A System for Design Decisions Based on Reliability Block Diagrams

A 'Reliability Block Diagram' (RBD) is a Design-for-Reliability (DfR) tool that is used for analysis of reliability allocation, availability, and maintainability. Furthermore, if applied systematically, RBD can also be used as an aid for making design decisions, especially during the improvement stage of a design. This paper discusses the application of RBD during the design improvement process of an electro-hydraulic control module. This electro-hydraulic control module provides the flow control capabilities which allow the well operator to optimize reservoir performance. The cost of failure of these control modules is very high as it directly effects the production and total losses could be millions of dollars. Therefore, this system is designed with reliability being paramount. RBD is an effective tool to perform system reliability analyses. When used as a design improvement decision tool, RBD is applied iteratively, and process steps for applying RBD in design decisions are described in detail in this paper. Once the RBD of the current system design was developed, the Reliability Importance (RI) metrics were evaluated to identify the relative importance of each subsystem with respect to the overall reliability of the system. RI is a valuable tool when used in this capacity to prioritize the design actions for reliability improvement. The Static RI diagram of the current design helped identify subassemblies with the highest RI index. The identified subassemblies then were prioritized for redesign to improve system reliability. Reliability improvement techniques considered for enhancing system reliability in this paper were (1) fault tolerance and (2) fault avoidance. Fault tolerance was achieved by redundancy, whereas fault avoidance was achieved by using high-quality and high-reliability components. Relatively, application of fault tolerance provides high reliability incremental vis-a-vis fault avoidance. However, fault tolerance methods generally are more expensive to use than fault avoidance methods, and thus, an RBD can be a vital tool in determining whether to improve the system reliability by fault tolerance, fault avoidance, or both. For each design alternative, an associated RBD was developed to capture system interactions, and system reliability was estimated. The electro-hydraulic control module presented here will demonstrate how RBDs are applied for both fault tolerance and fault-avoidance techniques. This paper highlights a successful application of RBD as a decision-making tool for design upgrades of a complex system.