Distributed Fiber Optic Strain Research Articles

Evaluation of the Structural Health Monitoring Results of the Applied Fiber Optics in the Pile-Raft Foundations of a High-Rise Building

Fiber optics are increasingly being used in structural health monitoring applications. In addition to the well-known advantages, they most reliably transmit the stress–strain condition of the structure in the long term. This article presents the results of structural health monitoring on the embedding of fiber optics in the upper layer of the raft over the entire area of a high-rise building in Astana city (Kazakhstan), including the 75-storey 320 m block R. Temperature and strain data were collected constantly via a distributed fiber optic strain sensing system (DFOSS). The analysis of strains in fiber optic chainage indicated that the compression strains were observed over the entire area of the building, with values from −20 to −40 µε and with progression of the compression loaded up to −80 µε along the core wall closer to the eastern facade of the building. Tension strains were concentrated along the extreme axes of the building, with a predominance in the southern part of the R block, as well as in the immediate vicinity of the columns, in a range of −20 to −40 µε, with separate spots from −40 to −60 µε. Individual patches of tension strains near the columns have been explained by the increased deflection of the raft under the application of a concentrated load (columns). Fiber optic monitoring results at the time of testing did not exceed the permitted values for high-rise building operation; however, they characterized the general picture of the strain in the raft plane and made it possible to determine the initiation of cracks in concrete at an early stage.

Distributed viscosity and flow velocity measurements using a fiber-optic shear stress sensor

The understanding and precise prediction of fluid and solid displacement is of great interest in many technical applications. Density and viscosity are two key parameters that govern process mechanisms. The possibility to measure transient processes over longer distances is desirable. We present a novel distributed shear stress sensor that allows to derive fluid rheological parameters such as the viscosity along a fiber-optic cable being exposed to a moving medium. This works because flow velocity and fluid viscosity directly translate to a shear stress and consequently to a tensile strain on the fiber optic cable. Using the technology of fiber-optic distributed strain sensing, strain changes (and temperatures) are detected in real-time at any location along the fiber. Given the cable mechanical properties and geometry of the flow path, the strain translates to a shear stress which can be correlated to either the flow velocity or fluid viscosity. We derive a theoretical characterization of the sensor based on the principles of fluid mechanics. Also, we perform laboratory experiments with the sensor and demonstrate that we can distinguish differences of 1 mPa s dynamic viscosities in a range as low as 1 – 7 mPa s. In the next phase, we are implementing this sensor into a real working environment in a wellbore application to investigate the applicability of this novel sensor technology.

Multi-physical field joint monitoring of buried gas pipeline leakage based on BOFDA

Determining the leakage location in an underground gas pipeline is a serious technical problem. Distributed fiber optic testing technology based on Brillouin optical frequency domain analysis (BOFDA) can be used for monitoring the change in the strain and temperature of the surrounding media caused by the gas leakage. In this study, a point source leakage model was built. The distributed fiber optic strain and actively heated fiber optic were installed around the pipeline to monitor the gas pipeline leakage. Variations in the strain and temperature characteristics of the pipeline under different gas pressure conditions were found to be in the range of −392–402 μϵ and 3.5 °C during the 1.0 MPa gas leakage, respectively. When the soil model was loaded with a gas leakage pressure of 0.3–1.0 MPa, a significant variation in strain and temperature was noticed. The copper mesh actively heated fiber optic can effectively identify the location of the leak. Meanwhile, the heating rate of the actively heated fiber optic cables in different media environments were obtained and ranked from fast to slow: air, followed by general soil, followed by leakage soil. This led to the generation of new technology to detect the leakage location of buried gas pipelines through actively heated optical cables.

Experimental investigation on pipe-soil interaction due to ground subsidence via high-resolution fiber optic sensing

Understanding the pipe-soil interaction is critical for the design and safety assessment of buried pipelines subjected to large differential ground motion. Laboratory model tests were conducted to study the mechanical response of polyvinyl chloride pipes to ground subsidence. The optical frequency domain reflectometry technique was employed to perform distributed strain sensing of the pipe-soil system. An analytical method was proposed to estimate greenfield settlements in terms of fiber optic strain measurements. The pipe-soil interaction mechanism due to ground subsidence was investigated concerning four aspects, i.e., displacement profile of the pipe-soil system, pipe bending, soil arching, and interfacial shearing. Accounting for three-dimensional soil arching effects, a limit equilibrium solution for the earth pressure imposed on the pipe crown was given. This solution was developed from the two-dimensional Terzaghi soil arching theory and was validated based on the measured earth pressures. The feasibility of distributed fiber optic strain sensing technique in monitoring pipe-soil deformation was discussed through the comparison with the data of displacement transducers. In addition, the change in relative pipe-soil stiffness with increasing ground movements was studied based on the bending behavior of the pipe. A simplified method was given for quantifying the axial pipe-soil interaction based on longitudinal strains. This paper presents new inspiration on the intricacies of pipe-soil interaction using high-resolution distributed fiber optic sensing, which can be applied to structural health monitoring and optimum design of buried pipelines.

