Fiber Optic Strain Monitoring Research Articles

Real-time monitoring of rock fracture by true triaxial test using fiber-optic strain monitoring in adjacent wells

The real-time monitoring of fracture propagation during hydraulic fracturing is crucial for obtaining a deeper understanding of fracture morphology and optimizing hydraulic fracture designs. Accurate measurements of key fracture parameters, such as the fracture height and width, are particularly important to ensure efficient oilfield development and precise fracture diagnosis. This study utilized the optical frequency domain reflectometer (OFDR) technique in physical simulation experiments to monitor fractures during indoor true triaxial hydraulic fracturing experiments. The results indicate that the distributed fiber optic strain monitoring technology can efficiently capture the initiation and expansion of fractures. In horizontal well monitoring, the fiber strain waterfall plot can be used to interpret the fracture width, initiation location, and expansion speed. The fiber response can be divided into three stages: strain contraction convergence, strain band formation, and postshutdown strain rate reversal. When the fracture does not contact the fiber, a dual peak strain phenomenon occurs in the fiber and gradually converges as the fracture approaches. During vertical well monitoring in adjacent wells, within the effective monitoring range of the fiber, the axial strain produced by the fiber can represent the fracture height with an accuracy of 95.6% relative to the actual fracture height. This study provides a new perspective on real-time fracture monitoring. The response patterns of fiber-induced strain due to fractures can help us better understand and assess the dynamic fracture behavior, offering significant value for the optimization of oilfield development and fracture diagnostic techniques.

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.

Online evaluation method for the performance of in-service fiber optic strain monitoring systems

Online evaluation method for the performance of in-service fiber optic strain monitoring systems

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.

Fiber Optic Sensing for Geomechanical Monitoring: (2)- Distributed Strain Measurements at a Pumping Test and Geomechanical Modeling of Deformation of Reservoir Rocks

In this study distributed fiber optic sensing has been used to measure strain along a vertical well of a depth of 300 m during a pumping test. The observed strain data has been used in geomechanical simulation, in which a combined analytical and numerical approach was applied in providing scaled-up formation properties. The outcomes of the field test have demonstrated the practical use of distributed fiber optic strain sensing for monitoring reservoir formation responses at different regions of sandstone–mudstone alternations along a continuous trajectory. It also demonstrated that sensitive and scaled rock properties, including the equivalent permeability and pore compressibility, can be well constrained by the combined use of water head and distributed strain data. In comparison with the conventional methods, fiber optic strain monitoring enables a lower number of short-term tests to be designed to calibrate the parameters used to model the rock properties. The obtained parameters can be directly used in long-term geomechanical simulation of deformation of reservoir rocks due to fluid injection or production at the CO2 storage and oil and gas fields.

Process-induced strain and distortion in curved composites. Part I: Development of fiber-optic strain monitoring technique and analytical methods

Process-induced shape distortion is generated in curved composite parts. Although the distortion mechanisms have been understood through considerable previous efforts, they were validated by indirect methods such as shape measurement, leaving some room for discussion. The present work developed a novel direct method using in-situ fiber-optic-based monitoring of through-thickness normal and shear strains. This method was applied to an L-shaped CFRP part and the result indicated that chemical cure shrinkage induces shear deformation. Moreover, the phenomenon gradually changed from shear-dominated to bending-dominated deformation as curing proceeded. The shear-lag analysis developed in previous work was then extended to L-shaped parts. The effects of flanges and edge constraints were investigated based on analytical and experiment results. The analysis also proposed a spring-in prediction scheme that uses strain monitoring results as inputs. The prediction agreed with the experiment result, indicating that cure-induced shear deformation should be considered for highly precise distortion prediction.

Analysis of Fiber-Optic Strain-Monitoring Data from a Prestressed Concrete Bridge

This paper presents data from fiber-optic strain monitoring of the Nine Wells Bridge, which is a three-span, pretensioned, prestressed concrete beam-and-slab bridge located in Cambridgeshire in the United Kingdom. The original deployment at the site and the challenges associated with collecting distributed strain data using the Brillouin optical time domain reflectometry (BOTDR) technique are described. In particular, construction and deployment issues of fiber robustness and temperature effects are highlighted. The challenges of interpreting the collected data as well as the potential value of information that may be obtained are discussed. Challenges involved with relating measurements to the expected levels of prestress, including the effects due to debonding, creep, and shrinkage, are discussed and analyzed. This paper provides an opportunity to study whether two commonly used models for creep and shrinkage, adequately model data collected in field conditions.

Evaluation of damage detectability in practical sandwich structure application conditions using distributed fiber optic sensor

Indentation loading or low-velocity impacts on foam core sandwich structures can leave only barely visible damage on the face sheets while causing notable damage in the core due to the thin face sheets and relatively weak core. In practical applications, the measurement and damage detection can also be notably affected by environmental conditions and relaxation of the foam core. To detect the damages, a Rayleigh-scattering-based fiber optic strain monitoring system with high spatial resolution was applied to foam core sandwich structures. Damage detection ability of the system was therefore tested by indentation tests on sandwich beam and panel specimens while also considering the aforementioned effects. Finite element analysis verified by the measurements was used to provide a method to estimate the damaged core area from the measured strain data. Finally, the monitoring ability of the system was demonstrated by low-velocity impact tests on a large scale sandwich panel. The used system provided detailed strain data during and after indentation and impact tests. Due to the high resolution of the system, the damage location and size could be estimated from the obtained strain data even when barely visible dent remained on the face sheet, thus demonstrating the damage detection ability even at various environmental conditions and after relaxation of the foam core.

Real-Time Strain Monitoring of a Navy Vessel During Open Water Transit

In this work we describe the installation of a real-time, fiber-optic strain monitoring system aboard a US Navy vessel undergoing sea trials. The system is lightweight, unintrusive, corrosion resistant, and provides dynamic strain response measurements from direct current (DC) up to 360 Hz. Also described are the data acquisition and subsequent analysis. The goals of the study were to demonstrate a fiber-optic strain monitoring system under real world conditions and to better understand the source of deck cracking above an area housing mission-critical components. The results of the two-day trial indicate that there exist large stresses in the deck plate during normal ship operation. Based on the results, it is clear that ship maneuvers, slewing of the radar located on the deck plate, and insolation-related effects are not the primary source of the observed stress.

FBG 센서를 활용한 온도와 스트레인 변화 모니터링

This paper reports on measurement method for the fiber optic strain monitoring of overhead contact line systems of trains, We used FBG (Fiber Bragg Grating) sensors to measure the strain variation of overhead contact line. FBG sensors can sensitively measure the variation of strain and! or temperature by the shift wavelength of reflected wave according to the lattice variation during the measurement. FBG sensor were attached on the contact line and connected to the monitoring system with optical fibers. The monitering system with FBG sensors showed very good sensitivity to measuring strain variation and this system could be applied to the overhead contact line of KTX (Korea Train eXpress).