Human factor analysis embedded in risk assessment of industrial machines: Effects on the safety integrity level

The study proposed in this paper consists of devising a method to account qualitatively and quantitatively for the human factor in verifying the Integrity Level of Safety system (SIL) assigned to the machinery. It is called "operational SIL", which may differ from the design SIL, due to the impact of human and organizational factors (H&OF) in the operational phase. There are two crucial aspects related to modelling man-machine interaction in Quantitative Risk Analysis (QRA) context: 1. the need to insert human interaction in the logical model of QRAs techniques; 2. the quantification of effect of human factors. During the study an Operability Analysis, which is usually known with the acronym HAZOP (Hazard and Operability studies) was initially used to support the assessment, but with some added features that enable one to accommodate systematically H&OF into the process called Integrated Recursive Operability Analysis framework (IROA). This first attempt to apply IROA methodology showed that this type of analysis highlights the position where in depth human factor analysis must be carried out. Once the point is identified in which the human erroneous action may occur it will be necessary to include the study of human factors and the assessment human error probability (HEP). Our efforts are aimed at defining an improved methodological framework encompassing the integration of H&OF into safety analysis by means of quantitative risk assessment schemes. In order to do that the adopted tool is the Integrated Dynamic Decision Analysis (IDDA). This tool allows modelling the logic of a complex system; it provides a representation of all the possible alternative states into which the system could evolve, as a real logical and temporal sequence of events. The proposed model is designed precisely with the aim of transferring the IDDA philosophy to the in-depth study of the deviations which may occur during human implementation of operational procedures. IDDA developed on the basis of a Task Analysis (TA) could allow to obtain a detailed quantitative analysis of human factors directly during the same risk assessment. Starting from the analysis of a technological system through IDDA it is possible to integrate in the logical model a task analysis describing where and why the operator can cheat or by-pass the safety system thus including in the assessment explicitly the human factors that allow evaluating the operational SIL.