3.2 The National Transmission System


The National Transmission System (NTS), operated and maintained by National Grid Transco, is a high pressure gas network consisting of terminals, compressor stations, pipeline systems and offtakes. The gas is transported through 6,300km of pipelines ranging in diameter from 63mm to 1200mm. The National Control Centre at Hinckley, Leicestershire remotely monitors and controls the system.


The NTS transports gas from six beach terminals to 120 offtakes from the system at pressures of up to 85 bar. These offtake installations supply eight regional domestic transmission systems, called Local Distribution Zones (LDZ), and over 40 large industrial consumers such as power stations. Gas removed from the network at these offtakes must meet a minimum pressure requirement, which is usually 25bar.


The beach terminals supplying gas to the UK are:


         Bacton, Norfolk

         Barrow, Cumbria

         Easington, East Yorkshire

         St. Fergus, Aberdeenshire


         Theddlethorpe, Lincolnshire


Bacton and St. Fergus process 60% of all gas entering the country.


Gas is received at the beach terminals where it is refined and processed so to meet the standards required to enter the network. Refining includes removing any liquids or contaminants that may be present and ensuring that the calorific value is at an acceptable level, usually around 39 MJ/m3. Almost all of the gas entering the terminals is supplied from gas fields located in British waters, although some is supplied by two import pipelines when demand is high. The main pipeline used for imports is the Bacton-Zeebrugge Interconnector, connecting the Bacton gas terminal in Norfolk to the terminal at Zeebrugge, Belgium. This pipeline is mainly used for exports out of Britain but is soon to be modified to provide increased import capacity, as Britain shifts from being a net exporter of gas to a net importer. The other import pipeline, the Vesterled pipeline, connects St. Fergus to Norwegian gas fields.


Gas pressure drops as it travels through pipelines due to forces caused by friction, reducing the quantity of gas that may flow through the pipe. 24 compressor stations, operating at powers of up to 27MW, are located throughout the network to maintain flow at high pressures, thus preventing the gas from dropping to dangerously low pressures. 61 industrial Rolls Royce, Orenda and General Electric jet engines, fuelled by natural gas from the pipelines, are used to power all but two of the compressors. Hot gasses generated by the jet engines drive power turbines which in turn drive the compressors. The compressors at Lockerley and Peterstow are electric and hence do not remove gas from the network.


Also located within the NTS are five storage facilities which assist in meeting demand when supply from gas terminals is insufficient. When supply can exceed demand, these facilities take in gas and covert it to liquefied natural gas (LNG) for storage. LNG occupies far less space than gas at ambient conditions and therefore allows greater amounts to be stored.


A map of the National Transmission System is shown on the next page. This map was drawn based on information from the Transco Ten Year Statement.