Accidental fire or explosion at an onshore major accident hazard pipeline 

Impact 5
upper risk error bar
risk indicator
lower impact error bar
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


This risk concerns an onshore major accident hazard pipeline (MAHP). These pipelines transport flammable and toxic materials with the potential to cause major accidents if accidently released. Operators of these pipelines have a legal duty to prevent accidents and to mitigate their consequences. The Health and Safety Executive develops and enforces legislation, standards, codes of practice and guidance to ensure that operators fulfil their responsibilities effectively.


The reasonable worst-case scenario is based on an accidental fire or explosion occurring at a MAHP situated close to an urban area. The ignition of flammable gas or liquids under high pressure would result in a loud explosion, which could cause a crater, building damage and require evacuation from homes. The fire may continue to burn until the pipeline is isolated. The fire or explosion would result in casualties and fatalities. Some specialist medical services such as intensive care or burns treatment may be required.

Key assumptions

It is assumed that the incident is accidental and would involve a loss of containment, producing a cloud of gas or vapour as well as the results of a fire or explosion. This is likely to result in substantial short-term demands on emergency responders, however this should not continue over an extended period of time as pipelines can be isolated.


The range and severity of anticipated impacts could be affected by the location of a pipeline, its contents, design, pressure and construction, the weather and the time of day/night.

Response capability requirements

The Pipelines Safety Regulations 1996 require both a local authority and the pipeline operator to prepare emergency plans for MAHP. There would be a need for specialist treatment, surge capacities and appropriate recovery and storage for no-notice mass fatalities and casualties. Temporary evacuation and shelter for displaced people, site clearance plans and infrastructure repair may be required.


The main health effects of exposure will be experienced in the initial incident. There may be long lasting effects for people with continued respiratory issues, but long-latency ill health effects are unlikely. Psychological support may need to be made available to those affected. If the incident involves a high-pressure gas pipeline, there may be some local disruption to supply while repairs take place.