Application research of distributed optical fiber sensing technology used in safety monitoring of coalbed methane pipelines

Abstract

In recent years, the demand for safe coalbed methane pipelines has dramatically increased in our society. With the superior advantages of large-scale monitoring, low cost, high sensitivity, quick response and immunity to electromagnetic interference, distributed optical fiber sensors provide an appropriate solution for large scale pipeline safety monitoring. Among them, Brillouin optical time domain reflectometer (BOTDR) and phase sensitive optical time domain reflectometer (Φ-OTDR) are fast developing scientific techniques and attract a significant amount of research attention. A multi-parameter measurement scheme based on BOTDR and Φ-OTDR is designed to monitor the strain and vibration conditions of the pipeline, respectively. In the BOTDR system, the pulse probe light modulated by an acoustic optical modulator (AOM) is launched into the sensing optical fibers and the beat frequency signal is obtained through coherent detection. Then, the Brillouin gain spectrum is figured out via heterodyne frequency sweeping. Ultimately the curve of Brillouin frequency shift versus sensing fiber length is plotted to realize the distributed strain measurement. In the Φ-OTDR system, light pulses from a laser with narrow linewidth and low frequency drift are gated into the fiber through an AOM, then amplified by erbium-doped fiber amplifier (EDFA), and finally coupled into the sensing fiber via a circulator. The Rayleigh backscattered light are monitored with a photodector (PD). The vibration position can be detected by subtracting the post-vibration Φ-OTDR trace from an earlier stored trace. The experimental results demonstrated that over a fiber sensing length more than 10 km, the accuracy of Brillouin frequency shift at the far end of the sensing fiber was ±0.88 MHz, which corresponds to the strain accuracy of 40 µe, and a vibration measurement with a positional accuracy less than 20m is also achieved, which can satisfy the practical requirements and shows considerable application potential for coalbed methane pipelines safety monitoring.