Distributed Fiber Optics Techniques for Gas Network Monitoring

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1 Distributed Fiber Optics Techniques for Gas Network Monitoring Carlo Edoardo Campanella Energy Research Institute, ERIAN Nanyang Technological University Singapore Gang Ai Energy Research Institute, ERIAN Nanyang Technological University Singapore Abhisek Ukil School of EEE Nanyang Technological University Singapore Abstract Robust and real-time condition monitoring of the underground gas distribution network (ca km) in Singapore is critical for interruption-free power generation. For this purpose, in this paper, we investigate optical techniques, based on distributed fiber optic sensors, recently proposed in the context of the gas pipeline monitoring. A detailed review of these techniques and their applications in terms of sensing range, spatial, temperature and strain resolutions has been generalized. During gas leaks, Joule-Thomson (JT) cooling effect takes places. temperature drops due to Joule-Thomson cooling effect for argon (has similar JT effect to methane) at various operating conditions are analyzed. It could be detected by the distributed temperature sensing (DTS), for monitoring leaks in the underground gas network. Keywords Condition monitoring, distributed temperature sensor, DTS, fiber optics, Joule Thomson cooling, gas network, underground gas pipeline, natural gas, argon I. INTRODUCTION re is a growing interest among Governments, engineering companies, for the safety and warranty of the gas pipeline networks. As Singapore s electricity is mainly generated from natural gas, robust and real-time condition monitoring of the underground gas pipeline network is critical for interruption-free power generation. Mechanical, electrical and electronic sensing technologies have been proposed for achieving this goal [1]. Among the existing sensing technologies, the fiber optic distributed temperature sensors (DTS) show many advantages, such as compactness, immunity to electromagnetic interference, rapid response for real time monitoring and the possibility to be embedded in the structures to be monitored. For this reason, in this paper, we investigate distributed fiber optics sensors, recently proposed in the context of the structural monitoring, that are suitable for gas pipeline monitoring. remainder of the paper is organized as follows. In Section II, a short introduction about the operating principles of the distributed fiber optics sensors, based on time domain and frequency domain technique, has been reported. In section III, a detailed review of these techniques and their applications in terms of sensing range, spatial, temperature and strain resolutions are introduced. In Section IV, the principle of Joule-Thomson cooling effect for different gases at various conditions is reported. Detection possibility using DTS is also highlighted. This is followed by conclusions in section V. II. DISTRIBUTED FIBER OPTIC SENSORS FOR SENSING TEMPERATURE AND STRAIN Standard distributed fiber optics sensors exploit the Physics of Rayleigh, and Raman scatterings, arising from the variation of the properties of the optical waveguiding medium, due to a strain or temperature change. In fact, if the optical features of the optical medium are changed, a modulation of the light wave occurs. With reference to the Rayleigh scattering peak, the anti-stokes component (on the left of the spectrum in Fig. 1) is temperature sensitive for the Raman scattering. However, the Stokes and the anti-stokes components of the scattering are temperature and strain sensitive. light modulation, due to temperature/strain sensitivity, is recorded by time domain or frequency domain approaches. time domain approach consists of launching a forward optical pulse of duration t, through the optical fiber. As shown in Fig. 2, the i-th scattering site is called Z i. Z i is equal to (1/2)vt i where v=c/n (c is the speed of the light in the vacuum while n is the group refractive index of the waveguiding medium) and t i is the pulse duration. If a perturbation source exists, which is responsible for the light modulation, it can be localized at the i-th scattering site by measuring the time delay between the forward pulse and the backward scattered pulse signal. pulse duration limited spatial resolution, R t, (i.e., the localization accuracy) is given by [2]: R t =tc/(2n) (1) This approach is called Optical Time Domain Analysis (OTDA) or Optical Time Domain Reflectometer (OTDR), because the backscattering corresponds to a back-reflection of the optical pulse. Usually this technique allows achieving resolutions of about 1m. Differently, the frequency domain measurement method,

