Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 72 studies the combination of various methods of designing for reliability, availability, maintainability and safety, as well as the latest techniques in probability and possibility modelling, mathematical algorithmic modelling, evolutionary algorithmic modelling, symbolic logic modelling, artificial intelligence modelling, and object-oriented computer modelling, in a logically structured approach to determining the integrity of engineering design. . | 694 5 Safety and Risk in Engineering Design d Systems Analysis with GAs and Fault Trees Commonly with mathematical optimisation problems such as linear programming dynamic programming and various other optimisation techniques an explicit objective function is derived that defines how the characteristic to be minimised is related to the variables. However in many design optimisation problems an explicit objective function cannot be formulated and system performance is assessed by fault-tree analysis FTA . This is often the case in safety systems design. The nature of the designvariables also adds difficulty to the problem. Design variables that represent the levels of duplication for fully or partially redundant systems as well as the period between preventive maintenance are all integer. Furthermore selecting component types is governed by Boolean variables . selection or non-selection. A numerical scheme is therefore required that produces integer values for these variables since it will not be appropriate to utilise a method where real numbers are rounded to the nearest whole number. Constraints involved in this problem fall into the category of either explicit or implicit constraints. Expected maintenance downtime for example can be represented by an explicit function of the design parameters however the number of spurious process trips can be assessed only via a full analysis of the system which will again require employment of the fault-tree analysis methodology. As no explicit objective function exists for most preliminary designs of safety systems particularly in redundancy allocation problems for design optimisation fault trees are used to quantify system unreliability and or unavailability of each potential design. It is however a time-consuming and impractical task to construct a fault tree for each design variation especially at the lower systems hierarchy levels. To resolve this difficulty select events can be used to enable the construction of a .
