Quantitative study on magnetic flux leakage internal detection signal of defects on the outer wall of bimetal composite pipes

Bimetal composite pipes are widely used in oil and gas gathering pipelines due to their exceptional anti-corrosion and high-pressure resistance properties. However, the magnetic flux leakage (MFL) internal detection device for single-layer pipes is unsuitable for the bimetal composite pipe. It is difficult to detect the location and size of pipe defects in the outer layer. To solve the problem of poor detection performance for the bimetal composite pipe, this paper conducted a series of studies. Firstly, in this paper, the multi-layer media magnetic dipole model for bimetal composite pipes is established. Then, the influence of the thickness and permeability of the inner layer on the MFL signal of the outer wall defects is quantitatively studied. What's more, a MFL detection simulation model for bimetal composite pipes is established, and the detection rate of defects on the outer wall of bimetal composite pipes is improved by optimizing the design of the magnetization device. Finally, a defect size inversion method in the outer layer of bimetal composite pipes is proposed, which is validated by an indoor experimental test. The results indicate that compared to the case with the inner layer, the amplitude of axial and radial MFL signals raise by 74 % and 76 % respectively in the absence of an inner layer pipeline. The amplitude of the MFL signal of the outer wall defect decreases exponentially as the thickness and relative magnetic permeability of the inner layer increase. After optimizing the magnetization device, the magnetic induction intensity of the outer layer is increased by 29 %, and the detection rate of defects is increased by 37 %. The maximum error of the defect size inversion model is 6.4 %, which meets the requirements of engineering applications.