During periods of low flow, steady state simulation shows that operating a pipeline system with no compression is possible. Operating experience suggests that some compression is necessary for control. Transient simulation has been used to reconcile these concepts. Simulations were conducted using adjusted historical data for a one month period with and without compression. The results show that some compression is required to maintain the historical level of system control.


Changing market conditions in both the spring of 1983 and spring of 1984 resulted in significantly reduced flow rates on the Western Alberta System of NOVA, AN ALBERTA CORPORATION and it became necessary to determine the effect of these reduced flow rates on the operation of the system. The first work done at these flows was carried out using steady state simulations, and the results showed that no compression on the mainline was required at these flow rates. This, of course, resulted in very low fuel costs to the transmission system and the question arose as to whether these low fuel costs were realistic, especially considering the large flow fluctuations which were being experienced at the same time. The purpose of this report is to examine system impacts and potential savings that would result from a decreased usage of compression for control purposes. To address this issue, transient simulations were performed at the Alberta-B.C. border with deliveries of 500-725 mmscf/d. The current level of system pressure control was compared to the level of service attainable with a significant reduction in compressor usage. The results of the analyses are summarized below.

- There were no indications that any of the alternatives studied would compromise system integrity at the flow ranges indicated above.

- Under the conditions studied, the potential saving that could be achieved by relaxing the level of service is small compared to the resulting degradation in service levels.

- A reduction in compressor running hours of 74% increases the Alberta-B.C. Border pressure fluctuation bandwidth by 67% and increases supply-demand flow changes to producers from 63 per month to 99 per month.

- The need for control facilities to maintain any given level of service at low flows is more dependent on fluctuations in delivery rates than on the delivery rate itself. The historic level of service at the Alberta-B.C. Border cannot be maintained without the use of compression or other control facilities.

- If WATEX is not used for control, the Alberta-B.C. Border pressure fluctuation bandwidth will increase by 80%. If neither WATEX nor compression are used for control, the bandwidth will increase by 200%, based on current projections of system instability.

The Nova System

The Alberta Gas Transmission Division of NOVA, AN ALBERTA CORPORATION operates a large gas gathering and transmission system in the Canadian province of Alberta. The pipeline system receives gas at about 650 receipt locations and delivers gas through 80 major delivery stations to customers within the province, and to large transmission systems delivering gas to other parts of Canada and the United States.

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