Smartec (Switzerland)

Distributed fiber optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of elongated structures such as pipelines, flow lines, oil wells, and coiled tubing. Sensing systems based on Brillouin and Raman scattering are used, for example, to detect pipeline leakages, to verify pipeline operational parameters and to prevent failure of pipelines installed in landslide areas, to optimize oil production from wells, and to detect hot spots in high-power cables. Recent developments in distributed fiber sensing technology allow the monitoring of 60 km of pipeline from a single instrument and of up to 300 km with the use of optical amplifiers. New application opportunities have demonstrated that the design and production of sensing cables are a critical element for the success of any distributed sensing instrumentation project. Although some telecommunication cables can be effectively used for sensing ordinary temperatures, monitoring high and low temperatures or distributed strain presents unique challenges that require specific cable designs. This contribution presents advances in long-range distributed sensing and in novel sensing cable designs for distributed temperature and strain sensing. This paper also reports a number of significant field application examples of this technology, including leakage detection on brine and gas pipelines, strain monitoring on gas pipelines and combined strain and temperature monitoring on composite flow lines, and composite coiled tubing pipes.

Many bridges worldwide are approaching the end of their lifespan and it is necessary to assess their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Fracture critical bridges are of particular interest since they have only little or no load path redundancy. Structural health monitoring (SHM) has recently emerged as a branch of engineering, which aim is to improve the assessment of structural condition. Distributed optical fiber sensing technology has opened new possibilities in SHM. A distributed deformation sensor (sensing cable) is sensitive at each point of its length to strain changes and cracks. Such a sensor practically monitors a one-dimensional strain field and can be installed over all the length of the monitored structural members, thereby providing with integrity monitoring, i.e. direct detection and characterization (including recognition, localization, and quantification or rating) of local strain changes generated by damage. Integrity monitoring principles are developed and presented in this article. A large scale laboratory test and a real on-site application are briefly presented.

Götaälvbron, the bridge over Göta river, was built in thirties and is now more than seventy years old. The steel girders were cracked and two issues are in cause of steel cracking: fatigue and mediocre quality of the steel. The bridge authorities repaired the bridge and decided to keep it in service for the next fifteen years, but in order to increase the safety and reduce uncertainties related to the bridge performance an integrity monitoring system has been mandatory. The main issue related to selection of the monitoring system has been the total length of the girders which is for all the nine girders more than 9 km. It was therefore decided to monitor the most loaded five girders (total length of 5 km approximately) and logically a fiber optic distributed sensing system have been selected. For the first time a truly distributed fiber optic sensing system, based on Brillouin scattering effect, is employed on such large scale to monitor new crack occurrence and unusual strain development. The monitoring system itself, the monitoring strategy, challenges related to installation and the data management are presented in this paper.

Submillimeter crack is detected with a dedicated fiber-optic strain cable, a 1-m-spatial-resolution ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</i> ) distributed Brillouin sensor and an advanced signal processing technique. The signal processing approach consists in spectrum shape analysis and multiple peaks detection.

All structures undergo deformations under the effects of loads or degradation of the constituent materials. The deformations of any structure (bridges, dams, frames, shells, tunnels, towers, wings, trusses,) contain a lot of information about its health state. By measuring these deformations it is possible to analyse the loading and aging behavior of the structure. The presented method analyses a structure by subdividing it into sections and cells. The deformation of each of these macro-elements is first analysed separately to obtain local information about the materials, and then combined to provide insight on the global behavior. Examples of these techniques applied to civil engineering structures fitted with long-gage-length fiber optic sensors show the variety of information that can be obtained using this powerful analysis technique.

