The Functional Reasoning Design Language (FRDL): Supporting Hazard Analysis with Graphical Models of Behavioral Interaction

Functional Hazard Assessment (FHA) is a key early-stage engineering process that supports the incorporation of safety in design by identifying the high-level functional hazards the system may encounter. Functional Hazard Assessment is often supported by models of system functionality that abstract system behavior at a functional level. However, the diagrams used to create these abstractions are limited in what types of behavior they can coherently represent–often leaving out or misrepresenting the key behavioral interactions that cause hazardous conditions to propagate from one function to another. To resolve these limitations, this paper introduces the Functional Reasoning Design Language (FRDL), a formal modelling language for describing the functional elements of a system and their behavioral interactions. To demonstrate the use of FRDL, an autonomous rover system is analyzed using FRDL, showing an improvement in the ability to identify hazard causes and effects when compared to a traditional Failure/Fault Analyses Procedure. This demonstration, along with a qualitative comparison of FRDL with other diagramming conventions used to support FHA, highlights the potential for FRDL to improve the analysis process by more rigorously representing the causal mesh of behavioral interactions that enable hazardous effects to arise from faults or adverse conditions.