Fiber optic sensing for monitoring structures and the health of railway infrastructures


Abstract

We will present our findings on the results of distributed acoustic measurements on optical fibers for the prevention and detection of anomalies along the railway infrastructures. This system can also be a new approach for Structural Health Monitoring (SMH).

Introduction

We use a commercial Distributed Acoustic Sensor (DAS) unit for these measurements. The DAS unit involves propagating a high frequency light pulse through a standard optical fiber and alayzing the backscattered signal (Ratleigh scattering). Any event occurring in the fiber that has an acoustic energy (vibration for example), will disturb the backscattered signal by shifting its phase [1].  This phase shift is analyzed by the DAS unit in order to provide the nature of the event. The exact location is then obtained by means of reflectometry (OTDR). 

Sample Presentation

We will briefly introduce the results of the acoustic signal processing collected from DAS measurements along two railway infrastructures: 

– A portion of high-speed line (40km monitored) which includes several railway viaducts.

– A subway line (2.53km monitored).

For these measurements, we connected the DAS unit on fiber optic cable, which was already installed by the rail operator. It’s a standard telecom optical fiber operated at a wavelength of 1550 nm. One of the advantages of this specific type of measurement is that is uses optical lines which already exist, this greatly reduces the cost of deployment. 

lo1

Figure 1 : Presentation of the experimental setup

In table 1, the parameters we implemented for each measurement can be seen.

Portion of the high-1

Table 1: Parameters of measurement set ups

Because of the fiber length deployed, we have preferred a low pulse repetition frequency (PRF). Moreover, having a high PRF may cause overlapping pulses and therefore a loss of information.

Results

In this section, we will show the acoustic data measured by the DAS during the passage of the train on the viaduct (see Figure 2). Both air and solid vibrations are digitized. Figure 3 displays the total accumulated spectral energy envelope. In other words, the intensity of the air and solid sounds recorded by the fiber during the passage of the train on the viaduct, and accumulated over the entire recorded frequency band.

lo2
lo3

Figure 2: Viaduct (480m long) monitored by Cementys


lo4

Figure 3: Behavior of the viaduct during the passage of the train and outside the passage of the train

Each point on the viaduct has a specific signature in comparison to the reference signal in blue. If one wishes to associate each signature with an element of the viaduct, they must subtract the reference signal to the signal measured during the passage of the train. By analyzing a 20-meter portion of viaduct located between two beams during the passage of the train, we have plotted in firgure 4 the acoustic spectrum before the passage of the train and after it. These plots exhibit specific excited frequencies. 

Doc2-1

Figure 4: Acoustic spectrum resulting from the passage of the train on viaduct

The acoustic spectrum corresponds to an average of 70 points of measurements, 1 point for every meter. We can conclude that the  frequency f1 (see Figure 4) corresponds to the first flexural mode of the viaduct [2]. Considering a distance between axles of the same bogie of 2.5m, the frequency associated with this speed can be estimated as: 𝑓 =V/D. Being f the frequency (Hz), V vehicle speed (km/h) and d the distance (m). Therefore, the frequency associated to the passage of the axles at 220km/h in a portion of 70m is 3.33Hz. It’s clear from this analysis that maintenance problems of both track and vehicle, situations of passengers discomfort or even lack of security would be derived from the excitation of the first flexural mode of vibration. A more complete dataset on different train passes can lead to further statistical studies and thus to better conclusions. Another analysis on the noise generated by the wheel-rail interaction can lead us to better differentiate the frequencies.


The following study, on a portion of subway line, presents results of the passage of two trains which we can compare. Some track devices have specific signatures as will be shown later:

Doc3-1

Figure 5: Comparison between 3 total accumulated spectral energy envelope displays

lo5

Figure 6: Correlation between 2 signals resulting from two different passages of train

A first conclusion can be drawn about the nature of the resulting signals on different track devices, a specific intensity to each device. This intensity (see Figure 7) can be monitored by comparing it with a reference signal which results from a train switch in good working order. We follow the evolution of the different signals resulting from different train passages. A decision support system concerning the state of the rail switch can then be put it place. Figure 8 shows the high correlation coefficient between two train passages. In this case, r = 0.78


lo6

Figure 7: Representation of the mechanism of generation of rolling noise

lo7

Figure 8: Acoustic spectrum during the passage and nonpassage of the train on the rail switch

Another conclusion can be made on the acoustic spectrum generated by the passage of the train on the rail switch. By analyzing these resonance frequencies and associating them with each element of the infrastructure, one can rule that each element is exited at a given frequency. Taking these frquencies as references; 26.36 Hz, 37 Hz, 57 Hz, 92 Hz (which are generated when the operating state of the switch is good) and following the evolution of these frequencies can be an alternative of monitoring this track and rolling device in addition to the intensity. For example, the sleepers vibrate at low frequencies, up to about 500 Hz [3]. Rolling noise covers a large frequency bandwidth, between 100 and 5000 Hz [4].