Despite the recent advances in sensor technologies and data-acquisition systems, interpreting measurement data for structural monitoring remains a challenge. Furthermore, because of the complexity of the structures, materials used, and uncertain environments, behavioral models are difficult to build accurately. This paper presents novel model-free data-interpretation methodologies that combine moving principal component analysis (MPCA) with each of four regression-analysis methods—robust regression analysis (RRA), multiple linear analysis (MLR), support vector regression (SVR), and random forest (RF)—for damage detection during continuous monitoring of structures. The principal goal is to exploit the advantages of both MPCA and regression-analysis methods. The applicability of these combined methods is evaluated and compared with individual applications of MPCA, RRA, MLR, SVR, and RF through four case studies. Result showed that the combined methods outperformed noncombined methods in terms of damage detectability and time to detection.

A large-scale lifetime building monitoring program was implemented in Singapore in 2001. The monitoring aims of this unique program were to increase safety, verify performance, control quality, increase knowledge, optimize maintenance costs, and evaluate the condition of the structures after a hazardous event. The first instrumented building, which has now been monitored for more than ten years, is presented in this paper. The long-gauge fiber optic strain sensors were embedded in fresh concrete of ground-level columns, thus the monitoring started at the birth of both the construction material and the structure. Measurement sessions were performed during construction, upon completion of each new story and the roof, and after the construction, i.e., in-service. Based on results it was possible to follow and evaluate long-term behavior of the building through every stage of its life. The results of monitoring were analyzed at a local (column) and global (building) level. Over-dimensioning of one column was identified. Differential settlement of foundations was detected, localized, and its magnitude estimated. Post-tremor analysis was performed. Real long-term behavior of concrete columns was assessed. Finally, the long-term performance of the monitoring system was evaluated. The researched monitoring method, monitoring system, rich results gathered over approximately ten years, data analysis algorithms, and the conclusions on the structural behavior and health condition of the building based on monitoring are presented in this paper.

Composite coiled tubing is an emerging technology in the oil and gas sector that presents important advantages compared to the steel coiled tubing and conventional drilling. The composite tube has reduced weight, allowing extended reach and improved fatigue life. An additional advantage resides in the fact that the coiled tube wall can contain and protect additional functional elements, such as electrical conductors and fiber optics for sensing and data communication. Sensing systems based on Brillouin and Raman scattering can be used to verify the pipe operational parameters, prevent failure, optimize oil production from the well, provide strain distribution along the tubing and detect hot-spots in high-power cables. The integration of such sensing elements into composite tubing presents additional advantages and challenges. On one hand the embedded sensors are protected by the composite material and can be installed during production, avoiding external installation that could interfere with the tubing operations. In the other hand, the integration of optical fiber sensors into the composite structure requires the development of appropriate packaging and installation techniques that allow easy handling during production and avoid and damage to the sensor and the composite structure itself. This contribution presents the sensing cable designs for temperature and strain sensing in a composite coiled tubing as well as testing results form initial field demonstrations.

Distributed fiber optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of large structures such as pipelines, flow lines, oil wells, dams and dikes. Sensing systems based on Brillouin and Raman scattering have been used for example to detect pipeline leakages, verify pipeline operational parameters, prevent failure of pipelines installed in landslide areas, optimize oil production from wells and detect hot-spots in high-power cables. The measurement instruments have been vastly improved in terms of spatial, temperature and strain resolution, distance range, measurement time, data processing and system cost. Analyzers for Brillouin and Raman scattering are now commercially available and offer reliable operation in field conditions. New application opportunities have however demonstrated that the design and production of sensing cables is a critical element for the success of any distributed sensing instrumentation project. Although standard telecommunication cables can be effectively used for sensing ordinary temperatures, monitoring high and low temperatures or distributed strain present unique challenges that require specific cable designs. This contribution presents three cable designs for high-temperature sensing, strain sensing and combined strain and temperature monitoring.

