Quasi-Distributed Low-Coherent Optical Sensors for In-Field Monitoring the Harmonic Settlement of Oil Tank

Abstract

Ground settlement (GS) developed around the base of an oil tank, also referred to as harmonic settlement, threatens the safety of the tank. However, the evolving process of the GS pattern is difficult to monitor due to its extremely slow changes, typically at a rate of a few millimeters per year. This requires a GS sensor bearing with excellent stability, and immunity to environmental temperature, by which it can work in temperature difference accord. This difference is created by the oil tank itself between its sunlit front and shadow occasionally over 30 °C. Therefore, until now, there still lacks of an effective method to approach it practically. In this article, a quasi-distributed low-coherent optical system composed of eight GS sensors is configured to monitor the GS evolving of a practical in-service oil tank. After one-year continuous practical observation, the long-term GS pattern determined by geographical variations is recognized. Combined with the results of dynamic loading caused by oil in or out, the periodical thermal load from solar irradiation, the long-term trend variation in the GS pattern is studied. A conclusion is derived that the dynamic loading and periodical thermal load overlap with its trend variation in a modulating way to affect the long-term GS pattern evolving.