Fiber-Optic Distributed Temperature Sensing Detects Pipeline Leaks

With increasing public consciousness and concern for the environment, recent pipeline-leak incidents across the globe 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 being introduced, cost-effective and reliable leak-detection systems are in demand.

According to the 2004 paper “Leakage Detection Using Fiber Optics Distributed Temperature Monitoring” by Marc Nikles et al., “In the past few years, innovative distributed temperature monitoring techniques using optical fibers have demonstrated to be an efficient way to detect and localize leakages along pipelines. These techniques use a concept similar to optical domain reflectometry for the localization, whereas the temperature information is extracted from the analysis of the scattered light through Raman or Brillouin scattering processes. Raman-based systems were first proposed and used in practical applications, while the Brillouin-based technique has been introduced in the early ’90s and offers longer distance ranges.”

This paper details the methodology adopted to monitor gas-pipeline leakages using distributed fiber optics sensing. Distributed temperature sensing has been introduced to UAE pipelines by GASCO through its first installation in the Habshan/Ruwais/Shuweihat gas pipeline followed by the Habshan/Maqta/Tawelah pipeline projects.

What is Distributed Sensing?

The term “distributed sensing” refers to the use of an optical fiber as a linear sensor that provides valuable measurement information from all along the fiber itself. The measurement is based on the analysis of back-scattered light when a laser pulse travels down the optical fiber.

The sensing fiber is integrated into the asset (pipeline, cable, or structure) using a dedicated fiber cable. A single optical fiber replaces thousands of single-point sensors, providing a significant reduction in installation, calibration, and maintenance costs. In addition, assets can be monitored in real time, whereas, previously, this was complicated by their distance, location, or environment. Optical fiber is inexpensive, lightweight, pliable, immune to electromagnetic interference, and has a life expectancy of several decades, which makes it a cost-effective, flexible, durable, and inert sensor medium.

Principle of Operation

Data from a stimulated Brillouin scattering (SBS) signal is processed using both time and frequency domain analysis. The analysis is performed by collecting the SBS information from every location along the sensing optical fiber, yielding an uninterrupted and fully distributed temperature or strain profile.

The leak-detection system uses a single laser to generate the two signals used to stimulate backscattering—an optical pulse (or pump) and a continuous-wave optical signal, known as the probe signal. Using a single laser gives stability because it self-compensates for drift.

When the frequency difference of the two signals reaches the Brillouin frequency shift, a resonance condition is established, leading to the efficient stimulation of Brillouin scattering. This stimulation induces an energy transfer from the pulse to the probe signal and an amplification of the probe signal. The technique enables the rapid and accurate identification of the local resonance condition at every location along the sensing fiber and computes the local temperature and strain conditions. The SBS stimulation occurs only if the pump and probe signals are counterpropagating inside the optical fibers. For this reason, two fibers are used. The first one brings the probe signal to the far end of the second optical fiber, which acts as the sensing fiber. These two fibers form a loop and are, most of the time, integrated in the same cable (i.e., the sensing cable). In some cases, the measurement can be performed over the entire loop distance.

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