Configuration Impacts Availability, Fuel Consumption And Pipeline Capacity

The number of compressors installed in each compressor station of a pipeline system has a significant impact on the availability, fuel consumption and capacity of the system. Depending on the load profile of the pipeline, the answers may look different for different applications.

Among the factors to be considered is the fact that gas turbines can produce a significant amount of additional power at lower ambient temperatures. So, even for constant load of the pipeline, the relative load of the driver changes, and thus influences fuel consumption and pipeline capacity.

In this article, a typical transcontinental pipeline with multiple compressor stations is evaluated. The determination of the exact hydraulic behavior of the pipeline is part of the modeling effort. The site ambient conditions, with a significant swing in ambient temperatures are considered. Further, the availability that can be achieved with various configurations, based on actually achieved reliability and availability numbers is discussed. Part 1 covers the more general thoughts and Part 2 will discuss the application of these principles to an actual situation.

With the worldwide demand for gas rising, new pipelines are required to bring gas over longer distances to the market. For long-distance pipelines, the transport cost of the gas will make up an increasing portion of the delivery cost to the customer, and can reach 30-50% of the total cost at the receiving terminal. This transport cost can be influenced by optimizing the fuel consumption, equipment first cost, equipment operating cost, as well as equipment reliability and availability. The pressure and flow characteristics of pipelines and other factors influence the arrangement of compressors in a station. The question about number of units, the spacing of stations, standby requirements or the use of series or parallel arrangements in a station arises, together with the type of driver and the type of compressor.

When planning a compressor station or, for a new pipeline, several stations, considerations include steady-state and transient capabilities and requirements of the system, growth requirements and capability, availability and total cost of ownership, and delivered cost to shippers and customers.

The first consideration involves the capability to cope with changes in flow capacity on all time scales (i.e., hourly, daily, seasonally) as well as changes in available power. The pipeline hydraulics relate pressure losses to the flow through the pipeline, determine the compressor operating conditions in terms of head and actual flow, and subsequently determine the required power from the driver. Contractual requirements and obligations, such as pressures and volumes at transfer points, have to be considered.

The second consideration deals with the fact that the nominal capacity of a pipeline may grow when additional customers demand a higher supply of natural gas. In fact, many new pipelines start out with 50% and less capacity and grow to full capacity over several years, or are sized for easy expansion. Often, the prediction of the rate of growth shows a significant degree of uncertainty. The growth scenarios, if foreseeable, drive a station layout to possibly allow additional power to be installed at the station level later or additional stations along the pipeline. The alternative scenario, where the pipeline usage declines over the years (e.g., because the gas supply from the field declines), is also a possibility.