Fiber Optic Strain Measurement Research Articles

A Numerical Model for Analyzing Mechanical Slippage Effect on Crosswell Distributed Fiber-Optic Strain Measurements During Fracturing

Summary Distributed fiber-optic (FO) strain measurements have become a widely used method for monitoring and diagnosing far-field strain induced during hydraulic fracturing (HF). However, field observations have revealed that the mechanical decoupling between the fiber and the rock often happens. Despite this phenomenon, much of the current research assumes that the fiber cable remains mechanically coupled to the rock formation. To improve the accuracy and reliability of field data interpretation, it is essential to better understand the effect of fiber decoupling on strain responses. The objective of this study is to simulate strain responses while taking into account the impact of fiber slippage. In this study, we developed a numerical model that allows for accurate prediction and analysis of strain responses of crosswell distributed FO experiencing mechanical decoupling. Our model first uses the 3D displacement discontinuity method to calculate displacement fields induced by hydraulic fractures. The model then accounts for the occurrence of slippage and calculates the fiber displacement distribution with slippage effect. Additionally, the model can predict fiber strain distribution in cases where the stick-slip behavior is present. To better understand the impact of friction on strain and strain-rate signals, we conduct a sensitivity analysis. This allows us to explore the effects of different friction levels on the signals, which can be critical in achieving more accurate and reliable results. Our model results reveal several key observations. First, when fiber slippage is detected, we typically observe a broadening of the extension zone near the fracture on the strain/strain rate waterfall plots, which cannot provide a well-defined fracture domain corridor. The simulated strain rate pattern matches the field observation. Additionally, the size of the decoupling zone increases as the friction between the fiber and formation decreases. Also, the decoupling generates negative cumulative strain change after a certain period of closure. Furthermore, when the dynamic friction is smaller than the static friction, sudden strain releases appearing as sticking portions were visible in the strain/strain-rate waterfall plots, indicating the occurrence of stick-slip behavior in the fiber. We also observed that the sudden strain releases in the strain rate waterfall plot occur more frequently when the maximum fiber strain gradient is smaller. Moreover, the longer sticking portion was observed when the dynamic/static friction ratio was smaller. The key innovation of this study is the development of a forward numerical model that simulates strain responses while taking into account the coupling effect between fiber and formation. To the best of our knowledge, this is the first time that such a model has been presented. Overall, our study provides an important contribution to the field of HF monitoring by enabling a more accurate and reliable interpretation of strain signals, which can ultimately improve the safety and effectiveness of HF operations.

Fiber-Optic Strain Measurements Aid Fracture Characterization

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 3864861, “Geomechanics Modeling of Strain-Based Pressure Estimates: Insights From Distributed Fiber-Optic Strain Measurements,” by Wei Ma and Kan Wu, Texas A&M University, and Ge Jin, Colorado School of Mines. The paper has not been peer reviewed. _ The combination of Rayleigh frequency shift distributed strain sensing (RFS-DSS) and pressure-gauge measurements has been reported recently in field applications. The main objective of the study detailed in the complete paper is to investigate the relationship between strain change and pressure change under various fractured reservoir conditions and provide guidelines for better using this novel strain/pressure relationship to estimate conductive fractures and pressure profiles. Introduction With a spatial resolution of 20 cm and a sensitivity of less than 1 με, RFS-DSS can measure mechanical strain changes along the fiber with higher accuracy and sensitivity than low-frequency distributed acoustic sensing measurements. The field applications of RFS-DSS have improved the understanding of near-well and far-field fracture characteristics and the relationship between stimulation and production in unconventional reservoirs. Although some numerical modeling works have been conducted to study the mechanisms of RFS-DSS data sets, the sensitivity, or influencing factors, of the relationship between strain change and pressure change along the fiber are still unclear. In this work, the authors use a coupled geomechanics and fluid-flow simulator to simulate the strain change and pressure change measured along the producing and monitoring wells during both stable production and shut-in periods. Methodology A 3D multilayer reservoir model with dimensions of 300×400×55.82 m was created using Permian Basin data sets. The reservoir was discretized into 553×129×5 gridblocks. To ensure accurate simulation of field RFS-DSS measurements, the mesh was refined around the fracture and wellbore. The fracture width was set to be the same as the RFS-DSS spatial resolution (0.2 m), and the grid size was set to 5 m except for the refined region. As shown in Fig. 1, the reservoir had 11 perforation clusters along the producing well and the monitoring well was 65 m away from the producing well. A fiber cable was installed on both wells to measure the RFS-DSS data set. The producing well was operated for 240 days before being shut in for 4 days, followed by a 1-day reopening and then continued production for 1 year. The pressure decline was 30–40 psi during the 1-day stable production period. Note that the moment after producing 239 days was taken as the reference time to calculate the strain change during the 1-day production (239–240 days) and the moment after 240 days as the reference time to calculate the strain change during the shut-in period (240–244 days).

