Memorandum submitted by National Grid
National Grid electricity and gas engineers
worked to maintain security of energy supply throughout the adverse
weather and heavy flooding affecting South Yorkshire and Gloucester
during late June and July 2007.
As outlined in the report below, the effects
of the weather conditions in South Yorkshire during the week of
25 June impacted on National Grid's operations and assets in a
number of ways:
Firstly, risks posed by the potential
breach of the Ulley Dam in South Yorkshire impacted on National
Grid's gas distribution operations at the Above Ground Installation
(AGI) at Guilthwaite and in addition Ulley Dam posed a risk to
National Grid's electricity substation at Brinsworth.
Secondly, as a result of severe weather,
National Grid's Neepsend electricity substation was affected by
flood waters, which led to the local electricity distribution
network, CE Electric losing electricity supplies from this supply
Thirdly, National Grid's gas distribution
network was affected in the Sheffield and Toll Bar region by a
number of weather related incidents affecting gas supply pipelines
at Kilham farm and Toll Bar detailed in the report below.
National Grid's operations and assets were also
affected in July 2007, as a result of flooding in the Gloucester
National Grid's Walham substation
was affected by the adverse weather conditions on 21 July which
led to parts of the substation becoming inundated with water.
National Grid engineers reconfigured electricity systems to ensure
continued electricity supply to Gloucester and South Wales.
National Grid has a number of measures in place
to assess flood risks to its electricity and gas assets and has
carried out a number of assessments on infrastructure in recent
years. Whilst flooding has not been a cause of significant unreliability
or unsupplied energy, with concerns over climate change and rising
sea levels, National Grid is working to understand the impact
of any consequent increase in flood risk, and the requirement
for further risk mitigation at our sites to ensure a high level
of reliability is maintained.
National Grid is participating in an Energy
Networks Association (ENA) review group, addressing electricity
resilience issues, which has been established at the request of
Energy Minister, Malcolm Wicks. This group will report to the
Energy Emergencies Executive Committee.
1. National Grid plc owns and operates the
high voltage electricity transmission system in England and Wales,
and operates the Scottish high voltage transmission system. National
Grid also owns and operates the gas transmission system in Britain
and distributes gas in the heart of England, to approximately
11 million offices, schools and homes. National Grid also manages
electricity and gas assets in the US, where it operates in the
states of New England and New York.
2. In addition, National Grid owns and operates
other energy infrastructure such as liquefied natural gas importation
facilities at Grain and the electricity interconnector with France.
National Grid owns around 20 million gas meters in Britain and
is at the forefront of gas and electricity smart metering competition.
3. National Grid is pleased to have the
opportunity to contribute to this inquiry and can report the following
account of the effects of the flooding that occurred during summer
2007 on its electricity and gas infrastructure.
4. The majority of the electricity transmission
system was designed and constructed in the 1960's. Subsequent
development has been incremental and generally in close proximity
to original installations. Flood risk was considered during the
original planning process in accordance with our procedures at
the time, using the best information available at that point.
5. The majority of the gas National Transmission
System (NTS) and Local Transmission System including fixed above
ground installations (AGI's) was constructed between 1965 and
1982. At construction, installations were frequently sited to
utilise brownfield sites from pre-existing gas distribution installations.
Flood risk was evaluated as part of their original design. As
with electricity transmission, in recent years the majority of
investment on the gas National Transmission System (NTS) has been
associated with the development of existing sites.
6. In developing existing sites or establishing
new sites National Grid is obliged to put forward designs which
take into account a range of technical, environmental and commercial
drivers with the aim of achieving:
The requirements of the connected
Compliance with the relevant planning
Minimal impact on the public, particularly
in terms of visual amenity or noise.
The most economic and efficient solution.
Compliance with the relevant National
Grid design standards.
7. Within the last 10 years only a relatively
small number of greenfield sites have been developed for either
gas or electricity transmission. As part of the design process
Environment Agency flood contour information is considered along
with other available information to inform site selection.
8. The introduction of Planning Policy Statement
25 (PPS25) in 2006 placed additional responsibilities on National
Grid as electricity substations and gas compressor stations are
defined as "Essential Infrastructure" within the statement.
Any proposed development in a Medium Probability Flood Zone or
above (greater than 1 in 1000 risk of flooding) is subject to
greater scrutiny with respect to site selection. Where development
is to go ahead in a high probability flood zone the design must
ensure that the site remains safe and operational during a flood.
