The aim of Functional Safety (FS) is freedom from unacceptable risk of physical injury or damage to health of people directly or indirectly (through damage to property or to environment). The Functional Safety is the part of overall safety of a system, called Safety Instrumented System that depends on the system operating correctly in response to its inputs, including the safe management of likely operator errors, hardware failures and environmental changes. The basic principles of Functional Safety were developed in the military field, nuclear and aerospace industry, and then taken up by rail transportation, process and Oil&Gas industries that have required a development of sector specific standards. According to the Standard IEC61508, the paper presents a case study concerning the evaluation of both the Safe Failure Fraction (SFF) and the Probability of Failure on demand (PFD) for a complex Safety Instrumented System in Oil & Gas application. After a preliminary presentation of the criteria for the SIL (Safety Integrity Level) verification, the work focuses the attention on the method to achieve the PFD. In particular, an approach based on Failure Modes, Effects and Diagnostic Analysis (FMEDA) and Reliability Block Diagram (RBD) is proposed and then a comparison with the approach described in the Standard. The paper aims to clarify how to demonstrate compliance to IEC61508 in OIL& GAS application, with a particular attention to steam turbine, and proposes a simplified technique for reliability analysis of complex Safety Instrumented System, taking into account that the methods should still be possible to comprehend by design engineers.