Development and Experimentation of Magnetostrictive Sensors for Inspection and Monitoring of Piping Systems

Nondestructive Evaluation – NDE, is an important aspect of the integrity management of industrial plants, where pipe systems are the dominant component. During the last decade Ultrasonic Guided Waves (UGW), have started to be used as a useful instrument for on-stream long range inspection of pipes. Various procedures and systems have been proposed for the generation and detection of UGW. Presently, they are based on piezoelectric (PZT) or magnetostrictive (MT) transducers or electromagnetic acoustic transducers (EMAT). It is generally known that PZT based systems have elevated diagnostic capacities due to their high transduction efficiency. However, the elevated costs of installation of such devices make their use for long-term monitoring of piping systems quite improbable. On the other hand, the MT based systems have the advantage of the reduced costs of the composing materials, simplicity of attaching it to the pipe wall and flexibility regarding the diameters of the pipes that can be inspected. Still, its single-element configuration limits the capacity to characterize the detected discontinuities in terms of geometry, thus being unable to distinguish between possible flaws from symmetrical features, normally located on pipes, like welds or flanges. Furthermore, its reduced capability to geometrically characterize flaws makes the classification of their severity particularly difficult. The improvement of the diagnostic capacity of MT based systems in order to make practically possible and economically convenient its use in monitoring applications is the main purpose of this thesis. In this dissertation multiple laboratory and field experiments are described and the magnetostrictive technology is evaluated. Furthermore, a new magnetostrictive transducer for UGW acquisition is presented. It allows step-by-step data acquisition around the pipe circumference revealing important information on the geometry and circumferential position of flaws. The new sensor was validated by computer simulations as well as further laboratory and field tests. The resulting data was used as input for various digital signal processing techniques to describe geometrically the features detected in the acquired signal. The final results outline the potential of MT based long-range inspection to reach also a good sensitivity and a good defect sizing and classification with respect to conventional techniques, making it an important candidate for monitoring activities for the integrity management of industrial plants

Monitoring system for long-distance pipelines subject to destructive attack

In an era of terrorism, it is important to protect critical pipeline infrastructure, especially in countries where life is strongly dependent on water and the economy on oil and gas. Structural health monitoring (SHM) using acoustic waves is one of the common solutions. However, considerable prior work has shown that pipes are cylindrical acoustic waveguides that support many dispersive, lossy modes; only the torsional T(0, 1) mode has zero dispersion. Although suitable transducers have been developed, these typically excite several modes, and even if they do not, bends and supports induce mode conversion. Moreover, the high-power transducers that could in principle be used to overcome noise and attenuation in long distance pipes present an obvious safety hazard with volatile products, making it difficult to distinguish signals and extract pipeline status information. The problem worsens as the pipe diameter increases or as the frequency rises (due to the increasing number of modes), if the pipe is buried (due to rising attenuation), or if the pipe carries a flowing product (because of additional acoustic noise). Any system is therefore likely to be short-range. This research proposes the use of distributed active sensor network to monitor long-range pipelines, by verifying continuity and sensing small disturbances. A 4-element cuboid Electromagnetic Acoustic Transducer (EMAT) is used to excite the longitudinal L(0,1) mode. Although the EMAT also excites other slower modes, long distance propagation allows their effects to be separated. Correlation detection is exploited to enhance signal-to-noise ratio (SNR), and code division multiplexing access (CDMA) is used to distinguish between nodes in a multi-node system. An extensive numerical search for multiphase quasi-orthogonal codes for different user numbers is conducted. The results suggest that side lobes degrade performance even with the highest possible discrimination factor. Golay complementary pairs (which can eliminate the side lobes completely, albeit at the price of a considerable reduction in speed) are therefore investigated as an alternative. Pipeline systems are first reviewed. Acoustic wave propagation is described using standard theory and a freeware modeling package. EMAT modeling is carried out by numerical calculation of electromagnetic fields. Signal propagation is investigated theoretically using a full system simulator that allows frequency-domain description of transducers, dispersion, multi-mode propagation, mode conversion and multiple reflections. Known codes for multiplexing are constructed using standard algorithms, and novel codes are discovered by an efficient directed search. Propagation of these codes in a dispersive system is simulated. Experiments are carried out using small, unburied air-filled copper pipes in a frequency range where the number of modes is small, and the attenuation and noise are low. Excellent agreement is obtained between theory and experiment. The propagation of pulses and multiplexed codes over distances up to 200 m are successfully demonstrated, and status changes introduced by removable reflectors are detected.Open Acces

