Estimation of permeability for magnetic flux leakage modelling

Non-destructive testing techniques are gen-erally used for the evaluation of material properties and the detection of defects in pipelines. Especially, the measure-ment of magnetic flux leakage (MFL) is one of the most used methods for in-service monitoring of corrosion in buried oil and gas pipelines. However, for the geometrical reconstruction of defects in a pipeline from measured signals (the inverse problem), the magnetic properties of the mate-rial need to be known accurately. Due to the pipe geometry and wall thickness measurement of the bulk properties is complicated. In particular, the permeability of ferromagnetic steels shows a non-linear and hysteretic behaviour.
Current research is focussed on the measurement and simulation of the permeability of pipeline material. The magnetic properties of a small ferromagnetic test sample is first investigated using a transformer set up in order to find a correct fitting equation for the non-linear permeability as a function of the magnetic flux density. This equation is then used for model simulations of a MFL test set up. Refitting the parameters of the permeability equation to the perme-ability of the pipeline material in the MFL set up proved that flux leakage signals could be predicted accurately for sev-eral defects by model simulation.