The catastrophic incident that 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 aforementioned 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 this article, a methodology is proposed whereby one should consider both the annualized risk and the peak risk involved in various scenarios 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 that will occur at the time of any impairment. For organizations where detailed risk assessments have been conducted for safety instrumented systems, such as those values generated through methods like layer of protection analysis, 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? © 2012 American Institute of Chemical Engineers Process Saf Prog, 2012
Quantitative risk assessments (QRAs) are used within the field of process safety to decide the allocation of resources and risk reduction investments. Typically risk assessments involve the evaluation of probabilistic measures that estimate the average expected value for the situation being considered across a range of potential outcomes. The resulting expected value is then used to determine if a situation represents an acceptable or unacceptable risk based on a threshold value allotted to the risk. This approach often gives guidance that is at odds with the thoughts and behaviors of some stakeholders as illustrated by the “but what if it does happen?” type of question. This inconsistency results from the inherent limitation associated with expected value approaches in that the methodology is based on whether or not a mean assessed risk represents an acceptable risk while overlooking the possibility that a single scenario could represent an intolerable event. This article looks at an adjustment to traditional QRAs so as to assess both the acceptability of risk and the tolerability of the associated consequences relative to risk criteria. These adjustments have been found to better represent stakeholder perceptions of risk, more closely relate risk tolerance to corporate values and resources, and to better justify the use of various risk transfer strategies. © 2008 American Institute of Chemical Engineers Process Saf Prog, 2009
Layer of protection analysis (LOPA) has quickly gained acceptance in the chemical processing industries and has risen to be one of the leading risk assessment techniques used for process safety studies. LOPA generally uses more rigor and science than what is encountered with qualitative risk assessments, while still not becoming overly onerous when compared with detailed quantitative risk assessments. In the interest of balancing time and resources against science and accuracy, certain tradeoffs and assumptions are made within the LOPA assessment. In turn, these tradeoffs and assumptions can lead to inaccurate conclusions. For example, one issue that arises is with the treatment of protection layers associated with mitigation of consequences. LOPA teams have a choice to account for mitigation layers in the consequence assignment or alternatively treat these layers as independent protection layers (IPLs). Although this may appear to be an inconsequential decision, it can in fact result in very different conclusions. In the course of treating mitigation layers as IPLs, organizations must ensure the necessary inspection, testing, and preventive maintenance practices are in place for these layers. Furthermore, recognizing this dichotomy in treatment, one can also show that these mitigation layers should be designed so as to achieve a balance between consequence reduction and desired reliability. This article discusses alternative treatments of risk mitigation layers that are commonly applied by LOPA teams and demonstrates their impacts through case studies. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011
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