ABSTRACT

Pipeline simulation tools include steady state, transient and on-line computer programs. The determining factor on how useful a simulation tool is is its ability to predict a line's capacity under a wide range of conditions. Southern Natural Gas had a simple model to predict firm flows during winter conditions. However, the model did not produce accurate predictions under conditions that varied significantly from a peak winter day. Recognizing that our modeling limitations were mostly self-imposed, SNG has transformed its model into a truly useful tool. In order for a model to be versatile, many details must be taken into account. An understanding of the equations pertaining to flow is an important first step towards building a good model. Observations will be made concerning the various gas, pipe and environmental variables and how they impact the flow through a line. In addition, several methods of determining the friction factor will be examined, with the focus on Colebrook and AGA. This paper will present a hierarchal need of a model for accurate data. The criterion used in creating this hierarchy is the extent to which a tuned model compensates for errors in the data set over a wide range of conditions. As a useful model depends on proper tuning, several tuning techniques are presented. They include: Steady state tuning Steady state tuning with transient factors Transient tuning On-line tuning Comparisons of the different tuning methods will be presented from actual studies performed on the SNG system. They will be evaluated on the basis of how consistent the resulting pipe efficiencies are over several studies. Tuning via pipe efficiency and roughness will also be addressed.

2 Background

Southern Natural owns and operates two pipeline systems in the southeast United States: Southern Natural Gas and South Georgia Co. Both are fully subscribed pipes that have a tariff provision that allows shippers to take gas at 6% of the daily quantity in any hour. Historically, winter was the only critical season as SNG served primarily heat sensitive loads. The fact that SNG was fully subscribed was moot as there was no summer market. As a result, the model was primarily a planning tool that was used to simulate flows during peak winter days. Operations would occasionally analyze the impact of a facility outage. However, flow studies were only a small part of their job function - due in part to the amount of time it took to conduct. Because of various assumptions and oversights, the model had to be tuned each time a study was conducted. Thus it provided little more benefit to operations than being a record base of pipe, compressor and connectivity information. With the growing power generation sector, SNG's world began to change. Loads in the summer were straining the system and maintenance windows were shrinking. As the Operational need for faster flow analyses became more prevalent, deficiencies in the model became intolerable.

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