Fibre Optic Leak Detection
Authors: Walker Ian , Carr Dennis 
Published: October 2008

Abstract
Fibre Optic Leak Detection Systems are based on Distributed Temperature Sensing (DTS) technology and use the principle of local temperature changes to detect and locate leaks.
For leaks in pressurised gas pipelines the escaping gas will generate a cold zone in the surrounding environment due to Joules- Thompson expansion.
For pipelines carrying heavy crude oils, the oil is often transmitted at temperatures significantly above ambient in order to maintain a reasonable viscosity. A pipeline leak would therefore create a local warming of the environment surrounding the pipeline.
Similarly, for cryogenic products, such as LNG, a pipeline leak would create a local cooling of the environment.
DTS systems have been monitoring processes in the hydrocarbon industry for more than 10 years. They deliver unequalled coverage and continuous performance to a multitude of applications, which demand an automatic, reliable and low maintenance monitoring system.
Distributed Sensing provides data spatially distributed over many thousands of individual measurement points along a single sensor - in this case an optical fibre.
The system comprises an opto-electronics unit and fibre optic sensor(s). The DTS unit is either located within a control room or in a ruggedised enclosure for field use with the optical fibre(s) being laid along the outside of the pipeline(s) within protective sheathing for mechanical and chemical protection.

The Technology
The DTS system is based on Optical Time- Domain Reflectometry (OTDR). In this technique, a pulse of light is launched into the sensing fibre and light is scattered as the pulse passes down the fibre due to various mechanisms. A proportion of the scattered light is retained within the fibre core and is guided back towards the source (the backscatter). This backscatter contains the Raman signal which comprises of two elements - the Stokes and anti-Stokes lines. The longer wavelength Stokes lines is only weakly temperature sensitive but the intensity of the backscattered light at the shorter anti-Stokes wavelength increases with an increase in temperature and vice versa. It is, therefore, this latter component which is used as the basis of temperature measurement.

Application Overview
Fibre Optic Leak Detection Systems are well suited to a wide range of single and multiphase liquids and gases including ammonia, ethylene, natural gas and heavy oil as well as cryogenic mediums such as LNG, LPG, etc. Such applications can equally be offshore as well as onshore.
The overall versatility, functionality and costeffectiveness of the system range lends itself to both short and long range pipelines as well as pipeline networks. A single DTS processor unit with multichannel capability at the centre of a pipeline network can cover up to 72 km of pipeline.
The versatility of alarm configuration allows these pipelines to be carrying different gases and liquids. Its remote communication and alarm capability is ideal for monitoring remote pipelines either to a discrete monitoring station or as part of a remote pipeline data collection service.
Multi-window Graphical User Interfaces are available which provide a straight-forward operator interface showing the status of the pipeline network.