Challenges in Circumferential Magnetisation II: A FEA Point of View

In order to design a Magnetic Flux Leakage (MFL) tool, knowledge of the magnetic field profile both within the pipe wall and around the sensor locations is required. The former is important since the magnitude of a defect signature will be maximised when the pipe wall is magnetically saturated. The latter is important for the reason that the background magnetic field profile will determine the sensor bias level for a given defect. Previous research using the Finite Element (FE) technique has investigated the magnetic field profile within the pipe wall for a generic circumferential MFL tool and showed how it varied depending upon the pipe wall thickness and tool speed. It was found that as the pipe wall thickness and tool speed increased, a plume of magnetic flux formed behind the tool. Furthermore, it was observed that as the pipe wall thickness increased, outer regions of the pipe wall exhibited substantially reduced levels of magnetic flux. A reduction of magnetic flux in these outer regions of the pipe wall suggests defects located here may be harder to detect and resolve compared to defects located closer to the inner pipe wall. This paper briefly reviews the previous research and extends the numerical analysis by considering the magnitude of the magnetic field profile for locations within the inner and outer pipe wall and possible sensor positions. It is shown how these profiles change with pipe wall thickness and tool speed and demonstrates the complex and non-linear nature of the magnetic field. The information obtained can be combined with previous research and will be useful for determining the optimal sensor location and yield predictions for background magnetic field magnitudes. The full complexity of magnetic materials is not incorporated into the modelling, however, the results obtained give a theoretical indication of operational limitations of the circumferential MFL technique.