Safety Instrumented Systems (SIS) reduce the risk of accidents and enhance overall plant availability and reliability in oil & gas, petrochemical and other industries. In fact, we are confronted with a complex systems, which the existing analytical methods are not able to provide the solutions. To resolve this problem, the advanced mathematic modeling tool called Petri net, recommended as the only feasible, proven and powerful method to handle such complex system for safety and reliability analysis by modelling its complex behavior and assess its performance using a chosen model. In this paper, we propose a standard Generalized Stochastic Petri Net (GSPN) model of a complex SIS used in oil & gas industry, serves as a reference to analyze the reliability and the safety performance. Based on Monte Carlo simulation technique, this propose generalized SPN model aims to calculate the PFDavg and to determine the safety integrity level (SIL) of an Emergency Shutdown System (ESDs) implemented in Algeria Edjeleh Gas Recovery (AEGR) DP Sonatrach In Amenas, to ascertain that the proposed model is easy to use and reliable.