National Transmission System
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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.
- 1 History
- 2 Operation
- 3 Ownership of the NTS
- 4 Northern Ireland
- 5 See also
- 6 References
- 7 External links
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. 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. 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, supplying gas to area boards.
|Area Board||Supply to||Diameter (inches)||Length (miles)|
|North Thames||Bromley/East Greenwich 'Tee'||14||15.5|
|Slough (from Reading spur line)|
|South Eastern||East Greenwich||12||3|
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.
UK natural gasEdit
Natural gas was discovered on the UK continental shelf in 1965 and production started in 1967. 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.
|Field||Field type||Licensee 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 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.
Building the NTSEdit
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).
Feeder pipelines - EnglandEdit
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.
|1||24-inch (600 mm)||90 miles (144 km)||Easington terminal||Scunthorpe and Totley near Sheffield, where it connected 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 connected to the original methane pipeline.||August 1968|
|3||36-inch||107 miles (171 km)||Bacton terminal||Roudham Heath, Cambridge, Whitwell near Hitchin, where it connected 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 connected 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||Former Viking (Theddlethorpe) terminal||Hatton||July 1972|
Feeder pipelines - ScotlandEdit
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 (Feeder No. 12) where a natural gas liquification plant was constructed.
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 (Feeder No. 10 and No. 11) 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 (Feeder No. 12) was completed in 1978, and a fourth 40-inch (1050 mm) pipeline (Feeder No. 13) in 1982.
Growth of the NTSEdit
The NTS was extended from Leeds to Newcastle upon Tyne in early 1969. 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 (an extension to Feeder No. 2). North Wales was also connected in 1969 via a 24-inch/18-inch pipeline from Audley Cheshire to Maelor near Wrexham (an extension to Feeder No. 4).
The South Western Gas Board received natural gas at the end of 1970 from a 24-inch/20-inch pipeline from Wormington to Exeter (Feeder No. 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.
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 (designated as part of Feeder No. 6), and the West Midlands Gas Board laid six similar 'spine' mains into industrial districts of Birmingham and the Black Country.
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.
|Years||NTS mileage||Operational Gas Terminals||Compressor stations|
|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|
|1983||3200||Canvey Easington Bacton Theddlethorpe St. Fergus||14|
Later (post-1983) feeder mains not described above include.
|Feeder No.||From||To||Year commissioned|
|Isle of Grain||Gravesend||2008|
|Elworth||Deeside power station||1994|
|25||Bridge Farm||Mickle Trafford||2001|
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 sitesEdit
In addition to the Canvey Island Liquefied Natural Gas (LNG) import terminal, further NGL storage sites were constructed from the late 1960s. 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.
|Site||LNG storage tank capacity||Commissioned||Decommissioned||Operations|
|Canvey Island, Essex||6 × 4,000 tonnes, 2 × 1,000 tonnes, 4 × 21,000 tonnes (underground)||1959, 1964, 1968, 1975||1984||Import of LNG from Arzew Algeria, original contract for 1 billion m3 (35 billion cu ft) per year of gas. Liquefication 205 tonnes/day, vapourisation 6 × 50 tonnes/hour.|
|Ambergate, Derbyshire||5,000 tonnes (1 × 12,000 m3) ||1967-1970||1985||Import of LNG by road tanker from Canvey, output 72 million cu ft (2.0 million m3) per day.|
|Glenmavis, Lanarkshire||20,000 tonnes (2 × 47,800 m3),||1972, 1974||2012||Liquefication 100 tonnes per day, vapourisation 250 million cu ft (7.1 million m3) per day|
|Partington, Cheshire||4 × 20,000 tonnes (4 × 49,800 m3)||1972 1972, 1974||March 2012||Liquefication 10 million cu ft (280 thousand m3) per day, vapourisation 8 × 75 tonnes/hour|
|Dynevor Arms, (Hirwaun) Rhondda Cynon Taf||2 × 20,000 tonnes||1972||March 2009||Liquefication 10 million cu ft (280 thousand m3) per day, vapourisation 2 × 75 tonnes/hour|
|Avonmouth, Bristol||3 × 50,000 m3||1974-1980; 1978, 1979, 1983||April 2016||Short-term storage, liquefication 205 tonnes/day, vapourisation 6 × 75 tonnes/hour|
|Isle of Grain, Kent||4 × 50,000 m3, 4 × 190,000 m3||1980-2010||Operating||Vapourisation 58 million m3/day. See Grain LNG Terminal|
High-pressure gas storageEdit
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.
- Isle of Grain, six 'bullets', 12 ft (3.6 m) diameter, 250 ft (76.2 m) long, capacity 8 million cubic feet (226,000 m3) of gas, operating at up to 1,000 psi (69 bar).
- Beckton gas works, eight 'bullets', 13.5 ft (4.1 m) diameter, 263 ft (80.1 m) long, capacity 5 million cubic feet (142,000 m3) of gas, pressure cycle 350-100 psi (24-6.9 bar).
- South Western Gas Board, for Bristol and Cheltenham, eleven ‘bullets’, 13 ft 6 in (4.1 m) diameter, 311 ft 8 in (95 m) long, total capacity 13 million cubic feet (368,000 m3), pressure cycle 450-40 psi (31-2.76 bar).
- Biggin Hill Kent, seventeen, 42-inch (1.07 m) diameter buried pipes, 1,040 ft (317 m) long, capacity 10 million cubic feet (283,000 m3), operating up to 1,000 psi (69 bar).
