Planned venting can arise from a number of sources around the network, including venting at compressor sites and pipeline decommissioning for repair, replacement or modification. Planned venting at compressor sites is monitored and recorded through the on-line control system. For 2011/12, this was reported as 2984 tonnes of natural gas.
For pipeline decommissioning current best practice employed by the Pipeline Maintenance Centre (PMC) involves transfer of gas from the decommissioned section to an active section by means of a compressor but this is only feasible until the decommissioned section pressure reaches 7 bar. At this pressure the Portable Recompression Equipment operation cannot increase the pressure sufficiently to transfer the gas to the active section. So the final operation during decommissioning is to then vent the remaining gas.
To improve the environmental performance of final stages of the decommissioning process several options are available including:
Collect the gas and use either on the decommissioned site or elsewhere with the network
Flare the gas. Methane is recognised as having a significantly greater “Global Warming Potential”
(GWP) than carbon dioxide, approximately twenty times. Thus flaring will reduce the environmental impact. The programme of works looks to address some of these issues arising from the need for National Grid to vent either pipelines or compressor pipe sections as part of the normal operation either for maintenance requirements or as part of the normal control sequence.
Its scope looks to cover the following areas:
Developed a venting decision/logging support tool
Conduct trials to demonstrate mobile recompression equipment
To develop and test the suitability of large scale adsorbed natural gas (ANG) technology to constructively reuse gas that would otherwise been vented to atmosphere.
This project is world leading with respect to the scale of the adsorbed natural gas (ANG) trials and will therefore deliver both a specific piece of new equipment and specific novel operational practices associated with the operation of the NTS.
The programme will provide a comprehensive set of experimental and design information relating to the behaviour of large scale ANG natural gas capture. The results will confirm whether ANG is a viable natural gas capture technology and will provide valuable design information for the assessment of suitable utilisation technologies for the captured gas.
The delivery of the ANG programme will provide National Grid with a fundamental building block towards a venting capture and fugitive emissions ‘road map’ giving the company the opportunity of employing a holistic natural gas capture strategy across the network.