Quantitative Study of MMM Signal Features for Internal Weld Crack Detection in Long-Distance Oil and Gas Pipelines

Abstract

The metal magnetic memory (MMM) inspection technology is an effective method to detect the stress damage of ferromagnetic materials, and it especially possesses great application potential in the field of long-distance oil and gas pipeline weld crack detection. In order to study the MMM generation mechanism and the signal feature of weld cracks, a model to quantitatively calculate the MMM signal of nonuniform magnetic charge of internal inspection is proposed. The effect of changes to the crack depth, the crack width, and the pipeline internal pressure on the MMM signal is studied, and the theoretical results have been proved by extensive experimental studies. The results indicate that the axial component has only one peak value, while the radial component has two pairs of independent and zero-crossing peak-to-peak values, of which the outside pair corresponds to the weld and the inside pair corresponds to the crack. The feature values change nonlinearly with the crack depth, and the growth rate decreases gradually. The feature values increase linearly with the crack width, and the radial component is more sensitive to width than the axial component. The internal pressure will increase the stress concentration around the weld and the crack, but the stress around the crack responds more sensitively. Therefore, the MMM signal is positively correlated with the internal pressure, and the feature value will increase linearly.