HAZOP method does not provide the completeness of risk analysis, especially indicating all causes of deviations of the process parameters which are the main risk. Particular difficulties occur when complex objects are concerned, which are decomposed into nodes for the needs of the HAZOP analysis. In the majority of cases, particular nodes are not independent - there are interactions between them, that may be overlooked. The presence of feedbacks between the nodes in the process is particularly dangerous. This paper proves that the completeness of this analysis may be increased by applying the qualitative model of the process in the form of a graph of a process. Definition of the graph of a process is provided, as well as discussion on the methodology of the process modelling with the use of such graph. Intelligent Accident Prevention System is also presented allowing for supporting HAZOP analysis by a model in form of a graph of a process. (C) 2017 Elsevier Ltd. All rights reserved.