2 called Optical Frequency Domain Reflectometer (OFDR), uses a tunable laser for scanning a frequency range of Δf. By using the Fourier transform, it leads to a spatial resolution expressed as [2]: Fig. 1. Spectral features of Raman, and Rayleigh scattering. R f =c/(2nδf) (2) which is limited by the tunability of the laser source (i.e., scanning range). By adopting laser sources with a high scanning range, the spatial resolution can be improved in order to achieve values below the millimeters. Fig. 2. Optical time domain reflectometer technique. III. DISTRIBUTED FIBER OPTICAL SENSORS: APPLICATIONS AND PERFORMANCE Recent review papers more focused on the physics of the distributed fiber optical sensors are reported in [3-5], while a review about the DTS has been presented in [6]. With reference to Table I, in this section we report the recent advances of distributed fiber optical sensors in those applications very close to the monitoring of the pipeline status. Moreover, Table I also reports the measurement range (i.e., sensing range), the spatial resolution (i.e., the accuracy for the localization of a scattering point as shown in Fig. 2), the temperature resolution (i.e., the minimum detectable temperature variation) and the strain resolution (i.e., the minimum detectable strain variation), defined in terms of ε [7]. In [8], distributed optical sensing technique based on Fiber Bragg Grating [7], have been applied for long range power system protection and control to be used in smart grid. For a similar purpose, the Raman based OTDR technique assisted by an optical fiber composite power cable (i.e., a power cable embedded with optical cables) has been used for monitoring underground distribution networks in a smart grid environment [9]. In [10] a OTDR system has been applied for monitoring the bridge loading conditions through its supporting cables while the same technique makes possible the detection of cracks and their locations in a beam [11-12]. A damage detection and localization in post tensioned concrete bridge and segmented concrete pipeline has been performed in [13]. Dynamic monitoring of a strain wave, propagating at a speed of 4km/sec, by using Slope-Assisted Optical Time Domain Analysis (SA-BOTDA) has been performed in [14]. By using based OTDR/OTDA bridge structural monitoring and remote monitoring of landslides (through an inclinometer) have been demonstrated in [15] and [16], respectively. OTDA has been also used for leakage monitoring in large diameter water pipes [17]. In [18], pipeline protection has been guaranteed by using Raman based OTDR DTS together with coherent Rayleigh based coherent OTDR for distributed acoustic sensing (DAS). A technique based on active optical fiber, doped with erbium, consisting in optical correlation-domain reflectometry (BOCDR) has been used for long-distance distributed strain and temperature sensing [19]. Finally, in [20] and [21], OTDR techniques based on Raman and, respectively, have been exploited for oil and gas pipeline monitoring. TABLE I. DIFFERENT APPLICATIONS AND SPECIFICATION OF FIBER OPTIC SENSORS Ref Applications Sensing techniques Measurement range [8] Power System Protection and Control (smart grid) Spatial Resolution [m] Temperature Resolution [ C] Strain Resolution Bragg Grating 1500 m [9] [10] Monitoring Underground Distribution Networks (smart grid) Monitoring Bridge Cable Loading Conditions Raman OTDR 1500 m OTDR m ± 20 µε

3 Ref Applications Sensing techniques Measurement range Spatial Resolution [m] Temperature Resolution [ C] Strain Resolution [11-12] Detection of Cracks and Locations in a Beam OTDR 15 m 1 - ± 30 µε [13] Damage Detection and Localization in Bridge and Pipeline OTDR Several Km [14] Dynamic monitoring of a Strain Wave [15] Bridge structural monitoring [16] [17] Inclinometer for remote monitoring of landslides Leakage monitoring for large diameter water pipes [18] Pipeline protection SA-BOTDA OTDR OTDA OTDA Raman OTDR (DTS); Rayleigh C-OTDR (DAS) Almost 100 m < 0.5 ± 1 µε 320 m m 1 ± 10 µε 7.5 m 1 ± 10 µε m [19] Long-distance distributed strain and temperature sensing OCDR MHz/C 479 MHz/% [20] Oil and gas pipeline