National Transmission System

The United Kingdom's National Transmission System (NTS) is the network of gas pipelines that supply gas to about forty power stations and large industrial users from natural gas terminals situated on the coast and to gas distribution companies that supply commercial and domestic users.



The National Transmission System (NTS) originated in the construction during 1962-3 of the 200 mile (320 km) long high-pressure methane pipeline from Canvey Island to Leeds.[1] Imported liquified natural gas (LNG) from Algeria was regasified at the Canvey terminal and supplied to the pipeline, this provided eight of the twelve Area Boards with access to natural gas.[2] The gas was initially used to manufacture town gas either as a feedstock in gas reforming processes or to enrich lean gases such as that produced by the Lurgi coal gasification process.

The pipeline was 18-inch (460 mm) in diameter and operated at 1,000 pounds per square inch (69 bar). The pipeline had 150 miles (240 km) of spur lines, ranging from 6 ⅝ to 14 inches (168–355 mm) in diameter,[3] to East Greenwich, Bromley, Slough, Reading, Hitchin, Dunstable, Coleshill, Sheffield, and Manchester. The Gas Council was responsible for this £10 million co-operative scheme and the construction details were a joint effort of the distribution engineers of the Area Boards.

LNG had first been imported to Canvey from Louisiana in February 1959 and piped to Romford gas works as feedstock to a reforming plant.[4]

UK natural gas

Natural gas was discovered on the UK continental shelf in 1965 and production started in 1967.[5] The development of offshore natural gas fields is shown in the following table. Shore terminals were built to receive, process, blend and distribute the gas.

UK sources of offshore natural gas, 1967-1985
Field Field type Licencee or operator Discovered First gas onshore Shore terminal
West Sole Gas BP September 1965 March 1967 Easington
Leman Gas Shell/Esso, Amoco/Gas Council, Arpet Group, Mobil April 1966 August 1968 Bacton
Hewett Gas Phillips Petroleum, Arpet group October 1966 July 1969 Bacton
Indefatigable Gas Shell/Esso, Amoco/Gas Council June 1966 October 1971 Bacton
Viking Gas Conoco/BNOC May 1968 July 1972 Viking (Theddlethorpe)
Rough Gas Amoco/Gas Council May 1968 October 1975 Easington
Forties Oil + associated gas BP October 1970 September 1977 St Fergus
Frigg (Norway) Gas Elf/Total June 1971 September 1977 St Fergus
Frigg (UK) Gas Elf/Total May 1972 September 1977 St Fergus
Piper Oil + associated gas Occidental group January 1973 November 1978 St Fergus
Tartan Oil + associated gas Texaco December 1974 January 1981 St Fergus
Brent Oil + associated gas Shell/Esso July 1971 1982 St Fergus
Morecambe Bay Gas + condensate Hydrocarbons (GB) September 1974 1985 Barrow

With the assured availability of natural gas a government White Paper on fuel policy[6][7] in November 1967 proposed that natural gas should be immediately and more extensively exploited The Gas Council and Area Boards began a ten-year programme to convert all users and appliances to operate on natural gas and consequently to discontinue the manufacture of town gas at local gasworks. In a pilot scheme users on Canvey Island had been converted to natural gas in 1966.[8]

Building the NTS

To exploit the availability of natural gas and to provide for more widespread distribution construction began of a major new transmission network which became the National Transmission System (NTS).[9]

Feeder pipelines - England

Gas from the West Sole field was first dispatched from the Easington terminal in July 1967, via Feeder No. 1 across the Humber to the East Midland Gas Board's gasworks at Killingholme. It was used to enrich low calorific value manufactured gas. Feeder No. 1 was extended to Totley near Sheffield where it connected to the 18-inch methane pipeline, UK natural gas first entered the NTS in July 1968.[10]

Feeder lines from the North Sea gas terminals to the spine of the NTS were laid and brought into use as the shore terminals were constructed.[11][12][13][14]

Feeder lines to the NTS
Feeder No. Diameter Length From To Operational
1 24-inch (600 mm) 90 miles (144 km) Easington terminal Scunthorpe and Totley near Sheffield, where it connects to the original methane pipeline. July 1967/ July 1968
2 36-inch (900 mm) 123 miles (197 km) Bacton terminal Brisley, Peterborough and Churchover near Rugby, where it connects to the original methane pipeline. August 1968
3 36-inch 107 miles (171 km) Bacton terminal Roudham Heath, Cambridge, Whitwell near Hitchin, where it connects to the original methane pipeline. October 1969
4 36-inch 154 miles (246 km) Bacton terminal Great Ryburgh, King's Lynn and Alrewas near Lichfield. Autumn 1970
5 36-inch Bacton terminal Yelverton, Diss, Chelmsford and Horndon, where it connects to the original methane pipeline Autumn 1971
6 30-inch (750 mm) 91 miles (146 km) Paull Pickering (see note), Westwood and Little Burden near Darlington Autumn 1971
7 36-inch Wisbech Hatton and Scunthorpe 1972
8 30-inch Viking (Theddlethorpe) terminal Hatton July 1972

