Branko Glisic

LONG-RANGE PIPELINE MONITORING BY DISTRIBUTED FIBER OPTIC SENSING

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.

Fiber optic method for health assessment of pipelines subjected to earthquake-induced ground movement

Natural disasters, in particular earthquakes, can cause damage to pipelines with disastrous humanitarian, social, economic, and ecologic consequences. Thus, real-time, automatic, or on-demand assessment of damage to pipelines after the earthquake is essential for early emergency response, efficient preparation of rescue plans, and mitigation of these disastrous consequences. This article presents the development of a method for buried pipelines health assessment based on distributed fiber optic sensors, which are sensitive to strain at every point along their lengths. The sensors are both bonded to pipeline and embedded in the soil, in the proximity of the pipeline. The research includes determination of sensor topology, identification of required sensor properties, selection of sensors, development of installation procedures, implementation, and validation. The validation of the method was made through a large-scale testing: a 13-m-long real-size concrete segmented pipeline was assembled in a large test basin filled with soil and was tested under simulated permanent ground displacement. The basin consisted of two parts: the movable north part and the fixed south part. The movable north part of the test basin was attached to four hydraulic actuators, which were used to apply controlled displacement of the basin, and it induced damage to the pipeline by crushing the joints between adjacent pipeline segments. As a part of validation, the results obtained from distributed sensors were compared with resistive strain gauges. Two validation tests were performed: the first in 2010 and the second in 2011. The method is presented in detail, and the most significant results of both tests are analyzed, compared, and discussed. The validation tests confirmed the capacity of the method to reliably detect and localize the damage on pipeline and the displacement in the soil.

MONITORING OF CONCRETE AT VERY EARLY AGE USING STIFF SOFO SENSOR

Abstract The SOFO system is based on low-coherence interferometry in single-mode fibres and allows the measurement of deformations in civil structures built with classical civil engineering materials (concrete, steel and wood). It has been successfully tested in different types of structures such as bridges, dams, tunnels and piles. In order to monitor behaviour of concrete at the very early age a stiff SOFO sensor has been developed. Using standard SOFO sensor it is possible to measure deformation of concrete at the very early age (thermal swelling and shrinkage). However, by coupling it with a stiff sensor, it is possible to determine the hardening time of concrete and to measure initial stress in the rebars. The stiff sensor and the results obtained using its first prototype are presented in this paper.

Submillimeter crack detection with brillouin-based fiber-optic sensors

Submillimeter crack is detected with a dedicated fiber-optic strain cable, a 1-m-spatial-resolution (w) distributed Brillouin sensor and an advanced signal processing technique. The signal processing approach consists in spectrum shape analysis and multiple peaks detection.

Neutral axis as damage sensitive feature

Structural health monitoring (SHM) is the process of continuously or periodically measuring structural parameters and the transformation of the collected data into information on real structural conditions. The centroid of stiffness is a universal parameter and its position in a cross-section can be evaluated for any load-carrying beam structure as the position of the neutral axis under conveniently chosen loads. Thus, a change in the position of the neutral axis within a cross-section can indicate a change in the position of the centroid of stiffness, i.e., unusual structural behaviors. This paper proposes a novel monitoring method based on deterministic and probabilistic determination of the position of the neutral axis under conveniently chosen conditions. Therefore, the method proposed in this paper is potentially applicable to a large variety of beam-like structures. Data from two existing structures were used to validate the method and assess its performance: Streicker Bridge at Princeton University and the US202/NJ23 highway overpass in Wayne, NJ. The results show that the neutral axis location is varying even when damage is not present. Reasons for this variation are determined and the accuracy in the evaluation assessed. This paper concludes that the position of the neutral axis can be evaluated with sufficient accuracy using static and dynamic strain measurements performed on appropriate time-scales and indicates its potential to be used as a damage sensitive feature.

Development of method for in-service crack detection based on distributed fiber optic sensors

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.

