Evaluation of fiber for fault detection in a Direct Electrical Heating System
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Direct electrical heating (DEH) is a field proven flow assurance technology which is based on controlling the temperature by electrifying the pipeline and utilizing the pipelines electrical resistance to generate heat. The main safety principle for a DEH system is to detect any faults in the piggyback cable (PBC) and immediately turn off the power to avoid potential pipeline damages. Break detection using fiber optics (FO) is a qualified fault detection method, patented by Nexans, and is installed in as a base case. Light is continuously going through a looped fiber configuration. In the event of a short circuit, the fiber will instantly burn off and trigger a shutdown of the system. The results from the qualification test concluded that break detection fulfils the safety requirements regardless of the surrounding medium of the pipeline, and fiber location relative to the pipeline and cable fault. Distributed temperature sensing (DTS) is a method to monitor the temperature profile along a continuous FO element. The measuring principles of Raman and Brillouin are commonly used and verified to be accurate and effective mechanisms to measure the temperature profile of the fiber. The DTS system performance is based on a trade-off between accuracy, measurement time and distance range. This thesis reviews the limiting factors and discusses the reliability of DTS measurements for a DEH system. Raman and Brillouin based DTS systems can be applied for distances up to 40 km and 75 km, respectively, and the literature survey shows that both have a localization accuracy better than 1 m and temperature accuracy within ±1°C. The possibility of using DTS for fault detection in DEH systems was explored experimentally by conducting Raman and Brillouin based DTS measurements. The correlation of distance, accuracy and measurement time was investigated. The results indicate that the measurement times for DTS are too long to detect a short circuit situation. The measurement time can be decreased by lowering the accuracy, which also will decrease the ability to detect hot-spots. The measuring units will display an alarm message immediately after a fiber break, which theoretically could be used to switch off the electricity. This would however then be a break detection system and a violation of patent.