9. National Grid's overhead line, cable
and buried pipeline assets are generally secure against flood
risk, with the main risks posed by land erosion, and movement
of structures caused by floods near transmission assets. However,
electricity substations and above ground installations (AGI's)
associated with gas transmission are not immune and inundation
can result in the need to disconnect critical plant and equipment.
10. Following the flooding that took place
in the South of England during 2001, National Grid undertook a
review of sites within "flood warning areas" as defined
by the Environment Agency at the time. Whilst events drew attention
to flood risk, National Grid's response was focussed on mitigating
the effects of our operations on the environment. To this end
we have an on-going environmental investment programme, spending
£108 million over the last 6 years.
11. Following the floods affecting Carlisle
in 2005, National Grid reviewed flood risk at its transmission
infrastructure sites using flood risk contour data available from
the Environment Agency. In total, 23 sites were identified and
considered to be at risk. This work has been revisited recently,
using the latest Environment Agency flood contour data. It is
now the case that 28 electricity substations and 7 gas installations
have been identified as at significant risk of flooding (greater
than 1 in 75 risk of flooding in any year).
12. In addition, during 2005, National Grid
commissioned engineering consultants to consider methods of assessing
actual flood risk at electricity substation sites. In 2006 National
Grid initiated more detailed site survey work with our engineering
consultants to establish indicative costs of mitigation against
flooding at high risk sites.
13. National Grid reviewed gas transmission
sites in 2006 to address installations at risk to flooding and
also commissioned an engineering review in July 2007 to consider
the risk posed by different levels of flood water.
14. Further work is on-going, for electricity
transmission, gas transmission and gas distribution, to determine
the level of flood waters necessary to render specific sites non-operational
and to determine the specific equipment that will first become
affected in the event of flooding.
15. Whilst flooding has not been a cause
of significant unreliability or unsupplied energy, with general
concerns over climate change and rising sea levels and the consequent
increase in flood risk, further consideration is being given to
risk mitigation at our sites to ensure a high level of reliability
16. Recent events have provided the opportunity
to test emergency response procedures and National Grid's ability
to react to unplanned events.
17. National Grid has initiated further
research and development work to improve the understanding of
the linkage between flood risk, weather events and levels of monitored
waters so that we can better identify flood risk. Most recently,
National Grid has collaborated and co-funded industry-wide research
and development with the Meteorological Office Hadleigh Research
Centre to understand and prepare for the wider impacts of climate
change, including assessment of likely flood scenarios on transmission
operations. The work is due to report initially in late 2008.
18. National Grid will take forward the
procurement of mobile flood defences to provide interim cover
catering for low probability events and to provide defence against
surface water flooding not covered by Environment Agency risk
models. The need for any further investment at our sites will
be discussed with our regulator, Ofgem.
19. National Grid is participating in the
Energy Networks Association review group, to examine the resilience
of electricity substations. The review has been established at
the request of Malcolm Wicks and will report to the Energy Emergencies
Executive Committee. National Grid will work with the Environment
Agency and other stakeholders to provide a co-ordinated input
to this review.
Floods in South Yorkshire Monday 25 JuneFriday
29 June 2007
20. The "Sheffield Ring" is a
group of circuits and substations which transports electricity
to the Sheffield conurbation and allows the transit of power more
generally across the country. The Sheffield conurbation is supported
by the broader transmission system through connections principally
at Brinsworth, Neepsend and Thorpe Marsh 275kV substations. (see
appendix i).The flooding affecting these sites on week commencing
Monday 25 June resulted in a loss of supply to the local electricity
distribution network, CE Electric which is fed from National Grid's
21. National Grid site staff became aware
of the flooding at Neepsend substation at midday on Monday 25
June and raised the issue with their local management team at
12:30. Water levels rose rapidly and as such National Grid's site
staff were evacuated for safety reasons at approximately 13:15.
22. Due to the risk of loss of supply caused
by the flooding, Electricity National Control Centre staff and
CE Electric Control coordinated efforts to transfer as much electricity
demand as possible away from Neepsend substation in order to ensure
security of supply to the area.
23. At 15:23, approximately 20MW of demand
was successfully transferred at which point circuit-breakers on
the transformer circuits between National Grid's substation and
CE Electric substation opened without instruction, due to the
effects of flood water on control and protection equipment. This
resulted in the loss of approximately 38MW of demand, affecting
an estimated 36,000 CE Electric domestic and commercial customers.