Detection of Steel Fatigue Cracks with Strain Sensing Sheets Based on Large Area Electronics

Reliable early-stage damage detection requires continuous monitoring over large areas of structure, and with sensors of high spatial resolution. Technologies based on Large Area Electronics (LAE) can enable direct sensing and can be scaled to the level required for Structural Health Monitoring (SHM) of civil structures and infrastructure. Sensing sheets based on LAE contain dense arrangements of thin-film strain sensors, associated electronics and various control circuits deposited and integrated on a flexible polyimide substrate that can cover large areas of structures. This paper presents the development stage of a prototype strain sensing sheet based on LAE for crack detection and localization. Two types of sensing-sheet arrangements with size 6 × 6 inch (152 × 152 mm) were designed and manufactured, one with a very dense arrangement of sensors and the other with a less dense arrangement of sensors. The sensing sheets were bonded to steel plates, which had a notch on the boundary, so the fatigue cracks could be generated under cyclic loading. The sensors within the sensing sheet that were close to the notch tip successfully detected the initialization of fatigue crack and localized the damage on the plate. The sensors that were away from the crack successfully detected the propagation of fatigue cracks based on the time history of the measured strain. The results of the tests have validated the general principles of the proposed sensing sheets for crack detection and identified advantages and challenges of the two tested designs.

Influence of the gauge length on the accuracy of long-gauge sensors employed in monitoring of prismatic beams

Depending on the geometric basis of measurement (gauge length), discrete strain sensors used in structural monitoring of civil engineering structures can be considered as short-gauge sensors or long-gauge sensors. Long-gauge sensors measure average strain over the gauge lengths and are used for global monitoring of structures, in particular, those built of inhomogeneous materials. However, the strain distribution along the sensors gauge length may be nonlinear and the measured average strain value that is commonly attributed to the midpoint of the sensor may be different from the real value of strain at that point. Consequently, excessively long sensors may feature significant errors in measurement. However, short-gauge sensors are more susceptible to other types of measurement error, most notably, error caused by discontinuities (open cracks) distributed in the monitored material. Thus an optimum gauge length is to be found. The error in average strain measurement inherent to the sensors gauge length introduced by the strain distribution and discontinuities in the monitored material is modelled for the most common applications met in civil engineering practice. The modelling takes into account the geometric properties of the monitored structure and various load cases. Guidelines for the selection of an appropriate gauge length are proposed, and tables for measurement error estimation are presented.

Large-Scale Sensing System Combining Large-Area Electronics and CMOS ICs for Structural-Health Monitoring

Early-stage damage detection for bridges requires continuously sensing strain over large portions of the structure, yet with centimeter-scale resolution. To achieve sensing on such a scale, this work presents a sensing sheet that combines CMOS ICs, for sensor control and readout, with large-area electronics (LAE), for many-channel distributed sensing and data aggregation. Bonded to a structure, the sheet thus enables strain sensing scalable to high spatial resolutions. In order to combine the two technologies in a correspondingly scalable manner, non-contact interfaces are used. Inductive and capacitive antennas are patterned on the LAE sheet and on the IC packages, so that system assembly is achieved via low-cost sheet lamination without metallurgical bonds. The LAE sheet integrates thin-film strain gauges, thin-film transistors, and long interconnects on a 50-μm-thick polyimide sheet, and the CMOS ICs integrate subsystems for sensor readout, control, and communication over the distributed sheet in a 130 nm process. Multi-channel strain readout is achieved with sensitivity of 18 μStrain RMS at a readout energy of 270 nJ/measurement, while the communication energy is 12.8 pJ/3.3 pJ per bit (Tx/Rx) over a distance of 7.5 m.

Sensing sheet: the sensitivity of thin-film full-bridge strain sensors for crack detection and characterization

Increasing concerns regarding the conditions of civil structures and infrastructure give rise to the need for efficient strategies to identify and repair structural anomalies. ‘Sensing sheets’ based on large-area electronics consist of a dense array of unit strain sensors. These are an effective and affordable structural health monitoring tool that can identify and continuously monitor the growth of cracks in structures. This paper presents a study on the quantitative relationship between crack width and strain, the latter measured by an individual sensor that would be part of a sensing sheet. We investigate the sensitivity of thin-film full-bridge strain sensors to concrete cracks by conducting laboratory experiments in temperature-controlled settings. The results show a distribution of near-linear relationships with an average sensitivity of 31 μe μm −1 . Experiments were also conducted to investigate the effect of crack position and orientation with respect to the sensor, and it appears that both variables affect the sensitivity of strain sensors to cracks. Overall, this study confirms that full-bridge resistive strain sensors can successfully detect and quantify cracks in structural materials and are therefore appropriate as part of a dense array of sensors on a sensing sheet.