cause consequence modelling

The principles of risk assessment detailed here are equally applicable to the qualitative as well as the quantitative approach to this discipline. They constitute a systematic framework within which, a broad spectrum of situations hazardous to health and safety of people may be identified, analysed and assessed.

 The qualitative risk assessment process broadly relies on expert judgement and past experience primarily within a subjective and coarse quantitative process. It is worth noting that mere use of quantification and numbers does not necessarily qualify an assessment as quantitative.  The Code-of-Practice on qualitative risk assessment encourages significant use of quantification. These are however mainly a reflection of judgement and lack the objectivity and accuracy to generate a detailed and reliable measure of risks.

The Quantitative Risk Assessment process in this Code-of-Practice satisfies the following requirements;

•       Extensive use of modelling

•       Predominate application of objective and validated data

•       Treatment of uncertainty associated with input data and results

•       Treatment of dependency between significant factors

•       Use of statistical simulation where appropriate

Modelling predominately represents a simplification and generalisation of reality but, enhances our understanding of causal relationships, highlights important factors and provides a useful tool for anticipation and potentially prediction of future.

The Risk modelling used in the study is Cause Consequence Analysis (CCA). Compatible with its title the method facilitates the analysis of an event (which is typically a hazard or failure mode of a piece of equipment) to investigate the range of possible outcomes or consequences. The particular strength of this method is that, when applied systematically with the appropriate experts, it logically develops the escalation scenarios that follow the hazardous event. The logic tree developed then reflects how situations develop in the operational environment.

This is achieved by starting the analysis from the hazard (termed the 'Critical Event'), then identifying the 'Barriers' than can mitigate the escalation of the event. A Barrier is any means which reduces the probability of the Critical Event developing into an undesirable consequence. The analysis continues until no more Barriers are identified and all significant Consequences are arrived at. The consequences from a hazard can range from a benign (safe) condition to a scenario that is associated with severe safety and economic losses.

The original CCA methodology, which was developed for the nuclear industry in the 1970s, has been enhanced to better suit the requirements of the rail industry. This has led to the development of a software package entitled Integrated Safety Assurance Environment  (ISAE) which operates in a windows environment and has proved CCA functionality.

The ISAE software enables three types of Barriers to be represented, these are as follows: 

Physical Barriers: These relate to Barriers which present hardware protective systems that impede the escalation of the hazard. Examples of hardware Barriers are; hoops on signal structure ladders that prevent maintainers falling from the ladder and a secured harness that would prevent a maintainer from falling from a structure.

Human or Circumstantial Barriers: These relate to Barriers that rely on the vigilance of personnel to prevent the escalation of a hazard. An example of this would be a member of staff noticing a fire. This particular category also includes Barriers that are circumstantial, the escalation of the hazard at this stage is predominantly exposed to chance. An example of this would be the chance that an object dropped from a signal structure does not hit a member of staff.

Procedural Barriers: Barriers within this category are where existing established procedures and work instructions if correctly followed prevent the escalation of a hazard. A Barrier of this type is the routine inspection of signal structures in OHL areas to ensure that earth bonds are not severed.

The icons used to represent the types of barriers and consequences are shown in Table 1, below. 

 At every Barrier, the probability of failure is entered based on historical data or calculations. Where validated numerical data were not available, best estimates based on the knowledge of the panel members were used. The probability of success is computed as the reciprocal of the failure probability.

GLOSSARY

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