# Modelling a Safety Management System Using System Dynamics at the Bhopal Incident
## 2 Assumptions Methodology Description  
In order to develop and implement an effective safety management system, many factors have to be considered to represent the whole system. Many authors in literature [13; 14; 15] have supported the effectiveness of system dynamics as a useful tool to highlight the interrelations among the components of a system: People, things, business functions, socio-technical elements, psychophysical characteristics, etc. System dynamics could be associated to a dynamic Bayesian network (DBN), which would allow association of a priority scale based on the temporal evolution of faults (as if a weight factor was associated with each fault) [16]. The causal loop diagram (CLD) is the graphical tool used to make cause and effect relationships among these elements immediately visible, so it is the starting point for the subsequent construction of the model. According to the aim of this paper and starting from the literary review, a causal loop diagram is proposed. The model highlights the main factors that influenced the Bhopal incident. The elements of the causal loop diagram and the cause-effect relations of each of them with the other elements of the system are described in Figure 1, in which the following variables are reported:  
* Failure risk: The risk of failure/breakage is based on probability of failure of an equipment/plant and the consequences deriving from it. This estimate is more accurate if the information about it is more precise relative to the type of failure/break (type of damage) [14]. This will depend on information derived from the historian of the inspections and maintenance carried out, on the planning data for inspection, on process data (temperature, pressure, etc. ), on the type of degradation to which the construction material is mostly subjected as well as the fault data. The risk of failure becomes the criteria of the decision-making process that leads to planning maintenance and inspection activities (plan). The more realistic risk estimation is, the more reliable the maintenance and inspection plans will be. * Type of damage: The type of damage that could occur on an equipment/plant depends significantly on the characteristics of construction, in terms of materials used and the history of breakdowns of the same type of equipment [15]. Based on the knowledge of the type of failure that could occur, it is possible to decide which inspection technique may be used. In this sense, the knowledge of the type of damage directly affects planning, in particular, with regard to the tools and specialized personnel used in conducting particular non-destructive tests. * Plan: In the planning phase, the decision-making process should lead to the optimal selection of interventions, considering the criteria of risk (risk failure) and the constraints of badges, which in turn are the result of an estimation process (cost estimation). As it is the time during which decisions are planned, it cannot be detached from human error that in turn could affect the reliability and the effectiveness of the maintenance activities [16]. * Execution: The execution of the maintenance/inspection plan is influenced by the planning (plan) in terms of priority of intervention and of personnel employed (cost estimation), as well as the knowledge of who conducts maintenance and inspection activities. Poor knowledge and experience, together with other factors such as cost reduction related to the acquisition of suitable instrumentation for conducting inspections, increases the probability of human error. * Cost estimation: The cost estimation sets constraints on the planning and the execution of maintenance and inspection activities. This process, if not supported by an adequate knowledge of the person who performs it, could have negative repercussions during both planning and execution. Vice versa, adequate capacity in estimating costs could lead to optimal planning in terms of cost-benefit [16].