The No. 6 feeder runs via Pickering which received gas from a treatment plant for the onshore Lockton gas field.[14]

Feeder pipelines - Scotland

North Sea gas first reached Scotland in Spring 1970 at Coldstream, this was via an extension of the Leeds-Newcastle pipeline. This pipeline was then extended to Glenmavis near Coatbridge Lanarkshire where a natural gas liquification plant was constructed.[14]

A major set of pipelines were constructed in Scotland in preparation for arrival of gas from the Frigg gas field in 1977. From the St Fergus terminal in Scotland, two 36-inch (900 mm) pipelines were laid via Bathgate to Partington and Bishop Auckland to connect to the NTS in England, a total pipeline length of 595 miles (950 km). These lines were commissioned in 1976 and cost £140 million. Initially these pipelines carried gas from southern England into Scotland until the Frigg field began production via St Fergus in September 1977. Compressor stations are provided at 40 mile (65 km) intervals along the pipelines. A third 36-inch pipeline from St Fergus was completed in 1978, and a fourth 40-inch (1050 mm) pipeline in 1982.[15]

Growth of the NTS

The NTS was extended from Leeds to Newcastle upon Tyne in early 1969.[14] This line was extended to Coldstream in Spring 1970 and then to Glenmavis near Coatbridge Lanarkshire.

The Wales Gas Board received natural gas supplies in 1969 from a 24-inch line from Churchover (Rugby) to Swansea via Wormington. North Wales was also connected in 1969 via a 24-inch/18-inch pipeline from Audley Cheshire to Maelor near Wrexham.[14]

The South Western Gas Board received natural gas at the end of 1970 from a 24-inch/20-inch pipeline from Wormington to Exeter.[14]

A 30-inch/24-inch extension of Feeder No. 3 runs to the west of London via Slough to Mogador Surrey and was commissioned in 1970. An extension of the Feeder No. 5 runs from Horndon-on-the Hill, crosses the Thames at Tilbury and runs via Shorne to connect to Mogador, thus completing the South London ring main, this became operational in early 1972.[14]

In addition to these distribution pipelines in 1971 Area Boards began to supply natural gas directly to major consumers. For example, a 24-inch 17 mile 'spine' pipeline was constructed to ICI Ltd at Billingham, and the West Midlands Gas Board laid six similar 'spine' mains into industrial districts of Birmingham and the Black Country.[14]

Most of the NTS was built from the late 1960s to the early 1980s, the growth of the system is shown in the following table.[16]

Growth of the NTS
Years NTS mileage Operational Gas Terminals Compressor stations
1966/7 320 Canvey 0
1968/9 688 Canvey Easington Bacton 0
1970/1 1898 Canvey Easington Bacton 1
1972/3 2199 Canvey Easington Bacton Theddlethorpe 4
1974/5 2308 Canvey Easington Bacton Theddlethorpe 9
1976/7 2915 Canvey Easington Bacton Theddlethorpe St. Fergus 10
1978/9 3047 Canvey Easington Bacton Theddlethorpe St. Fergus 11

The NTS now comprises over 7,600 km of welded steel gas pipelines. The Canvey to Leeds line is no longer part of the NTS.

LNG storage sites

In addition to the Canvey Island Liquefied Natural Gas (LNG) import terminal, further NGL storage sites were constructed from the late 1960s.[17] These were peak-shaving facilities used to support the NTS at times of high demand, and to ensure security of gas supplies at strategic locations. When demand was high liquefied natural gas was pumped from storage tanks, heated in vapourisers to a gaseous state and delivered into the NTS. When demand was low, gas was withdrawn from the NTS and liquefied by cryogenic cooling to minus 162 °C to replenish the storage tanks.

