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 Transco (formerly British Gas).
The product carried is natural gas.
Transco supply maps showing pipeline routes and
data to the Emergency Planning 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. Local Authority Planning and Emergency
Planning sections can request the Pipeline Major Accident
Prevention Document containing the above information from
Transco.
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 dependant on the location and topography of the
pipeline and the time for Transco personnel to arrive on site to
shut down valves not shut remotely from the Transco 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 Transco 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
effected.
Hazard Range and Emergency Planning
Distances - Transco 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 Transco 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.
There is a single national gas emergency number
for all notifications: 0800 111 999.
For Further Information:
- Contact: Senior Assistant
Director Community Services, Mansion House, Penrith CA11 7YG
- Email: engineer@eden.gov.uk
- Telephone: (01768) 212302
- Fax: (01768) 890732
