Distributed gas sensing using hollow core optical fibre

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Distributed optical fibre sensors can map measurements along an optical fibre, and are commercially deployed for the measurement of physical parameters such as temperature, strain and acoustic emission. They are used to monitor large structures such as pipelines, wind turbines, oil and gas wells, underground electricity cables, and geological sites used for carbon capture and storage. The fibre can be installed along or around the structure, and measurements read by a single interrogator box connected at one end. A large number of sensing sections can be monitored, e.g. every 1 m along a 100m - 1km length. The ability to map measurements in space and over time provides a rich source of information that has revolutionised the understanding and operation of large engineering structures.Distributed measurement is not currently available for chemical parameters. Our vision is to develop a new distributed sensor that can detect, locate and quantify gas concentrations along the length of an optical fibre. The sensor will combine the performance advantages of laser spectroscopic measurement of gas with the rich information content of distributed sensing. The process industries have well-established sensors for detection of potentially explosive hydrocarbons, including methane. For conventional point-by-point sensing, the UK Health and Safety Executive recommends that there should be no more than 5m between each sensor. If applied across an entire site, this would require thousands of sensors, therefore coverage is limited to safety-critical areas. Challenges include routine calibration, maintenance and replacement of sensors in difficult to access areas. Methane has a greenhouse warming potential of 32x that of carbon dioxide over a 100 year period. As the main constituent of natural gas and biogas, and a product of anaerobic processes within landfill sites and wastewater treatment, methane emission is an ongoing and significant problem. Reduction of fugitive methane leaks from industrial sites is recognised as an early target to reduce greenhouse emissions, but this requires a step change in measurement coverage. Site surveys can miss episodic leaks; to minimise emission requires a permanently available solution. The current generation of point sensors can miss fugitive leaks, which can originate from all over a site and be large enough to require mitigation, but may be below the detection threshold of existing flammable gas sensors. Many fugitive leaks are only detectable within a few metres downwind of the source, therefore an impractically large number of point sensors would be required for full site coverage.Currently available alternatives include passive thermal imaging of gas leaks, which has quantification / fail safety issues and requires a clear line of sight, and acoustic emission sensors, which can only find high-pressure leaks and are difficult to use in noisy environments. Site perimeter sensing using open-path optical (usually laser) beams can work well, but requires a clear line of sight and provides limited information on leak location.A new strategy is needed to complement these measures. Distributed optical fibre gas sensors have the potential to address the above issues; because gas can be measured along the sensor's entire length, with no significant gaps, leaks are less likely to be missed. Recent technology developments make a distributed gas sensor an enticing possibility. The Cranfield team has developed a novel interrogation instrument for gas detection in multiple sensing regions along a fibre, with high sensitivity. Southampton researchers have developed new hollow core fibres (HCFs); by drilling tiny side holes along the fibre length so that gas can enter the core, light can detect its presence. Together, we aim to develop a working demonstrator that can be field tested on simulated gas leaks.