Interacting effects of clustering defects on MFL signals using FEA

The magnetic flux leakage (MFL) technique is generally considered to be an efficient and effective non-destructive testing (NDT) method and has been widely used in the oil field inspection industry for the examination of oil and gas pipelines, both new and used. However, the interaction of clustering defects always affects the value of the MFL signal and further complicates the defect-induced MFL signal calibration. In this paper, a non-linear structural finite element analysis (FEA) model is created for the axial MFL tool and the interacting effects of clustering defects on the axial and radial MFL signals are investigated. According to the numerical simulations, the degree of interaction between two adjacent holes is analysed as a function of the holes' edge-to-edge separation by the MFL signal features of known geometries. It clearly illustrates that, at close-to-saturation magnetic flux density (1.8 T), an edge-to-edge separation of at least 4.5 × defect radius is found necessary for the MFL method to interpret the two holes independently.