Overview

Compressor stations play an important role in the successful design of gas pipelines; the right choice of centrifugal compressors and drivers is a key factor in the success of the entire project. The currently available design criteria for this type of equipment ensure generally high thermodynamic performance and, consequently, minimize the requirements for nameplate capacity and energy consumption, permitting substantial savings in operational costs over the project's entire economic life.

When using mechanical equipment in a gas pipeline compressor station, one of the main issues that needs to be solved is the number of units required to meet the pipeline's throughput capacity criteria. Various solutions can be used, depending on the pipeline's load profile. Another factor worth considering is the fact that gas turbines can generate significantly more power at lower ambient temperatures. This means that the driver's relative capacity can vary even under continuous pipeline load.

This study evaluates a typical trans-continental pipeline with a number of compressor stations. The simulation includes the determination of the pipeline's exact hydraulic indicators. The local environmental conditions are taken into account where there are cases of significant ambient temperature fluctuations. To calculate gas pipeline availability factors, the Monte Carlo system will be used.

As defined by Evans and Olson (1998), Monte Carlo simulation involves using statistical probability to evaluate the resulting distribution of a variable which, in turn, depends on several other independent variables. The Monte Carlo simulation is often used to evaluate the degree of risk posed by an event that is new but expected.

The focus for this publication is the gas pipeline availability factor that can be achieved using various configurations based on actual reliability and availability indices. Various statistical methods for calculating reliability and availability will also be considered. Another issue which will be examined that has a considerable impact on operational costs is fuel gas consumption. In this case, the number of installed units has a great impact on the annual volume of fuel consumption, as well as on the pipeline throughput based on different operational scenarios.

Introduction

The rising world demand for gas has created the need for new gas pipelines that can deliver gas to markets located a long distance from the supply source. For long-distance pipelines, gas transportation costs include increasing customer delivery costs, which can amount to 30-50% of the total costs for the receiving terminal. These transportation costs can be controlled by optimizing fuel consumption, equipment acquisition costs, equipment maintenance costs, and the equipment's reliability and availability. Pipeline pressure, flow, and other parameters influence the configuration of the compressors at any given station. The number of units, the distance between stations, the requirements for standby equipment, and the usage of series or parallel configurations at a certain station are all issues that arise along with the type of drives and compressors. Previous publications (Santos, 2004; Ohanian et al., 2002; Kurz et al., 2003) evaluated the relative advantages of series and parallel configurations, as well as the number of units per station. When designing a compressor station or a new pipeline, a definitive analysis of the number of stations needs to be performed. This includes: the steady-state and transient capacity conditions and system requirements, the requirements and ability to improve capacity and output, the system units' availability, the total cost of ownership, and the total cost of delivery to shippers and customers.

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