Dam Monitoring Using Long SOFO Sensor

Transcription

1 Hydropower, , Gmunden, Austria Dam Monitoring Using Long SOFO Sensor B. Glisic Ph.D. Student Swiss Federal Institute of Technology IMAC-EPFL CH-1015 Lausanne D. Inaudi Director of Smartec SA Via al Molino 6 CH-6916 Grancia P. Kronenberg Ph.D. Student Swiss Federal Institute of Technology IMAC-EPFL CH-1015 Lausanne S. Vurpillot Assistant Swiss Federal Institute of Technology IMAC-EPFL CH-1015 Lausanne Introduction In order to increase the safety of civil engineering structures many different monitoring systems have been developed. The majority of these systems are based on the measurement of the deformations, the displacements or the strains of the structure. The SOFO system is based on low-coherence interferometry in single-mode optical fibres and allows the measurement of deformations in civil structures with a resolution of a few micrometers and an excellent long-term stability. SOFO allows permanent, automatic and reliable monitoring and has been successfully tested in different types of structures such as bridges, tunnels and geostructures in, Europe and North America. The gage length of the standard SOFO sensors varies between 200 mm and 10 m. Monitoring of the large structures, such as dams or tunnels, requires however the use of sensors with a base length of several tenths of meters. To create such a sensor, certain modifications of the existing (standard) sensor are required. Two long sensors with gage lengths of 30 m and 39 m were designed, assembled and installed in the Emosson dam in. They are compared with two parallel rod extensometers during more than one year. The results indicate excellent agreement between the two systems and prove the soundness of the long sensor design. 1. The interest of dam monitoring using fibre optic sensors Monitoring is a very important task in appropriate dam management due to the economic, social and environmental significance of hydroelectric structures. It is fundamental in order to guarantee not only the safety of the structure and its users, but also to optimise the exploitation and the maintenance of the dam. Several "classical" monitoring systems, such as rod extensometers, penduls - normal or inverse, inclinometers, surveying networks, etc., exist and are applied on the dams. However, some of them have certain inconveniences: delicate manipulation, sensitivity to temperature, humidity or electromagnetic fields, difficult installation, manual treatment of each measurement. Using an optical fibre measurement system it is possible to avoid all these inconveniences. The fibre optic sensors could be used for deformation and/or displacement measurement of both, existing and new structures. In the first case they are used either to replace an existing system where the measurements don t satisfy any more, or to extend an existing monitoring network. The sensors are installed to look closer at a particular, initially underestimated structural behaviour (e.g. displacement of the dam s foundation) or simply to increase the redundancy of the monitoring network. Secondly there are new or the structurally upgraded (e.g. raised) dams where a new monitoring network has to be installed. For these a fibre optics deformation sensor network can be integrated in the project from the beginning, which is in fact much easier since the sensors have fewer constraints relative to their installation. In order to adapt the existing SOFO measurement system to dam monitoring, a special sensor, called long sensor, was developed. Following we will present an application of the SOFO measurement system on the existing Emosson dam in. Page 1 of 9

2 Hydropower, , Gmunden, Austria 2. Emosson dam The Emosson Dam is situated in the Swiss Alps, near the French border, 1930 metres above sea level, near the Swiss town of Martigny. Completed in 15, the dam is 180 m high and at its coping is 554 m long with a thickness varying from 9 m (coping) to 48.5 m (footing) (Fig.3.2.3). Water is collected during the warm seasons, when the snow and the glaciers melt, and used during the cold seasons, when the demand for electricity is greater (see Figure 5.1). The dam's maximum capacity is approximately 225 million cubic metres. There are three principal collectors that guide the water from the Alps glaciers to the dam: the west and south collectors guide water from the French Alps and the east collector from the Swiss Alps. Two power plants exploit the water, the Vallorcine in France (1125 m.a.s.l.) and the La Bâtiaz (462.5 m.a.s.l.) in. In 17/ the total annual production of electricity was GWh using million cubic metres of water [1]. The Swiss-French enterprise "Electricité d'emosson SA" manages and distributes the obtained electrical energy. 3. Sofo measurement system The SOFO measurement system consists of a reading unit, the fibre optic sensors and the appropriate software. The system is based on the low coherence interferometry[2]. Its functional principle is represented in Figure 3.1. The reading unit is composed of a light emitter (light emitting diode - LED), a low-coherence Michelson interferometer with a mobile scanning mirror, optical set up and an internal PC. Structure Under Test Reference Fiber Coupler Mirrors Measurement Fiber Portable Reading Unit Coupler Delay Line Portable PC µ A/D Filter Amplifier Internal PC Photo- Diode LED 1300nm Figure 3.1: Set-up of the SOFO System Generally, the sensor consists of two monomode optical fibres: the measurement and the reference fibre. The measurement fibre is in mechanical contact with the host structure and follows its deformation, while the reference fibre, placed close to the measurement fibre, is loose and independent of the behaviour of the structure. Any deformation of the structure will result in a change of the length difference between the two fibres. Infra red light is emitted by the LED, sent by the monomode optical fibre to the sensor, split by the coupler and introduced in to the two arms of the sensor. Then, the light reflects off the chemical mirrors deposed on the ends of both fibres and returns through the coupler to the reading unit, i.e. to the Michelson interferometer. The light is interfered in the coupler and contains the information of the length difference between the measurement and the reference fibre. This difference is analysed using the mobile mirror and transmitted to the external PC. By successively repeating the measurements, it is possible to determinate the evolution of the deformation of the auscultated structure. Page 2 of 9

