The temperature of the medium surrounding a pipeline significantly affects the pipeline's hydraulic capacity. However, often the ambient temperatures are approximated by statistical averages of historical data, hard-coded in a model file. Such data may at times deviate from the true temperatures by more than five degrees Fahrenheit. Especially for on-line systems the lack of up-to-date ambient temperature data may limit maximum utilization of the existing pipeline capacity. Integration of up-to-date ambient temperatures will therefore enable Gassco, and other pipeline operators, to improve the usage of the available network capacity and allow for better planning of the day-to-day capacity that can be offered for sale. This paper presents how up-to-date sea bottom temperatures were integrated in Gassco's Pipeline Modeling System. The increased modeling accuracy enables Gassco to commit approximately an additional 250 million SCF/d gas for any particular cold day.
Gassco supplies Norwegian gas to the European market through more than 4 850 miles (7 800 km) of large diameter high-pressure sub-sea pipelines. Contractual deliveries in 2005 amounted to nearly 3 000 billion SCF (84 billion Sm3). During winter, the request for gas exceeds the current estimated transport capacity of the pipelines. Consequently, Gassco constantly evaluates the accuracy of the margins applied when calculating the hydraulic capacity of the pipelines. Such studies concern in-situ capacity tests of the pipelines, evaluation of the theoretical aspects of flow models (such as the friction factor used in the pipeline models, see Langelandsvik et al., 2005), and improvement of data sources that are used as input in the pipeline models. Currently, capacity estimates are typically based on traditional on-line data from the transport network (pipeline inlet/outlet pressure, flow rates, gas temperature and composition) in combination with conservative estimates of the historical ambient temperatures. A preliminary study showed that when actual day-to-day ambient temperatures were integrated into the Pipeline Modeling System (PMS), Gassco would be able to more accurately predict and use the available transport capacity in its gas pipeline system. For example, Figure 1 shows the hydraulic capacity that is offered for sale (green line) for a particular pipeline. This capacity is an estimate based on a conservative scenario, i.e., high ambient temperatures based on statistical historical data (red line) combined with an additional margin. However, in case up-to-date temperature data shows that the ambient temperature in fact is lower, the extra margin in the pipeline capacity can be utilized. In this example, nearly 3% extra gas can be transported and sold on a cold day. Improved accuracy in the ambient temperature data thus makes it possible to offer an increased pipeline capacity and thus to better meet the increased demand for gas during the winter season. For instance, for each particular cold winter day, the increased modeling accuracy makes it possible for Gassco to commit approximately 250 million SCF gas per day extra.