Challenges in Circumferential Magnetisation: A FEA Point of View

Integrity of pipelines is of vital importance to their performance. This entails routine inspection using a variety of different inspection tools. One of the most popular type of tools is based upon Magnetic Flux Leakage (MFL). In this technique, the MFL tool magnetically saturates the pipe and in locations where defects exist, flux leaks out of the pipe and is detected by a set of on-board sensors. There are two different geometric configurations of an MFL tool, namely, axial and circumferential. Understanding the magnetic behaviour of these tools is critical is the MFL technique is to be successful. This paper details 2D and 3D finite element (FE) modelling of a circumferential magnetiser at rest and at typical operating speeds. 2D modelling of a stationary MFL tool indicates that pipe saturation can occur for a variety of pipe dimensions provided the magnets are correctly sized. However, this 2D modelling assumes a constant cross-sectional area and thus neglects tool and effects. This is rectified by the creation of several 3D models. It can be seen from these 3D models that end effects are significant and that pipe saturation is not as easily achieved as it was in 2D. Further, it is shown that when the MFL tool moves at constant speed down the pipe, the magnetic field profile in the pipe observed under stationary conditions is significantly altered. The significance of this effect depends on the operational speed of the tool, the material properties of the pipe and the dimensions of the pipe (pipe diameter and wall thickness). The 3D modelling presented suggests that under certain conditions the level of flux in the pipe is less than ideal (i.e. lacking the required magnetic intensity and homogeneity) and, for these conditions, the circumferential MFL technique may have difficulty detecting and sizing pipe anomalies.