Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 63 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. . | 604 5 Safety and Risk in Engineering Design an over-engineered solution to be less reliable than the original design because of inadequate testing and maintenance. Furthermore it is always advisable to take into account the level of training and experience of the personnel who will be operating the plant. Actions that call for elaborate and sophisticated protective systems are often wasted as well as being inherently hazardous if operators do not understand how they function. c Hazard and Operability Modelling A crucial step in support of a HazOp analysis is to find a suitable discrete event system DES representation for the physical system behaviour generally described by continuous dynamics. However systems modelling approaches have to be adapted to the information that is available at certain points in the design stage. To create a model that is appropriate for PHI a method must be developed that qualitatively maps the dynamics in state transition systems. This type of model is ideal for HazOp but is often not sufficient for controller verification especially if thresholds of timeouts have to be considered. Thus the initial model derived in the early design phases must be refined by adding quantitative information so that a timed discrete event system is obtained for controller verification in the detail engineering design phase. As a basis for a concept to check the safety of a process system in different design stages the physical systems behaviour is mapped into state transition systems given as a 6-tupel TS S Sq I O 0 9 where TS state transition system S finite set of states So set of initial states where Sq C S I finite input O finite output. Furthermore 0 S I 2s denotes the state transition function j S I 9 denotes the state output function. Application of the model in computerised form in a HazOp study relates system behaviour mapped into state transition systems to the HazOp guidewords of none more of less of reverse part of more than other than .
