This paper refers to optimizing fuel consumption and contract utilization for any type of gas network and operating conditions. Steady state optimization, in general, requires a non linear, mixed integer optimization problem to be solved. A solution method based on simplifications and sequential linear optimization techniques which has been previously presented will be shortly reviewed. The additional model components describing transient pipeline behavior will be discussed, simplified for optimization under usual operating conditions. It will be outlined, that once the steady state optimization problem can be modeled in a satisfying way, the extensions to transient conditions does not introduce major complications to the optimization procedure except for extra computing time.

1. Introduction
1. 1 The need for optimization

Gas network control is a complex task. A number of requirements have to be met originating from different areas like:

  • business process (long and mid term volume planning [resultant off-takes, target volumes for different planning horizons], availability of technical equipment [repair and maintenance]),

  • contracts (upper and lower limits in flow, pressure, and quality for different time levels),

  • features of technical equipment (compressor and blending stations [operating regimes for stations and units, minimum up and down times, local control logic], pipes [upper limits on pressure]),

  • random events (compressor failures, unpredictable changes in demand,…)

Certain additional aspects may influence how the gas grid is operated in detail, including business process strategy, market position, IT equipment and, not to forget, the skills and experience of dispatchers. This paper focuses on meeting some of the requirements mentioned above considering the aspects of feasibility and cost optimality. If the requirements can be formalized to such an extent that they can be described mathematically, then an optimization problem can be formulated and an optimum solution can be sought. Most requirements define target states at certain instants of time, described in detail by technical parameters like

  • pressure

  • flow

  • setpoints

  • compressor status

  • (control) valve status (incl. unit configuration in compressor stations)

  • gas volume in subsystems.

For long term planning (1 to 2 years) it is sufficient to consider steady state flow based on average values within each time period (weeks or months). Usually this is also the case for mid term planning (1 to 2 months). But for (short term) gas control it is necessary to respond to rapid changes in demands, supply or availability of equipment, and it is necessary to include the transient dynamics of gas flow into the decision process. Technical feasibility addresses the question whether there exists a path from a given or assumed state (e.g. the recent one) to a certain future state. Technologically spoken: find a sequence of control actions which leads to the target state taking into account the transient phenomena involved. In general more than one such path exists. This allows to include optimality as an additional concern. The resulting task is to find the path with minimal costs. The notion of cost is here not limited to energy costs of compressor operation and pressure control.

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