Abstract
Quantitative Risk Assessments (QRAs) are instrumental in critical safety decisions such as licensing operations, siting equipment and occupied buildings and periodically investing in safeguarding as part of continuous risk-reduction programmes. QRAs are mostly based on process information such as Piping and Instrumentation Diagrams (P&IDs) and Heat and Material Balances (H&MBs). This basic information is sufficient to identify inventories of hazardous substances within isolatable pressure-containing segments, and the number and type of mechanical component within the segments to estimate the likelihood of releases.
However, recent history is showing several accidents where mechanical components such as those identified in standard QRAs did not fail, prior to the release. We call these accidents operational accidents.
Operational accidents are rarely found in current QRAs. Consequently, this raises the question of whether operational risks are adequately covered in QRAs to ensure the calculated risk pictures are realistic and valid for the above mentioned critical decisions.
This paper looks at some of these accidents to illustrate operational risks and discusses some of the uncertainties present in current QRAs such those associated with failure rates provided in widely-accepted databases. The paper proposes a methodology for including operational risks in QRAs systematically using existing Process Hazard Analysis (PHA) techniques such as HAZID, HAZOP, Layers of Protection Analysis (LOPA) and Fault Tree Analysis (FTA).
