OFDR Distributed Temperature and Strain Measurements with Optical Fibre Sensing Cables: Application to Drain Pipeline Monitoring in a Nuclear Power Plant

International audienceThis study deals with the testing of innovative Optical Fibre Sensing (OFS) cables deployed on ducts, with the aim to perform distributed temperature and strain measurements. Such cables contain several optical fibres devoted to be interrogated by Optical Frequency Domain Reflectometry (OFDR). The methodology has first been developed and qualified in laboratory. Then, real tests have been performed on a Nuclear Power Plant (NPP) drain system to demonstrate the industrial feasibility of such technology. To do so, two small diameter sensing cables, compatible with distributed temperature and strain measurements, have been qualified and afterwards installed along a sodium drain line at Superphenix NPP (liquid sodium coolant fast breeder reactor in current dismantling). Measurements have been performed during the preheating operation. Recorded data were post-processed according to a semi-empirical model taking into account temperature dependence and thermo-mechanical sensing cable behaviour. Optical fibre distributed temperature measurements were then successfully compared to thermocouple reference measurements, whereas optical sensing cable data were processed to provide distributed strain, then distributed curvature radius, which will enable, after numerical integration, to compute distributed displacement data. The goal is to assess the use of OFS for monitoring both temperature and mechanical strain distribution along a pipe under heat stress

Détection répartie de la corrosion par capteur à fibre optique et OFDR sur les armatures de renforcement en acier des structures en béton armé

International audienceCorrosion is a major pathology for Civil Engineering structures, affecting long-term reliability. Huge direct costs (maintenance, rehabilitation) and indirect costs (lost productivity, litigation, outages, delays, downtimes) justify strategies to minimize the impact of corrosion. Regarding civil engineering (CE) infrastructures constructed of reinforced concrete, steel reinforcing bars (rebars) are naturally corrosion-protected when embedded into concrete (pH ~ 13). Concrete carbonation (“generalized” corrosion) and chloride ion penetration (“pitting” corrosion) both accelerate their corrosion rate. Corrosion products grow in volume and the increase in pressure at the steel-concrete interface leads to cracks of the concrete layer and acceleration of degradation. They may escape as well through cracks, thus leading to a reduction in rebar diameter and global structure weakening.Until now, inspections are carried out periodically and involve indirect measurement techniques (e.g. chemical-, impedance-, potential-based) that are time-consuming, costly and probabilistic in nature. Imaging techniques (e.g. ultrasonics) provide information about internal damage or voids within concrete but are of limited range of investigation (limited to accessible surfaces). Since it is impossible to predict where corrosion would start in large infrastructures, distributed monitoring techniques are desirable in order to early detect its onset, particularly in hidden or inaccessible areas.We investigated the OFDR technique in the perspective of a Condition-Based Maintenance (CBM), identified as a future challenge in order to provide safe operating conditions and cost savings. The use of telecom-grade fibers as sensors is motivated by their cost-effectiveness, electrochemical passivity, electromagnetic immunity and networking/multiplexing capability. We report on an original fiber-based corrosion sensor design employing usual steel rebars in order to avoid galvanic corrosion to occur. Since the sensor is a rebar, it also behaves as an extensometer and a dedicated design is proposed to discriminate between global thermomechanical loadings and local corrosion mechanical effects. Changes in OFDR signals with respect to reference signals provide localization, identification and direct measurement of corrosion. The sensing device was successfully tested under accelerated pitting corrosion as a proof-of-concept

State-of-strain evaluation with fiber Bragg grating rosettes: application to discrimination between strain and temperature effects in fiber sensors

International audienceAn optical rosette that incorporates fiber Bragg gratings as strain gauges has been designed, fabricated, and tested. We investigated it by measuring the state of strain of a thin plate as the test structure submitted to an increasing load in a four-point bending configuration and for various angular orienta-tions. This device has also been successfully investigated as a self-temperature-compensated in situ uniaxial strain sensor without any angular dependence and with high accuracy in recovery analysis, leading us to expect many industrial applications. Printed circuit processes or integrated optics on polymers would provide a means for accuracy, reproducibility, and integration in a mass-produced process

Applications of Fiber Bragg Grating sensors in the composite industry

International audienceOptical-fiber sensors based on fiber Bragg gratings (FBGs) provide accurate, nonintrusive, and reliable remote measurements of temperature, strain, and pressure, and they are immune to electromagnetic interference. FBGs are extensively used intelecommunications, and their manufacture is now cost-effective. As sensors, FBGs find many industrial applications in composite structures used in the civil engineering, aeronautics, train transportation, space, and naval sectors. Tiny FBG sensorsembedded in a composite material can provide in situ information about polymer curing (strain, temperature, refractive index) in an elegant and nonintrusive way. Great improvements in composite manufacturing processes such as resin transfer molding(RTM) and resin film infusion (RFI) have been obtained through the use of these sensors. They can also be used in monitoring the “health” of a composite structure and in impact detection to evaluate, for example, the airworthiness of aircraft. Finally, FBGs may be used in instrumentation as composite extensometers or strain rosettes, primarily in civil engineering applications

