ISO/TR 12489:2013 aims to close the gap between the state-of-the-art and the application of probabilistic calculations for the safety systems of the petroleum, petrochemical and natural gas industries. It provides guidelines for reliability and safety system analysts and the oil and gas industries. The elementary approaches (e.g. PHA, HAZID, HAZOP, FMECA) are out of the scope of ISO/TR 12489:2013. Yet they are of utmost importance as their results provide the input information essential to properly undertake the implementation of the approaches described in ISO/TR 12489:2013: analytical formulae, Boolean approaches (reliability block diagrams, fault trees, event trees, etc.), Markov graphs and Petri nets. ISO/TR 12489:2013 is focused on probabilistic calculations of random failures and, therefore, the non-random failures are out of the scope even if, to some extent, they are partly included into the reliability data collected from the field.