Safety barriers: Definition, classification, and performance

Abstract

In spite of the fact that the concept of safety barriers is applied in practice, discussed in the literature, and even required in legislation and standards, no common terminology that is applicable across sectors have been developed of the concept of safety barriers. This paper focuses on safety barriers and addresses the following aspects; definitions and understanding of what is a safety barrier, classification of safety barriers, and attributes of importance for the performance of safety barriers. Safety barriers are physical or non-physical means planned to prevent, control, or mitigate undesired events or accidents. Barrier systems may be classified according to several dimensions, for example as passive or active barrier systems, and as physical, technical, or human/operational barrier systems. Several attributes are necessary to include in order to characterize the performance of safety barriers; functionality/effectiveness, reliability/availability, response time, robustness, and finally a description of the triggering event or condition. For some types of barriers, not all the attributes are relevant or necessary in order to describe the barrier performance.

Introduction

Safety barriers have been used to protect humans and property from enemies and natural hazards since the origin of human beings. When human-induced hazards were created due to the industrialism, safety barriers were implemented to prevent accidents caused by these hazards. The concept of safety barriers is often related to an accident model called the energy model (see Fig. 1). Gibson (1961) pioneered the development of the energy model, while Haddon (1980) developed the model further as he presented his ten strategies for accident prevention. Safety barriers also play an important role in the Management Oversight & Risk Tree (MORT) concept (Johnson, 1980).

During recent years, an extended perspective on safety barriers has evolved. This is emphasized by Hollnagel (2004) who states that “whereas the barriers used to defend a medieval castle mostly were of a physical nature, the modern principle of defence-in-depth combines different types of barriers—from protection against the release of radioactive materials to event reporting and safety policies”. This development is also supported by Fleming and Silady (2002) who states that “the definitions of defence-in-depth have evolved from a rather simple set of strategies to apply multiple lines of defence to a more comprehensive set of cornerstones, strategies, and tactics to protect the public health and safety”. The concept of defence-in-depth was developed within the nuclear industry, but is also used in other high risk industries (e.g., the process industry where also the term multiple protection layers is used; CCPS, 1993).

The focus on the use of risk-informed principles and safety barriers in European regulations such as the Seveso II directive (EC, 1996) and the Machinery directive (EC, 1998), national regulations as the Management regulation from the Petroleum Safety Authority Norway (PSA) (PSA, 2001), and standards such as IEC:61508 (1998), IEC:61511 (2002), and ISO:13702 (1999) demonstrates the importance of safety barriers in order to reduce the risk of accidents. PSA has developed requirements to safety barriers, but has not given a clear definition of the concept. Discussions have emerged on what is a safety barrier. Specialists do not fully agree on this issue and it is difficult for the companies to know how to fulfil the requirements. It is also difficult for the PSA to manage the regulations without a clear definition and delimitation of the concept.

No common definition of the term safety barrier has been found in the literature, although different aspects of the term have been discussed, see, e.g., (CCPS, 2001; Duijm, Andersen, Hale, Goossens, & Hourtolou, 2004; Goossens & Hourtolou, 2003; Harms-Ringdahl, 2003; Hollnagel, 2004; Johnson, 1980; Kecklund, Edland, Wedin, & Svenson, 1996; Neogy, Hanson, Davis, & Fenstermacher, 1996; Rosness, 2005; Sklet & Hauge, 2004; Svenson, 1991), and applied in practice for several decades. Different terms with similar meanings (barrier, defence, protection layer, safety critical element, safety function, etc.) have been used crosswise between industries, sectors, and countries. Safety barriers are categorized in numerous ways by different authors and the performance of the barriers is described in several ways.

The extended use of the term safety barrier (and similar terms) and the lack of a common terminology imply a need for clarifying the terminology both in the Norwegian offshore industry and crosswise between sectors. This need is supported by the following statement from Kaplan (1990); “When words are used sloppily, concepts become fuzzy, thinking is muddled, communication is ambiguous, and decisions and actions are suboptimal, to say the least”. To clarify the terms will be useful in order to avoid misconceptions in communication about risk and safety barriers. The results should be of general interest, and furthermore, a clarification of the term will make it easier for the Norwegian offshore industry to fulfil the requirements from the PSA with respect to classification of barriers and analysis of the performance of different types of safety barriers and barrier elements.

The objectives of the paper are: (1) to present a survey of how the concept safety barrier and similar concepts are interpreted and used in various industries and various applications, (2) to provide a clear definition of the concept safety barrier, and associated concepts like barrier function, barrier system, and barrier element, (3) to develop a classification system for safety barriers, (4) to define attributes describing the performance of safety barriers, and (5) to give recommendations on how the concept of safety barrier should be interpreted and used in different contexts.

The paper is based on experience from a literature survey concerning the understanding of safety barriers in different industries, several projects focusing on analysis of safety barriers (e.g., the BORA project (Barrier and Operational Risk Analysis) (Aven, Sklet, & Vinnem, 2005; Sklet, Aven, Hauge, & Vinnem, 2005; Sklet, Vinnem, & Aven, 2005; Vinnem, Aven, Hauge, Seljelid, & Veire, 2004) and a project on behalf of PSA focusing on barriers during well interventions (Sklet, Steiro, & Tjelta, 2005), and a study of how safety barriers are analysed in different accident investigation methods (Sklet, 2004). The literature is identified in literature databases, from references in reviewed literature, and by attending international conferences.

The main focus in this paper is the use of the barrier concept within industrial safety, especially as applied to technical systems in the process and nuclear industry. Even though the main focus is on demands for clarification of the term safety barrier from the Norwegian offshore industry, the discussions are also relevant for other industries (e.g., the process industry) and application areas (e.g., the transport sector). The focus is on the risk of major accidents, i.e., occupational accidents have not been discussed in detail. The attention is directed toward safety issues, but the concepts may also be useful for security issues.

The concept of safety barriers is briefly introduced in this section together with the purpose of the paper. The next section discusses what a safety barrier is and gives an overview of some definitions applicable for explanation of the concept of safety barriers. Section three gives an overview of some schemes for classification of barrier functions and barrier systems. Several measures of barrier performance are presented and discussed in section four. Comments, a brief discussion, and recommendations are included in each section. Finally, some conclusions concerning the concept of safety barriers end the paper.