This document sets out our methodology calculating NTS linepack. In accordance with Special Condition 28B part 2 14 (9) (h) of National Gas Transmission's Gas Transporters Licence, this methodology is subject to approval by the Gas and Electricity Markets Authority. The methodology describes the basis on which we will calculate the volume of gas within the NTS for the purposes of determining opening and closing linepack values, on a day, for use within the Residual Gas Balancing Incentive as specified in our licence.
We determine NTS linepack by calculating the volume of gas within the NTS pipelines, using a network model and instantaneous measurements. The model contains a database of NTS pipeline diameters and lengths, updated as required to accommodate changes to the network.
The NTS is divided into pipeline sections, and the volume of gas within each section is determined by multiplication of the pipe free volume by the ratio of the gas density at actual conditions to the gas density at standard temperature and pressure.
The actual gas density is computed from a standard equation of state applying pressure, temperature and specific gravity data. The pressure is derived from telemetered measurements at the beginning and end of the pipeline section, and an average network gas temperature is applied. The specific gravity is determined from telemetered measurements at NTS entry and exit points according to a fixed mapping arrangement. The network model then summates the calculated volume of gas for each pipeline section to compute total NTS linepack.
The opening NTS linepack for a gas day, which is equivalent to the closing NTS linepack for the preceding gas day, will be the volume calculated by the network model as close to the start of the gas day as possible.