Safety System Impairments and the Need to Manage Peak Risk

The catastrophic incident which occurred at Bhopal is often referenced with respect to inherent safety issues. Did intermediate chemicals need to be present? Should the neighboring community start at the fence line? Although likely not as important as the above fundamental considerations, the impairment of safety systems also played a significant role with respect to this event. These impairments took what should have been an unlikely incident and made the catastrophic outcome a near certainty.

In the current paper, a methodology is proposed whereby one should consider both the annualized risk and the peak risk involved in various situations when managing process hazards. This approach looks at both the impacts of safety system impairments on the total risk and the fluctuations in short-term risk levels which will occur at the time of any impairment. For organizations where detailed risk assessments have been conducted for safety instrumented systems (SIS), such as those values generated through methods like Layer of Protection Analysis (LOPA), general impairment guidance can be readily assembled. This guidance looks at the type of protection layer and the potential amount of increased risk which will result from an impairment. The approach then considers both the duration of the impairment with respect to managing annualized risk and the need for interim risk reduction measures during the impairment to manage peak risk. Questions that arise from this approach include: how much fluctuation in risk is appropriate for an organization and how long should an impairment be allowed to go on for?