Evaluation of Safety Instrumented System in a Natural Gas Facility According to IEC 61508 Standard

The work presented was aimed primarily at contributing to the control of risks relating to an industrial installation by means of a safety instrumented system (SIS), through a global approach defined by the IEC 61508 standard which goes from risk analysis to the evaluation of SIS. The IEC 61508 approach consists of industrial risk analysis which is a process used to identify the risks inherent in an industrial system and estimate the levels of these risks using several methods, tools and software. Then, the reduction of risks deemed unacceptable in the risk analysis phase to an acceptable level using SIS. The extent of the reduction to be achieved determines the level of performance that the SIS must have to achieve this reduction assigned to it (required SIL). The real performance of SIS (real SIL) must meet the corresponding requirements. Otherwise, technical modifications must be made to the SIS. The application of the IEC 61508 approach addresses the risk analysis of butane product overflow stored in a tank and the verification of the ability of an SIS installed at this tank to control this risk. And if necessary, propose technical modifications to optimize the performance of the SIS. Risk analysis conducted using the HAZOP (HAZard and OPerability) method to identify critical accident scenarios, PHAST (Process Hazard Analysis Software Tool) software and risk matrix to estimate risk level of a given accident. Verification of the ability of an SIS installed at the tank to control the risk deemed unacceptable in the risk analysis phase is carried out by The LOPA (Layer Of Protection Analysis) method for the allocation of the required SIL for the SIS and SIL module of the GRIF (GRaphical Interface for reliability Forecasting) software to calculated real SIL of SIS. Depending on the results obtained (required SIL > real SIL), recommendations likely to strengthen the reliability of SIS are proposed (modification of the test policy and the architecture of the constituent elements of the SIS) in order to achieve the required SIL. And thus secure our system.