This paper is focused on Reliability Block Diagram (RBD) generation with the aim of providing a reliability prediction in the early stages of industrial product development, in particular the case study is focused on gas turbine auxiliary systems. These systems are made up of hydraulic items (valves, pumps, filters, etc.) and electronic ones (sensors, instruments, control logic); these complex structures can be decomposed to a collection of blocks and interconnections in order to achieve reliability performance of the whole system. System design based on reliability assessment allows project engineers to reduce timedelivery, reduce time for improvements, achieve reliability targets and guarantee availability performance to the costumers. Furthermore a new approach to standby redundancy architectures is proposed in order to achieve reliability assessment for complex systems, widely used in Oil&Gas applications.