Monitoring A) FBG B) Raman OTDR [21] Oil and gas pipeline Monitoring OTDR - A) B) 1-2 A) 0.1 B)1-2 40k-60k A) 1 µ B) - ± 20µε IV. JOULE-THOMSON COOLING EFFECT & DETECTION For the gas distribution network, anomalies due to cracks and leaks are difficult problems, which need to be solved urgently. physical signals (such as temperature change, leakage noise, etc.) caused by the low pressure gas leakage are weak and difficult to be detected. According to Ai et al. s study [21], there is a Joule-Thomson cooling effect during the process of gas leakage to cause gas temperature drop. This temperature drop could be detected by the DTS in the vicinity of the leaking points. And then the possible anomalies could be localized. In this section, calculations are carried out to evaluate the temperature changes of leaking gas caused by possible leaks at different sections of underground gas network. Joule- Thomson coefficients of different gases [22 25] are shown in Fig. 3. Argon gas has a similar Joule-Thomson coefficient to that of methane (the principal component of natural gas). Combining with safety considerations, argon gas is selected as the gas for the calculation to facilitate future possible experimental verification. evaluation procedure for leaking gas temperature is shown in Fig. 4. Joule-Thomson coefficient μ ( o C/bar) He CO 25%CH 50%CH 75%CH 2 N 2 Air CH 4 +75%N 2 +50%N 2 +25%N Different gases 2 Fig. 3. Joule-Thomson coefficient of different gases. Ar

4 (1) Start from the inlet of the crack: define initial gas temperature T (1) and pressure p (1) (2) Define crack geometry parameters: Crack surface angle, average flow direction, roughness, etc. (3) Find the pressure change of gas during leaking through the first small increment of the crack, which is the sum of pressure drop by frictional loss, inertial loss and recirculation loss: = + + TABLE II. INITIAL INPUT PARAMETERS FOR CALCULATION OF GAS TEMPERATURE Parameters Description Value T 0 Initial gas temperature inside pipes 30 C W c Crack width 0.25 mm R global Global roughness 3.6 μm α Crack surface angle π/6 OD Outside diameter of pipe 315 mm t Crack depth/pipe thickness 15 mm (4) Calculate the pressure p (2) of gas after leaking through the first increment of the crack (5) Calculate gas specific volume (i.e. the reciprocal of density) from the Redlich-Kwong (RK) equation: =. where = R2 2.5 c = R c c (6) Find the change of temperature of gas during leaking through the first increment of the crack. = μ d, where Joule-Thomson coefficient μ JT is calculated using the Redlich-Kwong equation (7) Calculate the temperature T (2) of gas after leaking through the first increment of the crack Develop iterative codes for the calculation of other increments c Fig. 4. Evaluation flowchart of leaking gas temperature. For the gas distribution network, the gas pressures span from 0.02 bar to 16 bar. In this paper, the calculations of temperature changes for different pressures (16 bar,12 bar,8 bar,4 bar, 0.5 bar, 0.02 bar) of gas are carried out. initial parameters are shown in Table II. roughness value shown in Table II is the crack surface roughness of polyethylene (PE) [26]. Currently, the PE pipes are the main stream in distribution network in pipeline companies. For the considerations of conservative evaluations, the largest pipe size (outside diameter of 315 mm) is selected. Reasonable initial crack parameters are estimated based on [22]. pressure and temperature changes of leaking argon along through-wall crack at different pipe pressures are shown in Figs. 4 and 5, respectively. results of the calculations are summarized in the following Table III. In the studies of these cases, it shows that the minimum temperature drop of the leaking argon through a crack is 0.27 C under the condition of the pipe pressure of 1.02 bar. Currently, the temperature resolution of commercialized DTS system with acceptable costs is up to 0.1 C. It means that even for the lowest pressure of 1.02 bar inside pipe (temperature drop of 0.27 C as shown in Table III), it could be possible for the DTS system to be able to detect the temperature changes caused by the Joule-Thomson cooling effect during leaks, and then locate the leaking points. Pressure of leaking argon (bar) Pipe pressure: 16 bar 12 bar 8 bar 4 bar 1.5 bar 1.02 bar Through-wall crack position (from crack inlet to outlet) (mm) Fig. 4. Pressure changes of leaking argon along through-wall crack.