24. Further reports from National Grid's
site staff indicated that the floodwaters inside Neepsend substation
had reached a depth of 1.2-1.5 m. The 275kV circuits connecting
Neepsend substation to the rest of the transmission system were
opened in order to de-energise the substation for safety reasons.
25. Following the flooding at Neepsend substation,
a flood risk assessment at other National Grid substations in
the Sheffield area was undertaken. The outcome of this assessment
showed no risk of breach of flood defences at Brinsworth substation
and whilst there were access problems at Thorpe Marsh, the sites
operations remained secure (the 275kV and 400kV substations are
raised relative to the local ground level and site access roads).
National Grid staff remained at Thorpe Marsh substation in order
to manage any potential further difficulties.
26. At 01:15 on the morning of Tuesday 26
June, National Grid managers joined the Gold Command and were
alerted to the possible risk to Brinsworth substation from the
Ulley dam. Staff were evacuated from Brinsworth substation and
the local area. The substation remained fully operational under
the control of National Grid's Electricity National Control Centre
27. The impact of dam breach on Brinsworth
substation was considered and contingency plans were prepared
for the eventuality of having to make Brinsworth substation dead
(in the event of deluge from the dam waters).
28. Concurrently, the flood risk at National
Grid's Thorpe Marsh substation was monitored through communication
with field staff and engagement with the Environment Agency. At
this time the advice was that National Grid's substation at Thorpe
Marsh was secure. However, water levels at Thorpe Marsh continued
to rise and by the evening of Tuesday 26 June water levels were
approached control and protection systems in the 400kV substation.
Sandbags were ordered to protect most critical parts of the site
and the army were engaged via Gold Command to provide boats to
facilitate movement around the site.
29. By early morning on the Wednesday 27
June, flood water started to affect vital site controls and protection
at National Grid's Thorpe Marsh 400kV substation. At 07:30 National
Grid engineers began to de-energise the 400kV substation circuits
in a controlled manner. The 275kV substation at Thorpe Marsh remained
fully operational as it is on higher ground than the 400kV substation,
and de-energisation of the 400kV substation did not result in
loss of supplies to any customers. A huge effort then ensued with
National Grid site staff, the military and fire brigade focussed
on constructing flood defences around critical plant and equipment.
At the same time, National Grid engineers monitored the situation
at National Grid's Brinsworth substation and the overhead line
circuit which was at risk from the potential breach of the Ulley
30. During Wednesday 27 June the protection
and control systems at Thorpe Marsh 400kV substation were secured
from flooding by National Grid field staff and the emergency services.
The substation was partially restored to operational service by
15:00 and fully restored by 01:50 on the morning of Thursday 28
31. By the evening of the Wednesday 27 June
water levels in the Ulley Dam had dropped significantly and the
risk of flooding at Brinsworth substation was significantly reduced.
32. At Neepsend substation, on the morning
of the Wednesday 27 June, co-ordinated efforts between CE Electric
and National Grid allowed restoration of electricity supplies
to all customers by Thursday 28 June. By Friday 29 June at 23:19,
National Grid restored sufficient electricity capacity to meet
all the local distribution company's energy demand.
33. National Grid engineers continued to
service the remainder of National Grid's Neepsend substation in
order to provide security of supply at Neepsend and restore resilience
on the electricity network.
34. National Grid operates the gas distribution
business serving the heart of England, where it transports gas
to over 11 million homes, offices and businesses. Events in Sheffield
during the week of 25 June-29 June impacted on gas operations
in a number of ways.
35. Following heavy rain on Monday 25 June,
fears that the Ulley Dam near Rotherham would breach posed a severe
risk to National Grid's AGI (Above Gas Installation) at Guilthwaite,
which is one of three primary supplies into the conurbation of
Sheffield. A number of measures were put in place by National
Grid managers and staff to ensure continuity of gas supply to
the Sheffield area.
36. An engineering solution was put in place
to protect gas supplies to Sheffield. Due to adverse weather conditions
and significant risk of the dam failing National Grid personnel
were not allowed on site at Guilthwaite, as such work took place
on Tuesday 26 June at locations either side of the AGI to reduce
pressure in both pipelines.
37. National Grid staff manned sites either
side of the Guilthwaite AGI right through until Friday 29 June
when the threat from the Ulley Dam subsided. At this point National
Grid engineers were able to re-establish pressure at the AGI.
Working with the Gold Command Unit, the Environment Agency and
other utilities, National Grid was able to ensure there were no
unplanned interruptions to gas supply as a result of the events
that took place at the Ulley Dam.