The Challenges and Advantages of Distributed Fiber Optic Strain Monitoring in and on the Cementitious Matrix of Concrete Beams

Distributed fiber optic strain measurement techniques have become increasingly important in recent years, especially in the field of structural health monitoring of reinforced concrete structures. Numerous publications show the various monitoring possibilities from bridges to special heavy structures. The present study is intended to demonstrate the possibilities, but also the challenges, of distributed fiber optic strain measurement in reinforced concrete structures. For this purpose, concrete beams for 3-point bending tests were equipped with optical fibers on the reinforcement and concrete surface as well as in the concrete matrix in order to record the strains in the compression and tension zone. In parallel, an analytical approach based on the maximum strains in the uncracked and cracked states was performed using the Eurocode 2 interpolation coefficient. In principle, the structural design correlates with the measured values, but the strains are underestimated, especially in the cracked zone. During load increase, structural distortions in the compression zone affected the strain signal, making reliable evaluation in this zone difficult. The information content of distributed fiber optic strain measurement in reinforced concrete structures can offer tremendous opportunities. Future research should consider all aspects of the bond, sensor selection and positioning. In addition, there is a lack of information on the long-term stability of the joint and the fiber coating, as well as the effects of dynamic loading.

Space tether system state estimation using distributed fibre optical strain measurement, inverse finite element method and Kalman filter

This paper proposes a robust finite element Kalman filter for the state estimation of flexible space tether systems, which are critical for space exploration and satellite missions. Due to the limited access to placing sensors along the flexible tether to measure them directly, the measurement of the state of these systems is challenging. To address this, a fully expandable, scalable, and robust approach is developed that fuses position/velocity sensors at tether ends, distributed fiber optic strain measurement along the tether, and a position/velocity estimation by a finite element-based Kalman filter to estimate the state of the tether system. The proposed estimator is mathematically proven to be observable, stable, and robust. The effectiveness of the estimator is demonstrated through numerical simulations of a space tether system in orbit, and the method can be extended to any flexible body system. The results of the simulations show that the proposed approach can accurately estimate the state of the tether system, even in the presence of disturbances and noise.

Experimental study on uplift mechanism of pipeline buried in sand using high-resolution fiber optic strain sensing nerves

Reliable assessment of uplift capacity of buried pipelines against upheaval buckling requires a valid failure mechanism and a reliable real-time monitoring technique. This paper presents a sensing solution for evaluating uplift capacity of pipelines buried in sand using fiber optic strain sensing (FOSS) nerves. Upward pipe-soil interaction (PSI) was investigated through a series of scaled tests, in which the FOSS and image analysis techniques were used to capture the failure patterns. The published prediction models were evaluated and modified according to observations in the present study as well as a database of 41 pipe loading tests assembled from the literature. Axial strain measurements of FOSS nerves horizontally installed above the pipeline were correlated with the failure behavior of the overlying soil. The test results indicate that the previous analytical models could be further improved regarding their estimations in the failure geometry and mobilization distance at the peak uplift resistance. For typical slip plane failure forms, inclined shear bands star from the pipe shoulder, instead of the springline, and have not yet reached the ground surface at the peak resistance. The vertical inclination of curved shear bands decreases with increasing uplift displacements at the post-peak periods. At large displacements, the upward movement is confined to the deeper ground, and the slip plane failure progressively changes to the flow-around. The feasibility of FOSS in pipe uplift resistance prediction was validated through the comparison with image analyses. In addition, the shear band locations can be identified using fiber optic strain measurements. Finally, the advantages and limits of the FOSS system are discussed in terms of different levels in upward PSI assessment, including failure identification, location, and quantification.

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.

Assessing the reliability of the cargo deck of a hovercraft through fiber optic strain measurements

While traditional sensing systems can suffer from low spatial sampling density and often face significant operational challenges in the maritime environments routinely experienced by naval craft, fiber optics provide a distributed sensing solution that is insensitive to many of these environmental stressors. Through the use of fiber optic sensing systems, strain data was collected along the cargo deck of two naval hovercraft during various maneuvers and loading conditions. This work presents a methodology for translating those strain measurements into deflection estimates, and ultimately reliability analyses, through optimization and analytical modeling tools. Specifically, by treating the cargo deck of the hovercraft as a large, thin plate, direct relationships between strain, bending curvature, and deflection can be reliably established. Comparisons of probability distributions of maximum absolute deflections experienced during set maneuvers and loading conditions reveal differences in the response between the two craft. These differences are further explored in the context of reliability indices based on limit deflections.