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 bar and 1050 mm 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:
- Offshore oil and gas fields on the UK continental shelf. These deliver gas via five (formerly six) UK coastal gas terminals (five in England: CATS Teesside; Easington/Dimlington; Bacton; Rampside Barrow and the former Theddlethorpe terminal and one in Scotland: St Fergus). Gas from the Liverpool Bay (Douglas) field formerly entered the NTS at Burton Point terminal in Cheshire, this terminal is now identified by National Grid as a NTS offtake to Connah's Quay power station.
- Onshore gas fields such as Saltfleetby Lincolnshire (production was via the former Theddlethorpe terminal); and Wytch Farm Dorset.
- Continental Europe. From Norway via the Langeled pipeline and the Easington terminal; from the Netherlands via the BBL pipeline; from Belgium via the Interconnector UK pipeline, both of the latter through Bacton gas terminal.
- Imported LNG. Gas is delivered from import terminals at the Isle of Grain LNG; Milford Haven (South Hook and Dragon). The Canvey Island gas terminal ceased importing LNG in 1984.
- Storage facilities. These include a mixture of salt cavity storage, onshore LNG storage sites, depleted onshore gas fields and the depleted offshore gas field at Rough (via Easington terminal). Storage facilities include: Holford Cheshire; Garton/Aldborough East Yorkshire; Hornsea East Yorkshire; Stublach Cheshire; Holford Cheshire; Hole House Farm Cheshire; Saltfleetby Lincolnshire; Hatfield Moor South Yorkshire; and Barton Stacey/Humbly Grove Hampshire. The NTS was formerly supplied by the following decommissioned LNG sites: Ambergate Derbyshire (closed 1985); Dynevor Arms Merthyr Tydfil (closed 2009); Glenmavis Lanarkshire (closed 2012); Partington Greater Manchester (closed 2012); and Avonmouth Bristol (closed April 2016).
- There is a salt cavity storage facility at Hornsea, East Yorkshire. Seven cavities at a depth of 1800 m each store up to 60 million m3 of gas at a maximum pressure of 240 bar. The releasable volume of gas is about half of the gross volume. During periods of low demand gas is compressed into the cavities by electrically driven compressors and fed back onto the NTS at times of peak demand.
Gas specification and compositionEdit
|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)|
|Incomplete Combustion Factor*||≤0.48|
Parameters marked * are specified in the Gas Safety (Management) Regulations 1996.
The composition of natural gas in the NTS is typically as follows.
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 km/h) 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.
Compressor stations include:
- England - Wooler, Bishop Auckland, Carnforth, Nether Kellett, Warrington, Hatton, Alrewas, Wisbech, King's Lynn, Peterborough, Churchover, Huntingdon, Cambridge, Diss, Chelmsford, Aylesbury, Lockerley and Wormington.
- Scotland - Aberdeen, Avonbridge, Kirriemuir, Moffat and St Fergus.
|Station||Gas turbines||Power rating (MW)||Commissioned|
|Alrewas||2 Rolls-Royce Avons||21.6||1970|
|Peterborough||3 Rolls-Royce Avons||35.4||1972|
|Scunthorpe||2 Rolls-Royce Avons||23.6||1973|
|Chelmsford||2 Rolls-Royce Avons||23.0||1973|
|King's Lynn||4 Rolls-Royce Avons||47.2||1973|
|Cambridge||2 Rolls-Royce Avons||23.0||1974|
|Bishop Auckland||2 Orendas||14.4||1974|
|Kirriemuir||4 Rolls-Royce Avons||47.2||1977|
|Bathgate||4 Rolls-Royce Avons||47.2||1977|
|Diss||3 Rolls-Royce Avons||34.5||1977|
|St Fergus I||4 Rolls-Royce Avons||47.2||1977|
|St Fergus II||2 Rolls-Royce Maxi Avons, 3 Rolls-Royce RB211s||65.3||1978|
|Moffat||2 Rolls-Royce RB211s||38.6||1980|
|Wisbech||1 Rolls-Royce RB211, 1 Rolls-Royce Maxi Avon||32.6||1980|
Offtakes from the NTS include those supplying industrial users, local distribution networks, storage sites and export pipelines.
- Offtakes to about 71 large users such power stations and industry either those on multi-business sites such as Billingham and Runcorn or to individual companies such as INEOS Teesside.
- NTS offtakes to inland storage sites (see 'Entry') and to the offshore Rough field storage site via the Easington gas terminal.
- The NTS supplies the Irish interconnectors; the 24-inch 135 km Scotland-Northern Ireland Pipeline (SNIP) to Ballylumford Northern Ireland and the two 24-inch UK-Eire Interconnectors to Dublin, they are supplied from an NTS offtake at Moffat. Gas can also be exported to Belgium and the Netherlands via the Interconnector UK and the BBL pipelines both via the Bacton terminal.
- Offtakes to the Gas Distribution Network of Local Distribution Zones.
|National Grid Area||Number of LDZ offtakes|
Gas distribution networkEdit
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, Cadent 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 NTSEdit
The changing ownership of the NTS reflects developments and corporate changes in the UK gas and energy industries.
- Gas Council and Area Boards, 1962 - 31 December 1972
- British Gas Corporation, 1 January 1973 – 24 August 1986
- British Gas plc, 24 August 1986 - 1994
- Transco plc, part of British Gas plc, 1994 - 17 February 1997
- Transco plc, part of BG plc,17 February 1997 - 1999
- Transco plc, part of BG Group plc, 1999 - 23 October 2000
- Transco plc, part of Lattice Group plc, 23 October 2000 – 21 October 2002
- Transco plc, part of National Grid Transco plc, 21 October 2002 – 10 October 2005
- National Grid Gas plc, part of National Grid plc, 10 October 2005 - date (2016)
NG is administratively based in Warwick. NG owns and operates the gas transmission system in all of Great Britain; in comparison it only owns the electrical transmission system in England and Wales but operates it for all of Great Britain.
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.
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