Temperature Resistant Fiber Bragg Gratings for Online Monitoring of Future Sodium cooled Fast Reactors: Paving the Way to SHM Implantation into the Nuclear Industry

International audienceAdvanced manufacturing techniques using thermal engineering (regeneration process) and ultrafast laser micromachining of silica optical fibers have tackled the challenges of making high temperature resistant Fiber Bragg Gratings (FBGs) sensors. Arrays of temperature and strain wavelength-multiplexed FBGs sensors for the online monitoring of future Nuclear Reactors (Generation IV) are becoming a reality. Both regenerated and femtosecond FBGs have been intensively tested up to 950 °C for temperature mapping and recently tested also up to 400 °C for strain measurements on metallic devices. These sensors are expected to perform continuous monitoring of sodium-cooled nuclear reactors and to improve their safety and pave the way to Structural Health Monitoring (SHM) implantation into the nuclear industry. We present the manufacturing of i) regenerated FBGs using an all-optical process and of ii) femtosecond laser-written point-by-point femtosecond (fs PbP) FBGs directly into free-standing singlemode optical fibers. The presented techniques may not only speed up the manufacturing process but also improve the mechanical reliability of the FBGs. The temperature stability of both kinds of FBGs is assessed through long term ageing up to 950 °C highlighting their temperature resistance. Both are appropriate candidates for online thermometry even in severe environments such as those encountered in future Sodium-cooled Fast Reactor. Strain sensing is under development using fs PbP FBGs to operate beyond 400 °C using a traction machine and standard metallic specimens

Temperature Resistant Fiber Bragg Gratings for On-Line and Structural Health Monitoring of the Next-Generation of Nuclear Reactors

The harsh environment associated with the next generation of nuclear reactors is a great challenge facing all new sensing technologies to be deployed for on-line monitoring purposes and for the implantation of SHM methods. Sensors able to resist sustained periods at very high temperatures continuously as is the case within sodium-cooled fast reactors require specific developments and evaluations. Among the diversity of optical fiber sensing technologies, temperature resistant fiber Bragg gratings are increasingly being considered for the instrumentation of future nuclear power plants, especially for components exposed to high temperature and high radiation levels. Research programs are supporting the developments of optical fiber sensors under mixed high temperature and radiative environments leading to significant increase in term of maturity. This paper details the development of temperature-resistant wavelength-multiplexed fiber Bragg gratings for temperature and strain measurements and their characterization for on-line monitoring into the liquid sodium used as a coolant for the next generation of fast reactors

Pantographe instrumenté de capteurs à réseaux de Bragg fibrés pour la surveillance de l’interface avec le fil de contact: Du concept de l’instrumentation aux essais TGV

La prise en compte de la saturation progressive de l’infrastructure européenne des transports est devenue, au fil des ans, une des priorités des Programmes Cadres de R&D européens. Ainsi, lors du 5ième PCRD, le projet SMITS (Smart Monitoring In Train Systems) a eu pour objectif d’adresser la problématique des perturbations du trafic ferroviaire causées par les chutes de pantographe du fait de leur usure excessive, entrainant de ce fait l’arrêt du train, par la mise en place d’un outil de contrôle des paramètres d’interface avec la ligne de contact (force de contact, température) afin de pouvoir, in fine, mettre en place une maintenance préventive.Dans le cadre de ce consortium( ) dont l’objectif était la mesure à haute cadence afin de satisfaire les besoins de la très grande vitesse (320 km/h sur les lignes TGV actuelles), le CEA LIST a développé un système de mesure innovant multivoies pour Capteurs à Fibres Optiques (CFO) à réseaux de Bragg, fondé en particulier sur une source laser accordable de type laser en anneau, permettant simultanément l’acquisition des longueurs d’onde de Bragg des transducteurs positionnés dans les bandes de contact d’un pantographe, et leur transformation, en temps réel sur sorties analogiques, en des grandeurs physiquement significatives pour les besoins des essais de qualification.Contrairement aux technologies traditionnelles reposant sur l’utilisation des jauges électriques de déformations positionnées aux points d’appui de la tête de pantographe et dont le principe de mesure repose sur un équilibre de forces et de moments, le choix du consortium, tirant parti de l’isolation galvanique intrinsèque des fibres optiques, s’est porté sur l’équipement de jauges à réseaux de Bragg des bandes de contact portées au potentiel de la ligne de contact (jusqu’à 25 kV), les transformant chacune en des capteurs de mesure en flexion 3 points “au plus près” du point de contact.La chaîne de mesure complète a ainsi été testée pendant 1 semaine, sur plus de 5000 km, entre Paris et Vendôme courant mars 2005 à bord d’un TGV Duplex spécialement affrété pour l’occasion. Ont ainsi pu être mesurés, à une cadence de 500 Hz et sur 16 points de mesure simultanés, les efforts de contact verticaux, la position du fil de contact, les températures et leurs gradients régnant au sein des bandes de contact.L’intérêt d’une telle cadence de mesure provient de fait qu’à 300 km/h, un TGV parcourt chaque 500e de seconde un peu moins de 20 cm ; en cas de détection d’un défaut sur la ligne, le couplage avec un GPS permet de localiser le secteur correspondant. La résolution spectrale du système, meilleure que le picomètre, a permis d’évaluer les variations de la force de contact à quelques newtons près.La présentation s’attachera à rappeler les concepts de l’instrumentation mis en œuvre, mais également de relater les essais à grande vitesse, tout en replaçant la démarche en perspective avec les besoins des utilisateurs finaux de ce secteur

