National Transmission System

The National Transmission System (NTS) originated in the construction during 1962–3 of the 200-mile (320 km) high-pressure methane pipeline from Canvey Island to Leeds.[2] 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 gas boards with access to natural gas.[3] 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.[4]

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.[5]

Natural gas was discovered on the UK continental shelf in 1965 and production started in 1967.[6] 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.

With the assured availability of natural gas a government white paper on fuel policy[7][8] 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.[9]

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).[10]

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.[11]

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.[12][13][14][15]

The No. 6 feeder runs via Pickering which received gas from a treatment plant for the onshore Lockton gas field.[15]

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.[15]

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.[16]

The NTS was extended from Leeds to Newcastle upon Tyne in early 1969.[15] 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).[15]

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).[15]

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.[15]

In addition to these distribution pipelines in 1971 the 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.[15]

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.[17]

Later (post-1983) feeder mains not described above include.[18]

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.

In addition to the Canvey Island Liquefied Natural Gas (LNG) import terminal, further NGL storage sites were constructed from the late 1960s.[19] 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.

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.[28]

• 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).

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.[21]
• 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).[27]
• 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.[22]

The specification of gas transported within the NTS is typically within the following parameters.[30][31][32][33]

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

The composition of natural gas in the NTS is typically as follows.[34]

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.[35]

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.

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.[36]

Companies that own part of this gas network, also known as the Local Transmission System (LTS), are known 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, which was a department of National Grid before it 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.