Terje Aven

Stochastic models in reliability

Introduction.- Basic Reliability Theory.- Stochastic Failure Models.- Availability Analysis of Complex Systems.- Maintenance Optimization.

On risk defined as an event where the outcome is uncertain

In the social sciences, two prevailing definitions of risk are: (1) risk is a situation or event where something of human value (including humans themselves) is at stake and where the outcome is uncertain; (2) risk is an uncertain consequence of an event or an activity with respect to something that humans value. According to these definitions, risk expresses an ontology (a theory of being) independent of our knowledge and perceptions. In this paper, we look closer into these two types of definitions. We conclude that the definitions provide a sound foundation for risk research and risk management, but compared to common terminology, they lead to conceptual difficulties that are incompatible with the everyday use of risk in most applications. By considering risk as a state of the world, we cannot conclude, for example, about the risk being high or low, or compare different options with respect to risk. A rephrasing of the two definitions is suggested: Risk refers to uncertainty about and severity of the consequences (or outcomes) of an activity with respect to something that humans value.

The risk concept—historical and recent development trends

Abstract This paper reviews the definition and meaning of the concept of risk. The review has a historical and development trend perspective, also covering recent years. It is questioned if, and to what extent, it is possible to identify some underlying patterns in the way risk has been, and is being understood today. The analysis is based on a new categorisation of risk definitions and an assessment of these categories in relation to a set of critical issues, including how these risk definitions match typical daily-life phrases about risk. The paper presents a set of constructed development paths for the risk concept and concludes that over the last 15–20 years we have seen a shift from rather narrow perspectives based on probabilities to ways of thinking which highlight events, consequences and uncertainties. However, some of the more narrow perspectives (like expected values and probability-based perspectives) are still strongly influencing the risk field, although arguments can be provided against their use. The implications of this situation for risk assessment and risk management are also discussed.

Some considerations on the treatment of uncertainties in risk assessment for practical decision making

This paper discusses the challenges involved in the representation and treatment of uncertainties in risk assessment, taking the point of view of its use in support to decision making. Two main issues are addressed: (1) how to faithfully represent and express the knowledge available to best support the decision making and (2) how to best inform the decision maker. A general risk-uncertainty framework is presented which provides definitions and interpretations of the key concepts introduced. The framework covers probability theory as well as alternative representations of uncertainty, including interval probability, possibility and evidence theory.

On how to define, understand and describe risk

The prevailing perspectives and definitions of risk, at least in the engineering community, are based on probabilities. In this paper we argue that such perspectives and definitions are too narrow. The probability component of the risk concept should be replaced by uncertainty. By jumping directly into probabilities, important uncertainty aspects could easily be overlooked or truncated. In the paper we point at several extended risk definitions, and a formal structure for the various perspectives and definitions is developed. Fundamental concepts such as second-order probabilities and uncertainties are discussed. Examples are provided showing the importance of the choice of risk perspective in a risk assessment and decision-making context. The examples cover offshore operations, security and market price risks.

On Some Recent Definitions and Analysis Frameworks for Risk, Vulnerability, and Resilience

Recently, considerable attention has been paid to a systems-based approach to risk, vulnerability, and resilience analysis. It is argued that risk, vulnerability, and resilience are inherently and fundamentally functions of the states of the system and its environment. Vulnerability is defined as the manifestation of the inherent states of the system that can be subjected to a natural hazard or be exploited to adversely affect that system, whereas resilience is defined as the ability of the system to withstand a major disruption within acceptable degradation parameters and to recover within an acceptable time, and composite costs, and risks. Risk, on the other hand, is probability based, defined by the probability and severity of adverse effects (i.e., the consequences). In this article, we look more closely into this approach. It is observed that the key concepts are inconsistent in the sense that the uncertainty (probability) dimension is included for the risk definition but not for vulnerability and resilience. In the article, we question the rationale for this inconsistency. The suggested approach is compared with an alternative framework that provides a logically defined structure for risk, vulnerability, and resilience, where all three concepts are incorporating the uncertainty (probability) dimension.

A unified framework for risk and vulnerability analysis covering both safety and security

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Models and model uncertainty in the context of risk analysis

Abstract This paper addresses the concept of model uncertainty within the context of risk analysis. Though model uncertainty is a topic widely discussed in the risk analysis literature, no consensus seems to exist on its meaning, how it should be measured, or its impact on the application of analysis results in decision processes. The purpose of this paper is to contribute to clarification. The first parts of the paper look into the contents of the two terms ‘model’ and ‘uncertainty’. On this platform it is discussed how focus on model uncertainty merely leads to muddling up the message of the analysis, if risk is interpreted as a true, inherent property of the system, to be estimated in the risk analysis. An alternative approach is to see the models as means for expressing uncertainty regarding the system performance. In this case, it is argued, the term ‘model uncertainty’ loses its meaning.

Risk assessment and risk management: Review of recent advances on their foundation

Risk assessment and management was established as a scientific field some 30–40 years ago. Principles and methods were developed for how to conceptualise, assess and manage risk. These principles and methods still represent to a large extent the foundation of this field today, but many advances have been made, linked to both the theoretical platform and practical models and procedures. The purpose of the present invited paper is to perform a review of these advances, with a special focus on the fundamental ideas and thinking on which these are based. We have looked for trends in perspectives and approaches, and we also reflect on where further development of the risk field is needed and should be encouraged. The paper is written for readers with different types of background, not only for experts on risk.

Perspectives on risk: review and discussion of the basis for establishing a unified and holistic approach

There exist many perspectives on risk, including safety engineering, social scientist perspectives, risk perception research and economic decision analysis. Traditionally, some of the different perspectives have been viewed to represent completely different frameworks, and the exchange of ideas and results has been difficult. Much of the existing discussions on risk perspectives have in our view lacked a sufficient level of precision on the fundamental ideas of risk assessments and management. For example, there is more than one line of thinking in risk analysis and assessment and mixing all approaches into one, gives a rather meaningless discussion. In this paper, we review some of the most common perspectives on risk. We show that it is possible to establish a common basis for the different perspectives, by looking at risk as the full spectrum of the dimensions (a) possible consequences and (b) associated uncertainties. Examples from the offshore oil and gas industry are included to illustrate ideas.