GASSCO transports approximately 100 billion standard cubic meters (3.5 10·MMSCF) of Norwegian natural gas to the European market through nearly 8000km (5000 miles) of large diameter high-pressure subsea pipelines each year. The two key accuracy requirements of the pipeline network operator are flow rate capacity utilization and on spec. gas quality, the latter both for contractual and pipeline integrity reasons. Most subsea pipelines do not have any intermediate measurements available due to the high cost of installation and maintenance. To manage and plan the operations of such pipelines effectively hydraulic simulation software is used to provide the calculated flow, pressure, temperature and inventory (linepack). Thus the accuracy of such calculations becomes an essential requirement of pipeline operations and control. This paper addresses the challenge of obtaining accurate models of subsea pipelines such as one of 42" diameter, more than 600 KM (375 mile) length, with online composition, flow rate, pressure and temperature measurements at the inlets and outlets only. To reduce the effort a tuning assistant program has been developed to automate the whole tuning process. In addition, the application of Maximum Likelihood State Estimation will be discussed to demonstrate its effectiveness in overcoming measurement errors.
The Norwegian gas is transported in seven large diameter sub-sea pipelines to the United Kingdom and continental Europe, covering around 15% of the European natural gas consumption. The Norwegian export pipelines are between 300 km (190 miles) and 900 km (560 miles) long, and have diameters up to 44". The state of the gas in these pipelines has to be calculated by computer models, as measurements are only made at the inlets and outlets. Consequently, these computer models are essential when monitoring the operation of the pipelines as well as performing predictive simulations for future operational conditions and transport capacity calculations.