Per Hokstad

An overall model for maintenance optimization

This paper presents an approach for identifying the optimal maintenance schedule for the components of a production system. Safety, health and environment objectives, maintenance costs and costs of lost production are all taken into consideration, and maintenance is thus optimized with respect to multiple objectives. Such a global approach to maintenance optimization requires expertise from various fields, e.g., decision theory, risk analysis and reliability and maintenance modelling. Further, a close co-operation between management, maintenance personnel and analysts is required to achieve a successful result. In the past this has been a major obstacle to the extensive use of proper maintenance optimization methods in practice, and techniques to promote the communication between the involved parties of the optimization process is an essential element in the suggested approach. A simple step by step presentation of the required modelling is provided. Contrary to most current methods of RCM (Reliability Centered Maintenance), the approach is based on an analytic model, and therefore gives a sound framework for carrying out a proper maintenance optimization. The approach is also flexible as it can be carried out at various levels of detail, e.g., adopted to available resources and to the managements willingness to give detailed priorities with respect to objectives on safety vs production loss.

A method for risk modeling of interdependencies in critical infrastructures

Failures in critical infrastructures may be hazardous to population, economy, and national security. There can be strong interdependencies between various infrastructures, but these interdependencies are seldom accounted for in current risk and vulnerability analyses. To reduce probability and mitigate consequences of infrastructure failures, these interdependencies have to be assessed. The objective of this paper is to present a method for assessing interdependencies of critical infrastructures, as part of a cross-sector risk and vulnerability analysis. The method is based on a relatively simple approach applicable for practitioners, but may be extended for more detailed analyses by specialists. Examples from a case study with the Emergency Preparedness Group of the city of Oslo, Norway, are included.

Loss of safety assessment and the IEC 61508 standard

Abstract The standard IEC 61508 contains a lot of useful information and guidance for safety improvement regarding the use of safety systems. However, some of the basic concepts and methods for loss of safety quantification are somewhat confusing. This paper discusses the failure classification, the various contributions to the safety unavailability, and in particular the common cause failure (CCF) model presented in this standard. Suggestions for clarifications and improvements are provided. In particular, a new CCF model is suggested, denoted the Multiple Beta Factor model.

Overall strategy for risk evaluation and priority setting of risk regulations

Abstract This paper presents the framework of an approach to support planning and priority setting for risk control. Such an approach could assist government/regulatory authorities in their allocation of resources among different sectors. The term risk will here be used in a very wide sense, and it will include, but not restrict to, the traditional HES (Health, Environment and Safety) concept. An overall classification of risk (‘loss categories’), to be used across sectors and directorates is suggested. The risk evaluation includes a number of factors not accounted for in a standard risk assessment, but should be taken into account when authorities set priorities regarding risk control. Sociological, psychological and ethical perspectives are included, and the need for a discourse during the decision process is pinpointed. The paper also discusses the potential inclusion of cost benefit analyses in such an approach. The indicated approach is denoted Risk Across Sectors (RAS), and suggestions regarding the process to implement it are given. Such an implementation process will by itself increase the knowledge and competence of the involved parties.

Common Cause Failure Modeling: Status and Trends

This chapter presents a status of common cause failure (CCF) modeling. The well known betafactor model is still the most commonly used CCF model. The strengths and limitations of this model are therefore outlined together with approaches to establish plant specific beta-factors. Several more advanced CCF models are also described with a special focus on the new multiple beta-factor model. Problems relating to data availability and estimation of the unknown parameters of the various models are discussed, and ideas for further research are suggested.

Practical experience with a data collection project: the OREDA project

Abstract Experience data on the reliability of equipment has become vital to many types of engineering and maintenance analyses. The consequences of incorrect design or poor maintenance may adversely affect: safety, the environment or cost in most categories of process industries, and, in particular, offshore exploration and production industries. The OREDA project is a data collection programme for the offshore industry which has been operating since the early 80s. A high level of knowledge has been gained from this programme on: specification of data, data collection methods and the utilization of data. Some of the results and the knowledge gained from this project are presented in this paper.

A shock model for common-cause failures

Abstract This paper deals with the modeling of common-cause failures in redundant systems. Two of the well-known models for common-cause failures, the β-factor and the binomial failure rate (BFR) model, are discussed and evaluated. Applicability and shortcomings of these models are pinpointed. Further, a new parametric model for common-cause failures, here denoted the random probability shock (RPS) model, is suggested. The main feature of this model is the ability to model various degrees of dependence between the components of the system in a rather straightforward way. The β-factor and BFR models are special cases of this new model.

The modelling of degraded and critical failures for components with dormant failures

Abstract The failures reported in reliability data bases are often classified into s severity classes, e.g., as critical or degraded failures. This paper presents models for the failure mechanism causing the degraded and critical failures, and estimators for the failure intensities of the models are provided. The discussions mainly focus on dormant (hidden) failures of a standby component. The suggested models are based on exponentially distributed random variables, but they give non-exponential (phase-type) distributions for the time to failure, and thus provide alternatives to the more common Weibull model. The main model is adapted to the information available in modern reliability data bases. Using this model it is also possible to quantify the reduction in the rate of critical failures, achieved by repairing degraded failures. In particular the so-called ‘naked failure rate’ (defined as the rate of critical failures that would be observed if no repair of degraded failures was carried out) is derived. Further, the safety unavailability (Mean Fractional Deadtime) of a dormant system is obtained for the new model.

A risk influence model applied to North Sea helicopter transport

Abstract Offshore helicopter transport represents a quite complex, socio-technical system. A number of factors affect the risk related to this activity, e.g. maintenance, design of helicopter/helideck, and the competence/training of the crew. The present paper presents an overall, holistic risk model, which aims at presenting the total risk picture and the risk influencing factors (RIFs) for helicopter transport. The main objective is to provide a tool for identifying the most effective risk reducing measures. The RIFs both for accident frequency and accident consequence are arranged hierarchically in a type of influence diagram, illustrating the effect of the various RIFs. Accident and incident statistics from North Sea helicopter transport are used in combination with expert judgements to assess the weight (importance) and the present status of the RIFs. The model is also used to provide numerical results.

The failure intensity process and the formulation of reliability and maintenance models

Abstract A unified approach to the formulation of failure event models is presented. This provides a common framework for the analysis of both repairable and nonrepairable items, preventive as well as corrective maintenance, and it also applies for items with dormant failures. The suggested procedure is supported by a set of graphs, thereby identifying the significance both of the inherent reliability (i.e., hazard rate) and of the maintenance/repair policy. The definition/interpretation of various failure intensity concepts is fundamental for this approach. Thus, interrelations between these intensities are reviewed, thereby also contributing to a clarification of these concepts. The most basic of these; concepts, the failure intensity process, is used in counting processes (Martingales), and is the rate of failures at time t, given the history of the item up to that time. The suggested approach is illustrated by considering some standard reliability and maintenance models.