Monitoring Flexure Behavior of Compacted Clay Beam Using High-Resolution Distributed Fiber Optic Strain Sensors

Abstract Understanding the mechanical characteristics of clayey soil is critical to estimate the stability of clay-related geotechnical engineering works. However, the distributed information of the internal deformation of clay is hard to capture through traditional methods. In this paper, a set of four-point bending tests were performed on a compacted clay beam instrumented with fiber optic strain sensors to investigate the flexure and cracking behaviors. The optical frequency domain reflectometry technique with high spatial resolution and precision was employed to establish a distributed strain sensing (DSS) system in this study. The results show that the DSS system exhibits a high accuracy in the collection of the clay deformation in terms of compression, tension, and cracking. The comparison of the internal strain profiles with the observation of particle image velocimetry analysis system verifies the feasibility of fiber optic sensing technology in the quantitative assessment of clay deformation. The performance of fiber optic sensors in the rebound deformation and creep deformation monitoring was preliminarily issued in the study. In addition, the progressive failure on the fiber–soil interface was analytically studied to evaluate the deformation compatibility between the fiber and surrounding clayed soil. The conclusions drawn in this paper can help to guide the state interpretation of clayey soil instrumented with fiber optic sensors.

Fibre optic measurements and model uncertainty quantification for Fe-SMA strengthened concrete structures

This investigation aimed at studying a novel strengthening method using ribbed memory-steel bars by means of quasi-static load experiments and computational modelling of large-scale concrete members in flexure. The first part of the study focussed on the strain in activated and non-activated memory-steel bars measured with a distributed fibre optic measurement system during external loading of strengthened slabs. In the second part, a computational cross-section analysis model was developed and enhanced with two uncertainty quantification methods: sensitivity analysis and Bayesian model calibration.The distributed fibre optic strain measurements enabled detailed monitoring of the peak strains owing to concrete cracking. The strain data was post-processed to obtain axial stress and bond shear stress in proximity to the largest bending crack. The sensitivity analysis provided the relative importance of the calibrated input parameters and their interdependencies. Model calibration quantified the uncertain parameters such as prestress, concrete parameters, and memory-steel constitutive laws, resulting in good accordance between the model and the experiments. The models were validated by independent strain measurements. Parametric studies enabled prediction of the effects of the investigated strengthening method on similar concrete members with different prestress, materials and geometries.

The Effect of Fault Architecture on Slip Behavior in Shale Revealed by Distributed Fiber Optic Strain Sensing

AbstractWe use Distributed Strain Sensing (DSS) through Brillouin scattering measurements to characterize the reactivation of a fault zone in shale (Opalinus clay), caused by the excavation of a gallery at ∼400 m depth in the Mont Terri Underground Laboratory (Switzerland). DSS fibers are cemented behind casing in six boreholes cross‐cutting the fault zone. We compare the DSS data with co‐located measurements of displacement from a chain potentiometer and a three‐dimensional displacement sensor (SIMFIP). DSS proves to be able to detect in‐ and off‐fault strain variations induced by the gallery excavated 30–50 m away. The total permanent displacement of the fault is ∼200 microns at rates up to 1.5 nm/s. DSS is sensitive to longitudinal and shear strain with measurements showing that fault shear is concentrated at the top and bottom interfaces of the fault zone with little deformation within the fault zone itself. Such a localized pattern of strain relates to the architecture of the fault that is characterized by a thick “layer” including an anastomosing network of polished surfaces where clay‐rich rock splits into progressively smaller flakes conferring the entire zone very little cohesion and friction (weak zone), and favoring slipping at the edges of the “layer”. Overall, DSS shows that slow slip may activate everywhere there is a weak fault within a shale series. Thus, our work demonstrates the importance of shear strain on faults caused by remote loading, highlighting the utility of DSS systems to detect and quantify these effects at large reservoir scales.

Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring

Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring

Design a Building Information Modeling (BIM) project concept in combination with foundation monitoring on the example of the Abu Dhabi Plaza construction project

With the advent of new technology in the monitoring of buildings and highrise structures it has become possible to monitor damage and defects in existing structures and infrastructures. The need for timely detection of critical points of building structure defects through the use of sensors, as well as the lack of description tools for understanding, visualizing and documenting sensor output data increasingly encourages designers, builders and facility managers to use the capabilities of such a powerful system as the Building Information Modeling System (BIM). The primary goal of any construction project is safety for people, which can only be ensured by proper monitoring of building health, where the presentation and management of the received information about changes in the system and structure can be ensured by using BIM. The purpose of this paper is to describe the application of a distributed fiber optic strain gauge sensor system, and the possibility of integrating the data obtained from the sensors into the BIM system.

Real-time monitoring of the development of brittle fracture in hard rock tunnels based on physical model test

Brittle fracture in deep hard rock tunnels in the Jinping laboratory, China, can often endanger the stability of engineering structures. This study describes physical model testing and real-time observations on the evolution process of v-shaped brittle fracture, failure characteristics, onset conditions, and influencing factors in hard rock tunnels. Real-time observations are presented integrated a high-speed camera, acoustic emission system, a digital image correlation, and distributed fiber optic strain sensing technologies. The results show that the process of spalling failure can be divided into two main stages, namely, the instantaneous cracking process in the sidewall surface, and the spalling process of the thin rock chips developing progressively from the excavation boundary to the far field, finally forming stable v-shaped notches. The deviatoric stress of spalling failure in the physical model test satisfies the constant deviatoric stress criterion. The critical strain at spalling failure is consistent with that calculated by the Hoek–Brown strength criterion. Spalling is a form of brittle failure caused by deferential tensile strain. The working face effect is observed in the physical model excavation process. The highest strain caused by overloading occurs near the free surface of the sidewall. The relationship between stress and geometric similarity constants in deep rock engineering is derived. The failure characteristics revealed in the physical model tests are verified by numerical simulations based on cellular automaton theory, which agree well with the experimental results. The research findings can provide theoretical support for the prevention and control of spalling disasters in deep engineering practice.

Distributed fiber optic strain sensing of bending deformation of a well mockup in the laboratory

Well integrity is critical to the safety and success of subsurface energy exploration and management, as leakage of fluids from subsurface reservoirs is often induced by compromised wells. This study investigates bending deformation of a mockup of an oil/gas well that is subjected to loads expected in deviated wells under reservoir compaction and assesses the feasibility of utilizing distributed fiber optic strain sensing to monitor the deformation. A well mockup, which consists of outer and inner steel pipes with a cemented annulus, is tested under three-point bending loading and its strain and curvature development is monitored by Brillouin optical time domain reflectometry/analysis (BOTDR/A) as well as optical frequency domain reflectometry (OFDR). The primary objective of this research is to assess the strain sensing performance of newly fabricated fiber optic cables and to identify key cable characteristics which could improve the quality of distributed strain measurements with BOTDR/A. Results show that the tight-buffered cable is best suited for strain sensing as its maximum errors in the strain measurement were −36% and −24% against conventional sensors at the maximum elastic and plastic bending loads, respectively, whereas those of the non-tight-buffered cable were −45% and −71%, respectively. Similar trends were obtained in the bending curvature measurement. The detailed design of the tight-buffered cable is presented to elucidate key characteristics of such a cable, which will facilitate accurate distributed strain sensing in oil and gas wells.

Swelling-Based Chemical Sensing With Unmodified Optical Fibers

We use distributed fiber optic strain sensing to examine swelling of the fiber’s polymer coating. The distributed sensing technique that uses unmodified low-cost telecom fibers opens a new dimension of applications that include leak detection, monitoring of water quality, and waste systems. On a short-range length scale, the technology enables “lab-on-a-fiber” applications for food processing, medicine, and biosensing for instance. The chemical sensing is realized with unmodified low-cost telecom optical fibers, namely, by using swelling in the coating material of the fiber to detect specific chemicals. Although generic and able to work in various areas such as environmental monitoring, food analysis, agriculture or security, the proposed chemical sensors can be targeted for water quality monitoring, or medical diagnostics where they present the most groundbreaking nature. Moreover, the technique is without restrictions applicable to longer range installations.

