Evaluating an SH Wave EMAT System for Pipeline Screening and Extending Into Quantitative Defect Measurements

Guided waves are now commonly used in industrial NDT for locating corrosion in pipelines in the form of wall thinning. Shear Horizontal waves generated by EMATs are used in a screening arrangement in this work to locate and size corrosion in terms of axial extent and circumferential positioning. This is facilitated by propagating SH waves circumferentially around the pipeline whilst moving a scanning rig axially, keeping transducer separation constant. This arrangement is preferential in that it can operate through thin(up to 1mm) coatings and does not require full access to the pipe's circumference and is useful for detecting corrosion in difficult to access regions, such as below pipe supports and in subsea applications. The performance of the system in terms of screening capability and the possibilities of extension into more quantitative measures are assessed. The behaviour of different wave modes as they interact with defects is investigated via experimental measurements on artificially induced corrosion patches and measurements on samples with in service corrosion. Measurement of the axial extent of corrosion patches, circumferential positioning and a range of possible remaining thickness is assessed. Finite element modelling of SH mode interaction with defects is used to understand what happens to different wave modes when they interact with defects in terms of reflection, diffraction and mode conversion.