Structuring Safety Policy Decomposition

Safety policy is a collection of rules that govern the behaviour of entities such that they do not cause accidents. It has been suggested that policies in general can be expressed at various levels of abstraction and organised as a hierarchy of goals. In developing policy, it is desirable to decompose from top-level objectives down to rules in a structured manner. The Goal Structuring Notation (GSN) allows us to model the policy decomposition in order to scrutinise and better understand the development process. In so doing, a number of issues arise concerning reusable patterns of decomposition and the assumed models of the system whose behaviour the policy is intended to govern. This paper discusses the need to structure a safety policy decomposition and howmodelling techniques and patterns can aid in this.