ABSTRACT

Natural gas is increasingly being used as an energy source. Natural gas transmission pipelines transport large quantities of natural gas across long distances. They operate at high pressures and utilize a series of compressor stations at frequent intervals along the pipeline to move the gas over long distances. The operating costs of transmission pipelines can be significant because of compressor station fuel costs, emission minimization, etc. The analysis of these pipelines is very complex. This paper details techniques that can be used to determine optimal operating regions, schedule changes to move the pipeline from one optimal state to another, and automatically implement these changes using model predictive controllers. The optimal operating conditions that meet all constraints and minimize fuel consumption for the pipeline are determined by deciding (1) which compressor units need to be run at each compressor station and (2) the best suction pressure set point for each compressor station. The optimizer used is capable of running through a multitude of cases. It then selects the most optimal unit allocation and pressure profile found. Automated controls designed to move a pipeline towards optimal conditions are a very new feature in the process control industry and non-existent in the gas pipeline industry. Each controller sets the conditions, if necessary, for a unit to come on or go off. Each controller then moves a segment of the pipeline from one set of optimal conditions at one flow to the next set of optimal conditions at the new flow. Process limitations are considered and the Scheduler provides a transition schedule to facilitate the flow change gracefully. The Gas Dispatcher either approves or rejects the schedule. Upon approval, the controllers, over a few hours, move the pipeline to the new set of optimal conditions for the new flow with the correct allocation of units. If necessary, whole stations are brought up or down.

INTRODUCTION

On September 17, 1898, for a fee of $61, West Virginia's secretary of state granted Standard Oil of New Jersey a certificate of incorporation for the Hope Natural Gas Company. Now, 101 years later, the transmission pipeline segment of Hope, after many organization changes, has evolved into CNG Transmission Corporation (CNGT) a subsidiary of Pittsburgh-based Consolidated Natural Gas Company, one of the largest producers, transporters, distributors and marketers of natural gas in the United States. CNG Transmission Corporation (Figure 1) owns and operates 10,000 miles of pipeline, 68 compressor stations, 3,362 production wells, 1,510 storage wells in 15 storage pools, 300 pipeline interconnects, and an extraction and fractionating plant. All the above is operating by two busy Gas Dispatchers around the clock. Lebanon, Ohio to the Leidy hub located in north central Pennsylvania. It resulted from an agreement CNG Transmission had signed with the Transco Energy Company to provide 250 million cubic feet per day of firm capacity on the company's lines from the Texas Gas terminus at Lebanon to Transcontinental Gas Pipeline Corporation's Leidy line.

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