5 Temperature of leaking argon ( o C) Pipe pressure: 16 bar 12 bar 8 bar 4 bar 1.5 bar 1.02 bar Through-wall crack position (from crack inlet to outlet) (mm) Fig. 5. Temperature changes of leaking argon along through-wall crack. TABLE III. SUMMARY OF CALCULATION RESULTS OF LEAKING ARGON S TEMPERATURE pressure of gas inside pipe pi (bar) pressure of leaking argon at the crack outlet pressure drop of leaking argon through the crack Δp (bar) temperature of leaking argon at the crack outlet To ( C) temperature drop of leaking argon through the crack ΔT ( C) po (bar) As long as there is enough source gas, the Joule-Thomson cooling effect would occur continuously. Once the gas leaks out, its flow will be retarded by surrounding earth, and it will further diffuse into the porous soil as shown in Fig 6. eruption of long-time continuous leaking gas could finally lower the temperature of air in the certain region near the crack through the heat transfer process. Eventually, the temperature of surrounding atmosphere in the vicinity of the crack could approach the lower temperature of leaking gas induced by Joule- Thomson cooling effect. Thus, the ambient temperature change could be detected by the distributed temperature sensor mounted on the pipe and then the warning signals might be sent to control room for engineer s quick response to avoid possible hazard and further property losses. For a better detection of gas leakage, the alarm system of DTS system should be further improved. Expanding cooled gas diffused into soil Low temperature signal DTS Through-wall crack Fig. 6. Detection of leaks on gas pipeline employing DTS system. response time shouldn t be regarded as the first important parameter as a certain amount of leakage is acceptable. focus should be placed on the accurate localization of gas leaking points. Quick response time can compromise the temperature resolution which is the critical parameter for the detection of minor temperature changes as shown in Fig. 7. key point of the alarm system of DTS system should be put on the identification of the signal of relatively long time stable minor temperature drop in order to avoid the interference of unnecessary noise signals and the sending of wrong alarms and further enhance the correctness of alarming response for gas leaks. FIG. 7. Performance curve of distributed temperature sensing system [27]. V. CONCLUSION distributed fibre optic sensing systems is on the way to the practical implementation for the protection of the pipeline. Especially, the fibre optic temperature sensing is a promising and sensitive technique for surveying underground gas pipelines. Even for small leaks at distribution gas network with low

6 pressure (such as 1.02 bar), it could be able to detect minor gas leaks. sensor cables can be installed directly into the earth along the pipelines. Leaking gas is recognized through a temperature anomaly that is caused by the Joule-Thomson effect. possible danger originating from a leak can be evaluated from the spatial and temporal development of the leak-induced temperature variation. For achieving a more accurate detection of gas leakage, the alarm system of the commercialized DTS system should be further improved to adapt to the specific characteristic of Joule-Thomson effect occurred in the gas leaking process through cracks, which is expected to capture the minor signal changes coming from stable temperature drop for a relatively long time. ACKNOWLEDGMENT This work was supported by Energy Innovation Programme Office (EIPO) through the National Research Foundation and Energy Market Authority (EMA), Singapore (Project LA/Contract No.: NRF2014EWT-EIRP ). authors are grateful to the generous technical support by the engineers of Singapore PowerGrid Powergas, and research staffs of ENGIE, France. REFERENCES [1] Avateq Corp. Press Release. New leak detection and monitoring technology ensures safety of pipelines. [2] X. Biao, L. Cheng, Recent progress in distributed fiber optic sensors, Sensors, vol. 12, no. 7, pp , [3] M. A. Soto, L. Thévenaz, Modeling and evaluating the performance of distributed optical fiber sensors, Optics Express, vol. 21, no. 25, pp , [4] M. Tur, A. Motil, I. Sovran, A. Bergman, Recent progress in distributed scattering fiber sensors (invited), Proceedings of IEEE Sensors, Valencia, pp , [5] A. Ukil, H. Breandle, P. 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