Velocity Effects and their Minimization in MFL Inspection of Pipelines - A Numerical Study

A wide variety of methods are used for the inspection of the 448,000 kilometers of gas pipelines currently in operation in the United States. Speed and accuracy are the prime concerns in inspections of this magnitude. Magnetic flux leakage (MFL) inspection of pipelines [1], using a magnetizer moving at velocities up to 30 kilometers per hour, is currently the most commonly used inline inspection method. At these velocities the leakage field signal is significantly distorted due to motionally generated currents in the pipeline. Experimental measurements of the velocity effects is expensive and possible for only very limited choices of parameters such as geometry and dimensions of the probe, defect, etc. Analytical, closed form solutions for electromagnetic (EM) non-destructive testing (NDT) problems including velocity effects can be found for only the simplest examples and are impractical for most NDT problems. Numerical analysis techniques for the modeling of velocity effects in a variety of EM areas are developing rapidly [2], [3]. In modeling the MFL inspection, the numerical model is required to be capable of modeling non-uniform geometries in order to simulate defects. Also, for accurate predictions, nonlinearity in various regions of the geometry must be incorporated. A numerical model with these capabilities is an invaluable asset both in terms of studying in detail the total physics of the situation, and also to aid in the magnetizer design