This paper gives a general overview of the gas transmission system in Great Britain that is owned, maintained and operated by Transco, a part of BG plc. In recent years, legislation has steadily introduced competition, beginning with the opening up of the large industrial market allowing limited third party access. This year saw the arrival of full competition in the domestic market. Legislation and the impact of competition have produced technical challenges for network simulation. The company has had to declare publicly the way it plans, designs and operates gas networks. It is shown how network simulation has responded or in some cases has aided the change in the industry; the demand for network simulation is greater than ever before.

2. Description of the transmission system
2.1 The National Transmission System (NTS)

The main transporter of gas in the UK is Transco, a part of BG plc. Figure 1 shows a schematic representation of Transco's main high pressure supply system, known as the National Transmission System (NTS). Gas is input at high pressure, up to 1085 psig (75 bar), into national transmission pipelines and with the aid of compressors it is transported to 13 geographic areas, known as Local Distribution Zones (LDZs), via approximately 120 offtakes. The average daily sendout is about 7500 mmscf (220 mcm) and peak sendout is about twice that. The NTS consists of: gas reception terminals; a high-pressure pipeline system; compressor stations; in-line regulator stations; pipeline junctions; offtakes to 13 geographic areas; and large industrial and power station loads. A short description of some of these follows.

Gas Reception Terminals

These installations receive gas from producers' sub terminals - a number of these may feed into one Transco terminal. The terminals perform quality monitoring, mixing, control gas flows into different pipelines leaving the terminal, provide emergency shutoff facilities and in the case of St Fergus, compress gas to raise the pressure to Transco system requirements. Transco operates seven terminals: St Fergus, Easington, Theddlethorpe, Bacton, Barrow, Teesside and Burton Point. In network simulation terms, gas reception terminals are usually modelled as supplies or sources. Gases of differing qualities come from different offshore fields and are mixed at the terminals and also within the NTS itself.

High Pressure Pipeline System

This system consists of around 3,750 miles (6,000 km) of steel pipeline, operating up to 1085 psig (75 bar). The pipelines are between 6 inches and 42 inches (150 and 1,050 mm) in diameter, with the large sizes making up the bulk of the system. The system receives gas from the reception terminals and transmits it through the pipeline system to the 13 LDZs and to a number of very large loads, mainly power stations. Gas is also input to the high pressure system from salt cavity storage and LNG sites at a number of locations. At various points on the NTS there are pipeline junctions consisting of pipework, valve and pigging facilities. These are sometimes combined with compressor and regulator stations.

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