6 th Pipeline Technology Conference 2011

Transcription

1 6 th Pipeline Technology Conference 2011 Leakage Detection using Fibre Optics Distributed Temperature Sensing Ashim Mishra, Ashwani Soni Engineers India Limited, New Delhi, India Abstract Pipelines have been a vital component of the energy supply chain in India; have to be laid in harsh surroundings; crossing mountain ranges characterized by unstable grounds; where seasonal soil texture changes increase the probability of hazards and uncertainties. Therefore, pipeline monitoring systems for leakage, ground movement, and intrusion detection are part of new pipeline projects. Leakage detection using distributed fibre-optic sensors can be a comprehensive solution for continuous, in-line, real-time monitoring of various pipelines. The monitoring of temperature profiles over long distance by means of optical fibres represents a highly efficient way to perform leakage detection along pipelines. Different techniques have been developed taking advantages of the fibre geometry and of optical time domain analysis for the localization of the information. Raman-based systems have been envisaged for one of the very first projects of India where leakage detection using Distributed Temperature Sensing has been envisaged. The paper presents and discusses the possibility to actively and automatically monitor leakages using distributed fibre optics sensing techniques. The second part of the paper focuses on the monitoring of leakage and third party intrusion detection of petroleum product pipelines. The key features and performances of the technology are reviewed in this paper. Keywords: pipeline leakage detection, intrusion detection and temperature monitoring, Raman Scattering, fibre optics sensor, database management 1

2 Introduction With increasing public consciousness and concern for the environment, recent pipeline leak incidents have proved that the cost to a company can be far more than the downtime and clean up expenses. As more stringent statutory regulations are getting introduced, cost effective and reliable leak detection systems are in demand. The paper presents and discusses the possibility to actively and automatically monitor leakages using distributed fibre optics sensing techniques. The second part of the paper focuses on the monitoring of leakage and third party intrusion detection of 20 petroleum product pipelines with lengths varying from 7-10 kms approx. This is one of the very first projects of India where leakage detection using Distributed Temperature Sensing has been envisaged. Distributed temperature sensing systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1 C at a resolution of 0.01 C [1, 5]. Physical measurement dimensions, such as temperature or pressure, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the damping of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor. Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre [4, 5]. The Raman scattered light is caused by thermally influenced molecular vibrations. Consequently the backscattered light carries the information on the local temperature 2

3 where the scattering occurred. In fact the Raman backscattered light has two frequency shifted components: the Stokes and the Anti-Stokes components [1]. The amplitude of the Anti-Stokes component is strongly temperature dependent whereas the amplitude of the Stokes component is not. Therefore Raman sensing technique requires some filtering to isolate the relevant frequency components and consists in the recording of the ratio between Anti-Stokes amplitude by the Stokes amplitude, which contains the temperature information. Figure 1 shows the spectrum of the scattered light in optical fibres assuming that a single wavelength λ o is launched in the fibre. Brillouin-based sensing techniques rely on the measurement of a frequency as opposed to Ramanbased techniques which are intensity based [1]. Project defined in this paper, adopts Raman based scattering for sensing, as maximum pipeline length is limited to 10 kms. Figure-1 Schematic representation of the scattered light spectrum from a single wavelength signal propagating in optical fibres. An increase of the fibre temperature has an effect on the both Raman and Brillouin components The temperature measuring system consists of a controller (laser source, optical module, HF mixer, receiver and micro-processor unit) and a quartz glass fibre as line-shaped temperature sensor (figure 2). The fibre optic cable is passive in nature and has no individual sensing points and therefore can be manufactured based on standard telecom fibres. Because the system designer/ integrator does not have to worry about the precise location of each sensing point, the cost for designing and installing a sensing system based on distributed fibre optic sensors is reduced from that of traditional sensors [4]. Additionally, because the sensing cable has no moving parts and design life of more 3

