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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

6th Pipeline Technology Conference 2011

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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

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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 λo is launched in the fibre. Brillouin-based

sensing techniques rely on the measurement of a frequency as opposed to Raman-

based 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

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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].

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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].

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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

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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

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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.

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Figure-6 Typical peak during gas leak detection

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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

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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 non-

threat, 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.

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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, 2003

5. Daniele Inaudi and Branko Glisic, Fibre Optic Sensing for Innovative Oil &

GasProduction and Transport Systems

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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