HAZOP System Safety: HAZOP and Software HAZOP, by Felix Redmill, Morris Chudleigh, James Catmur, John Wiley & Sons, 1999

What is HAZOP? Technique for identifying and analyzing the hazards and operational concerns of a system. Central activity – a methodical investigation of a system description (design representation).

What this presentation does not cover: The book puts a LOT of emphasis on –Selecting the study initiator –Selecting the study leader –Planning the study –Roles during the study –Questions vs. follow-up –Completion criteria (P.S. It also tells how to conduct the study itself :-)

Reasonable Limits for this class This is a human-intensive activity As such, the details on the previous page are of extreme importance – authors are experienced and therefore recognize this You won’t be able to conduct a HAZOP study on the basis of these slides Goal: Understand what it is – set the bar higher

Study process itself in a nutshell Introductions Presentation of design notation Examine design methodically one unit at a time Is it possible to deviate from design intent here? Examine both consequences and causes of the possible deviation YES NO Document results Define follow-up work Time up?Agree on documentation Sign off YES NO

Examine design methodically each unit in turn Suppose the design representation is a collection of state transition tables: Units are states, transitions, event/action pairs For EACH, list the recommended attributes (see table from the Hazop book) For each attribute, use the guide words to trigger the questions about ways to deviate

The suggested guide words –No: negation of design intention; no part of design intention is achieved but nothing else happens –More: Quantitative increase –Less: Quantitative decrease –As well as: Qualitative increase where all design intention is achieved plus additional activity –Part of: Qualitative decrease where only part of the design intention is achieved –Reverse: logical opposite of the intention –Other than: complete substituion, where no part of the original intention is achieved but something quite different happens

When timing matters Add the following guide words: –Early: something happens earlier in time than intended –Late: something happens later in time than intended –Before: something happens earlier in a sequence than intended –After: something happens later in a sequence than intended

Guide words chosen Match the system being examined to appropriate table or modify the closest Match the design representation Note: not all guide words apply to all attributes –For attribute “speed” of an electric motor, omit guide word “as well as” and “part of” –For attribute “data flow” on a dfd, “less” is not used because meaning covered by “part of” Generally, study leader selects from the guide words, provides interpretations based on chosen design representation and context, distributes to team in advance of the study

Applications Originally developed for chemical plants Book has detailed examples for –Software using data flow diagrams –Software using state transition diagrams Includes timing attributes of response time and repetition time –Software using various OO models –Digital electronics –Communication systems –Electromechanical systems Same guide words, different interpretations

See book excerpts More detailed outline of the HAZOP process – Figure 9.2 –For all entities For all attributes –For each guide word »Is deviation credible? Example matrices

Fig 9.2 HAZOP meeting process