Novel Filtering and Quantifying Method for Magnetic Flux Leakage Signals of Defects in Bimetallic Composite Pipes

L360QS/Incoloy 825 bimetallic composite pipe (BCP) is widely used for high-sulfur gas transportation in the oil and gas industry. The outer layer of BCPs is susceptible to external soil erosion. However, conventional magnetic flux leakage (MFL) testing has a low recognition rate for defects in the outer layer due to noise interference. In this paper, a Butterworth filter–variational mode decomposition–discrete wavelet transform (BW-VMD-DWT) denoising method is established. Then, the MFL pull test and finite-element simulation are carried out to obtain the original magnetic signal of cylindrical and round pit defects. Finally, the defect detection rate before and after noise filtering is compared via the BW-VMD-DWT method. The results show that the average noise amplitude of cylindrical and round pit defects decreased by 0.35 and 0.20 mT, and the signal-to-noise ratio (SNR) increased by an average of 14.5% and 24.5%, respectively. In addition, the detection rates of cylindrical and round pit defects have increased by an average of 5.5% and 8.0%.