Detecting pipeline leaks using fiber optic sensing1 Temperature-based leak detectionFiber optic sensing can detect and localize leaks continuously and accurately all along the length of the pipeline. It does this by detecting the change of temperature which occurs when a pipe leaks. Leakage from a compressed gas line is identified by the development of a cold spot due to the pressure release (known as the Joule-Thomson effect). Whilst liquids such as crude oil, brine or heating system fluids are at a different temperature (typically higher) than the surrounding soil temperature, so a leak produces a temperature increase around the pipeline. The occurrence of local temperature event (1-2 meter long) encountered by the sensing cable anywhere along the pipeline could be the signature

of a leak. Information about size and location of the temperature event is compared with seasonal and daily temperature norms to check that it is in fact an abnormal event, if so an alarm will be sent to the control system.

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Technical Note1

Fig. 1: Numerical simulation of the temperature change occur-ring in soil surrounding a pipe leak. The pipeline surface is cools rapidly and a temperature gradient develops around the pipeline.

Typical Joule-Thomson effect figures for natural gas are -0.5°C/bar x delta p which indicates that small pressure changes result in significant temperature variations. The pipeline outer surface is rapidly cooled by the Joule-Thomson effect and a tempera-ture gradient develops in the soil around the pipeline surface. The speed of the temperature gradient development depends on the type of soil and may vary from a few seconds to a few minutes.

It is important to note that the cooling effect is independent of the soil temperature and that the magnitude of the cooling effect remains the same whatever the soil temperature is.

Where the temperature monitoring cable (TMC) is located rela-tive to the pipeline is important to ensure fast and accurate leak detection. The ideal TMC position is in the backfill, within 10 cm of the pipeline.

Fig. 2: TMC temperature sensing cable location for compressed gas pipeline.

1.1 Compressed gas pipelineLeak detection is based on the Joules-Thompson effect. The fluid being in adiabatic regime, any pressure change, as caused by a leak for example, induces a temperature drop which cools down the surrounding soil as shown in figure 1.

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Presence of gas leakAbsence of gas leak

Omnisens SARiond Bosson 3, 1110 Morges – SwitzerlandT: +41 21 510 21 21 - F: +41 44 274 20 31sales@omnisens.comwww.omnisens.com

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Technical Note

1.2 Liquid phase pipelineIn the case of a liquid phase pipeline, leaking liquid changes the local surrounding soil temperature which affects the temperature of the TMC cable. To optimize the detection of the temperature event resulting from the leaking fluid, the TMC cable is placed underneath the pipe.

In this case, the TMC cable should be placed underneath the pipe in order to optimize the detection of the temperature event introduced by the leaking fluid. Typically, this would be about 15 cm under a 22” pipe.

Fig. 3: TMC temperature sensing cable location

2 Pipeline leak detection performance The DITEST LTM system detects and locates small leaks with meter accuracy over 50 km range in each direction from one DITEST interrogator unit. Each DITEST interrogator unit measures two channels as standard, so can measure 100 km (50 km in each direction).

Over a distance of 50 km all the monitoring parameters can be maintained, so that leaks are quickly and accurately detected and localised.

2.1 Deciding on performance criteria for a given project The extremely low detection limit means early detection of small leaks (< 0.1% of the transported volume) before they develop into large, potentially catastrophic ones. The DITEST LTM system is at least one order of magnitude more sensitive than conventional mass/volume and computational loss detection techniques, with the additional advantages of event localization and no false alarms.

Fig. 4 This figure shows typical achievable performances in terms of temperature and strain resolution (2 sigma repeatability) ob-tained with the typical recommended spatial resolution from 1 m to 3 m relative to the distance range.

A further advantage of the DITEST LTM is that environment variability and the pipeline operation conditions do not affect the leak detection performance. The monitoring is continuous, non-intrusive and not affected by pipeline operational changes, such as flow rate changes, run up and even shutdown.

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Omnisens operates a quality policy, which seeks continuous improvement in all aspects of the business, in pursuance of which this document, and all products may be modified without notice.

DITEST™ is a trademark of Omnisens.Laser safety notice: The DITEST™ Series is classified as Class 1M Laser Product according to IEC/EN 60825-1 (2001) and IEC/EN 60825-2 (2005) under normal operating conditions and under those of reasonably foreseeable single-fault conditions, subject to use as described in the user documentation. The product complies with 21 CFR 1040.10 except for deviations pursuant to Laser Notice No. 50, dated 2001-July-26.

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Omnisens SARiond Bosson 3, 1110 Morges – SwitzerlandT: +41 21 510 21 21 - F: +41 44 274 20 31sales@omnisens.comwww.omnisens.com

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Technical Note