NTS Liquefied Natural Gas (LNG) Sites
Site LNG storage tank capacity Commissioned Decommissioned Operations
Canvey Island, Essex 6 x 4,000 tonnes, 2 x 1,000 tonnes, 4 x 21,000 tonnes[18] (underground) 1959-1967 1984[19] Import of LNG from Arzew Algeria, original contract for 1 billion m3 of gas a year (c.100 MMSCFD)
Ambergate, Derbyshire 5,000 tonnes[20] 1967-1970[18] 1985[21] Import of LNG by road tanker from Canvey, output 72 MMSCFD (2.04 million m3/d)[22]
Glenmavis, Lanarkshire 20,000 tonnes[20] 1971 2012 Liquefication 100 tonnes per day, vapourisation 250 MMSCFD (7 million m3/day)
Partington, Cheshire 2 x 20,000 tonnes[20] 1972[18] March 2012 Liquefication 10 MMSCFD (283,000 m3/day)[23]
Dynevor Arms, (Hirwaun) Aberdare, Glamorganshire 2 x 20,000 tonnes[20] 1972 March 2009 Liquefication 10 MMSCFD (283,000 m3/day)[23]
Avonmouth, Bristol 3 x 50,000 m3 1974-1980[18] April 2016[24] Short-term storage
Isle of Grain, Kent 4 x 50,000 m3, 4 x 190,000 m3 1980-2010 Operating Vapourisation 58 million m3/day. See Grain LNG Terminal

High-pressure gas storage

In addition to LNG storage for peak-shaving, several sites had storage facilities for high pressure gas that could be released into, and pressurised from, the NTS. The following sites were operational by 1972.[25]


NTS is the starting point for UK gas distribution. The pipeline system for houses is not part of the NTS, but is part of the Gas Distribution Network of Local Distribution Zones, the two systems combine to form the UK's gas distribution network.

The two types of gas pipelines in the UK are: large diameter high-pressure (up to 85[26] bar and 1050 mm[26] diameter) pipelines - the type that the NTS uses, and smaller diameter lower pressure pipelines that connect to users who burn gas for heat. The wall thickness of the high pressure pipelines is up to 0.625-inches (18mm).


Gas currently enters the NTS from a number of sources:

Gas specification and composition

The specification of gas transported within the NTS is typically within the following parameters.[27][28][29][30]

Specification of gas in the NTS
Content or Characteristic Value
Gross Calorific Value 37.0 – 44.5 MJ/m3
Wobbe Number* 47.2 – 51.41 MJ/m3
Water Dewpoint <-10 °C @ 85barg
Hydrocarbon Dewpoint <-2 °C
Hydrogen Sulphide content* ≤5 mg/m3
Total Sulphur content (including H2S)* ≤50 mg/m3
Hydrogen content* ≤0.1% (molar)
Oxygen content* ≤0.2% (molar)
Carbon Dioxide content ≤2.0% (molar)
Nitrogen content <5.0% (molar)
Total Inerts <7.0%
Incomplete Combustion Factor* ≤0.48
Soot Index* ≤0.60

Parameters marked * are specified in the Gas Safety (Management) Regulations 1996.

The composition of natural gas in the NTS is typically as follows.[31]

Composition of natural gas
Component Volume %
Methane 93.63
Ethane 3.25
Propane 0.69
Butane 0.27
Other hydrocarbons 0.20
Nitrogen 1.78
Carbon Dioxide 0.13
Helium 0.05

Compressor stations

There are twenty five (mostly gas turbine driven) compressor stations and over 25 pressure regulators. Gas moves through the NTS at speeds up to 25 mph (40 m/s) depending on pressures and pipeline diameters. Compressor stations generally operate at a pressure ratio of 1:1.4, this ratio is a balance between maintaining pressure and hence flow, and the capital and running cost of the compressors. It also ensures that the temperature rise across the compressors is not high enough to require after-coolers to prevent damage to the pipeline protective coatings. On the pipelines from St Fergus, compressor stations are provided at 40 mile (65 km) intervals; each compresses the gas from about 48 bar at 5 °C to 65 bar at 45 °C.[32]

Compressor stations include:


Offtakes from the NTS include those supplying industrial users, local distribution networks, storage sites and export pipelines.

NTS Gas Distribution Network offtakes
National Grid Area Number of LDZ offtakes
Scotland 19
Northern 15
South West 13
East Midlands 13
West Midlands 12
North West 11
East Anglia 11
North East 9
Southern 8
North Thames 5
South East 5
Wales 3
Total 124

Gas distribution network

Companies that own part of this gas network, also known as the Local Transmission System (LTS), are known officially as Gas Transporters. Gas enters this network via the NTS through a pressure reduction station to the twelve gas distribution zones in England, Scotland and Wales within eight distribution networks. The network covers 275,000 km (171,000 mi). The LTS is managed from Hinckley, Leicestershire (former headquarters of the NTS). Financial transactions between gas transporters are managed by Xoserve, based in Solihull. It was formerly an internal department of National Grid and then became an independent company.

