Study Of Circumferential Magnetic Flux Leakage Internal Inspection Technology In Pipeline

Pipeline transportation is the primary way of transporting oil-gas resource, and the safety of oil-gas pipeline has become a very important problem related to the national economy and the people's life. Regular pipeline inspection is the main method to avoid the pipeline accident and ensure the pipeline safety. Magnetic flux leakage (MFL) internal inspection technology is most widely used in many on-line oil-gas pipeline inspections. Concerning the difficult problems of MFL internal inspection combining with the practical inspection demand, after theoretical analysis and analysis on the experiment systems, this dissertation has summarizes the MFL internal inspection technology of pipeline, and has done thorough research into axial MFL and circumferential MFL.After the detailed analysis on the formation mechanism of defect MFL, the theory models of defect MFL field are established. In accordance with the drawbacks in the magnetic dipole model, the finite element model is applied to the analysis on defect MFL.The dissertation analyzes principle of axial MFL inspection in pipelinend MFL signal features. It studies the law for the effect of such affecting factors of MFL signal features as defect geometry parameters (length, width and depth) and the extension direction of the main characteristics of defects. The experimental platform of axial MFL inspection in pipeline is designed, pipeline traction test is implemented, and MFL signal samples are collected and analyzed. The experimental results verify that the axial MFL inspection method can detect the circumferentially orientated defects with good confidence. The circumferentially orientated defects, such as spiral weld, hole, circumferential groove, produce clear MFL signal. Each channel signal measured from different sensors has a very good consistency, stability and relatively high detection accuracy. The axial MFL inspection can identify a minimum depth of defects in 10% of wall thickness, and minimum size of defects in the hole of 2mm diameter. However, the axial MFL inspection method is not ideal for detecting axially aligned defects(such as narrow crack).The dissertation studies principle of circumferential MFL inspection in pipeline, and comparison of the finite element simulation, the magnetizer structure that has the circular geometry of quadrupole separation is proposed which meets engineering design requirements. After the application of magnetic circuit theory, a mathematical modeling of magnetic circuit of magnetizer is established, a magnetic circuit calculation program is developed, the influence of the geometric size of permanent magnets on MFL signal is determined, these provide a method for the magnetizer design. MFL signal features are analyzed, the features of signals are extracted from the recorded flux leakage response and characterizing definition is introduced as well; It is much more difficult to saturate the pipe material in the circumferential direction, which is caused by the circular geometry providing alternative flux paths. The strength of the magnetic field is greatest near the magnetizer poles and smallest at the center, and this non-uniform field makes defect sizing more difficult, for these problems, compensation methods of MFL signal are proposed to eliminate the non-uniform magnetic field. The affecting factors of MFL signal features, such as the distance between the defect and magnetic pole, and defect geometry parameters(length, width and depth), are researched, then get some important law and offer theoretical direction for defect evaluation and defect signal compenstation. By adopting the multiple linear regression theory, the regression equation is established between the defect geometry parameters and the features of MFL signals to implement the quantitative identification of the defect parameters.The circumferential tool measures the MFL signal by using the sensors which are installed between the magnets in parallel, by selected Hall element as the sensor to measure the MFL signal, sensor arrays made up of multiple sensors with samller spacing intervals are constructed, and high-density measurement probe is produced which completely covers the narrow crack thus improves the inspection precision need to ensure axially oriented anomalies were detected efficiently. The data acquisition system software with modular structure is developed so that real-time signal acquisition and processing are achieved. MFL signal processing and compensation methods are implemented, such as detection signal interpolation smoothing method, measuring method of singular points of the amendment, defect-free magnetic flux leakage signal compensation method, as well as differential magnetic flux leakage signal processing method, the disadvantage of too few spatial sampling points caused by insufficient number of sensors is eliminated, the adverse effects on the detection caused by the non-uniform magnetic field in wall and the background magnetic field near the magnetic poles too strong are overcome. It has design the experimental platform of circumferential MFL inspection in pipeline, and has conductsed pipeline traction test, MFL signal samples collected by pipeline traction test are analyzed. The experimental results verify the relationship between the defect geometry parameters and the signal characteristics. The experimental results show that the circumferential MFL inspection method can detect axially aligned defects (such as axially oriented groove). The circumferential MFL inspection can identify a minimum depth of defects in 10% of wall thickness and a minimum width of defects in 3mm. These results and conclusions solve some difficult problems of not detecting axially aligned defects in the field of pipeline MFL inspection, and determine the technical foundation for the development of circumferential MFL inspecting equipment in pipeline.