National Transmission System
The United Kingdom's National Transmission System 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.
History
Origins
The National Transmission System originated in the construction during 1962-3 of the 200 mile long high-pressure methane pipeline from Canvey Island to Leeds. Imported liquified natural gas 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 in diameter and operated at 1,000 pounds per square inch. The pipeline had 150 miles of spur lines, supplying gas to area boards.
| Area Board | Supply to | Diameter | Length |
| North Thames | Bromley/East Greenwich 'Tee' | 14 | 15.5 |
| North Thames | Bromley | 10 | 2.5 |
| North Thames | Slough | ||
| South Eastern | East Greenwich | 12 | 3 |
| Southern | Reading | 8 | 49 |
| Eastern | Hitchin | 8 | 7 |
| Eastern | Dunstable | 6 | 8.5 |
| West Midlands | Coleshill | 14 | 10.5 |
| East Midlands | Sheffield | 12 | 10 |
| North Western | Manchester | 14 | 24 |
| North Eastern | Leeds | 12 | 8 |
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 gas
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 |
| Rough | Gas | Amoco/Gas Council | May 1968 | October 1975 | Easington |
| Forties | Oil + associated gas | BP | October 1970 | September 1977 | St Fergus |
| Frigg | Gas | Elf/Total | June 1971 | September 1977 | St Fergus |
| Frigg | 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 | 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 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.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.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.
| Feeder No. | Diameter | Length | From | To | Operational |
| 1 | 24-inch | 90 miles | Easington terminal | Scunthorpe and Totley near Sheffield, where it connected to the original methane pipeline. | July 1967/ July 1968 |
| 2 | 36-inch | 123 miles | Bacton terminal | Brisley, Peterborough and Churchover near Rugby, where it connected to the original methane pipeline. | August 1968 |
| 3 | 36-inch | 107 miles | Bacton terminal | Roudham Heath, Cambridge, Whitwell near Hitchin, where it connected to the original methane pipeline. | October 1969 |
| 4 | 36-inch | 154 miles | 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 | 91 miles | Paull | Pickering, Westwood and Little Burden near Darlington | Autumn 1971 |
| 7 | 36-inch | Wisbech | Hatton and Scunthorpe | 1972 | |
| 8 | 30-inch | Former Viking terminal | Hatton | July 1972 |
The No. 6 feeder runs via Pickering which received gas from a treatment plant for the onshore Lockton gas field.
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.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 pipelines were laid via Bathgate to Partington and Bishop Auckland to connect to the NTS in England, a total pipeline length of 595 miles. 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 intervals along the pipelines. A third 36-inch pipeline from St Fergus was completed in 1978, and a fourth 40-inch pipeline in 1982.
Growth of the NTS
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 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.
The South Western Gas Board received natural gas at the end of 1970 from a 24-inch/20-inch pipeline from Wormington to Exeter.
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, 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 |
| 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 |
| 1983 | 3200 | Canvey Easington Bacton Theddlethorpe St. Fergus | 14 |
Later feeder mains not described above include.