Innovative FBG sensing techniques for the railway industry: application to overhead contact line monitoring

International audienceCurrent collection is a key issue in modern electrical railway, and its performances are mainly determined by the pantograph/catenary interactions. Whilst being the less investigated topic, the pantograph/catenary mechanical interface is the most crucial one. Many incidents and traffic interruptions are due to degraded, or even damaged, electrical contacts between current collectors and contact wires. During the 6th European Framework Program (FP6) CATIEMON project (CATenary InterfacE MONitoring), both FBG-based sensors and systems dedicated to the pantograph/catenary interaction monitoring - directly from the high voltage contact wire - have been developed and tested. This paper describes their design and installation but also results coming from field-tests. It highlights their advantages for the railway stakeholders and end-users in term of train operation enhancement

Instrumentation de lignes caténaires par capteurs à réseaux de Bragg fibréspour la surveillance du trafic ferroviaire - Du concept de l’instrumentation aux essais terrain

Le captage du courant électrique est au centre des préoccupations des sociétés de chemin de fer et des exploitants d’infrastructures ferroviaires. Il conditionne les vitesses de circulation des trains commerciaux mais également la disponibilité des infrastructures. La dérégulation du secteur ferroviaire en Europe a en outre conduit la Commission Européenne à lancer des initiatives de Recherche visant à améliorer la fiabilité et la disponibilité des réseaux ferrés transnationaux. Dans ce contexte, l’Institut LIST du CEA participe au projet CATIEMON – pour CATenary InterfacE MONitoring. Le rôle du CEA LIST consiste à développer des Capteurs à Fibres Optiques reposant sur la technologie des réseaux de Bragg et des instrumentations dédiées. L’objectif est de caractériser l’interaction entre les lignes caténaires et les pantographes des trains empruntant un réseau donné. Les mesures obtenues serviront à optimiser la maintenance età accroître la disponibilité des infrastructures.Dans cet article, nous décrivons les différents capteurs et systèmes de mesure à réseaux de Bragg développés et testés dans le cadre de ce projet, tant en laboratoire que sur site. Enfin, ces résultats sont analysés et discutés au regard des besoins des utilisateurs finaux

Instrumentations et Capteurs à Fibres Optiques à réseaux de Bragg dédiés aux besoins du secteur ferroviaire : du concept aux essais terrain

International audienceIn answer to the deregulation of the railway sector to come, the European Commission launched R&D programs. Since any company will make its trains circulate in any country of the Union, it arouses anxieties of the historical companies. Two projects, SMITS and CATIEMON dealing with the pantograph-catenary interface monitoring are presented: the first deals with Fiber Bragg Grating-based sensors (FBG) embedded into the head of pantographs, and the second with a control gate equipped with FBG sensors and installed at strategic locations of a railway grid. In these projects, the FBG sensing technology has proved its potential by answering needs of innovative measurement, in harsh conditions (high-speed measurements, under high voltage, in severe climatic conditions). Developed instrumentations and sensors open the way to the use of this new technology for monitoring in railway sector, as well as for many other industrial applicationsDu fait de la prochaine dérégulation du secteur ferroviaire, la commission européenne a lancé plusieurs initiatives de R&D. En effets, dès lors que toute compagnie pourra faire circuler ses trains dans n’importe quel pays de l’Union, cela suscite des inquiétudes de la part des compagnies historiques. Deux projets, SMITS et CATIEMON traitant de la surveillance de l’interface pantographe-caténaire sont présentés : l’un fondé sur des mesures par Capteurs à Fibres Optiques (CFO) à réseaux de Bragg intégrés dans la tête de pantographe, et l’autre sur une ‘porte’ de contrôle équipée de capteurs et placée en des nœuds stratégiques du réseau. Grace à ces projets, la technologie de mesure par CFO à réseaux de Bragg a prouvé son potentiel en répondant à des besoins en mesures innovantes, dans des conditions difficiles (mesure à grande vitesse, au potentiel, climat hivernal). Les instrumentations et capteurs développés ouvrent la voie à l’utilisation d’une nouvelle technologie pour la surveillance ferroviaire, et à bien d’autres applications industrielle