Stress monitoring of cylindrical structures using guided waves

This thesis presents some approaches for guided wave based stress monitoring as a part of Structural Health Monitoring (SHM). SHM systems include different levels, from damage detection to prognosis, however, this work is focused on detection and on an estimation of the actual stress. The proposed stress monitoring strategies are based on different statistical and signal processing approaches such as Principal Component Analysis and Residuals. These techniques are applied on signals of elastic guided waves generated and sensed via Piezoelectrical (PZT) or Magnetostrictive transducers. Transducer devices are chosen in this work to generate longitudinal, flexural and torsional guided waves in cylindrical specimens, since their high performance, low energy consumption, weight and reasonable price. In order to guarantee the efficacy of the proposed techniques, they are tested in laboratory by emulating real installations and abnormal conditions. Experimental tests revealed that temperature and bonding layer between the PZT and the specimen influence on the performance of the monitoring scheme by changes in the guided wave propagation. Thus, the temperature effect on guided wave propagation was examined by checking the sensitivity of the PCA-based proposed approach. Then, a temperature compensation strategy is applied to improve stability and robustness of the scheme for structures subjected temperature changes. On the other hand, since the acoustoelasticity effect is predominant in the propagation of stressed guided waves, it was observed its incidence on the dispersion curves by using a SAFE method (Semi-Analytical Finite Element) to generate stressed dispersion curves via Effective Elastic Constants (EEC). Finally, as a consequence of some observations in the experimentation stage, it is proposed a scheme for monitoring the supports rigidity in pipelines based on a guided waves energy leakage perspective. The proposed approaches may promise the ability and capability of being implemented in different fields such as aerospace and gas/oil industry.En esta tesis se presentan algunos enfoques para el monitoreo de esfuerzos usando ondas guiadas como parte de un sistema de evaluación de integridad. Estos sistemas incluyen diferentes niveles de monitoreo que van desde la detección de daños hasta su predicción; sin embargo este trabajo se enfoca solo en la detección y estimación de un valor probable de esfuerzo. La estrategia de monitoreo propuesta se basa en diferentes enfoques estadísticos y de procesamiento de señales tales como Análisis de Componentes Principales y Residuos. Estas técnicas se aplican en señales que corresponden a ondas guiadas generadas por transductores piezoelectricos (PZT) o magnetostrictivos. Estos elementos tienen la capacidad de generar ondas guiadas longitudinales, de flexión y torsionales en especímenes cilíndricos, con alto desempeño, bajo consumo de energía, bajo peso y a un costo razonable. Para garantizar la efectividad de las técnicas propuestas, estas se prueban en laboratorio emulando una instalación real y bajo condiciones anormales de esfuerzo. Los resultados experimentales revelaron que la temperatura de los alrededores y la capa adhesiva entre el piezoelectrico y el espécimen influyen en el desempeño del esquema de monitoreo debido a los cambios que se producen en la propagación de onda . Por lo tanto, se estudia el efecto desde una perspectiva analítica el efecto de la temperatura en la propagación de la onda guiada y en consecuencia en el desempeño del enfoque de monitoreo propuesto. Basado en lo anterior, se aplica una estrategia de compensación del efecto de la temperatura para mejorar la estabilidad y la robustez del esquema propuesto ante escenarios de cambios de temperatura. Por otro lado, debido a que el efecto predominante en la propagación de ondas guiadas bajo esfuerzo es el efecto acoustoelastico, se estudia su influencia en las curvas de dispersión usando un método semi-analitico basado en elementos finitos (SAFE del inglés) en combinación con las Constantes Elásticas Efectivas (EEC del inglés) para estimar las curvas de dispersión de ondas guiadas bajo esfuerzo. Finalmente, como un resultados de la experimentación, se propone un esquema de monitoreo de rigidez de soportes de tubería cilíndrica basado en un perspectiva de energía de ultrasonido transferida al entorno vía contacto de superficies. El enfoque propuesto puede ser extendido al monitoreo de rigidez o contacto en otros sistemas en campos tales como el aeroespacial y en la industria del Gas/PetróleoPostprint (published version

