Fiber Optic Leak Detection Systems for Subsea Pipelines

Abstract

Abstract This paper summarizes leak detection technologies available for use on subseapipelines and the potential role of fiber optic cable (FOC) integritymonitoring systems to improve leak detection capabilities. Available systemswhich monitor offshore pipelines for potential leaks based on internalflowrates and pressures are described along with their limitations for rapidlydetecting small leaks. Alternative pipeline leak detection technologies whichhave been used on some specialized subsea pipeline projects to supplement thecapabilities of these flow-based systems are also described along with theirlimitations. The potential for modern FOC distributed sensing technologies tofill the remaining leak detection capability gaps is then addressed. There is increased interest in improving leak detection system capabilitiesthroughout the pipeline industry. However, the primary application of thispaper is for offshore pipeline projects which may not be adequately covered byconventional flow-based leak detection systems and supplemental monitoring forpotential oil sheens visible on the sea surface. These applications includedeep water pipelines, where potential oil leaks may not reach the surface untilmiles away from the source, and subsea arctic pipelines, which could slowlylose significant oil volumes under the cover of winter sea ice. Other potentialapplications include subsea field developments and pipelines installed inunusually sensitive marine environments. Distributed FOC sensor systems can monitor real-time temperature, acousticnoise/vibration and strain along many miles of pipeline. Changes in any one ofthese monitored parameters can indicate a leak event or other potentialintegrity threats to a pipeline. An overview of FOC systems already installedon offshore pipeline projects and the current testing status of FOC systems forpipeline leak detection are provided. Recommendations for installation andimplementation of the available FOC technologies for subsea pipeline leakdetection are summarized. The limited applications of distributed fiber optic cable (FOC) monitoringsystems on subsea pipelines have not yet been used specifically for leakdetection purposes. The leak detection thresholds defined for FOC systems andthe procedures for design, installation and system repair will facilitate theuse of FOC systems to help ensure leak free pipeline operations. Introduction Internal leak detection systems have historically been used for permanentlyinstalled, long-term leak detection monitoring of subsea pipelines. Thesesystems monitor internal parameters, such as internal pressures and/orflowrates based on internal pressure, temperature, density, and/or flowrateinstrument measurements. Discrepancies between the measured parameters and theoperating trends or calculated operating predictions are used to alert thepipeline operator of a potential leak event. However, the instruments aretypically located only at the ends of a pipeline segment, remote from an actualleak location. This has some effect on sensitivity. Instrument inaccuraciesalso introduce error in the measured parameters. Selecting the alarm settingsof these internal type leak detection systems for very small leaks may resultin an unacceptable number of false alarms (nuisance alarms) due to theirachievable sensitivity and this inherent error. Therefore, alarm settings forthese systems are set-high enough (for large enough leaks) to minimize falsealarms. Leaks that are smaller than the alarm settings (minimum leak detectionthresholds) may continue undetected by these systems. Periodic over-flights orvessel observations from areas with subsea pipelines are then relied upon toobserve oil sheens from potential leaks that fall below a system's minimumdetection threshold. A chronic leak from a deep-water pipeline may manifest as an oil sheen on thewater surface miles away from the leak source due to currents. There may be atime delay in determining the source of the oil sheen, especially if multiplepipelines are located near the observed oil sheen or if there are local subseafield developments.