Although structural health monitoring and patient monitoring may benefit from the unique advantages of optical fiber sensors (OFS) such as electromagnetic interferences (EMI) immunity, sensor small size and long term reliability, both applications are facing different realities. This paper presents, with practical examples, several OFS technologies ranging from single-point to distributed sensors used to address the health monitoring challenges in medical and in civil engineering fields. OFS for medical applications are single-point, measuring mainly vital parameters such as pressure or temperature. In the intra-aortic balloon pumping (IABP) therapy, a miniature OFS can monitor in situ aortic blood pressure to trigger catheter balloon inflation/deflation in counter-pulsation with heartbeats. Similar sensors reliably monitor the intracranial pressure (ICP) of critical care patients, even during surgical interventions or examinations under medical resonance imaging (MRI). Temperature OFS are also the ideal monitoring solution for such harsh environments. Most of OFS for structural health monitoring are distributed or have long gage length, although quasi-distributed short gage sensors are also used. Those sensors measure mainly strain/load, temperature, pressure and elongation. <i>SOFO</i> type deformation sensors were used to monitor and secure the Bolshoi Moskvoretskiy Bridge in Moscow. Safety of Plavinu dam built on clay and sand in Latvia was increased by monitoring bitumen joints displacement and temperature changes using <i>SMARTape</i> and <i>Temperature Sensitive Cable </i>read with <i>DiTeSt</i> unit. A similar solution was used for monitoring a pipeline built in an unstable area near Rimini in Italy.

A full-scale on-site test represents an ideal way to check a hypothesis and to determine the real behavior of structures, especially in cases in which some uncertainties cannot be reduced otherwise. To perform the test successfully it is necessary to monitor the parameters that representatively describe the structural behavior. In the case of piles, axial compression, pullout, and flexure tests cover all load combinations that may appear in service. To assess the foundation performance at a semiconductor production facility, two sets of piles with three piles in each set were tested. The monitored parameters were average strains, registered in several segments over the whole length of each pile using long-gauge fiber optic sensors. This type of sensor, combined in appropriate topologies, gives rich information concerning the piles’ behavior and soil properties. The monitoring method is presented and its performances through the results of the tests are discussed. This method allowed the determination of the Young modulus of the piles, the occurrence of cracks, the normal force distribution, and the ultimate load capacity in the case of axial compression and pullout tests, as well as the curvature distribution, horizontal displacement, deformed shape, and damage localization in the case of the flexure tests. Moreover, the pile–soil friction distributions, the quality of soil, and the pile tip force were estimated. The advantage of the presented method resides in the use of long-gauge sensors, which are insensitive to local structural defects like crack openings or air pockets and allow the collection of data on a global structural level and not on a local material level.

In many bridges, the vertical displacements are the most relevant parameters to be monitored in both the short and long term. Current methods (such as triangulation, water levels or mechanical extensometers...) are often tedious to use and require the intervention of specialized operators. The resulting complexity and costs limit the temporal frequency of these traditional measurements. The spatial resolution obtained is in general low and only the presence of anomalies in the global structural behavior can be detected and warrant a deeper and more precise evaluation. To measure bridge vertical displacements at low cost and frequently in time, one solution consists of installing a network of fiber optic sensors during concrete pouring or installing them on the surface of the structure. By subdividing the whole structure into structural elements and those elements into cells that are analyzed by the sensors, it is possible to obtain information about the average cell deformation (e.g., mean curvature) that can then be combined to obtain the global structural displacement field. In 1996, a concrete highway bridge near Geneva (Switzerland) was instrumented with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30-cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the interaction between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor the long term deformations of the bridge. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The vertical and horizontal curvatures of the bridge are measured locally at multiple locations along the bridge span by installing sensors at different positions in the girder cross-section. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformations of the bridge. This measurement methodology was also applied to the Lutrive Highway Bridge in Switzerland in order to measure the variation in vertical bridge displacements due to a static load test. The results obtained using the low coherence interferometric sensors of the SOFO system were then compared with the displacements obtained through an optic leveling system. In the case of this cantilever bridge of 60 meters half-span equipped with 30 fiber optic sensors, a discrepancy of less than 7% was obtained between the two measuring systems.

