Probability Bow-Ties

The 'bow-tie' diagram is a constructive risk management tool, providing a pictorial representation of the relationship between hazards, initiating events, controls and consequences. It is easily understood by management, engineers, OHSE professionals, process operators and maintenance personnel involved in risk management. Rapid risk ranking methods are commonly used to evaluate the risks of simple likelihood-consequence pairs and are straightforward in application. Rapid risk ranking uses descriptors rather than numbers for likelihood, severity and risk, avoiding the common aversion to numerical values. Nonetheless, it is based on an underlying mathematical construction. These two tools have been combined to provide a simple and transparent risk management tool, the Probability Bow-tie, which can be applied to more complicated systems usually found in the process industries. With adherence to conservative assignment rules, the Probability Bow-tie provides a robust method for determining likelihoods for complicated systems, and assists in assessing the adequacy of controls to achieve a company's tolerable risk criteria. This paper presents a practical methodology using probability bow-ties for risk assessment using standard spreadsheet computer programs normally available in even the smallest operating companies. Probability bow-ties were constructed for a new hazardous chemical marine terminal, using spreadsheets to depict the Bow-tie structure and incorporating lookup tables for likelihoods, outcome severities and risk levels. The risk level for individual outcomes and aggregated risk levels for the terminal are calculated by spreadsheet linkages. The methodology is simple in application, is transparent, is easily maintained by the operating company, provides a useful training tool for hazard awareness and presents a realistic appreciation of the value of controls in place.