Study On Key Technology Of Distributed Optic-Fiber Sensing System Based On Sagnac Interfering Principle For Long-Distance Pipelines Sabotage Acts

Long distance oil/gas pipelines have the features of high temperature, high handed, flammability and explosive. If they are leakage, it will trigger great safety misadventure and lead to huge economic losses. For the past few years, the breakage acts such as stealing oil and barbarous construction threat the safety of pipelines seriously. The existing monitoring method can only detect the leakage position passively after it happened, therefore it cannot avoid losses. So it has big social and economic benefits to realize the early alarm of sabotage which would threat the safety of pipelines. The distributed fiber sensing base on Sagnac interference early warning system has become the research emphases because it has lots of advantages such as high sensitivity, easy to fix and maintain. But if it's asked to use in true pipeline safety monitoring, it has many technology problem to resolve. The paper concretely researches several key technologies of system as bellow:①To aware of the characteristics of sabotage, the hypocenter vibration model and earth vibration model are formed after analyzing the generation and transmitting mechanics of the earth vibration signals caused by the destroying behavior such as digging and boring. According to the function style Of P wave on the fiber, optical sensing model is proposed. It is tested and analyzed from time domain and frequency domain that the characteristics of typical sabotage agree with the theoretical model. It's proved sabotage of multi-frequency will induce the phaseshift with the same frequency ingredients in distributed fiber sensor, and proposed methods to recognize the type of sabotage acts.②To raise the sensitivity of Sagnac interference system, it derives the characters of 2×2 coupler and 3×3 coupler from the transverse coupling theory of optical waveguide. It's proved that the sensitivity of Sagnac interference system can be raised when the 2×2 coupler is replaced by 3×3 coupler.③It proposed an innovative interference early warning system based on Saganc theory applying a 3×3 coupler to overcome the demodulation method which depends on the frequency characteristic of breakage acts. It adds a delay fiber ring, uses pulse light source, and it can be proved through theoretical derivation and simulated experiment that: the position of vibration happened to sensing fiber can be detected by comparing the amplitudes of multiple response, the mathematical model can be posted exactly, and the localization is independent on the characteristics of external influence.④Based on the theoretical research, the experimental system is constructed which has 10 kilometers sensing length. Using phase modulator to simulate the breakage acts, it's derived the localization accuracy which is less than 100 meters. It's proved from the result that through the key technologies researching of distributed optical sensing and early warning system, the distributed optical fiber sensing system becomes more sensitivity, the structure is reasonable, its localization is more accurately and the system would have prodigious utility value.