In a national and worldwide context, countless reinforced concrete structures are in an advanced state of deterioration. A principal cause of such degradation is chloride induced corrosion of reinforcement bars. This phenomenon is accentuated in countries where de-icing salts are used for road safety, as well as in maritime zones. To date, no non-destructive method quantifying chloride content during the corrosion initiation phase has been established. Measurement of such a parameter is important for the development of a better understanding of the complexity of corrosion phenomena and, more practically, for better management of existing structures. This paper proposes a new method for non-destructive measurement, for monitoring continuously and in real time free chloride content in concrete pores. In this context, a chemical sensor that employs optical fibers was developed and tested. The sensor functions using the fluorescence of an indicator dye that is sensitive to chlorides. Through fluorescence spectroscopy, variations in the concentration of free chlorides are related to intensity fluctuations of fluorescence. The use of optical fibers also provides an advantage compared with existing electric non-destructive detection systems due to superior electromagnetic stability. Theoretical and experimental studies calibrated and validated the sensor for implementation within mortar samples. Free chloride concentrations between 30 and 350 mM can be detected. Two experiments reproduced climatic variations in a controlled environment. The first test simulated a hot maritime climate and the second test simulated a cold continental climate. These tests confirmed that it is possible to determine with precision the free chloride content. Also, fluorescence spectroscopy with optical fibers offers an innovative means for early and non-destructive detection of free chloride content in concrete. As a result, this new method has potential for improving the science of corrosion process understanding and for planning appropriately for preventive action in practical situations.

Distributed fiber optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of elongated structures such as pipelines, flow lines, oil wells and coiled tubing. Sensing systems based on Brillouin and Raman scattering are used for example to detect pipeline leakages, verify pipeline operational parameters, prevent failure of pipelines installed in landslide areas, optimize oil production from wells and detect hot-spots in high-power cables. Recent developments in distributed fiber sensing technology allow the monitoring of 60 km of pipeline from a single instrument and of up to 300 km with the use of optical amplifiers. New application opportunities have demonstrated that the design and production of sensing cables is a critical element for the success of any distributed sensing instrumentation project. Although some telecommunication cables can be effectively used for sensing ordinary temperatures, monitoring high and low temperatures or distributed strain present unique challenges that require specific cable designs. This contribution presents advances in long-range distributed sensing and in novel sensing cable designs for distributed temperature and strain sensing. The paper also reports a number of significant field application examples of this technology, including leakage detection on brine and gas pipelines, strain monitoring on gas pipelines and combined strain and temperature monitoring on composite flow lines and composite coiled-tubing pipes.

Civil structural monitoring by optical fiber sensors, require the development of reliable sensors that can be embedded or surface mounted in concrete, mortars, steel, timber and other construction materials as well as in rocks, soils and road pavements. These sensors should be rapid and simple to install in order to avoid any interference with the building site schedule and not to require specialized operators to accomplish the task. The sensors have to be rugged enough to withstand the harsh conditions typically found in civil engineering including, dust, moisture, shocks, EM disturbances and unskilled workman. It is also desirable that the instrumentation survives for tens of years in order to allow a constant monitoring of the structure aging. This contribution presents the results of a four-year effort to develop, test and industrially produce a palette of sensors responding to the above requirements and adapted to different applications and host materials. These sensors include a small version (length up to 2 m) adapted for embedding in mortars, grout and glues, an intermediate version of length between 20 cm and 6 m adapted to direct concrete embedding or surface installation and a long version adapted to measure large deformations (up to 2%) over length up to 30 m and especially adapted for geostructures monitoring.

The security of civil engineering works demands a periodical monitoring of the structures. The current methods (such as triangulation, water levels, vibrating strings or mechanical extensometers) are often of tedious application and require the intervention of specialized operators. The resulting complexity and costs limit the frequency of these measurements. The obtained spatial resolution is in general low and only the presence of anomalies in the global behavior urges a deeper and more precise evaluation. There is therefore a real need for a tool allowing an automatic and permanent monitoring from within the structure itself and with high precision and good spatial resolution.In many civil structures like bridges, tunnels and dams, the deformations are the most relevant parameter to be monitored in both short and long-terms. Strain monitoring gives only local information about the material behavior and too many such sensors would therefore be necessary to gain a complete understanding of the structure’s behavior. We have found that fiber optic deformation sensors, with measurement bases of the order of one to a few meters, can give useful information both during the construction phases and in the long term.

