Traditionally, the environmental risk assessment (ERA) has been limited to discharge probabilities with a qualitative description of potential environmental impact. This paper presents an ERA methodology which introduces algorithms for quantification of environmental damage which can be directly evaluated against well defined acceptance criteria. Moreover, the methodology includes three levels which are constructed so that more conservative risk estimates are obtained the less data and knowledge that are applied in the analysis. The same acceptance criteria are applied for all three levels. The presented methodology is limited to assessments of acute oil discharges in relation to ecological components. Aesthetical and economical components are not included.


Norwegian legislation requires that the offshore petroleum operators define acceptance criteria for the risks in their offshore activities. Such criteria are required to be defined for human safety as well as environmental impact.

Further, the legislation requires that a risk assessment has to be carried out in order to evaluate whether the acceptance criteria are met or not. This requirement for environmental risk assessment is a new practice in risk management and define needs for practical and quantitative methods.

The legislation as defined today is directed towards assessing the probability for environmental damage in particularly sensitive areas. The presented methodology focus on particularly valued and sensitive environmental components.

Damage Classification

A major problem in environmental risk analysis is to define a taxonomy for quantification of damage to the range of environmental components. The presented methodology is limited to assessments of acute oil discharges in relation to ecological components. Effects on human health and aesthetical and economical components are not included.

The ecological components are assessed on a population or community/habitat level. For a given oil contamination, they may experience unwanted effects (acute mortality, reduced reproduction, decreased biodiversity, habitat loss) to a given extent (fraction of population affected, length or area of coastal zone contaminated).

Experience from earlier oil spills has shown that most ecological components will start the restitution process after the incident. The time period until the component is back to a healthy pre-spill level or a new stable situation (e.g. new carrying capacity) is referred to as the restitution time. The restitution time will depend on the environmental component considered and the magnitude and extent of the effects.

The restitution time is used as an expression for the damage to environmental components considered. Five damage categories are defined and presented in Table 1. The range for the damage categories shown in Table 1 has been defined to cover the range of restitution times seen after historical spills.

Acceptance Criteria

The risk acceptance criteria are defined by the industry itself. The criteria are given by means of upper frequency limits for the defined damage categories. Acceptance criteria may be related to fields, installations and operations. Restitution time as consequence parameter forms a rational basis for the development of such criteria. The acceptance criteria may be presented in a consequence-frequency matrix as shown by the example in Fig. 1. In the area between intolerable and negligible risk, risk reduction means may be implemented to reduce the risk to As Low As Reasonable Possible (ALARP).

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