily covered by the exciting field of the excitation device,which makes the microcracks difficult to identify.In this paper,the weak magnetic method is used to detect pipeline cracks.The principle of weak magnetic microcracks detection is that the stress concentration degree of the in-service pipeline crack is detected to determine the size characteristics of microcracks in the geomagnetic field.The weak magnetic method has the characteristics of non-contact,fast acquisition of signals,support for dynamic detection,and no excitation structure.It has practical significance to improve flexibility and reduce the cost of the inspection in oil and gas pipelines.In this paper,a numerical model of non-uniform crack magnetic charges is established based on the stress distribution laws of pipeline cracks under internal pressure.The crack internal pressure increasing factor is introduced as the characteristic value to quantify the crack.The relationship between the internal pressure increasing factor and crack size is analyzed.The eigenvalues of cracks in different directions with internal pressure are compared and analyzed.Changes of internal pressure increasing factor before and after crack propagation are analyzed.The influence of external magnetic fields on weak magnetic signals is analyzed.The systematic experiment is carried out.The results show that the axial component of the weak magnetic signal at the crack has a maximum value near the tip,and a minimum value appears in the middle of the crack.The internal pressure increasing factor is introduced to quantify the weak magnetic signal,the crack is in a safe state(not expanding)when the internal pressure increasing factor is positive,the weak magnetic signal has a linear relationship with the crack size.However,the crack is in a dangerous state when the internal pressure increasing factor is negative,and the pipeline crack will expand as the internal pressure increases.When the external magnetic field increases,the field weakening signal at the crack also increases,while the field weakening signal characteristic at the stress concentration area decreases.