Investigating the Use of Argument Modularity to Optimise Through-Life System Safety Assurance

Safety cases are now regularly used to communicate the argument about the achievement of acceptable levels of safety for safety critical systems. Increasingly, safety standards such as Defence Standard 00-56 require the scope of the safety case to cover not only the development of the system, but also operating and maintenance through life, including decommissioning. This involves two dimensions of safety case management: safety case development and safety case maintenance. The development of modular safety cases is considered to address to some extent a number of challenges during development and maintenance. In modular safety cases, the safety case argument and evidence are organised into separate but interrelated and cross-referenced modules. However, the adoption of modular safety case development approach is not a panacea. Alongside the advantages listed, modular safety cases can bring a number of new challenges that need to be taken into account. This paper discusses some of these difficulties, together with suggested mitigation strategies.