DETECTION AND LOCALIZATION OF MICRO LEAKAGES IN MULTIPHASE PIPELINES USING DISTRIBUTED FIBER OPTIC SENSING (2012)

Gas and liquid pipelines often cross hazardous environmental areas, from the point of view of natural exposures such as landslides and earthquakes, and from the point of view of third party interferences such as vandalism or obstruction. These hazards can significantly change the original functioning of the flowline, leading to damaging, leakage and failure with serious economic, social and ecological consequences. Furthermore, the operational conditions of the pipeline itself can induce additional wearing or even damage due to corrosion, erosion and fatigue. The distributed and functional monitoring can significantly improve the pipeline management and safety. Providing regularly with parameters featuring the functional condition of the flowline, monitoring can help (1) prevent the failure, (2) detect in time the problem and its position and (3) undertake maintenance and repair activities in time. Thus the safety is increased, maintenance cost optimized and economic losses decreased. The most interesting functional parameters are temperature distribution, leakage and third-party intrusion. Since the flowlines are usually tubular structures with kilometric lengths, functional monitoring of their full extent is an issue itself. The use of the discrete sensors, short- or long-gage is practically impossible, because it requires installation of thousands of sensors and very complex cabling and data acquisition systems, raising the monitoring costs. Therefore, the applicability of the discrete sensors is rather limited to some chosen cross-sections or segments of flowline, but cannot extend to full-length monitoring. Other current monitoring methods include flow measurements at the beginning and end of the pipeline, offering an indication of the presence of a leak, but limited information on its location. Recent developments of distributed optical fiber strain and temperature sensing techniques based on Raman and Brillouin scattering provide a cost-effective tool allowing monitoring over kilometric distances. Thus, using a limited number of very long sensors it is possible to monitor structural and functional behavior of flowlines with a high measurement and spatial resolution at a reasonable cost. Unlike electrical and point fiber optic sensors, distributed fiber optic sensing offers the ability to measure temperatures and strain at thousands of points along a single fiber. This is particularly interesting for the monitoring of pipelines, where it allows the detection and localization of leakages of much smaller volume than conventional mass balance techniques. Fiber optic sensing systems are used to detect and localize leakages in liquid, gas and multiphase pipelines, allowing the monitoring of hundreds of kilometers of pipeline with a single instrument and the localization of the leakage with a precision of 1 or 2 meters. This contribution presents recent testing results on controlled field trials. The tests demonstrate that it is possible to reliably detect oil leakages of the order of 10 liters to 1’000 liters per hour, corresponding to 0.01% to 0.1% of the pipeline flow. Tests were performed with small temperature differences between liquid and ground. The detection time was between 1 minute and 90 minutes. All simulated leakages were detected and localized to better than 2m accuracy. The paper describes the main parameters that affect the response time and detection volume, including the relative position of the leak to the sensing cable, temperature contrast and instrument performance.