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.

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.

A mechanical model to interpret distributed fiber optic strain measurement at displacement discontinuities

Distributed fiber optic (strain) sensing, which provides the unique advantage of sensing damage (e.g. cracking) at locations that are not known a priori, has been increasingly used in civil engineering. Quantitative crack measurement requires the translation of a discontinuous displacement field at the crack to a continuous strain deformation in the fiber. The main purpose of this article is to develop a mechanical model to explain the fiber deformation in the presence of a displacement discontinuity. The proposed mechanical model is validated with experimental results from cable calibration tests and concrete cracking tests. The model is extended to simulate the effects of multiple closely spaced cracks on fiber optic strain measurement, and this model is used to create an algorithm to automatically distinguish multiple cracks in distributed fiber optic (strain) sensing strain distributions. Using the model and two shape parameters, kurtosis and standard variation, the effects of cable properties (i.e. shear stiffness between cable and fiber, cable radius, elastic modulus, and interface cohesion) on the shape of fiber optic strain distributions across cracks are also quantified. The results provide an indication of beneficial cable properties for various measurement objectives.

Flexible gauge length intrinsic fiber-optic strain sensor using broadband interferometry [Invited

Monitoring strain is important in precision engineering applications that require maintaining the precise alignments of structures over time, such as those found in machine tools and metrology frames. We present a fiber-optic strain measurement technique based upon broadband interferometry that is variously configurable in terms of gauge length and sensitivity. This is achieved by the use of an unbalanced interrogation interferometer configuration that alleviates the cavity length limit imposed by the temporal coherence of the system. We also demonstrate that dispersion in fiber sensors based on intrinsic Fabry-Perot interferometers must be considered to optimize performance. The possibility of multisensor interrogation using the same optical system is also reported.

Comparison of different fiber coatings for distributed strain measurement in cementitious matrices

Abstract. Distributed fiber optic strain measurement based on Rayleigh scattering has recently become increasingly popular in automotive and mechanical engineering for strain monitoring and in the construction industry, especially structural health monitoring. This technology enables the monitoring of strain along the entire fiber length. This article addresses integrating optical fibers of different coatings into the concrete matrix to measure the shrinkage deformations. However, previous studies do not give a clear statement about the strain transfer losses of fiber optic sensors in this application. In this context, three different coating types were investigated regarding their strain transfer. The fibers were integrated into fine-grained concrete prisms, and the shrinkage strain was compared with a precise dial gauge. The analysis shows a high correlation between the reference method and the fiber measurement, especially with the ORMOCER® coating. The acrylate coating used is also consistent in the middle area of the specimen but requires a certain strain introduction length to indicate the actual strain. The main result of this study is a recommendation for fiber coatings for shrinkage measurement in fine-grain concretes using the distributed fiber optic strain measurement. In addition, the advantages and disadvantages of the measurement method are presented.

Methods of mechanical noise impact suppression during streamer towing process using fiber Bragg gratings

Subject of Research. The paper deals with the problem of seismic streamers towing process during marine geological exploration. Classification of the mechanical noise effects during towing process is presented. A brief overview of the main technical solutions is performed for suppression of each impact type. Method. A method is proposed for calculating the parameters of elastic sections for suppression of noise effects, which takes into account towed body tension level. We show the possibility and prospects of applying for this purpose a fiber-optic strain measurement system based on Bragg gratings inscribed in the G.657.A1 standard SMF-28 telecommunication optical fiber with a sampling rate of 5 kHz as part of a fiber-optic seismic streamer design. Main Results. Experimental data on the prototype research are given. With the relative elongation of the uniform-strength bar in the range up to 1030 μm/m, an average sensitivity of 0.68 pm (μm/m) is achieved, and the spread of values is ±1 pm. It is shown that the work of fiber Bragg gratings is practically non-inertial itself. The response time and dynamic range of the sensor amplitude is largely determined by the properties of metal bars and tool set where optical fibers are fixed. Practical Relevance. Application of the method provides opportunities for the solid construction of the towed body. The possibilities for calculation of the elastic section parameters to the specific conditions of marine geological exploration and continuous state monitoring of the towed body are presented. The method makes it possible to control the quality of the recorded seismic data and partially compensate the undesirable effects during signal processing. Proposed measures will increase significantly the accuracy and reliability of the obtained geological data.

Fiber-optic simultaneous distributed monitoring of strain and temperature for an aircraft wing during flight.