3 Hydropower, , Gmunden, Austria The main characteristics of the SOFO system are given in Table 3.1. Parameter Gage length Resolution Dynamic range of the sensors Dynamic range of the reading unit Precision Measurement speed Stability SOFO characteristics 20cm to 10m for standard sensors Up to 50m with special (long) sensors 2µm, independently from gage length 1% elongation, 0.5% shortening for standard sensors Up to 70mm in elongation and shortening Better than 1% of the measured deformation Less than 10 seconds Drift not observable over at least four years Table 3.1:Specifications of SOFO Measurement System The SOFO system is very adapted to the building site conditions. The reading unit is portable, battery powered and waterproof, making it ideal for dusty and humid environments as usually found in most building sites. 3.1 Standard SOFO sensor The standard SOFO sensor [3] is composed of two zones, the active zone that is used for the measurement of deformation, and the passive zone that serves as guide of information. The sensor is schematically represented in Figure Mirrors Reference Fibre Measurement Fibre PVC Protection Tube Coupler Connector Anchor Pieces Active Zone (20cm - 10m) Passive Zone (Unlimited) Figure 3.1.1: Schema of SOFO Standard Sensor The active zone is limited by two anchor pieces and consists of two optical fibres placed in a protection tube. The anchor pieces have a double role: to attach the sensor to auscultated structure and to transmit the deformation from the structure to the active zone. The deformation of the structure induces a change of the distance between the anchor pieces, and this change is registered by the measurement fibre. The measurement fibre is pre-tensioned between the anchor pieces in order to measure the shortening of the structure as well as its elongation. The reference fibre is independent of both, the measurement fibre and the deformation of the structure, and its purpose is only to annul the temperature influences to the sensor. Both fibres leave the active zone and continue to the passive zone. The length of the active zone of the standard sensors is limited between 20cm and 10m. Exceeding these limits the independence of the reference fibre can not be guaranteed. The passive zone transmits the information from the active zone to the reading unit. It is composed of one monomode optical fibre, a connector and a coupler, all protected by a plastic tube. The coupler is placed in the passive zone of the sensor, close to the anchor piece in order to increase the precision and to facilitate the manipulation during the measurement. The length of the passive zone is nearly unlimited and depends only of distance between the sensor emplacement position and the reading unit. If this distance is very long (several tenth of metres) the passive zone can be extended by a simple fibre optic cable. The sensor is linked to the reading unit by means of E2000 connector. The sensors are protected by PVC tube that allows an easy manipulation, fast installation and very good resistance during the pouring and the vibrating of concrete. Due to the high softness of the sensor it is possible to measure the deformation of concrete from the pouring, including the very early age deformation of concrete. Typical measurement made during the first ten days of concrete life is represented in Figure Page 3 of 9

