Dynamic Optical Fiber Sensing With Brillouin Optical Time Domain Reflectometry: Application to Pipeline Vibration Monitoring

Brillouin distributed optical fiber strain sensors are often limited to static events because of necessary long acquisition time due to the frequency sweep technique. Dynamic distributed strain monitoring is necessary to ensure the integrity of linear structures submitted to fatigue loading or vibrations, such as sub-sea risers or flowlines, overhead pipelines, long bridges, railways, or high-rise towers. In this paper, a novel dynamic Brillouin optical time domain reflectometer for single ended truly distributed strain measurement based on the slope-assisted method is presented. The optical local oscillator frequency is adjusted to measure Brillouin backscattered power at the maximum slope of the Brillouin gain spectrum along the fiber length. Any strain variation inducing Brillouin frequency shift will be detected by amplitude variation. To demonstrate the performance of our acquisition system, three optical fibers have been implemented along a 10 m steel pipe submitted to vibration. Through this mockup testing, we were able to measure the oscillation damping time and the geometrical displacement of the pipe in real time. A 7.6 Hz acquisition rate was achieved with a strain error of +/- 40 mu strain and with a spatial resolution of 1 m. From integration of longitudinal strains, pipe displacement is calculated with an error of +/- 12 mm. A performance comparison between frequency sweep and slope assisted method is made with a 2 km fiber sensor.