Multi-parameter optical fiber sensing of humidity, CH4, CO2, and corrosion

Abstract

Internal corrosion in natural gas transmission pipelines can occur through the condensation of water droplets onto the interior of the pipe. Use of optical fiber sensors (OFS) has been demonstrated to be very successful for distributed sensing of humidity and corrosion under pressurized natural gas pipeline conditions. In this work, the capability of the OFS for successful monitoring of humidity has been extended to monitor the humidity, CH4, and CO2 with different gas composition based on the strain produced along the single-mode fiber (SMF) sensor. This is enabled by absorption of H2O/gases onto the commercially available polyacrylate coated jacketed portion of the fiber resulting in a change in strain. Under equilibrium, a differential microstrain was observed along the jacketed portion of the SMF with N2, CH4, and CO2 at different relative humidity (RH) conditions, while the unjacketed portion of the fiber was used only for sensing pressure/temperature induced strain. In the case of N2 at 800 psig pressure, the observed microstrain (µε) was approximately 80, 65, 50, and 35 µε at 100, 75.0, 46.8, and 23.4 RH%, respectively. Comparatively, a microstrain of approximately 95 µε was observed with 100% RH CH4 which demonstrates that SMF produces a measurable CH4 response alongside water. Similarly, the observed microstrain with CO2 was approximately 105, 90, 85, 80, and 70 µε at 100, 75.0, 46.8, 23.4, and 0 RH%, respectively. The strain response of the SMF under various mixed gas composition and different RH conditions were also measured and made calibration curves accordingly. Linear regression and principal component analysis of these datasets provided deconvolution of the impact of strain from H2O, N2, CH4, and CO2. Additionally, modified OFS comprised of the Fe coated fiber section was employed to monitor corrosion based on the increase in backscattered intensity amplitude of the light being passed once corrosion of Fe occurs. The corrosion rates were studied by monitoring the rate at which the intensity of backscattered light amplitude attains a steady state value when complete corrosion of Fe with a specific coating thickness occurs.