Conclusions

The acoustic measurement distributed by optical fiber seems to be a new alternative for the monitoring of railway infrastructures. It’s a low-cost technique and it provides a good prediction of the state of the infrastructure and some track devices. It’s also expected that with a larger data set, such as; the average age of the trains, the weight and the speed of the trains, the state of the rails, several passages of trains (to deepen the statistical measurement) and using the PCA method (principal analysis component), for example, will provide for better predictive maintenance.

References

[1] Zhou Sha., et al., Phase demodulation method in phase sensitive OTDR without coherent detection. Opt.
Express, Vol. 25, Issue 5, pp. 4831-4844 (2017)
[2] Xia, H., Zhang, N., Guo, W.W.: Analysis of resonance mechanism and conditions of train-bridge system.
Journal of Sound and Vibration 294, 810–822 (2006)
[3] Virginie Delavaud, “Time modeling of wheel / rail interaction, for rolling noise application train” Thesis in
applied physics, p30, (2011)
[4] D.J. Thompson. Railway noise and vibration: mechanisms, modelling and means of control. Elsevier, 2009

Read more...

Instrumentation of tunnels by distributed optical fiber sensors: the Fréjus Tunnel, France


Summary

The distributed measurement done by optical fiber is a new technique that has opened many possibilities in the monitoring of deformed tunnels. In this specific case of monitoring the behavior of the Frejus tunnel (located on the France-Italy border) before and after extensive digging, Cementys used its "SensoLuxTM®" sensor to measure the deformation of the tunnel vault. It is a cable containing four optical fibers, which makes it possible to measure the backscattering deformation Brillouin (sensitive to deformation and temperature), and Raman backscattering temperature (temperature sensitive). The sensor is completely responsive over its entire length. Once integrated onto the structure, it measures the deformation of the concrete in tension and compression, therefore determining the convergence phenomenon of the tunnel. Data can then be transmitted over long periods of time and distances thanks to the optical fiber, thus allowing deformation tracking in real
time.

Introduction

Auscultation of underground infrastructures is crucial in order to know its structural state and to be able to mitigate the potential risk of accidents during the construction or operation phase along with optimizing long-term maintenance. In tunnels, visual inspections can be difficult to achieve because of restricted access. A distanced real-time vault monitoring system detects and follows the movements of the structure.

Distributed measurement, unlike point sensors, can monitor a tunnel over its entire length. It makes it possible to precisely locate and identify potential malfunctions, with only one only optical fiber connected.

Wide ranges of fiber optic sensors have been developed for different uses. In our article, we will introduce you to one of these major technologies: the distributed measure of pressure and temperature by Brillouin / Raman coupled optical backscattering.

What is an optical fiber?

In order to understand fiber optic sensors, let’s first look at what an optical fiber is. An optical fiber is a silica flexible wire that transmits light from end to end. It is extremely thin; it nearly has the same diameter as a strand of hair. In addition, it is completely passive: it carries no energy other than light. The heart of the fiber can be integrated inside a sheath. We can therefore obtain an optical cable as resistant as an electric cable.

An optical fiber is composed of two layers of silica, with different optical indices. This difference means that the light is totally reflected inside the heart, which ensures its transmission without loss (Figure 1).

Figure 1. Section of a fiber optic cable

Fiber optic distributed measurement by reflectometry

Distributed measurement called “répartie” in French is a major advance in terms of monitoring linear structures: a single fiber telecommunication can provide measurements every meter for several kilometers worth of measurements! Having no punctual sensor, the fiber itself is impacted by its
thermomechanical environment and transmits information through the light directly to the measuring device. The fiber therefore serves as a sensor and as a means of data transmission. (Figure 2)

SensoLux-™-Optical-Fiber-Sensor_Cementys-1-300x200

Figure 2. SensoLuxTM® Sensor

The measuring device analyzes backscattered light in the fiber (Figure 3) by time domain reflectometry (radar technology).

pi1

Figure 3. Measurement principle by reflectometry

A laser emits a short light pulse of about ten nanoseconds through the fiber. This transmitted flash is backscattered back to its source in the fiber, which is due to different physical phenomena (Rayleigh, Raman and Brillouin backscatter) (Figure 4). The received signal is then analyzed in the time and frequency domains. The location of the information is thanks to the signal return time (measured for a known speed of light in the fiber).

