Risk Assessment, 2nd Edition

Chapter 14 Barriers and Barrier Analysis

14.1 Introduction

Most well‐designed systems have protection equipment or other features to protect people, the environment, and other assets against harm should failures or dangerous deviations occur in the system. The equipment and features that are implemented for this purpose are called safety barriers, or simply barriers. Many other names are used in the literature, such as countermeasures, safety functions/systems, safety critical functions/systems, defense measures, defenses, lines of defense, layers of protection, protection layers, and safeguards. In this book, we use mainly the term barrier, but other terms are also used when we discuss specific methods, such as layer of protection analysis (LOPA).

Barriers are mentioned in previous chapters, e.g. Chapters 2, 8, and 12, without a proper definition. This chapter defines and discusses the barrier concept and shows how barriers can be classified into categories. A special type of barrier, called safety‐instrumented systems (SISs), is described in more detail, and five methods for barrier analysis are introduced:

• Hazard–barrier matrices are used in a simple qualitative method that identifies all hazards in a system and all the barriers that have already been, or are planned to be, introduced to protect against these hazards. The hazards and the barriers are displayed in a matrix diagram that can be used to evaluate the adequacy of the barriers.
• Safety barrier diagrams