4 Hydropower, , Gmunden, Austria Deformation [mm] Figure 3.1.2: Typical Measurement in Fresh Concrete Element Done by SOFO Standard Sensor 3.2 Long SOFO Sensor Deformation Measurements of Concrete at Early Age Shrinkage -0.1 Thermal Swelling Time After Pouring [Days] As mentioned above, the SOFO standard sensor consist of two fibres: the measurement fibre which is fixed on the structure and follows its behaviour (deformation or displacement), and the reference fibre that is independent of both, the structure and the measurement fibre, and whose purpose is to compensate the influence of the thermal variations. Both fibres are protected by means of a PVC tube. 1 Reference Fibre Measurement Fibre PVC Protection Tube 1 Section 1-1 Figure 3.2.1: Measurement and Reference Fibre in Protection Tube Figure represents the measurement fibre and the reference fibre placed in the PVC protection tube in the case of a standard SOFO sensor (see also Fig ). The reference fibre has a helicoidal shape in the tube, and is independent of the measurement fibre and deformation. In the case of a sensor with a measurement basis longer then ten metres, the independence of the reference fibre regard to the measurement fibre can not be provided by applying the concept shown in Figure It is necessary to separate the fibres, and place them in different tubes [3, 4]. Two solutions are shown in figure PVC Protection Tube A) B) PE Protection Tube Reference Fibre in Protection Micro-tube Measurement Fibre in Protection Micro-tube PVC Protection Tube PE Protection Tube Reference Fibre Without Micro-tube Measurement Fibre in Protection Micro-tube Figure 3.2.2: Cross-section of Long Sensor A) First Version -Applied on Sensor B3 B) Final Version - Applied on Sensor F3 Page 4 of 9

5 Hydropower, , Gmunden, Austria The difference between two solutions is protection of the reference fibre. In the first version the reference fibre was protected by plastic micro-tube, but a creeping of this micro-tube occurred during the measurements and perturbed the functioning of the reference fibre. Hence, the micro-tube is removed in the final version. Being relatively fragile, the measurement fibre was firstly protected by a plastic micro -tube (diam. ext. 0.9mm). Then, in order to separate from the measurement fibre, the reference fibre was put in the polyethylene (PE) tube (diam. int./ext. 3.0/5.0mm), and finally the whole assembly was protected inside a PVC tube (diam. int./ext. 8.7/12.2mm). The reference fibre in the PE tube has a helicoidal shape, identical to standard sensors (Figs and 3.2.1). Two long sensors, named B3 and F3, are built in order to replace two rod extensometers [5] in "Emosson" dam in. Some properties of the external PVC protection tube were imposed by the in-situ conditions: these are the external diameter of protective PVC tube being limited by the dimensions of the hole in which the sensor had to be placed, and the tube had to be sufficiently stiff to allow the anchorage, but sufficiently flexible to be winded on the transporting spool (Figure 3.2.3). 4. Long sensor installation Figure 3.2.3: Long Sensor on Transporting Spool Since the two sensors are installed inside the dam, we had to respect the local accessibility situation (dimension of elevator, doors, gallery, ). For the transport from the lab to the site, the sensors had been spun up on a wooden cross (Figure 3.2.3). In the dam, once the old mechanical extensometer was dismounted, the installation of the long sensors processed rapidly. The sensors had to be pre-tensioned in situ because of the manner of fixation: the upper anchor piece is fixed to the dam by means of a wedging screw. The lower anchor piece is fixed to the rock using a bayonet anchor, the same as used on the dismounted rod extensometer. Within half a day the sensor was fit to measure (Figure 4.1). When building a sensor in the lab it is very important to be aware of the climatic conditions present on the site. Especially for the Emosson dam gallery where the relative humidity goes up to 90% and the temperature is all the year round at about 5 C. This information, as well as the length of pre-stressing (approx. 0.5% of the length of the active zone of the sensor), have to be considered when the dimensions of the components of a sensor are defined. In order to centralise the monitoring of the new sensors, situated at opposite ends of the dam, optic extension cords have been installed. In this way the two sensors can be measured from a common point without having to displace the reading unit. Using this set-up it is even possible to monitor the sensors automatically over a longer period of time without any human intervention. Page 5 of 9

