Field validation of a novel, near real-time methane monitoring system for vent and leak detection

National Grid Plc is incentivised by the Office of Gas and Electricity Markets (Ofgem) to conduct research into new techniques to enable National Grid Gas (NGG) to improve understanding and transparency, and allow for cost-effective mitigation, of greenhouse gas (GHG) venting on the National Transmission System (NTS). The mechanism by which this is done is special condition 8J of the Gas Transporters Licence, the Greenhouse Gas Investigation Mechanism (GHGIM).

Since 2008 NGG has conducted 4 yearly fugitive emission surveys of ground level accessible valves and pipework on all 23 compressor stations and terminals of the National Transmission System (NTS). However, these surveys only provide a 'snap-shot' of emissions and are limited to accessible pipework not including pits and vents. In addition, long term boundary fence-line methane monitoring cannot be used to locate sources of emissions. 

The aim of this project was to develop a cost effective methodology to enable NGG to monitor and control fugitive emissions from above ground installations (AGI) on the National Transmissions System (NTS), and to understand both planned and unplanned venting events. The project involved trialling a method to assess the practicality, performance and cost effectiveness of the approach. In addition there was a validation of the "portability" of the equipment to expand application beyond AGIs. 

The technique trialled was the use of a highly accurate and sensitive real-time methane gas analyser to monitor at an array of sample points from locations around the boundary fence of an AGI. These measurements combined with a continuously updated reverse dispersion model provided hourly mass emission measurements for the facility. The results of the reverse dispersion model provided a focus for operational staff on the installation to investigate the detected leaks through physical inspection of the highlighted area and sniffing techniques. Where available optical gas imaging (OGI) was used and leaks quantified using high-flow samplers in order to prioritise leak repair.

The project is ongoing but it is hoped it will provide a more accurate picture and understanding of total site fugitive emissions on a continuous basis. The continuous monitoring should allow the investigation of different valve configurations, process sequences and isolations to minimise emissions while operating and maintaining the equipment, whilst maintaining availability of compressor units. The improved understanding and quantification of the emissions should enable a better cost/benefit analysis for investment plans for areas such as asset health and defect remediation. This is an area where it has always been difficult to factor in the cost of emissions both from a resource loss and an environmental impact perspective as they have not previously been fully identified or costs fully quantified.