4 than 30 years, the maintenance and operation costs are also expected to be considerably less than for conventional sensors. Advantages of having fibre optic sensing technology includes large number of monitored points over a single optical fibre sensor, immunity to electromagnetic interference, vibration, insensitiveness to humidity and corrosion, no active electronic circuits along the cable, long-term reliability and is safe for use in hazardous zones (the laser power falls below the levels that can cause ignition), thus making these sensors ideal for use in industrial sensing applications [1, 4, 7]. Figure 2 Schematic arrangement for light traveling through fibre Project Definition Project targets to detect leakages along the whole length of the pipelines to increase knowledge, to plan maintenance interventions and to ensure safety. The monitoring parameters are average temperature distribution and leakage detection of various petroleum product pipelines with lengths varying from 7-10 kms approx. feeding jetty. Leak detection for fluids like Crude oil, Naphtha, DPK, MS, HSD, Paraxylene, Propylene, Service water, Nitrogen, LPG, ATF, SKO as depicted in figure 3, for pipelines ranging from 4 to 38 has been envisaged [8]. 4

5 Figure-3 Pipelines schematic Each section consists of one temperature sensing cable with four single mode fibres to be buried above or below the pipe. Each section can be connected through optical connectors or spliced together. The project concentrates on the Continuous Monitoring. Here the monitoring system is based on Raman scattering technology, is selected for distributed temperature monitoring. As detailed above, the distributed fibre optic sensors shall detect temperature changes with resolutions up to 0.05 C. Spatial resolution depends on sensor cable length, and is typically one meter for the present maximum lengths of up to 10 km [8]. 5

6 System Description & Monitoring Strategy The system consists of reading unit, sensing cable and accessories (connection boxes, extension cables, splice protectors etc.). The optical fibres, which are integrated into robust cables, are the temperature sensitive elements and allow the measurement of temperature profiles at arbitrary times, quasi-continuously with a high spatial resolution along the cable. Liquid leak detection monitoring will be performed indirectly below the pipe (temperature cable at 6 O clock position) by the temperature increase in the ground. Gas leak detection monitoring will be performed indirectly on the top part of the pipe (temperature cable at 12 O clock position) by the temperature decrease in the ground induced by the decompression of the leaking gas caused by the Joule-Thompson effect. Intrusion detection will be performed indirectly on the top part of the pipe (temperature cable at 12 O clock position) by the temperature change in case of removal of covering material. Figure 4 shows the typical trench layout for laying of optical fibre cables for different product lines. Figure-4 Trench Details Cross Section 6

7 Following parameters are to be monitored using this methodology: Average temperature along the sensor with spatial resolution of 1-2 m Average temperature threshold detection along the sensor Measurement of temperature variation along the sensor Leakage detection Third party intrusion detection The present monitoring strategy contains a certain sensor redundancy, which is necessary for cases where the sensors are damaged during installation or later. Hence, even if one sensor in one cable gets damaged after a certain time, the global performance of the system would not be decreased. To install the sensors at proposed location it is necessary to be sure that no physical or constructive obstacle is presented. The origin of the temperature disturbance around the pipeline depends on the type of pipeline and its surroundings. The most typical effects are the following: The released liquid is warmer than the surrounding soil (typical for buried oil and liquid pipelines) The released hydrocarbon liquid changes the thermal properties of the soil, in particular thermal capacity, and influences the natural day/ night temperature cycles Gas leakage is detected by the temperature decrease in the ground induced by the decompression of the leaking gas caused by the Joule-Thompson effect The above effects influence the ideal cable placement around the pipeline. Ground temperature: leakage of oil/ water is detected because of punctual temperature increase In the case of a buried oil pipeline the best location for the sensing cable is below the pipe, but not in direct contact. At that position there is a maximum probability of collecting the released liquid, independently from the leakage location. The Distributed Temperature Sensing cable has therefore to be installed, approximately between 0.2 m 7

8 and 0.5 m, below the pipeline along its whole length. Figure- 5 demonstrates the typical peak received at the point of liquid leak detection. Figure-5 Typical peak during liquid leak detection Leakage of gas detected because of strong punctual temperature decrease In the case of gas leak detection in buried pipelines the best location for the sensing cable is above the pipe. At that position there is a maximum probability of collecting the released gas, independently from the leakage location. Figure-6 demonstrates the typical peak received at the point of gas leak detection.. Figure-6 Typical peak during gas leak detection 8