6 Hydropower, , Gmunden, Austria REFERENCE PLATE LOWER ANCHORING PIECE (BAYONET-TYPE) MOUNTING TUBE REFERENCE Fiber (loose) UPPER ANCHORING PIECE (INCL. COUPLER, SPLICES) MEASUREMENT Fiber (pre-stress 0.5%) WEDGING SCREW Monitoring zone 3.178" Mobile mirror Coupler A/D FilterAmpli µ Photo- LED Diode 1300nm Internal PC Portable PC Figure 4.1: Installation Set-up of Long Sensor Portable reading unit 5. Results of measurements The long sensors B3 and F3, 39 m and 30 m long respectively, are mounted side by side with the rod extensometers B4 and F4. The monitoring started in October 16. As mentioned in 3.2, creeping of the sensor B3 caused us to improve the conception of the long sensor. The improved design has been successfully applied to sensor F3, therefore only the results of this sensor are presented and analysed. The 30 m long sensor F3 is placed close and parallel to the 60 m long extensometer F4. In order to compare the results of the measurements the deformation measured by extensometer is divided by 2. The improved F3 sensor is installed in October 17. Its measurement data as well as the measurement data of the extensometer, the difference between the long sensor and the extensometer and the stored water level altitude are represented in Figure 5.1. The measurements are performed periodically once a month, with, however, some exceptions. Deformation [mm] o- Stabilisation Period Extensometer and Long Sensor Measurements n- d- f- a- Dam Level a- s- o- n- Date [month-year] Extensometer F4/2 Long Sensor F3 Difference F3-F4/2 d- f- a Dam Level [m.a.s.l.] Figure 5.1: Measurements of Long Sensor F3 and Extensometer F4 Page 6 of 9

7 Hydropower, , Gmunden, Austria 6. Analysis of results Some creeping of the long sensor F3 has been noticed during the first two months following the installation. This period is called the stabilis ation period of the long sensor (see Figure 5.1). The creeping is due to residual stresses in the plastic parts of the sensor, the PVC and PE tubes. The stresses are caused by transport and by the temperature and humidity differences between the sites of the long sensor construction (laboratory of IMAC, 20 C, ~50% RH) and implementation (Emosson dam, 5 C, ~90% RH). The stabilisation period is unavoidable if plastic pieces are used in the sensor design. After two months the difference between the compared sensors begun to alternate around a constant value depending on the water level. The long sensor worked fine. The variations of the difference is due to the different sensitivities of the sensors, but also due to their different gage length. Comparison of measurements after the stabilisation period is represented in Figure 6.1. Deformation [mm] n- d- Extensometer and Long Sensor Measurements after the Stabilisation Period f- a- a- Dam Level Extensometer F4/2 Long Sensor F3 Difference F3-F4/2 o- n- d- Date [month-year] Figure 6.1: Measurements of Long Sensor F3 and Extensometer F4 after the Stabilisation Period Comparison shown in Figure 6.1 shows a very good agreement between the two compared systems and proves the correct functioning of the SOFO long sensor. Moreover, the response of the long sensor is faster than the rod extensometer response, and consequently the long sensor is more sensitive than the extensometer. The lagging of the extensometer in regard to the long sensor was expected as this is a consequence of its friction on the walls of the borehole in which it is placed. The biggest delay of the extensometer is registered just after the dam level starts to increase. At this time the force of friction changes sign, increasing the duration of inactivity of the extensometer. This effect is even more pronounced in the current year (see encircled area in Figures 6.1). Figure 6.2: Measurements of Long Sensor F3 and Extensometer F4 with Respect to Dam Level f- a Dam Level [m.a.s.l] A) Deformation - Dam Level Dec Dec B) Deformation - Dam Level Dec July Deformation [mm] Extensometer Long Sensor 4 Deformation [mm] Extensometer Long Sensor Dam Level [m.a.s.l.] Dam Level [m.a.s.l] A) From Dec. 17 to Dec. 19 and B) from Dec. 19 to July 19 Page 7 of 9

