Nondestructive evaluation of ferromagnetic critical water pipes using pulsed eddy current testing

University of Technology Sydney. Faculty of Engineering and Information Technology.Thesis (PhD)Modern day maintenance of infrastructure demands significant attention to structural health monitoring. Assessment of surface condition alone is insufficient for health and strength assessment, creating the necessity to evaluate the integrity of subsurface regions through Nondestructive Evaluation (NDE). This thesis focuses on approaches to solving the problem of condition assessment of critical pipes, i.e., large diameter high-pressure pipes owned and managed by water utilities to distribute consumable fresh water to customers, by developing techniques for representing the geometry of electrically conductive ferromagnetic materials via Pulsed Eddy Current (PEC) sensors. The main contribution of this thesis is a novel detector coil voltage decay rate based PEC signal feature, the fundamental behavior of the feature is analytically described and experimentally validated. The feature has a convenient advantage in practical application since it is directly extractable from raw PEC signals and demonstrates significant invariance to sensor shape, size, and lift-off. The feature behavior is exploited in two estimation approaches, in situ measurements on pipes are performed and pipe wall thickness is inferred with uncertainty. Firstly, an analytical approach to learning a function mapping the decay rate feature to test piece thickness with the aid of signals captured on calibration blocks is presented. The requirement of fabricating calibration blocks to have material properties matching those of pipes is extremely challenging. Thus, combining ultrasound measurements together with PEC is proposed to address material variations. Secondly, a numerical NDE semi-parametric estimation approach is presented, PEC sensor signals are simulated taking into account measured electrical and magnetic properties of materials being tested. The thickness-feature function is learned probabilistically using Gaussian Process. Unlike in the analytical approach, the function is learned non-parametrically, therefore, variations and marginal nonlinearities are captured. The advantages over the analytical approach are demonstrated in terms of improved accuracy of inferred material thickness. Finally, the resolution of commercial PEC sensors employed on pipes is identified as a limiting factor for structural integrity assessment. A numerical study on optimizing PEC sensor architecture to achieve higher resolution while maintaining sufficient penetration capability is carried out and a framework which can be used to perform 3D profiling by means of joint inference of thickness and lift-off is proposed

Magnetic Flux Leakage techniques for detecting corrosion of pipes

Oil and gas pipelines are subjected to corrosion due to harsh environmental conditions as in refinery and thermal power plants. Interesting problems such as internal and external corrosion, emerging from the increasing demand for pipeline protection have prompted this study. Thus, early detection of faults in pipes is essential to avoid disastrous outcomes. The research work presented in this thesis comprises investigations into the use of magnetic flux leakage (MFL) testing for pipe in extreme (underwater and high temperature) conditions. The design of a coil sensor (ferrite core with coil) with a magnetic circuit is carried out for high temperature conditions. The sensor thus developed lays the ground for non-destructive evaluation (NDE) of flaws in pipes through the MFL technique. The research focusses on the detection and characterization of MFL distribution caused by the loss of metal in ferromagnetic steel pipes. Experimental verifications are initially conducted with deeply rusted pipe samples of varying thicknesses in air. AlNiCo magnets are used along with Giant Magneto Resistance (GMR) sensor (AA002-02). The experiment is further repeated for saltwater conditions in relation to varying electrical conductivity with radio frequency identification (RFID) technique. A further study carried out in the research is the correlation between magnetic and underwater data communication. The study has resulted in the development and experimental evaluation of a coil sensor with its magnetic response at room and high temperatures. This makes the system effective under high temperature conditions where corrosion metal loss needs to be determined

Magnetic Flux Leakage techniques for detecting corrosion of pipes

Oil and gas pipelines are subjected to corrosion due to harsh environmental conditions as in refinery and thermal power plants. Interesting problems such as internal and external corrosion, emerging from the increasing demand for pipeline protection have prompted this study. Thus, early detection of faults in pipes is essential to avoid disastrous outcomes. The research work presented in this thesis comprises investigations into the use of magnetic flux leakage (MFL) testing for pipe in extreme (underwater and high temperature) conditions. The design of a coil sensor (ferrite core with coil) with a magnetic circuit is carried out for high temperature conditions. The sensor thus developed lays the ground for non-destructive evaluation (NDE) of flaws in pipes through the MFL technique. The research focusses on the detection and characterization of MFL distribution caused by the loss of metal in ferromagnetic steel pipes. Experimental verifications are initially conducted with deeply rusted pipe samples of varying thicknesses in air. AlNiCo magnets are used along with Giant Magneto Resistance (GMR) sensor (AA002-02). The experiment is further repeated for saltwater conditions in relation to varying electrical conductivity with radio frequency identification (RFID) technique. A further study carried out in the research is the correlation between magnetic and underwater data communication. The study has resulted in the development and experimental evaluation of a coil sensor with its magnetic response at room and high temperatures. This makes the system effective under high temperature conditions where corrosion metal loss needs to be determined

Coupled Finite Element Modelling and Transduction Analysis of a Novel EMAT Configuration Operating on Pipe Steel Materials