9 Intrusion detection detected by temperature change in case of removal of covering material ( T = T1-T2) as depicted in figure 7 where T is the difference of T1 and T2 which is detected by fibre optic cable. Figure-7 Intrusion Detection The system can detect the removal of earth from the optical fibre cable. This results in an immediate change in the recorded temperature that can be used to generate an alert. The position of the event can clearly be identified in all situations. Intrusion detection through temperature anomalies analysis Change of cable temperature due to digging and cable exposure Change of pipeline temperature due to exposure to air System Features Major system requirements/ features for the project include: Reading unit with data acquisition software to show the results locally and remotely and in form of warnings and pre-warnings depending on the measurements. Distributed Data Management and Analysis Software- an integral and fully compatible part of distributed monitoring system for data storing, processing, representation and analysis, as well as for the control of single or multiple reading units. The main functions of the software are aimed to measure sensors automatically. The operator shall view in real time the sensors measurement history in graphical form. Software shall provide the platform to monitor various trends, graphs for the entire length of pipeline as depicted in figure-6 and 7. The software shall trigger alerts (SMS, mail and phone call) and show warnings on the display. The software shall combine measurements from different sensing cables to obtain complex results. The software stores all information related to a sensor in a single 9

10 data-base structure. All data to be exported to third party software is depicted in figure 8 including MS Excel and MS Access. Multiple users to access the software simultaneously from different PC (locally or remotely over a modem or LAN). Figure-8 Data Transfer in DTS As continuous monitoring is vital to this project, alarms to be classified as nonthreat, possible threat/ leak, and a threat / leak. Each event classification to be colour coded (i.e. green no threat / leak, yellow possible threat / leak, red (flashing) threat / leak) for easy identification. Intelligent Alarms - The software shall also include assignment of zones to each pipeline varying in length as depicted in figure 9. It shall be possible to change the sensitivity or isolate alarm and events based on the pipeline zone. Each zone can be individually tuned to the local environmental conditions and have parameters set to distinguish the differences in the identification of possible noises. 10

11 Figure-9 Different Alarm Criteria Conclusion Recognizing the importance of leak detection in the prevention of oil spills and the need for a more thorough understanding of the use and effectiveness of leak detection technologies has led major oil companies to adopt the best possible technologies available. Often it is difficult for a pipeline company to discern, what is the best solution for their particular pipeline and philosophy of operation. Distributed Temperature Sensing is one of the prominent/ emerging technologies which offer several advantages and posses clear advantages over other existing conventional sensors. With this project a new initiative has been taken. References: 1. Marc Nikles, Bernhard Vogel, Fabien Briffod, Stephan Grosswig, Florian Sauser, Steffen Luebbecke, André Bals, Thomas Pfeiffer - Proceedings of the 11th SPIE Annual International Symposium on Smart Structures and Materials, March 14-18, 2004, San Diego, California, USA, 2. Dr Jun Zhang, Designing a Cost Effective and Reliable Pipeline Leak Detection System 3. E. Tapanes, Fibre optic sensing solutions for real time pipeline integrity monitoring 4. Dr. Stuart L. Scott, Dr. Maria A. Barrufet, - Worldwide Assessment of Industry Leak Detection Capabilities for Single & Multiphase Pipelines, Daniele Inaudi and Branko Glisic, Fibre Optic Sensing for Innovative Oil & GasProduction and Transport Systems 11

12 6. Daniele Inaudi, Branko Glisic, Distributed Fiber optic Strain and Temperature Sensing for Structural Health Monitoring 7. Dawn K. Gifford, Brian J. Soller, Matthew S. Wolfe, Mark E. Froggatt- Distributed Fiber-Optic Temperature Sensing using Rayleigh Backscatter 8. Engineering Design Document, South Jetty Project, EIL, New Delhi 12