Knowing the safety level of a system in real-time: An extended mathematical model of the STAMP-based RealTSL methodology

Abstract

The RealTSL methodology is based on STAMP and aims at determining the safety level of complex systems in real-time. RealTSL utilizes accident scenarios derived from STPA, together with: (a) The time durations required between system state transitions, (b) Managerial decisions regarding the sensitivity of the mathematical model, and (c) Real-time system data. This manuscript proposes an extension of the RealTSL mathematical model that allows analysts to distinguish system-level accidents according to their severity, and introduces a new expression of the safety level. In addition, this paper shows how the information contained in the graphical form of the acyclic diagram can be used as an interface so that managers can: (a) Comprehend the current systems state against possible accidents, (b) Interpret the results of the mathematical model, and (c) Help them decide whether mitigation actions are needed. The safety level of a system in time t, therefore, is defined as SL=p→wAi>p→wAj>⋯>p→wAm ordering, where m is the number of system accidents and p→wAi are the most detrimental to safety sequences of safety constraint violations for every accident and ordered according to the severity of their corresponding accidents. The extended mathematical model is demonstrated using a fictitious system, providing a graphic rendition of how unsafe situations progress to accidents through time. This paper aims to open a new perspective in the STAMP literature, on discussions on the problem of measuring what the actual safety level of a system is at a certain moment in time and in a certain context.