ABSTRACT

The quality of gas entering the UK's National Transmission System (NTS) is controlled to ensure the energy content is within an acceptable range and that gas is within the strict safety limits set out by the Gas Safety (Management) Regulations (GS(M)R)7. Although the exact limits are negotiated between the NTS operator, National Grid, and the connecting party, once agreed, the Network Entry Quality Specification must be adhered to.

n the UK the major source of gas delivered through the NTS is from the gas fields in the North Sea. The composition and quality of the produced gas varies significantly depending on the field, location, well depth and also on the production age of the field. The many hundreds of miles of interconnected large pipe diameter gas gathering pipeline systems which are used to transport the produced gas to processing terminals are sufficiently complex to make optimum operations challenging; the required entry specifications add a further level of complexity to the operations of the pipeline system.

The role of balancing the commercial needs to maximize delivery whilst adhering to the NTS entry specifications falls on the pipeline operator. Given the unsteady production rates and varying qualities of produced gas at the offshore platforms the pipeline/platform operator needs advisory tools to help facilitate decisions about both plant and pipeline operations.

This paper we discuss the development of such a tool based on a Computational Pipeline Model (CPM): a real time pipeline modeling system with predictive and look ahead capability that tracks the necessary gas qualities. The paper will discuss the specific gas qualities tracked (CO2 mole fraction, Wobbe Index, Incomplete Combustion Factor and Soot Index), what role they play and how they are controlled. The paper will also demonstrate how predictive tools can also be used to provide plant output rates for both NGLs and Sales Gas.

The gas quality prediction tool has recently been deployed on the Frigg System and Frigg Terminal Processing plant which is owned and operated by Total E&P UK Ltd. The Frigg Pipeline System comprises over 625 miles (1,000 km) large bore pipeline and gathers rich gas from around 8 different areas in the Northern North Sea to the processing plant at St. Fergus. The Vesterled Pipeline System comprises 225 mile (363 km) large bore pipeline transporting gas from the Heimdal platform to the Frigg Terminal Processing plant. The Vesterled Pipeline System is owned by the Gassled partners and operated by Gassco with Total E&P UK acting as a technical service provider. The Frigg Pipeline System and Vesterled Pipeline System each have the capacity to transport around 1275 MMSCF of gas per day (36 MSCMD).The paper will report how the tool is to be used by the pipeline operators and how the system will be developed further.

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