HAZOP Methodology and Guidewords

As explained earlier, the HAZOP method relies on brainstorming. Brainstorming as a technique is very powerful, but it is difficult to ensure that it is sufficiently rigorous to meet our objective of not missing any significant safety or operability issues. One requirement to meet this need has already been discussed, that is the size and capability of the team. The other requirement is a suitable set of 'prompts'.

HAZOP methodology relies on a set of seven guidewords, which should be applied to all the important parameters of the process under study, to identify deviations from design intent which could lead to the issues we seek. The problem of completeness lies in the identification of the 'parameters of importance'. The method adopted by many proponents of the HAZOP technique, including DNV Technica, is first to ask the team to identify this set of 'parameters of importance', then to apply to them the HAZOP guideword list to generate a much longer checklist of guidewords to be used as our 'prompts'.

In practice, for most oil or chemical industry process systems, a single, generic set of some 20-30 guidewords is used, as for example the list of Table 1.

The Leader should always check before using a generic guideword list that the team believes it to be sufficient. When non-standard equipment is involved, or batch operations, or a high level of operator involvement in the process, a special exercise will be required to generate an appropriate guideword set. The use of a guideword set to identify scenarios is illustrated by the flowchart of Figure 1.

The completeness and appropriateness of the guideword set is a key to the completeness of scenario identification, and the choice of guidewords is a mandatory step in our HAZOP procedure.

The leader will divide the equipment under study into 'study sections', and will ensure that these sections are not too large or complex, so that the team does not become confused when applying the guidewords, leading to missed scenarios.

In order to meet our requirement to identify all causes of problems, all guidewords will be applied to each 'study section', and their use will be recorded to aid later quality control. During this process it is vital that the action of all safety systems is ignored by the team, ie that the identification process assumes the failure of safety systems. The reliability and effectiveness of these systems is considered when judging the seriousness and likelihood of each scenario (see the section on Judgement Criteria below).

TABLE 1 Generic HAZOP Guidewords for Continuous Chemical Processes