Civil structures are important for any society and it is necessary to monitor their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Structural health monitoring (SHM) recently emerged as a branch of engineering with a great potential for addressing the above mentioned challenges. In spite of its importance and promising benefits, SHM is still relatively infrequently used in real structures. A possible reason for this is a lack of understanding of the SHM process, which is often considered to be a supplemental activity that does not require detailed planning. However, the opposite is true - only proper and detailed development and implementation of each SHM step can ensure its successful and maximal performance. The aim of this paper is to present the SHM process through more than 350 projects. Basic concepts are introduced, and the purpose, requirements and benefits of SHM are discussed. The importance of monitoring over a life span is highlighted. Core activities such as creating monitoring strategy, installation and maintenance of hardware, and data management are presented and discussed. The involved parties are identified and their interaction with the monitoring process is analyzed. Finally, important SHM challenges are identified.

The monitoring of dams represents an important task in the management of hydroelectric systems. Their economic, social and environmental value imposes to know well the real behavior of the structure and its foundations. This paper shows in two practical cases the possibility to improve the quality of deformation measurements by an appropriate fiber optics sensor network. The first case is a study showing the technical and economical feasibility to install an extended, spatial fiber optics deformation sensor network to detect the relative deflection of an entire shell dam. At this purpose of theoretical study has been evaluated on the base of typical load situations with their effective deflections on the Schiffenen dam, a shell-shaped concrete structure near Fribourg. The second case concerns the development and realization of two long fiber optics deformation sensors anchored in the rock to monitor the displacement of the dam relatively to its underground. These sensors have been installed in the Emosson shell dam.

Distributed fiber optic sensing offers the ability to measure temperatures and strain at thousands of points along a single fiber. This is particularly interesting for the monitoring of pipelines, where it allows the detection and localization of leakages of much smaller volume than conventional mass balance techniques. Fiber optic sensing systems are used to detect and localize leakages in liquid, gas and multiphase pipelines, allowing the monitoring of hundreds of kilometers of pipeline with a single instrument and the localization of the leakage with a precision of 1 or 2 meters. This contribution presents recent testing results on controlled field trials. The tests demonstrate that it is possible to reliably detect oil leakages of the order of 10 liters to 1’000 liters per hour, corresponding to 0.01% to 0.1% of the pipeline flow. Tests were performed with small temperature differences between liquid and ground. The detection time was between 1 minute and 90 minutes. All simulated leakages were detected and localized to better than 2m accuracy. The paper describes the main parameters that affect the response time and detection volume, including the relative position of the leak to the sensing cable, temperature contrast and instrument performance. We also briefly report on relevant full-scale installations for the permanent monitoring of oil, brine and natural gas pipelines.

The Siggenthal Bridge is a concrete arch bridge with an arch span of 117 m, being built over the Limmat River in Baden, Switzerland. This bridge has been instrumented with 58 long- gage SOFO fiber optic deformation sensors, 2 inclinometers and 8 temperature sensors to monitor its deformations, curvatures and displacements during construction and int eh long-term. The sensor have been built installed successfully and the arch was monitored during the removal of the formwork and supports. It was therefore possible to observe the deformations of the arch wen being loaded by its dead load and by the daily temperature fluctuations. The measurements have shown that the temperature changes produce deformations of the same order of magnitude as the dead loads. The out-of-plain displacements obtained by double- integration of the measured curvatures are in good agreement with the direct triangulation measurements. Monitoring was also carried out during the construction of the superstructure, with the associated change of the load distribution in the arch. This paper briefly introduces the functional principle of the long-gage sensors used in this application, illustrates their installation and discusses the measurement results obtained during the bridge construction.