2. Risk acceptance criteria
Every year ship collisions and grounding events cause loss of hundreds of lives, economic losses, environmental damages and other unwanted events. It is indispensable that collision and grounding events are considered to be so rare that the benefit of the ship operation to the owner and the public exceeds their sensitivity to risk. Therefore, one of the many performance goals during the design phase of ships should be to ensure that serious accidents and service disruptions are low enough to be acceptable to all stakeholders, i.e. owners, the public and those responsible for public safety. On the other hand, the required risk levels should still allow construction and operation of the ship at feasible cost levels.
The risk involved in a given activity is a function of the possible hazards related to the activity and the probabilities and consequences related to these hazards.
Traditionally, risk acceptance criteria must be established for three main types of risks:
•
Fatalities
•
Pollution of the environment
•
Loss of property or financial exposure
A much discussed problem with risk evaluation and risk mitigation measures is that the consequences may be of very different nature such as fatalities, pollution of the environment, and economic losses.
Criteria for regulation of the risk associated with fatalities have been analysed by Ditlevsen and Friis-Hansen [15] and a detailed proposal for consequence measurement of oil spill pollution is made by McGregor et al. [16]. In a very comprehensive report by Skjong [17] criteria used by IMO and other organisations for fatalities and for environmental damages caused by accidental release of oil and oil products are described in detail.
The acceptance criteria for fatalities are normally based on two principles:
•
The individual fatality risk shall be approximately the same as typical for occupational hazards
•
The frequency of accidents with several fatalities, that is the societal fatality risk, shall not exceed a level defined as unconditionally intolerable
The latter societal risk acceptance criterion must be introduced because society is more concerned about single accidents with many fatalities than many accidents with few fatalities per accident. To kill 100 people in one accident every 1000 years is considered more serious than to kill one person every ten years due to risk aversion of the society.
To risks which do not significantly exceed this upper bound the general ALARP (As Low As Reasonably Practicable) risk management shall be applied. Fig. 4 illustrates this criterion. Thus, in principle for all non-negligible risks, it is required that all possible measures for risk reduction should be identified and analysed and their societal value assessed.