8 Hydropower, , Gmunden, Austria The fact that the long sensor is more sensitive than the extensometer is more noticeable in Figure 6.2 representing the dependence of deformation with respect to the water level. The hysteresis of the long sensor being smaller than the one of the extensometer indicates a better sensitivity of the long SOFO sensor. 7. Conclusions The following conclusions can be drawn of this project: -The long sensor starts creeping right after the installation because of the deformation of the sensor's plastic parts during transportation. The creeping stabilises after approx. 60 days. -Once stabilised, the long sensor continues to work fine and achieves a very good agreement with the rod extensometer. -The long sensor is more sensitive than the rod extensometer. It is also more precise than the rod extensometer (less hysteresis). -It is the first time that a 30 metres long SOFO sensor has been implemented in a dam and compared with a classical monitoring system. The comparison proves its good functioning. -Due to the general advantages of the SOFO monitoring system (long term stability, no calibration, insensitivity to electro-magnetic fields, possibility of permanent remote-monitoring, etc.), the use of the long sensor can be recommended for dam monitoring. 8. Acknowledgements The author of this report would like to thank to "Électricité d'émosson SA" for financing this project, Mr. J.-M. Rouiller, Mr. J. Hugon, Mr. B. Gay des Combes and Mr. A. Stein mann from "Électricité d'émosson SA" for their collaboration, kindness, helpfulness and interest in this research and new technologies. Mr. R. Délez from IMAC-EPFL whose help and mechanical knowledge in all phases of project (from the preparation to the measurements in-situ) was precious, and to Mr. Y. Putallaz for aid in the preparation phase of the sensor. Finally, thanks to Miss R. Stalker for corrections to this report. 9. References 1. "Rapport annuel de gestion 17/19", Annual Report of Management, Electricité d'emosson SA, Martigny 19, 2. Inaudi D., "Fiber Optic Sensor Network for the Monitoring of Civil Engineering Structure", Thesis N 1612, Swiss Federal Institute of Technology, Lausanne 17, 3. Inaudi D., Vurpillot S., Casanova N., Osa-Wyser A., "Development and Field Test of Deformation Sensors for Concrete Embedding", Smart Structures and Materials, SPIE Vol.2721, pages , San Diego 16, USA 4. Kronenberg P., Casanova N., Inaudi D., Vurpillot S., "Dam Monitoring with Fiber Optics Deformation Sensors", Smart Structures and Materials, SPIE Vol.3043, pages 2-11, San Diego 17, USA 5. "Suggested Methods for Monitoring Rock Movements Using Borehole Exstensometers", Int. Society for Rock Mechanics; Oxford a.o., 18, International Journal of Rock Mechanics and Mining Sciences, vol.15, no.6, pp Biographical details of the authors Branko Glisic has studied at the Belgrade University and received his B.Sc. degrees 14 in civil engineering, and 16 in mathematics. During high school he won second and third prize in federal mathematics competitions. He was also awarded as the best student of the year at the Faculty of Civil Engineering. From February 15 to October 16, he worked at the Faculty of Civil Engineering in Belgrade as teaching assistant in the static of structures and the theory of plates and shells. His research domain was the dynamical behaviour of cooling towers. Since 17, he is a Ph.D. student at the Swiss Federal Institute of Technology in Lausanne researching and developing different types of fibre optic sensors, and studying the behaviour of fibre optic sensors embedded in concrete at the very early age. Page 8 of 9

9 Hydropower, , Gmunden, Austria Daniele Inaudi received a degree in physics at the Swiss Federal Institute of Technology in Zurich (ETHZ). His graduation work was centred on the theoretical and experimental study of the polarisation state of the emission of external grating diode lasers and was prized with the ETHZ medal. In 17 he obtained his Ph.D. at the Laboratory of Stress Analysis (IMAC) of the Swiss Federal Institute of Technology in Lausanne for his work on the development of a fibre optic deformation sensing system for civil engineering structural monitoring. Daniele Inaudi is co-founder and director of SMARTEC SA (Grancia, ), a company active in the domain of fibre optic smart sensing. Pascal Kronenberg accomplished his studies at the EPFL (Swiss Federal Institute of Technology in Lausanne) with a degree in Civil Engineering. The graduation work was centered on the measurement of deflection of beams and shells (such as hydroelectric dams) by monitoring deformations with optical fibers (SOFO project) and was awarded by the ZSCHOKKE prize for its multidisciplinary character to optimize established approaches in civil engineering. Since 16 he is active as a research assistant at the Laboratory of Stress Analysis (IMAC) of the EPFL. At the beginning of 19 Pascal Kronenberg has started a Ph.D. work in the field of non destructive monitoring with fiber optic sensors of structural environment parameters such as humidity, chloride concentration and ph. Samuel Vurpillot received a degree in Civil Engineering at the Swiss Federal Institute of Technology. His graduation work was centered on the stress analysis of a buttress dam under cyclic temperature loading and the results have been published in "Dam Engineering". Since 13 he is active as researcher at the Laboratory of Stress Analysis (IMAC) of the Swiss Federal Institute of Technology in Lausanne. His area of interest is the application of fiber optic sensors in the field of civil engineering smart structures. In 19 he obtained his Ph.D. at the laboratory of Stress Analysis (IMAC) of the EPFL on the development of automatic analysis of deformation measurement for structure. He is actually working also for SMARTEC SA (Grancia, ), a company active in the domain of fibre optic smart sensing. Page 9 of 9