ABSTRACT

The "Transmission Capacity" of a Natural Gas Pipeline is generally a definition based on certain assumptions made by the operator. Meanwhile, the "Design Capacity" is an assumption made before the pipeline construction. "Transmission Capacity" is defined after the commercial pipeline operation, verifying real performance, and assuming sometimes arbitrarily, certain hypothesis. In theory, the main difference is based on time and amount of the available information. Any definition of "Capacity" is a picture of a vision in steady state of pipe operation. When somebody mentions 400 Mcf/d of Capacity (millions of cubic feet day), it means "…in sometime of the pipe operation … under certain circumstance assumed…" Clearly, a research to define a framework, the precedent conditions and the meaning of capacity definition, could be a useful instrument to show a common understanding of the "Nominal Capacity" concept. This paper analyzes the main variables involved, contractual services obligations, demand curve, border conditions to be specified, transient vs. steady state, cost benefit analysis, peak hourly vs. average daily flow rate design, operational policy, etc. Finally, conclusions and recommendations are presented as a procedure to define "Nominal Transmission Capacity" and to produce a "Nominal Transmission Capacity Technical Report" under the concept of arbitrary specific assumptions for further analysis and discussions.

INTRODUCTION

In any pipeline designed and operated around the world, "Transmission Capacity" definition is a controversial issue. The life of a pipeline suffers a lot of changes and transformations in its own structure and in its border conditions. At the time of design a lot of assumptions are made about demand, peak and average flows, gas specifications, environmental constrains, climate, route classifications, performance estimations, operational philosophy, next expansions, regulatory changes, etc. However, this circumstances generally changes and clearly this are unpredictable in the design time, thus the pipeline became incapable to comply with new demands or border condition changes. As the design capacity is an assumption made before the pipe construction, the "real capacity" is a performance result obtained after the start-up and the beginning of the commercial operation. Clearly, any definition of the transmission capability requires a complete description of the framework and operational environment in which that capacity is estimated. The system of TGN (Northern Gas Transmission Company) in Argentina is one of the several cases that could be taken as an example. TGN is mainly composed by two main pipelines called "North" and "Central West" with a nominal capacity of 1,984 Mcf/d (56.2 Mm3/d) (43% North - 57% C.West) covering the Northern part of the country along 2,000 miles of R/W and more than 300,000 HP of installed power in compression stations. Along the time a lot of expansions were made to fulfill the customers' requierements, starting from 870 Mcf/d in 1993 to the current 1,984 Mcf/d.

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