Stress monitoring of cylindrical structures using guided waves

This thesis presents some approaches for guided wave based stress monitoring as a part of Structural Health Monitoring (SHM). SHM systems include different levels, from damage detection to prognosis, however, this work is focused on detection and on an estimation of the actual stress. The proposed stress monitoring strategies are based on different statistical and signal processing approaches such as Principal Component Analysis and Residuals. These techniques are applied on signals of elastic guided waves generated and sensed via Piezoelectrical (PZT) or Magnetostrictive transducers. Transducer devices are chosen in this work to generate longitudinal, flexural and torsional guided waves in cylindrical specimens, since their high performance, low energy consumption, weight and reasonable price. In order to guarantee the efficacy of the proposed techniques, they are tested in laboratory by emulating real installations and abnormal conditions. Experimental tests revealed that temperature and bonding layer between the PZT and the specimen influence on the performance of the monitoring scheme by changes in the guided wave propagation. Thus, the temperature effect on guided wave propagation was examined by checking the sensitivity of the PCA-based proposed approach. Then, a temperature compensation strategy is applied to improve stability and robustness of the scheme for structures subjected temperature changes. On the other hand, since the acoustoelasticity effect is predominant in the propagation of stressed guided waves, it was observed its incidence on the dispersion curves by using a SAFE method (Semi-Analytical Finite Element) to generate stressed dispersion curves via Effective Elastic Constants (EEC). Finally, as a consequence of some observations in the experimentation stage, it is proposed a scheme for monitoring the supports rigidity in pipelines based on a guided waves energy leakage perspective. The proposed approaches may promise the ability and capability of being implemented in different fields such as aerospace and gas/oil industry.En esta tesis se presentan algunos enfoques para el monitoreo de esfuerzos usando ondas guiadas como parte de un sistema de evaluación de integridad. Estos sistemas incluyen diferentes niveles de monitoreo que van desde la detección de daños hasta su predicción; sin embargo este trabajo se enfoca solo en la detección y estimación de un valor probable de esfuerzo. La estrategia de monitoreo propuesta se basa en diferentes enfoques estadísticos y de procesamiento de señales tales como Análisis de Componentes Principales y Residuos. Estas técnicas se aplican en señales que corresponden a ondas guiadas generadas por transductores piezoelectricos (PZT) o magnetostrictivos. Estos elementos tienen la capacidad de generar ondas guiadas longitudinales, de flexión y torsionales en especímenes cilíndricos, con alto desempeño, bajo consumo de energía, bajo peso y a un costo razonable. Para garantizar la efectividad de las técnicas propuestas, estas se prueban en laboratorio emulando una instalación real y bajo condiciones anormales de esfuerzo. Los resultados experimentales revelaron que la temperatura de los alrededores y la capa adhesiva entre el piezoelectrico y el espécimen influyen en el desempeño del esquema de monitoreo debido a los cambios que se producen en la propagación de onda . Por lo tanto, se estudia el efecto desde una perspectiva analítica el efecto de la temperatura en la propagación de la onda guiada y en consecuencia en el desempeño del enfoque de monitoreo propuesto. Basado en lo anterior, se aplica una estrategia de compensación del efecto de la temperatura para mejorar la estabilidad y la robustez del esquema propuesto ante escenarios de cambios de temperatura. Por otro lado, debido a que el efecto predominante en la propagación de ondas guiadas bajo esfuerzo es el efecto acoustoelastico, se estudia su influencia en las curvas de dispersión usando un método semi-analitico basado en elementos finitos (SAFE del inglés) en combinación con las Constantes Elásticas Efectivas (EEC del inglés) para estimar las curvas de dispersión de ondas guiadas bajo esfuerzo. Finalmente, como un resultados de la experimentación, se propone un esquema de monitoreo de rigidez de soportes de tubería cilíndrica basado en un perspectiva de energía de ultrasonido transferida al entorno vía contacto de superficies. El enfoque propuesto puede ser extendido al monitoreo de rigidez o contacto en otros sistemas en campos tales como el aeroespacial y en la industria del Gas/Petróle

On-line measurements of contents inside pipes using guided ultrasonic waves

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