Distributed Fiber Optic Strain Sensing for Geomechanical Monitoring: Insights from Field Measurements of Ground Surface Deformation

In recent years, distributed fiber optic strain sensing (DFOSS) technology has demonstrated a solution for continuous deformation monitoring from subsurface to surface along the wellbore. In this study, we installed a single-mode optical fiber cable in a shallow trench to establish an effective technique for ground surface deformation mapping. We conducted three experimental field tests (iron plate load, water tank filling up load, and airbag inflation) in order to confirm the strain sensitivity of DFOSS for static loads, dynamic overload, excavation, subsidence, and uplift. This paper also presents two installation methods to couple the fiber cable with the ground under various environmental conditions; here, the fiber cable was installed in a shallow trench with one part buried in the soil and another part covered with cement. Our results suggest that covering the cable with cement is a practical approach for installing a fiber cable for ground surface deformation monitoring. By combining this approach with wellbore DFOSS, accurate surface–subsurface deformation measurements can be obtained for three-dimensional geomechanical monitoring of CO2 storage and oil and gas fields in the future.

Numerical analysis and geomonitoring of behaviour of foundation of Abu-Dhabi Plaza in Nur-Sultan

In similarity to every civilization in history originated from the riverside, the city of Nur-Sultan, which the new capital of Kazakhstan developed around the Ishim River. During the last 25 years, many high-rise buildings supported by pile foundations are rising in Nur-Sultan. The paper presents the monitoring of development for the unique residential building of 310 m high with 4-story underground parking. The pile raft foundation (for block R) analyses had designed using Geocthenical soft, which can assess in precise of the deformations, occurs in soil. In the paper presidents, method monitoring raft foundations for block R, the height of this block R is 320 meters. Design of distributed fibre optic strain sensing system (DFOSS) has been employed for measuring the strain in civil engineering structures for over a decade. It is now harnessed worldwide for monitoring a wide range of structures slab pile foundation. Geotechnical monitoring data of adjacent building and utility systems settlement caused by the construction of presented high-rise buildings were compared to numerical modelling results, predicted, and permissible values.Finally, the raft foundation recommended for future high-rise buildings constructed of Nur-Sultan city on complex soils.

Repeatability precision error analysis of the distributed fiber optic strain monitoring

The precision error of the Brillouin optical time domain reflectometry (BOTDR)-based distributed fiber optic strain measurement is normally evaluated based on strain change from the initial zero strain state. In many structural health monitoring applications, however, there is initial strain caused by the installation process of a fiber optic sensor cable to a structure. Engineers are interested in the incremental strain profile from the initial strain profile to assess the performance of the structure. The initial strain profile is often not constant throughout the cable length due to the manner that the fiber optic cables are installed (e.g., gluing, clamping, or embedding). This uneven strain distribution causes precision error in the strain incremental values, which in turn leads to difficulty in data interpretation. This paper discusses why large initial strain variation (or initial strain gradient) increases the precision error of the subsequent incremental strain reading and how to evaluate the magnitude of such precision error. A relationship between strain gradient and precision error is demonstrated. A sectional shift method is proposed to minimize the precision error. Results from laboratory tests and a field case study show that the method can reduce the precision error approximately 50% when the strain gradient is large.

Fusion-based damage diagnostics for stiffened composite panels

Conducting damage diagnostics on stiffened panels is commonly performed using a single SHM technique. However, each SHM technique has both its strengths and limitations. Rather than straining the expansion of single SHM techniques going beyond their intrinsic capacities, these strengths and limitations should instead be considered in their application. In this work, we propose a novel fusion-based methodology between data from two SHM techniques in order to surpass the capabilities of a single SHM technique. The aim is to show that by considering data fusion, a synergy can be obtained, resulting in a comprehensive damage assessment, not possible using a single SHM technique. For this purpose, three single-stiffener carbon–epoxy panels were subjected to fatigue compression after impact tests. Two SHM techniques monitored damage growth under the applied fatigue loads: acoustic emission and distributed fiber optic strain sensing. Four acoustic emission sensors were placed on each panel, thereby allowing for damage detection, localization, type identification (delamination), and severity assessment. The optical fibers were adhered to the stiffener feet’ surface, and its strain measurements were used for damage detection, disbond localization, damage type identification (stiffness degradation and disbond growth), and severity assessment. Different fusion techniques are presented in order to integrate the acoustic emission and strain data. For damage detection and severity assessment, a hybrid health indicator is obtained by feature-level fusion while a complementary and cooperative fusion of the diagnostic results is developed for damage localization and type identification. We show that damage growth can be monitored up until final failure, thereby performing a simultaneous damage assessment on all four SHM levels. In this manner, we demonstrate that by proposing a fusion-based approach toward SHM of composite structures, the intrinsic capacity of each SHM technique can be utilized, leading to synergistic effects for damage diagnostics.

