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Modelling As A Tool For Quantitative RA

This chapter has the goal to present a general description on quantitative risk assessment based on available methodologies on the topic, which use

  • event trees or other equivalent approaches
  • approaches available for handling large databases as well as
  • transfer of information from the design documents to the safety/reliability models

The QRA approaches are illustrated with results from available case studies, by underlying the specifics for the hydrogen cases. QRA is considered as part of an integrated evaluation of a given installation (deterministic and probabilistic) designed to give a complementary set of insights. The main strong feature of QRA results resides in the fact that it gives a set of insights based on risk ranking and evaluation.

Quantitative Risk Analysis Objectives

The objectives of a QRA may include

  • Estimating risk levels and assessing their significance. This helps decide whether or not the risks need to be reduced.
  • Identifying the main contributors to the risk. This helps understanding of the nature of the hazards and suggests possible targets for risk reduction measures.
  • Defining design accident scenarios. These can be used as a design basis for fire protection and emergency evacuation equipment, or for emergency planning and training.
  • Comparing design options. This gives input on risk issues for the selection of a concept
  • Evaluating risk reduction measures. QRA can be linked to a cost-benefit analysis, to cost-effective ways of reducing the risk.
  • Demonstrating acceptability to regulators and the workforce. QRA can show whether risks have been made 'as low as reasonably practicable'.
  • Identifying safety-critical procedures and equipment. These are critical for minimize risks, and need close attention during operation.
  • Identifying accident precursors, which may be monitored during operation to provide trends in incidents?
  • Taken together, these possible uses of QRA provide a rational structure for monitoring guidance for decision-making about safety issues.

QRA strengths

The main strengths of QRA are:

  • QRA is one of the few techniques able to provide guidance to designers and operators on how best to minimize the risks of accidents.
  • QRA combines previous experience with structured judgments to help anticipate accidents before they occur.
  • QRA is most effective when applied to major accidents. These are difficult to address subjectively, because they lie outside the experience of most designers, operators and regulators. The chances of such accidents occurring are low, but their consequences can be catastrophic, involving the potential for massive loss of life, damage to the environment, financial loss, and on occasions leading to the failure of the company or major changes to the entire industry. Thus there is a moral and practical incentive to use the best-available methods to minimize these risks.
  • QRA is used for comparative ranking of risks
  • QRA is usually applied to activities where there is operating experience to provide a statistical base for the analysis (e.g. semi-submersible drilling rigs). However, safety in these areas can be managed reasonably well on the basis of accident experience. The added value of a QRA is usually greatest in relatively novel applications (e.g. early concrete platforms, floating production systems, tension leg platforms, extension of using from one area-nuclear ¡V to another-non ¡¥nuclear etc) with little operating experience, especially where standard technology is applied in novel environments.
  • There are specific energy related areas with extensive use of QRA, e.g. onshore process industries by evaluating the hydrocarbon release forming fires and explosions and predicting risk of process or pipeline operations,in hydrogen installations, chemical production plants, space industry, aircraft industry , nuclear industry etc.

QRA Limitations

The main QRA limitations are:

  • Since QRA is a relatively new technique, there is a lack of agreed approaches and poor circulation of data, resulting in wide variations in study quality.
  • On the other side, because it is quantitative, QRA appears to be objective, but in reality it is very judgmental, using in a very intensive manner subjective type of probabilities, dependent on expert judgment. Expert judgments may be explicit in areas where data is unavailable, but there are also many implicit judgments in the analysis and application of data that is available, and these are often unrecognized. Therefore the evaluation of the sensitivity and uncertainties of the results is actually a decisive factor of the analysis. QRA thus has rather a higher impact and significance from the point of view of risk ranking, than from the point of view of absolute risk values.
  • The QRA results provide valuable insights and support for the decision-making about safety issues and in general it is recognized that a sound decision making on issues involving subjective or objective probabilities cannot be made without inputs from the QRA.
  • The previous general accepted conclusions is also valid for the decision-making about hazardous activities, which are influenced by economic, social and political factors.

Quantitative Risk Assessment (QRA)- methods and examples

Irrespective of the detailed approach adopted for QRA it has some specific features, as follows

  • A QRA type of analysis is a systematic process of modeling the installations based on barrier approach and considering all the challenges potentially leading to undesirable effects to public, environment or workers
  • Any QRA is structured on some key steps, which are present in any QRA analysis, e.g.:
    • Definition of risk sources,
    • Scenarios and mitigation systems (like ventilation ones),
    • Implementation of results from deterministic (e.g.thermohydraulic) analyses,
    • Use of extensive diverse databases,
    • Definition of the list of postulated initiating events (IE). The IE frequency analysis estimates how likely it is for the IE to occur. The frequencies are usually obtained from analysis of previous operating history of the installation. If no data is available a set of FT are built to derive expected frequencies for IE.
    • Definition of scenarios and failures in detail, leading to the development of ET. The definition of the consequences of each scenario is part of this task. Consequence modeling evaluates the resulting effects if the accidents occur, and their impact on personnel, installation, environment and population, based on the adopted set of acceptable final states of the installation, given a set of challenges to it. Definition of end states-states in which we expect to have potential hazardous situation is also part of building ET.
    • Definition of a set of systems operating as barriers for various challenges to the installation, for which FT¡¦s are built. The action includes system definition, defining the system boundaries, part of the installation or the activity whose risks are to be analyzed and detailed system failure mechanisms.
    • Integration of the FT into the ET is performed so that to derive the full set of paths leading to various potential dangerous from risk perspective end states.
    • Risk quantification and assessment of acceptability is finally performed for the risk metrics adopted, which usually is related to:
      • Individual risk - the risk experienced by an individual person.
      • Group (or societal) risk - the risk experienced by the whole group of people exposed to the hazard. Up to this point, the process has been purely technical, and is known as risk analysis.
    • Writing of the QRA report based on the steps performed. QRA reporting includes also documentation on actions performed for all steps during the process.
    • The use of the QRA results as input to the design and/or ongoing safety management optimization based on risk criteria for the installation, depending on the objectives of the study.andout text is modified after ¡§A guide for Quantitative Risk Assessment for Offshore Installations, CMPT publication¡¨, Aberdeen, UK, John Spouge, (1999). This feedback is done so that to incorporate risk control measures. The use of QRA report conclusions is correlated with other types of analyses for further steps in risk management activities. QRA and other risk assessment methodologies as part of risk management process] []
  • QRA models are based on the assumption that the models are defined by using a systemic approach, e.g. by considering the installation compose of a system ofsystems. Those systems with their components and the interdependencies between them define the installation reaction during various operation regimes. From this point of view a model for which QRA type method is applied has some specific features, e.g.
    • The model consists of systems and subsystems performing various functions, acting as barriers to prevent undesired effects from various challenges to the whole installation
    • The model response to a given challenge is evaluated by using response combination of scenarios (usually called Event Trees ¡V ET) and barrier failures (usually called Function events defined by failures in Fault Trees ¡V FT).
    • Modeling is performed at various levels, in relationship with the boundaries defined for the whole installation, leading to models of
      • QRA/PRA level 1, in which the potentially risk inducing states for the installation itself, without its confinement envelope is considered.
      • QRA/PRA level 2, in which the potentially risk inducing states for the installation with its confinement envelope is considered
      • QRA/PRA level 3, in which the potentially risk inducing states for the installation with the consideration of protection by distance, zoning etc is considered.

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Page last modified on February 20, 2009, at 03:22 PM