We applied a fiber optic distributed simultaneous strain and temperature measurement technique to the structural monitoring of the main wing of a middle-sized passenger jet aircraft during flight. We used 40 10cm long fiber Bragg gratings (FBGs), inscribed in a highly birefringent polarization-maintaining fiber. The FBGs were interrogated by optical frequency domain reflectometry, which could measure Bragg wavelength distributions at a sampling rate of 151Hz. The simultaneous measurement technique could detect structural behaviors of the wing during flight under temperature-changing conditions. In addition, we discuss the effect of the polarization mode-coupling and the apparent position shift of the FBGs over time, which occurred during flight.

A kinematic method for calculating shear displacements of landslides using distributed fiber optic strain measurements

Deformation monitoring of landslides is of great significance to characterize and understand their evolutions. Recently the distributed fiber optic strain sensing (DFOSS) technique has emerged as a powerful tool for landslide monitoring by enabling distributed and real-time measurement along a sensing optical fiber (SOF) over dozens of kilometers with high strain accuracy. However, the correlation between landslide displacements and axial strains exerted on an SOF remains elusive. Here we present a preliminary attempt to calculate shear displacements of landslides based on distributed strain measurements via a kinematic method. Parametric studies on the sliding direction, width of shear zone and magnitude of shear displacement indicate that, under certain circumstances, the shear displacements of a landslide can be well estimated without going much deep into the pattern of shear zone or the slope mass–SOF coupling condition. The proposed calculation method is validated through field shear tests of geologic granular materials, and has immediate application to the analysis of the Majiagou landslide, Three Gorges Reservoir region, China. The distributed strain measurements captured by a borehole-embedded SOF allow two sliding surfaces of this landslide to be located; one occurred at the contact between surface deposits and bedrock, whereas the other was the main sliding surface occurring within bedrock. The shear displacements along the main sliding surface are calculated using the proposed method. It is revealed that the landslide responds more aggressively to the fluctuation of reservoir water level than to the rainfall over a 1-year period.

Shape Sensing Monitoring System Based on Fiber-Optic Strain Measurements: Laboratory Tests

In the context of the characterization of the shape of flying sails whilst racing, a novel method for real-time form-finding of flexural beams based on strain measurements has been developed and implemented. The very large displacements and deformations occurred on the beam pose specific demands both to the algorithm for shape detection and to the materials and sensors to use in the practical implementation, which are described and addressed in this paper. The developed method is implemented by installation of fiber Bragg grating (FBG) sensors on a beam, which are used to evaluate the curvature at specific cross-sections. The deformation curve is reconstructed by integration of local curvatures and interpolation. The effectiveness of the developed method is verified by tests on a physical model. An image-based monitoring system is further employed to validate the concept and the tests conducted with the FBG sensors. This allows the performance of an additional series of sensitivity tests in order to define the most relevant measurement parameters and characterize the accuracy of the developed method.

分布式光ファイバーセンサーによる静水圧環境下におけるひずみ測定

分布式光ファイバーセンサーによる静水圧環境下におけるひずみ測定

Distributed Strain Behavior of a Reinforced Concrete Bridge: Case Study

To effectively assess and manage aging infrastructure, sensing technologies that produce accurate and useful quantitative data are required. Distributed fiber optic strain measurement systems are one potential technology that could fulfill this requirement. This case study investigates the performance of a distributed fiber optic strain measurement technology with high accuracy and spatial resolution during a load test on a RC bridge. The measurements from the fiber optic system are compared with conventional strain gauges and linear transducers, and good agreement between the sensors was found. The fiber optic measurements are also used to examine the bridge support conditions as well as the load distribution between beams. The results show that assumptions made about the support conditions during design do not match the actual bridge behavior, although the load distribution between beams is as expected.

Dynamic fiber optic strain measurements and aliasing suppression with a PDA-based spectrometer

Dynamic fiber optic strain measurements at 3.3 kHz were performed employing a photodiode array spectrometer. The occurrence of aliasing components in the spectral data is investigated, and their dependence on the spectrometer integration time is analyzed. A theoretical model describing the aliasing components is discussed and could be confirmed by the experimental data. It is shown that the aliasing components can be partly suppressed by maximizing the duty cycle of the spectrometer.

Fatigue testing of a composite propeller blade using fiber-optic strain sensors

The performance of surface-mounted extrinsic Fabry-Perot interferometric (EFPI) sensors during a seventeen-million-cycle, high-strain fatigue test is reported. Fiber-optic strain measurements did not degrade during the test. The sensors were applied to a composite propeller blade subject to a constant axial load and a cyclic bending load. Strain measurements were taken at four blade locations using two types of EFPI sensors and co-located electrical resistance strain gages. Static and dynamic strain measurements were taken daily during the 65 days of this standard propeller-blade test. All fiber-optic sensors survived the fatigue test while most of the resistive gages failed. The suitability of fiber-optic monitoring for fatigue testing and other high-cycle monitoring is demonstrated.