For retail distribution, National Grid plc (this division is based in Northampton) owns the network in North West England, the West Midlands, the East Midlands, the East of England and North London. In the North of England, local distribution is owned by Northern Gas Networks; in the Wales and West by Wales and West Utilities; and in Southern England and Scotland by SGN.

Ownership of the NTS

The changing ownership of the NTS reflects developments and corporate changes in the UK gas and energy industries.

NG is administratively based in Warwick. While NG owns the gas transmission system in England, Wales and Scotland, electrical distribution in Scotland is owned by two Scottish companies.

Northern Ireland

Northern Ireland is not part of the NTS per se and gets its gas via the Scotland-Northern Ireland pipeline (SNIP) owned by Premier Transmission and built between 1994 and 1996. The gas network in Northern Ireland is split, with one area owned by Phoenix Natural Gas and the other by Firmus Energy.

See also


  1. Williams, Trevor I. (1981). A History of the British Gas Industry. Oxford University Press. p. 147.
  2. "Origins and growth of the British Gas Plant Operations Department" (PDF).
  3. Wilson, D. Scott (1969). The Modern Gas Industry. Edward Arnold Ltd. p. 43.
  4. Falkus, Malcolm (1988). Always under Pressure: A History of North Thames Gas since 1949. Macmillan. pp. 63 & 74.
  5. Falcus, Malcolm (1988). Always under Pressure: A history of North Thames Gas since 1949. Macmillan. pp. 76 & 108.
  6. Fuel Policy, Cmnd. 3438, HMSO, London (1967).
  7. Williams, Trevor I. (1981). A History of the British Gas Industry. Oxford University Press. pp. 210–11.
  8. Williams, Trevor I. (1981). A History of the British Gas Industry. Oxford University Press. pp. 182–9.
  9. John Ellis 'The Origins and Growth of the British Gas Plant Operations Department', 2014.
  10. Tiratsoo, E.N. (1972). Natural Gas. Beaconsfield: Scientific Press Ltd. pp. 216, 221, 222.
  11. "Origins and growth of the British Gas Operations Department" (PDF).
  12. Williams, Trevor I. (1981). A History of the British Gas Industry. Oxford University Press. pp. 177–8.
  13. Wilson, D Scott (1974). North Sea Heritage: the story of Britain's natural gas. British Gas. p. 27.
  14. 1 2 3 4 5 6 7 8 Tiratsoo, E.N. (1972). Natural Gas. Beaconsfield: Scientific Press Ltd. pp. 221–2.
  15. Cassidy, Richard (1979). Gas: Natural Energy. London: Frederick Muller Limitted. pp. 39–47.
  16. Williams, Trevor I. (1981). A History of the British Gas Industry. Oxford University Press. pp. 225–30.
  17. Tiratsoo, E.N. (1972). Natural Gas. Beaconsfield: Scientific Press Ltd. pp. 159, 183, 224.
  18. 1 2 3 4 "Whessoe LNG tanks".
  19. 1 2 "Methane gas terminal Canvey Island".
  20. 1 2 3 4 Tiratsoo, E.N. (1972). Natural Gas. Beaconfield: Scientific Press Ltd. p. 183.
  21. "Testing of redundant LNG tank" (PDF).
  22. Tiratsoo, E.N. (1972). Natural Gas. Beaconfield: Scientific Press Ltd. p. 162.
  23. 1 2 Tiratsoo, E.N. (1972). Natural Gas. Beaconsfield: Scientific Press Ltd. p. 224.
  24. 1 2 "UK's Avonmouth LNG storage site to stop operations on April 30".
  25. Tiratsoo, E.N. (1972). Natural Gas. Beaconsfield: Scientific Press Ltd. p. 174.
  26. 1 2
  27. "Gas Safety (Management) Regulations, 1996".
  28. "Shell Bacton Infrastructure" (PDF).
  29. "Perenco Bacton & Dimlington Infrastructure" (PDF).
  30. "Gas Ten Year Statement (GTYS) | National Grid". Retrieved 2016-09-20.
  31. Cassidy, Richard (1979). Gas: Natural Energy. London: Frederick Muller Limited. p. 14.
  32. Cassidy, Richard (1979). Gas: Natural Energy. London: Frederick Muller Limited. pp. 46–47.
  33. "Gas Transportation Charges Tables 3&5".
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