From EMAT to image: practical guided wave tomography

Abstract

The detection and characterisation of corrosion type defects on pipelines is a major challenge for the petrochemical industry, especially in regions with poor accessibility. Guided wave tomography is one feasible approach to inspect areas with restricted access by transmitting guided waves through the area and then processing the measured wave field into a thickness map of the pipeline wall, without having to take measurements at all points on the surface. The key objective of this research project is to develop, implement and test a prototype guided wave tomography system based on the A0 Lamb mode.

For the development of a guided wave tomography system a low-frequency, omni-directional A0 Lamb wave ElectroMagnetic Acoustic Transducer (EMAT) is developed, and operates at 0.50MHzmm on a steel plate. For that, a parametric Finite Element (FE) model is implemented in a commercially available FE software and a numerical optimization process employing a genetic algorithm is set up to optimised the EMAT design for an improved A0 mode selectivity. The FE model is validated against measurements on an aluminium plate and on a steel plate. A two-step model-based design approach is proposed whereby only the Lorentz force is used in the first step for the optimisation and then in a second step, a realistic estimate of the mode selectivity can be obtained by additionally considering the magnetisation force. The optimised design fulfils the S0 suppression design requirement and is integrated into the guided wave tomography system consisting of two ring arrays.

The developed guided wave tomography system is tested on two steel pipes with smooth and well defined defect. The repeatability of measurements is assessed and the robustness of the guided wave tomography measurements to sensor position errors is investigated. It is demonstrated that there is a small influence on the thickness reconstruction for fairly large systematic and unsystematic position errors. Similar results are obtained for single sensor failures or gaps in the arrays and an increase in sensor spacing is found to increase reconstruction artefacts. With Golay complementary sequences, a signal processing technique is presented that allows for a significant increase in the data capture speed with the same performance as time averaging.

Three areas with restricted access, support locations, pipe clamps and STOPAQ(R) coatings, are considered and their influence on the thickness reconstruction is investigated relative to a reference configuration and only a small influence is found in the experiments.