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonElectromagnetic Acoustic Transducers (EMATs) are advanced ultrasonic transducers that generate and detect acoustic waves in an electrically conducting material without making physical contact with the material unlike its counterpart, the piezoelectric transducers (PZT). The conventional EMAT consists of copper coil that generates the dynamic field when excited with a sinusoidal current, a permanent or electromagnet that provides the bias field and the conducting material specimen. The complex interaction between the bias field and the Eddy current induced within the skin depth of the conducting material by the dynamic field gives rise to the acoustic wave that then propagates within the surface of the material. Within the research a finite element EMAT model was developed using commercial software Comsol Multiphysics, to study and compare the Eddy current density and Lorentz force density generated by three EMAT configurations: The Meander-line, Spiral and Key Type EMAT configuration respectively. It was observed that apart from the ease of fabrication and simplicity of connectivity when stacked in layers, the Key Type coil EMAT showed a high tendency to generate higher amplitude of Eddy current and Lorentz force test materials especially when stacked in layers. Also, the effect of varying some key EMAT parameters was investigated to determine the optimal performance of Key Type EMAT configuration on CS70 pipe steel plate. The research further developed a coupled finite element model using the same software, Comsol Multiphysics to account for the generation, propagation and detection of acoustic wave by the Key Type EMAT configuration on CS70 grade of pipe steel. The model can solve the magnetostatic, electrodynamic and elastic equations that give rise to acoustic wave generation, propagation and detection on the test material. The developed coupled finite element model was validated both analytically and experimentally to establish the validity of the finite element model. The analytical and experimental results obtained were consistent with the numerical result with an average discrepancy less than 9 % percent. Finally, the research developed a novel modelling strategy to decouple and quantify the various transduction forces in operation when normally-biased EMAT and magnetostrictive EMAT configurations are used on various grades of pipe steel materials. The strategy established the value of the critical excitation current beyond which acoustic wave is generated solely by the dynamic Lorentz force mechanism. The critical excitation currents when Magnetostrictive EMAT configurations are used to generate acoustic wave was found to be; 268A, 274A, 279A, 290A and 305A for CS70, L80SS, L80A, TN80Cr3 and J55 respectively. While for Normally-Biased EMAT configurations, the critical excitation current was found to be 190A, 205A, 240A, 160A and 200A respectively. This work also compared the critical excitation current of the two EMAT configurations studied and established that normally-biased EMATs are more efficient in the generation of acoustic waves than their magnetostrictive counterpart due to their lower value of critical excitation current.Petroleum Technology Development Fund (PTDF) Nigeri

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

PhD ThesisEddy 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.Petroleum Technology Development Fund (PTDF) for sponsoring this research work through the overseas scholarship scheme

Inspection and structural health monitoring techniques for concentrated solar power plants

Parabolic trough concentrators are the most widely deployed type of solar thermal power plant. The majority of parabolic trough plants operate up to 400 °C. However, recent technological advances involving molten salts instead of oil as working fluid the maximum operating temperature can exceed 550 °C. CSP plants face several technical problems related to the structural integrity and inspection of critical components such as the solar receivers and insulated piping of the coolant system. The inspection of the absorber tube is very difficult as it is covered by a cermet coating and placed inside a glass envelope under vacuum. Volumetric solar receivers are used in solar tower designs enabling increased operational temperature and plant efficiency. However, volumetric solar receiver designs inherently pose a challenging inspection problem for maintenance engineers due to their very complex geometry and characteristics of the materials employed in their manufacturing. In addition, the rest of the coolant system is insulated to minimise heat losses and therefore it cannot be inspected unless the insulation has been removed beforehand. This paper discusses the non-destructive evaluation techniques that can be employed to inspect solar receivers and insulated pipes as well as relevant research and development work in this field

Current deflection NDE for pipe inspection and monitoring

The detection of corrosion on insulated and/or coated pipes in the oil and gas industry remains a challenge. Routine inspection, which is commonly achieved with in-line tools known as "pigs", is not possible where there is any risk of the pig becoming stuck. There are thousands of kilometers of pipe worldwide deemed ``unpiggable'' whose safety must be ensured using Non-Destructive Evaluation (NDE) external to the pipe if potentially catastrophic failure is to be avoided. Many NDE techniques lack sufficient sensitivity due to the coating thickness producing a high standoff distance between the pipe and the sensor and therefore require costly and time-consuming removal of the coating. A method capable of detecting and/or monitoring of defects (e.g. one-third-wall depth corrosion) while leaving the insulation/coating intact would be highly attractive. This thesis documents the development of a technique in which a low-frequency AC current is directly injected into the pipe at distant locations, and perturbations in the magnetic field caused by "current deflection" around defects are measured using solid-state magnetic sensors. Two methods of applying this novel technique were investigated. Firstly, scanning the sensors to measure perturbations in the field and screen for defects, and secondly, permanently installing sensors outside the pipe for Structural Health Monitoring (SHM). A Finite Element (FE) model has been developed and used to investigate the practical challenges that are faced by the technique and how these may be overcome. The sensitivity of the technique for defect detection by external pipe scanning in a practical scenario has then been evaluated using a model-assisted Probability of Detection (POD) framework that combines the measurements of the signal from an undamaged pipe with synthetic damage profiles and contributions from general corrosion and sensor misalignment. The results indicate that good performance is expected for damage detection by scanning above a typical insulation thickness with just a few amps of injected current. A similar framework has then been used to evaluate the sensitivity of the technique as an SHM solution which suggests excellent corrosion detection performance with the permanent installation of inexpensive magnetic sensors. The technique has potential advantages over competing methods in both scanning and monitoring modes and there are many opportunities for future development.Open Acces