Spectre_de_rétrodiffusion_par_la_silice

Figure 4. Backscattering spectrum of light

Application: experimental instrumentation of a section of the Fréjus tunnel

The SensoLuxTM® sensor was installed on the Fréjus tunnel, which is the longest road tunnel in Europe, linking France and Italy across the Alps. The fiber optic sensor was placed on areas near sensitive and important work, such as the digging of a new gallery. (Figure 5)

tunnel

Figure 5. Frejus Road Tunnel

The system consists of gluing the fiber optic cable to the bottom of a millimeter groove dug on the surface of the concrete, found in the coating of the reinforced concrete structure. The instrumentation was made on three rings with a distance of 5 m from each other. (Figure 6)

pi2

Figure 6. Optical fiber installation plan: the fiber labeled in red is glued to each vault

The small diameter of the cable (2mm) makes it easy to integrate on the vault by structural reinforcement in grooves milled in concrete. Therefore, the fiber optic cable is protected for long-term monitoring without maintenance of the measuring system and without drifting the data. This solution also has the advantage of perfectly integrating with the material and not being intrusive. (Figure 7)

pi3

The sensor detects local disorders such as cracks as well as general deformations such as convergence or bending phenomena. SensoLogger® measures the deformations of concrete without a blind zone and with a spatial resolution of 50 centimeters along with an accuracy of ± 10 μdef (10 "micro deformation" or micrometers of deformation per meter of reinforced concrete).

The Brillouin optical backscattering deformation measurement is a relative measure because the fiber has intrinsic mechanical stresses to the cable and its installation: the initial measurement is therefore necessary and constitutes the reference. The following measurements will be compared to this first one. Consequently, the initial deformation variations inflicted on the fiber during laying have no impact on the measurement.

In several studies carried out on concrete structures with thermal gradients, we have developed a decoupling methodology for phenomena measured by fiber optic sensors. This method is based on the principle of superposition of deformations during the life of the work.

pi4

Figure 8. Deformation of arches 1, 2 and 3 (μɛ)

pi5

Figure 9. Tunnel diameter on a profile – magnified 1000 times

Civil engineering concrete structures often have as much thermal deformation as mechanical deformations. In addition, the photo-elastic Brillouin effect is also impacted by temperature. Overall, it can be assumed that a change of one degree Celsius in the SensoLuxTM® fiber embedded in a "free"
concrete will generate a change of deformation measured by the Brillouin effect of about 30 micro deformations. That is why we must systematically measure in the same optical cable the Raman backscattering temperature distribution on a multi-mode fiber distinct from the single-mode fiber used
for Brillouin backscattering.

Therefore, with an accuracy of ± 0.1 ° C, we can measure mechanical deformations with an accuracy of about ± 15 micro deformations. The variations of deformations measured during the digging phase of the gallery are presented by profile on a color scale. (Figure 8)

In figure 9 we observe a phenomenon of convergence that is very weak and fairly regular on the vault. We found in these digging tests that the profile of the tunnel was very slightly deformed in the instrumented areas.

The analysis of the deformations of the structure in the long term is possible thanks to the installed fibers. To interpret these measurements, the “free” part related to concrete shrinkage will have to be evaluated if one wishes to calculate the mechanical stresses that evolve over the long term.

Conclusions

Fiber-optic distributed measurement has a unique ability to instrument large-scale structures, compared to conventional gauges or extensometers; optical fiber provides comprehensive measurement over a wide area to diagnose complex deformation phenomena, in heterogeneous environments.

This technology makes it possible to represent the spatial deformation state of the tunnel in real time, which is accessible by the work manager on a secure server.

The small size of the sensor allows it to follow the behavior of the structures from the inside of the materials (keying mortar, voussoirs, shotcrete …).
Its ability to detect and locate punctual phenomena (cracking, water inflow, fountains, etc.) is also noteworthy.

References

Lamour V., Haouas A., Dubois J.-Ph. & Poisson R. (2009) Long term monitoring of large massive concrete structures : cumulative effects of thermal gradients. 7th International Symposium on Non
Destructive Testing in Civil Engineering (NDTCE’09), Nantes, France.

