INTRODUCTION.

CIMSA-SINTRA, a branch of Thomson-CSF has developed a large control system for the trans-Siberian russian gas pipeline. This pipeline is one of the largest ever built. It is 3000 Miles long, with & 55 inches lines in parallel and the corresponding compressor stations spanned every 70 miles. Among numerous equipment and pieces of software, Planning and Modelling software is provided by CIMSA-SINTRA. This paper focuses on the optimization methods developed for fuel use and steady state planification. A quick look at the PSIG bibliography on optimization shows that the domain has been recently very active. Standard optimization methods as dynamic programing and reduced gradient optimization have been used. The purpose of this paper is to describe a fully integrated method using both dynamic programming and reduced gradient techniques (*). We want to stress the decisive advantage of using both methods in a hierarchical way and also show that both methods cannot be implemented by a straightforward derivation from theory but require a large amount of refined adaptations more suitable with the practical problem to be solved. We also focus on the multi-criteria optimization problem and show that very different solutions with respect to fuel cost can be found, depending on the selected criteria and that optimized solution can result in large savings.

1 - PROBLEM POSITION.
1.1 The Pipeline.

The type of pipeline on which we want to perform a steady state fuel use optimization is seldom found except in the USSR, where huge gas and oil fields are now opening to extraction in Siberia. But networks of gas pipelines are much more common in OCDE countries, and, to some extend, can be compared to the system we deal with. The above introduction gave some figures about this pipeline, but, to be more precise, we can add the following : DEFINITIONS: Compressor stations and workshops (see Fig. 2) As mentioned above our system comprises six parallel lines. Every seventy miles we find six compressor stations (one per line). In this paper we call each of these individual compressor station a workshop and the six compressor stations is called g compressor station. Compressor stations-The whole pipeline comprises 40 compressor stations (defined as above), spanned every 70 miles. Each compressor station can control up to six compressor workshops (defined as above). The number of lines ranges from two to six depending on the geographical area but is mostly equal to five or six). The overall number of workshops is 196. Compressors - The compressors are driven by gas turbines ("gas compressors") or by electric motors ("electric compressors"). There can be up to eight compressors per workshop. Electrical compressors can be arranged in parallel or in a two stage configuration.

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