A Pattern to Argue the Compliance of System Safety Requirements Decomposition

Safety case is a structured argument aimed to argue the system is acceptably safe to operate in a specific environment. Safety cases have been successfully used as documentation to support the certification process of safety-critical systems. Safety case patterns have been used to document recurrent safety argumentation solutions that have evolved over time by providing a simple and efficient solution to a particular problem. There are in the literature safety case patterns providing recurrent solutions for arguing the safety of product and process, and the compliance of development processes with safety standards. These patterns may require information provided by safety analysis to be instantiated. The decomposition of system safety requirements (SSRs) throughout component-level failures is one of the activities of the system safety analysis. Existing model-based safety analysis tools provide the capability of automatically allocate SSRs to component failures. In this context, there is a need for an structured safety argument arguing the conformance of the allocation of SSRs provided by these tools with requirements allocation rules prescribed by safety standards as ISO 26262 and ARP 4754A. In this paper, we propose safety case pattern describing a reusable argument structure to argue the compliance of the requirements allocation with the assignment rules prescribed by safety standards. The pattern addresses to support safety analysts in developing safety arguments to justify sufficiency and compliance of the requirements decomposition with the target safety standard. This pattern was applied in the design of arguments of two systems from automotive and avionics domains.