Read more...

Charles de Gaulle Express Project

The Charles De Gaulle Express project is part of  Le Grand Paris Express which revolves around a public transportation network plan consisting of four metro lines around Paris, along with the expansion of two existing lines. The goal of this railroad project in the Ile-de-France region is to connect the Paris-Est train station to the Paris-Charles-de-Gaulle airport in less than 20 minutes. With a budget of more than 1.7 billion euros, the CDG Express is a major project that will shape Parisian mobility in the near future.

 

Once again, Cementys has been recognized for our reliability, responsiveness and technological solutions by providing a high-precision automatic topographical monitoring for one of the largest Parisian railway projects dating back to the beginning of the century while accompanying Eiffage and SNCF Réseau on one of the first works of the CDG express in Tremblay, Seine-Saint-Denis.

 

Cementys monitors, in real time, the impact of work on the RER line B. We do this through our 6 fully automated theodolites, 500 prisms located on the rails and our innovative data processing softwares.

Over the course of 3 months, Cementys will accompany SCNF Réseau on monitoring the impact of the sewage clean-up operation on the RER B North line.

Read more...

New Monitoring project : Grand Paris Express Line 16 Work Package 2

We are pleased to collaborate with Salini ImpregiloNGE consortium on the construction of the new line 16 of Grand Paris Express Project.

The monitoring job of Line 16 Work Package 2 deals with the Tunnel portion between the connection structure at the Aulnay operation center and the Bel-Air in Chelles (Seine Saint Denis), that will be commissioned by 2024.

The construction of a 11.1 km tunnel and four railstations – Aulnay, Sevran-Beaudottes, Sevran-Livry, Clichy-Montfermeil will be monitored in real time through our latest technologies (DeltaLOG robotized total stations, Vibrating Wire Sensors, LoraLOG data loggers, Fiber Optic Sensors, InSAR and GNSS remote sensing, THMInsight real time database)

Monitoring tasks are numerous :

  • Existing Building subsidience monitoring in the geotechnical influence area during construction works
  • Existing Building subsidience in the geotechnical influence area during tunelling (TBM)
  • Railroad geometry monitoring in the geotechnical influence area during tunneling
  • Existing underground Network monitoring in the geotechnical influence area during tunneling
  • Geotechnical Monitoring with the our GEOTYS subsidiary (In place Inclinometers, Extensometers, Piezometers)
  • Data Visualization and Interpretation though our THM-Insight database with 24/7 alarm systems (data managers in France and Vietnam)

Combined with other Grand Paris Express & EOLE projects, Cementys will monitor 11 TBM (13 drives) and the construction of more than 40 railway stations or ventilation shafts around Paris in the next couple of years.

Read more...

Offshore Riser Tension Monitoring

Late 2018, Cementys engineering team finalized its first deepwater riser real-time monitoring project.

The instrumentation operation took place in the Gulf of Mexico, on an existing Tendon Leg Platform (TLP), exposed to frequent Loop Current events.

Vortex Induced Vibrations (VIV) of a riser in severe loop currents can decrease the riser fatigue life and thus may have significant cost impacts.

VIV suppression devices are essential to minimize those events, eventhough VIV behavior presents one of the biggest simulation uncertainties facing the riser engineers.

This shows that full scale monitoring is essential to improve our understanding of VIV response of risers equipped with VIV suppression devices.

Cementys designed and set up a system to measure the strain variation of Top Tension Risers (TTRs), focusing on one of the regions where fatigue damage is expected to be greatest : the top of the riser, around 20 meters above sea level. The objective is to compare efficiency of the different currently installed VIV suppression devices and estimate the induced fatigue damage during a single Loop Current event.

The system is based on the ExtensoVib vibrating wire strain sensors, allowing for accurate strain measurements, with a resolution below 0.1 microstrain.

Each riser is equipped with six ExtensoVib extensometers with a 60° separation: 3 sensors for one monitoring system, 3 others for redundancy.

Sensors are connected to two dynamic data loggers, making a high frequency measurement (20Hz) for high frequency riser oscillations.

 

ExtensoVib risers are bonded directly on the riser metal surface, without the coating, to measure directly the surface strain, without drift or recalibration for a period of several years.

Cementys has a remote access to check the incoming data. The real-time axial loads and bending moments are analyzed in correlation with loop current conditions to follow the fatigue damage accumulation.

ExtensoVib sensors represent a cost-effective solution for riser monitoring projects.

Read more...