Gas pipeline emergencies
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The Pipeline Safety Regulations 1996 task local authorities with the production of emergency plans or the modification of existing plans, to cover certain Major Accident Hazard Pipelines (MAHP) within their area. The Health and Safety Executive notify the Council of those pipelines that require plans in the region.
The following gives an overview of the emergency response to a pipeline incident, showing how the existing procedures of various organisations would be applied and co-ordinated. It follows the recommended national format for Pipeline Safety Plans.
Pipeline System
The operator for the notified Major Accident Hazard Pipelines in this area is National Grid. The product carried is natural gas.
National Grid supply maps showing pipeline routes and data to the Emergency Planning & Business Continuity Team. This information is shared with the Fire Service. The details recorded include data on bore diameter, operating pressure and above ground equipment such as valves and pumping stations. For security reasons this information is not available to the public. The Local Resilience Unit can request the Pipeline Major Accident Prevention Document containing the above information from National Grid.
Hazard and Effect
An incident involving a high pressure gas pipeline is readily identified by the following features:
Release of gas - Significant damage to a Major Accident Hazard Pipeline that results in a pipeline puncture or rupture will lead to a pressurised release of natural gas. All pipelines operating under 75 bar (which is used for the National Transmission System) contain odorised gas.
If ignited this may give rise to a thermal radiation hazard to individuals in the vicinity. Ignition can be immediate, delayed local ignition, delayed remote ignition, or no ignition at all. Each may have different consequences, hazard ranges and duration and for this reason time scale and sequence of any incident will vary.
Duration of pipeline leaks - When a high-pressure pipeline fails, immediate and rapid de-pressurisation occurs over a matter of seconds, and is followed by relatively stable flow as the pipeline unpacks due to the leak and continued pumping of gas into the pipeline. Flow may last for several hours dependent on the location and topography of the pipeline and the time for National Grid personnel to arrive on site to shut down valves not shut remotely from the National Grid Control Centre.
Blast Effects and Projectiles - The pressure blast at the time of failure can be significant in close proximity to the pipeline, cover material over the pipeline may be thrown into the air at high velocity, but the serious effects will diminish with distance. Delayed ignition in the vicinity of buildings may result in loss of window glass as a result of blast over pressure.
Fire and Explosion - The ignition of any release of gas will cause a flare, which may have serious effects due to thermal radiation. People can be shielded indoors but radiation levels may be sufficient for the buildings to catch fire. Techniques are available for estimating the thermal radiation from an estimated quantity of gas released over time. Any failure of pipelines carries the risk of ignition, but experience has shown that in the majority of cases ignition does not occur.
If a release of gas does not ignite immediately, it will form a cloud, which will disperse over large distances. If a cloud of gas ignites it may burn back as a flash fire to the point of origin. As it disperses it will be diluted with air, the concentration falling below the lower explosive level (LEL) when it will no longer present a fire hazard. The distance over which such a release may disperse depends on the type of release and the prevailing weather conditions.
Concentrations and duration may be estimated using plume modelling.
It is important that ignited gas is not extinguished unless specifically requested by the National Grid on site controller.
Noise - The release of high-pressure gas creates a great deal of noise, which can be very intense leading to temporary hearing damage. High noise levels can also be disorientating and may cause unexpected behaviour in people affected.
Hazard Range and Emergency Planning Distances - National Grid have calculated hazard information related to thermal radiation. This is supplied to the Fire Service and is also available to planning departments in local authorities.
Plan Activation
In view of the extreme nature of a high-pressure pipeline failure it is likely that initial notification will be by a member of the public either by 999 call or to the gas emergency number.
The Emergency Services and National Grid will follow their standard procedures to investigate reports. If at any stage they identify a requirement for additional resources, a mutual decision is taken to notify other organisations and undertake wider co-ordination. This process is the same as that employed in response to other major incidents.