Decision process architecture selection

How are these decisions made The selection process usually involves extensive tradeoff analysis that compares the different feasible architectures with respect to some important system property or properties. Cost, not surprisingly, usually plays a large role in the selection process while other properties, including system safety, are usually left as a problem to be addressed later in the development lifecycle. Many of the early architectural decisions, however, have a significant and lasting impact on safety and may not be reversible after the basic architectural decisions have been made. For example, the decision not to include a crew escape system on... 

After equipment is selected, the next step in the conceptual design process is the decision to use multiple instruments to serve the same purpose -redundancy. Redundancy is configured to provide continued system operation even though one or more specific instruments may fail - fault tolerance. Some redundant architectures provide fault tolerance against a... 

When no hazard exists, continued operation of the process is generally preferable to initiating an unnecessary shutdown. Shutdown can cause additional hazards, such as large flaring events or cascade shutdowns of other process units, and does result in the need to restart the process unit with all of its potential hazards. The decision to shut down versus continued operation is made by balancing the risk posed by the potential process hazard if a process demand occurs and the risk associated with shutdown and start-up. This decision is greatly influenced by the relative robustness of the SIS architecture and system utilities. The path can be selected at the channel, sub card, card, or CPU level of the system. The device safety manual may contain specific requirements that should be followed, unless a detailed analysis of the final system architecture demonstrates that deviation from the safety manual is acceptable. 

Because there is no unique solution to satisfy the stakeholder requirements, the Architectural Design Process is an iterative trade-off study, performed at a macroscopic scale, leading to a selected system architecture baseline as a final output. When alternative solutions present themselves, technical analysis and decisions are made as part of this process to identify a set of system elements. The overall objective is to create a System Architecture (defined as the selection of the types of system elements, their characteristics, and their arrangement) that meets the following criteria ... 

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