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‘QRA’ is used as the abbreviation for ‘Quantified Risk Assessment’ or ‘Quantitative Risk Analysis’. The context usually has to be considered in order to determine which of these two terms is applicable. Risk assessment involves (see Abbreviations, Page 555) risk analysis as well as an evaluation of the results. ‘QRA’ is one of the terms used for a type of risk assessment frequently applied to offshore operations. This technique is also referred to as: In spite of more than two decades of use and development, no convergence towards a universally accepted term has been seen. QRA and TRA are the most commonly used abbreviations. The nuclear industry, with its origins in the USA, particularly favours the terms Probabilistic Risk Assessment or Probabilistic Safety Assessment.

This chapter defines risk quantitatively and presents the dimensions and elements of risk. The chapter further gives an extensive illustration of how risk is expressed. Further discussion about interpretation of risk may be found in Aven and Vinnem (2007) and Aven (2003).

There is a long tradition in the Norwegian offshore industry of the use of risk acceptance criteria. At present however, there is an ongoing debate about whether one should continue to use risk acceptance criteria or not. The author actually favours an alternative approach, whereby generation of alternatives and ALARP assessments replace the more mechanistic use of risk acceptance criteria. This is discussed at length in Aven and Vinnem (2007).

Experience from major accidents in the past is an important source of information to prevent the occurrence of similar accidents in the future. There have been a number of major accidents in worldwide operations in the second half of the twentieth century. There has been a positive trend during the last 15–20 years, resulting in fewer and fewer major accidents.

Although risk may be assessed in both a qualitative and quantitative way, this book primarily deals with quantitative methods. Thus qualitative risk assessment and safety review methods are not covered explicitly in this chapter, which provides a brief overview of QRA. Further details are presented in the following chapters.

Chapter 5 has presented an overview of all the steps involved in performing a QRA. This chapter is devoted to the main analysis techniques that may be used.

Analysis of fatality risk is one of the crucial elements of a full Quantified Risk Analysis. There is insufficient amount of data available for important parts of this analysis, and there is extensive uncertainty about some of the results. It is therefore an important element requiring thorough discussion, and consideration of the various options available to assess the risk of fatalities.

Hydrocarbon leaks and subsequent fire and/or explosion are one of the main threats to offshore exploration and production/processing of hydrocarbon oil and gas. The Piper Alpha catastrophe in 1988 is ‘proof’ of this. A severe fire on the Bombay High production installation in 2005 reaffirmed the severe threat implied by leaks.

Explosions on offshore installations have come very much into the focus in the last few years because new insight has revealed that previous knowledge about blast loads is obsolete. Even worse, the blast loads that result from the latest tests are so high that they cannot be designed against, in many cases.

The most frequent collisions , contacts between offshore supply vessels and the platform to which they are delivering supplies, generally cause only minor damage, although significant impacts have been reported in rare cases. Impact from external, non-platform related vessels is probably the most location dependent hazard. This is however, not reflected in general design criteria, which are not related to impact from non-offshore traffic. When it is considered that the level of this hazard may vary considerably, it is primarily the collision which will be location dependent, whereas the consequences are not. In addition, the collision probability is also more or less (but not entirely) independent of the concept that is chosen.