38. National Grid's gas distribution teams
also worked on a number of river crossings around the Sheffield
area affected by adverse weather and rising water levels in the
river Don during the week of 25 June and the following week. Maintaining
the safety of gas pipelines was the key issue with floodwaters
potentially affecting critical infrastructure.
39. A National Grid 24 inch diameter seven
bar pipeline at Kilham farm became exposed as a result of floodwaters
rising, and was found by National Grid gas engineers floating
in a field as a result of the breach of river Torne (appendix
ii). National Grid engineers worked in difficult conditions in
conjunction with control room and planning staff to prepare appropriate
contingency arrangements to ensure continuity of gas supply was
maintained along this route. In addition National Grid engineers
worked in close co-operation with the Environment Agency throughout
the operation at Kilham farm.
40. In spite of the significant flooding
at Toll Bar during the first week in July, National Grid, through
configuration of the gas network was able to prevent significant
water ingress (ie. flooding of gas main) into the gas network
thereby protecting supplies to customers in the Toll Bar area.
Thursday 21 July-Tuesday 24 July Walham
41. Walham substation provides approximately
470MW of electricity supplies via four transformers to 450,000
domestic and commercial customers of Central Networks Power distribution
company in the Gloucester area. Transmission circuits emanating
from Walham form a part of the interconnected transmission network
to South Wales supplying approximately 3,000 MW of demand (appendix
42. National Grid engineers and staff working
at Walham on Saturday 21 and Sunday 22 July carrying out planned
repair work were alerted by Electricity National Control Centre
(ENCC) staff that there was a risk of flooding in the area caused
by rising river levels.
43. During Sunday 22 July, ENCC staff developed
a contingency strategy in the event that National Grid's Walham
protection equipment was inundated by rising water. Through re-configuring
transmission circuits at Walham substation, National Grid ensured
that in the event of inundation of Walham substation, only one
of the two circuits feeding into South Wales from Walham would
be lost. This route supplemented the three other available transmission
circuits into South Wales ensuring that all demand in South Wales
would be secure even in the highly unlikely event of the worst
44. On Sunday 22 July water levels started
to rise. National Grid staff placed sandbags around the critical
circuit control cubicles and entry points to the protection (relay)
rooms at around 14:00. The fire brigade were also in attendance
to provide pumps working closely with National Grid staff to protect
45. Gold Command (Resilience) was established
by the authorities in Gloucester at on the Sunday 22 July. The
first Resilience meeting was held with National Grid in attendance.
Following this meeting the Environment Agency provided emergency
flood defence systems to be used at Walham later on the evening
of Sunday 22 July. The military attended Walham substation to
assist with the installation of the systems.
46. At 21:00, on the Sunday 22 July, one
of the four Walham transformers was de-energised and isolated
due to rising water levels.
47. The temporary flood defences withstood
the peak water level which occurred at around 05:30 on the morning
of Monday 23 July.However, it was expected that the river level
would peak again during the night of Monday 23/Tuesday 24 July
at the same or a higher level than had been seen the previous
day. National Grid engineers began switching operations to implement
the contingency strategy to protect electricity supplies to the
48. On Tuesday 24 July National Grid took
the decision to implement a more resilient flood defence system
at Walham substation, thus freeing up temporary resources which
were in place. This entailed the construction of the Hesco Bastion
barrier with the help of the armed forces (see appendix iv). This
structure was completed at 21:15 on the Saturday 28 July and remains
in place as a more permanent barrier. No further flooding was
experienced at National Grid's Walham substation after water subsided
on Tuesday 24 July.
49. The circuits at Walham were returned
to their normal operational configuration by 18:15 on the Tuesday
24 July. Through the implementation of contingency strategies,
and as a result of a significant effort by the armed forces, fire
brigade and National Grid engineers, electricity supplies were
maintained to the areas of Gloucester and Cheltenham.
(a) Geographic map of Sheffield and neighbouring
citiesNational Grid electricity substations marked in yellow.
(b) National Grid diagram of its electricity
system configuration in SheffieldMonday 25 June.
(ii) Photograph of National Grid gas pipeline
floating at Kilham Farm.
(a) Geographic map of Gloucester and neighbouring
citiesNational Grid Walham substation marked in yellow.
(b) National Grid diagram of its system configuration
including Walham substation.
(iv) Photograph of Hesco Bastion wall erected
at National Grid's Walham substation.
(v) National Grid Network magazine for employees
"Flood response special" July 2007
1 Not printed. Back