Numerical modelling and experimental investigations of eddy current systems for non-destructive testing

Abstract

Corrosion under the insulation of process pipework is a serious problem for oil & gas industries and is difficult to detect because of the insulation and covering material that masks the corrosion problem. This type of corrosion can cause failures if not inspected and treated. Eddy current testing (ECT) is a non-contact inspection method and meets the requirement of measuring wall thickness through insulation and covering. Pulsed eddy current testing (PECT) uses broad-band excitation for sufficient electromagnetic penetration into test objects. To detect and characterise corrosion under insulation this thesis proposes a novel pulsed eddy system via numerical simulation and experimental studies. Finite element method (FEM) numerical simulations for eddy current non-destructive evaluation (ECNDE) can provide information on how the induced eddy current interacts with defects. In this thesis, finite element eddy current models were developed using commercial software COMSOL Multiphysics. Through the analysis of the simulation results, links were established between the measurements and information relating to the defect, such as 3-D shape, size and location. Firstly, developed 3D models were used to study and compare the eddy current model and experimental response for different flaw types in plates and pipes using pancake and encircling coils. Having validated the models, depth of penetration of the eddy currents for different scenarios and coil types were studied. It was found that an encircling type probe provides a larger depth of penetration compared to pancake probes. A novel encircling pulsed eddy current (PEC) probe for detection of material loss type corrosion under non-conductive insulation was proposed to increase the depth of penetration and sensitivity. 3-D FEM simulations were utilised to provide the guidelines for experimental designs and specifications; understanding of the underlying physics surrounding a particular defect; and means for feature extraction from the acquired responses. Through the study, defect detection and characterisation using the PEC encircling probe was investigated and the features extracted were used to establish quantitative information from the defects. Furthermore finite element models were used to investigate the introduction of conductive coverings over the nonconductive insulation. Time and frequency domain analysis were carried out on the acquired data. Moreover, an experimental platform was built to verify the simulation results and the feasibility of the proposed technique and extracted features to be used in obtaining information about the defects. The work concludes that the proposed system is a potential tool and paves the path to enhanced corrosion under insulation detection. Features extracted from the numerical investigations have provided limited means for the QNDE of corrosion under insulation. The system is limited to cylinder structures and does not address problem of corrosion near bends and attachments to the pipe. The system is only capable of detecting corrosion patches and is not suitable for crack and pitting detection.