Experimental Study on the Displacement Sensor of Coal Roadways Roof Settlement Based on Distributed Fiber Optic Sensing

Roof displacement monitoring is an important means to ensure roof stability and prevent roof accidents of Coal Roadways (CRs). In this paper a new sensor of roof settlement displacement (RSD) monitoring of CRs' roof by the pre-stretched optical fiber (PSOF) laid out horizontally at fixed points was proposed. The principle of the sensor is to qualitively and quantitively characterize the RSD by the Fiber Optic Strain (FOS) which is measured based on distributed fiber optic strain measurement. Through theoretical analysis, the mathematical expression for quantifying RSD is derived. The correctness of the sensor is verified by the indoors FOS experiments and the field monitoring of CRs' roof in Yinying Coal Mine. This new sensor can be applied not only to the roof of CRs, but also to the displacement monitoring of other structures with the similar settlement presentation.

Laboratory Investigation of the Structural Performance of a Corrugated Steel Culvert under Increasing Cover Depth

The performance of large diameter corrugated steel culverts/pipes under varying cover depth up to deep burial has previously not been investigated in a laboratory setting due to limitations associated with the application of overburden pressures. Using the new deep burial simulator at Queen’s University, it is now possible to test flexible pipes up to 3 m in diameter up to a simulated burial depth of approximately 17 m. In this study, a 2-m diameter corrugated steel pipe (CSP) culvert was instrumented with displacement transducers, strain gauges, and distributed fiber optic strain sensors and was tested under increasing overburden pressure up to a total of 358 kPa (equivalent to a depth of 17.9 m of soil with a unit weight of 20 kN/m3). The diameter change values along the length of the culvert were within 10% of each other, suggesting that the simulator produces approximately uniform loading. The Canadian bridge design code produced thrust estimates within 10% of the measured response but did not predict the development of localized plastic hinges seen in the experiment.

Active Reservoir Pressure Management with Guidance and Surveillance by Distributed Fiber-optic Strain Sensing

CO2 injection and storage in aquifers can cause pore pressure build up and deformation of the aquifer-caprock system due to extra-fluid accumulation. Active reservoir pressure management, a concept that using brine production before or during CO2 injection to reduce reservoir pressure build up, has been suggested to avoid potential risks posed by geomechanical deformation during large-scale CO2 injection [1], [2]. Sedimentary formations usually develop heterogeneities in fluid properties and mechanical properties. When conducting CO2 injection or brine production, acquiring the in-situ information of pore pressure and fluid distribution and rock deformation would be much helpful for the project operating. In this presentation, we propose to give guidance and surveillance of the CO2 injection and brine production operations in real time using a distributed fiber optic strain sensing (DFOSS) system. We show the field and lab studies and numerical simulation results to demonstrate the feasibility and the advantages when using DFOSS for pressure management in a storage reservoir. Existing difficulties when using DFOSS are also presented.

Strain and displacement measurement based on distributed fibre optic sensing (DFOS) system integrated with FRP composite sandwich panel

An increase in application of advanced materials and high-tech monitoring systems is being observed in bridge engineering in recent years. The main goal is aimed at optimizing maintenance costs spent during entire lifecycle of a bridge. The paper describes the concept of the smart fibre reinforced polymer (FRP) sandwich deck panel, dedicated for newly-designed and renovated bridges. This panel is equipped with the distributed fibre optic sensing (DFOS) system, integrated with composite laminates. The DFOS system is provided to control strain and displacement measurement, further used in the structural health monitoring of a bridge. The DFOS system is characterized by the following features: accurate, reliable and distributed strain measurements, possibility of assessing shape and displacements, detection of local damages, reliable protection of the sensors, no need for surface installations, high durability, measurements from the real zero state of the structural element. Exemplary results of distributed fibre optic strain and displacement measurements performed under laboratory conditions on laminate specimens as well as the beam cut from the prototype panels are presented and compared to conventional measurements and FEM predictions.