Methods for Increasing the Survival Probability and Availability

An important objective of probabilistic safety analyses is to point out the way towards increasing the survival probability of technical systems. In what follows several possibilities for doing this are described. 

The principle of redundancy was already presented. Redundancy implies that more components than strictly required for a task are available. Redundancies enable one to design a technical system in such a way that it is more reliable than the components of which it is made up. 

A further measure which is frequently encountered in the process industry is the installation of reserve components. These are in stand-by and take over the function of the main component in case that should fail. They often allow plant operation to continue virtually without interruption. 

Finally, safety systems like the trip system or the emergency power supply have to be mentioned. These are essential for avoiding the destmction of a plant and the consequential major accident. It goes without saying that they must be highly available. If the components of these systems must function for a certain period of time in order to fulfil their mission, they must additionally have a high probability of survival. 

In what follows mathematical models for failure probabilities and unavailabilities of components and sub-systems, which are equipped with reserve components or subject to functional tests and repair, are presented. The expressions derived are then used to calculate the expected value of the structure function in order to determine reliability parameters for the system under investigation, as shown in the preceding section. 

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