Multiple parameters based pulsed eddy current non-destructive testing and evaluation

Abstract

Eddy current sensing technique is widely used primarily because of its high tolerance to harsh environments, low cost, broad bandwidth and ease of automation. And its variant, pulsed eddy current offers richer information of target materials. However, accurate detection and characterisation of defects remains a major challenge in the petro-chemical industry using this technique which leads to spurious detection and false alarm. A number of parameters are contributory, amongst which is the inhomogeneity of the materials, coupling variation effect and relatively large lift-off effect due to coating layers. These sometimes concurrently affect the response signal. For instance, harsh and dynamic operating conditions cause variation in the electrical conductivity and magnetic permeability of materials. Also, there is the increased need to detect defects and simultaneously measure the coating layer. In practice therefore, multi-sensing modalities are employed for a comprehensive assessment which is often capital intensive. In contrast to this, multiple parameter delineation and estimation from a single transient response which is cost-effective becomes essential. The research concludes that multiple parameter delineation helps in mitigating the effect of a parameter of interest to improve the accuracy of the PEC technique for defect detection and characterisation on the one hand and for multi-parameter estimation on the other. This research, partly funded by the Petroleum Technology Development Fund (PTDF), proposes use of a novel multiple parameter based pulsed eddy current NDT technique to address the challenges posed by these factors. Numerical modelling and experimental approaches were employed. The study used a 3D finite element model to understand, predict and delineate the effect of varying EM properties of test materials on PEC response; which was experimentally validated. Also, experimental studies have been carried out to demonstrate the capabilities of the proposed to estimate multiple parameters vis-à-vis defect depth (invariant of lift-off effects) and lift-off. The major contributions of the research can be summarised thus: (1) numerical simulation to understand and separate the effect of material magnetic permeability and electrical conductivity in pulsed eddy current measurements and experimental validation; (2) proposed the lift-off point of intersection (LOI) feature for defect estimation invariant of lift-off effects for ferromagnetic and non-ferromagnetic samples; a feature which is hitherto not apparent in ferromagnetic materials (a primary material used in the oil and gas industry); (3) separation and estimation of defect and the lift-off effects in magnetic sensor based pulsed eddy current response; and (4) application of the LOI feature and demonstration of increased defect sensitivity of the PEC technique with the proposed feature in both ferrous and non-ferrous conductive materials.