GasTerra B.V. has a supply obligation to all costumers. This obligation is translated to a required (very small) failure probability rate. There is failure if total supply is less than total demand. The "small field policy" of the government states that the small gas fields have to be emptied first. Total supply of all small fields can be expressed in a combined probability distribution. Total demand can also have a combined probability distribution. The large Groningen field acts as a balancing item. The capacity of the Groningen field must therefore be at least the difference between total demand and total supply (of the small fields) at the required failure probability. COMO is a simulation program that is used to determine the capacity of the Groningen field. The required capacity is calculated out of the cumulative probability distribution of the supply surplus (or deficit) is calculated. It is embedded in the PLATO suite. The probability of the capacity of a supply is dependant on supplies and operation elements (compressors, scrubbers etc.) in the production network with a certain failure behavior. The failure behavior of supplies and operation elements is described by a mean time to failure (MTTF) and mean times to repair (MTTR) with a degradation level of capacity. Out of these parameters the individual probability distribution of each element is determined. The system combines two elements that are pairwise connected in parallel or in series. The new element has a combined probability distribution. The two elements are replaced by the new element, so the network becomes more compact. This is repeated until ultimately the entire supply network is reduced to just one supply! The cumulative probability distribution of the demand is calculated by combining all (temperature dependant) demands at all considered offtakes in a network. The demand of each offtake is calculated. This is done for every hour (1-24), for both a working day and a weekend day and for several temperatures. These temperatures are taken from a given temperature probability distribution. By adding up all calculated individual demands at a hour/day type/temperature combination one gets the total demand. Each total demand has the probability of the corresponding combination. Combining the supply and demand distribution, again by convolution provides the cumulative probability distribution of the supply surplus. Finally, the required capacity of the Groningen field can be found as being the supply-demand deficit at the required failure rate.

1 Introduction
1.1 Background Company

In the early 1960s a large reservoir of natural gas was found in the Netherlands. In the article this field will be referred to as the Groningen field. The estimated extractable volume of gas was 2*1012 m3 (_70*1012 ft3), which would be sufficient to cover the national need for decades. De N.V. Nederlandse Gasunie, was founded to organize national transport and trading of gas. Within a few years the majority of households switched to natural gas. Many industries and power plants were connected to the system as well.

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