Using Importance Measures in the improvement of complex process safety

The aim of this work is the assessment of the safety level of a complex industrial process by using the LOPA (Layers Of Protection Analysis) method. The estimation of Probability of Failure on Demand (PFD) of independent protection layers (IPLs) is made using Fault Tree Analysis (FTA). The combining of LOPA and FTA methods is used to estimate in a first time, the PFD of IPLs that are implemented in the process. In a second time, Importance Measures related to the developed FTA are computed in order to establish the importance of the events associated to the elements of each IPL in terms of their contribution to the probability of failure of this IPL. The results given by this analysis will contribute to the improvement of complex process safety by improving the performance of IPLs, both on their architectures and proof-tests intervals related to their elements. An optimization of a particular IPL refereed as “Safety Instrumented Systems (SIS)” architecture and the tests periods of its elements is proposed in order to reduce its PFD_avg and therefore to improve the availability of this barrier. Finally, the proposed methodology is applied to a case study on an operational process, a combustible gas balloon (G01-VN-45-01) within the unit of gas treatment (UTGA), Sonatrach/Hassi Messaoud).