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.

Event Tree Analysis (ETA) is a method that examines the consequences of a particular event. Starting from an initial event, the tree is divided into two branches, whereby the upper one represents a positive and the lower one a negative development (event trees are usually drawn from left to right). Repeated branching of the tree shows possible effects of an initial event on a system. The aim of ETA is the identification of possible damage events. 

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. 

The FTA (fault tree analysis) is a scientific method for detailed fault analysis. It serves the systematic identification and documentation of possible causes for errors or misconduct. Found causes are broken down as precisely as possible to get a comprehensive fault analysis.