| Feeder No. | From | To | Year commissioned |
| 9 | Easington | East Ilsley | 1983-86 |
| 15 | Longtown | Warburton | 1984 |
| 16 | Barrow | Lupton | 1983 |
| 17 | Theddlethorpe | Hatton | 1988 |
| 18 | Peterborough | Cambridge | 1988–94 |
| 18 | Isle of Grain | Gravesend | 2008 |
| 18 | Matching Green | Tilbury | 1990 |
| 19 | Easington | Paull | 1991 |
| 20 | Ilchester | Choakford | 1989 |
| 21 | Mawdesley | Alrewas | 1992–2001 |
| 21 | Carnforth | Burscough | 1992 |
| 21 | Elworth | Deeside power station | 1994 |
| 22 | Goxhill | Peterborough | 1993 |
| 23 | Churchover | Honeybourne | 1998-2001 |
| 23 | Peterstow | Gilwern | 2000 |
| 23 | Wormington | Corse | 2000 |
| 24 | St Fergus | Lochside | 2001 |
| 24 | Easington | Paull | 2010 |
| 24 | Hatton | Silk Willoughby | 2001 |
| 25 | Bridge Farm | Mickle Trafford | 2001 |
| 26 | Huntingdon | Steppingley | 2001 |
| 27 | Bacton | King's Lynn | 2003 |
| 27 | Cambridge | Matching Green | 2002 |
| 28 | Herbrandston | Corse | 2007 |
| 29 | Easington | Nether Kellett | 2006–08 |
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 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 | 1959, 1964, 1968, 1975 | 1984 | Import of LNG from Arzew Algeria, original contract for per year of gas. Liquefication 205 tonnes/day, vapourisation 6 × 50 tonnes/hour. |
| Ambergate, Derbyshire | 5,000 tonnes | 1967-1970 | 1985 | Import of LNG by road tanker from Canvey, output per day. |
| Glenmavis, Lanarkshire | 20,000 tonnes, | 1972, 1974 | 2012 | Liquefication 100 tonnes per day, vapourisation per day |
| Partington, Cheshire | 4 × 20,000 tonnes | 1972 1972, 1974 | March 2012 | Liquefication per day, vapourisation 8 × 75 tonnes/hour |
| Dynevor Arms, Rhondda Cynon Taf | 2 × 20,000 tonnes | 1972 | March 2009 | Liquefication 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 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.- Isle of Grain, six 'bullets', 12 ft diameter, 250 ft long, capacity 8 million cubic feet of gas, operating at up to 1,000 psi.
- Beckton gas works, eight 'bullets', 13.5 ft diameter, 263 ft long, capacity 5 million cubic feet of gas, pressure cycle 350-100 psi.
- South Western Gas Board, for Bristol and Cheltenham, eleven ‘bullets’, 13 ft 6 in diameter, 311 ft 8 in long, total capacity 13 million cubic feet, pressure cycle 450-40 psi.
- Biggin Hill Kent, seventeen, 42-inch diameter buried pipes, 1,040 ft long, capacity 10 million cubic feet, operating up to 1,000 psi.
Operation
The two types of gas pipelines in the UK are: large diameter high-pressure 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.
Entry
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 UK coastal gas terminals. Gas from the Liverpool Bay field formerly entered the NTS at Burton Point terminal in Cheshire, this terminal is now identified by National Grid as a NTS of
ftake to Connah's Quay power station. - Onshore gas fields such as Saltfleetby Lincolnshire ; 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. The Canvey Island gas terminal ceased importing LNG in 1984.
- Storage facilities. These include a mixture of storage, onshore LNG storage sites, depleted onshore gas fields and the depleted offshore gas field at Rough. 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 ; Dynevor Arms Merthyr Tydfil ; Glenmavis Lanarkshire ; Partington Greater Manchester ; and Avonmouth Bristol.
- 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 composition
| 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 * | ≤50 mg/m3 |
| Hydrogen content* | ≤0.1% |
| Oxygen content* | ≤0.2% |
| Carbon Dioxide content | ≤2.0% |
| Nitrogen content | <5.0% |
| Total Inerts | <7.0% |
| Incomplete Combustion Factor* | ≤0.48 |
| Soot Index* | ≤0.60 |
Parameters marked * are specified in the .
The composition of natural gas in the NTS is typically as follows.
| 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 compressor stations and over 25 pressure regulators. Gas moves through the NTS at speeds up to 25 mph 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 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 | Commissioned |
| Alrewas | 2 Rolls-Royce Avons | 21.6 | 1970 |
| Peterborough | 3 Rolls-Royce Avons | 35.4 | 1972 |
| Churchover | 3 Orendas | 18.4 | 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
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 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 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 |
| 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, 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. The LTS is managed from Hinckley, Leicestershire. 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 NTS
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