Repository of DUTH

A real-time safety level calculation method for complex engineering systems based on systems theory and Fuzzy Logic (Doctoral thesis)

Ζελεσκίδης, Απόστολος/ Zeleskidis, Apostolos

The present doctoral work concerns itself with providing an evaluation of safety as well as its fluctuation over time in today's complex systems. The Real Time Safety Level (RealTSL) methodology is presented as well as its extended and fuzzy iterations. The methodology is based on the Systems Theoretic Accident Model and Processes (STAMP) accident model and aims at providing a framework for determining the safety level of complex systems in real-time. RealTSL utilizes sequences of unsafe system states derived from the Systems Theoretic Process Analysis (STPA) hazard analysis, together with: a) The time durations required between system state transitions, b) Managerial decisions regarding the sensitivity of the mathematical model, c) Real-time system data, d) fuzzy logic as the mathematical backbone to identify the degree of confidence in the information about the occurrence of unsafe system states. The safety level of a system in time t in the RealTSL methodology is defined as the ordered set of the most detrimental to safety sequences of unsafe system states that could lead to a loss, ordered according to the severity/consequences of their resulting loss. The iterations of the methodology and how it can be used are presented through 4 case studies in a wide variety of systems (railway systems, collaborative robot (cobot) systems, renewable energy generation systems). This work aims to open a new perspective in the safety and STAMP literature, on discussions about the problem of measuring what the actual safety level of a system is at a certain moment in time and in a certain context.