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

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 800psig pressure, the observed microstrain (με) was approximately 80, 65, 50, and 35με at 100, 75.0, 46.8, and 23.4 RH%, respectively. Comparatively, a microstrain of approximately 95με 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 70με 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.