Towards Effective Hazard Analysis: Qualitative Modeling Approach For Diagnosing Control Function Failures Using Multilevel Flow Modeling

Failures of control systems including wrong interactions with control systems by operators can lead to accidents. Modelbased techniques can model control systems to diagnose such failures. However, traditional mathematical models of control systems do not consider the relationships between process and control on a system level. It also requires detailed information after the detailed design stage. Instead, functional models can be used for the early stage of system development. Multilevel Flow Modeling technique (MFM), as a representative functional modeling method, was applied to modeling control systems. However, the modeling of failure modes of control systems and reasoning rules regarding the control functions were not completely implemented. Consequently, it creates a problem when MFM Models are used for safety analysis, i.e., Hazard and Operability Study (HAZOP) for identification of hazards related to control functions and reasoning about their consequences. To enable MFM to diagnose such hazards, a modeling method for two typical input devices of control systems: switches and analogues is proposed in this paper. A process HAZOP study of a water injection system is used as a modeling input for examining consequences due to the failures of control systems. The scope of the paper is limited to diagnosing control function failures qualitatively for a single failure mode in one mode of operation, i.e., the normal operational mode.