Semi-Quantitative Methods

The ALARP (As Low as Reasonably Practicable) principle is based on the fundamental thinking of ‘acceptable’ or ‘tolerable’ risks. It allows analysts and decision makers to define boundaries to combine probability-consequence scales. These boundaries can be used to delineate acceptable and intolerable risks. This allows decision makers to evaluate whether a system or process poses certain risks which need to be treated using risk-control options. The ALARP principle can easily be combined with tools such as Risk Matrices to graphically represent the boundaries of risk tolerability.

The Cause-Consequence Diagram is developed from a certain initiating top event, i.e. an event that initiates a specific operational sequence or an event which activates certain safety systems. The Cause-Consequence Diagram consists of two reliability analysis methods previously elaborated, the FTA and ETA methods.

The preliminary hazard analysis (PHA) is a semi-quantitative risk analysis methods that is used to identify potential hazards and the corresponding top events (risks). The PHA sheet identifies as well the required measures and follow-up actions to control the hazard sources. The PHA should consider hazardous components, facilities, safety-related equipment, and environmental constraints.

In a qualitative approach, risks are usually categorized into “high”, “medium” and “low”, and are represented in a risk matrix. This risk matrix represents the possibility and severity in order to integrate the potential accident scenarios that were identified in the threat identification stage. Inside the matrix, the criticality of a risk, through the use of colors, can identified. The results provided by this matrix can be used to compare risks and identify improvement and mitigation measures.

The Failure Mode and Effects Analysis is a cross-industry established method to identify and eliminate potential failures, problems, errors and risks of a system, design or process before adverse consequences reach the internal/external customers. The basic idea of the FMEA is thus the preventive risk identification and error prevention instead of a subsequent correction.