(106a) Do You Know Your Engineered Systems?

The process industry has been adopting different scenario-based methods for risk assessment. As a function of the requirements and available resources, scenario-based approaches vary from qualitative to quantitative in nature. Layer of Protection Analysis (LOPA), and Safety Integrity Level Analysis (SILA) are examples of hands-on methods that use scenario-based approaches. This paper takes a closer look at scenario-based approaches, analyzes the underlying principles, and explains the basic concepts in terms of engineered systems. Engineered systems are defined as the systems necessary to operate process plants productively and safely; thus, all equipment and facilities (i.e., the hardware associated) as well as procedures, and their combinations constitute engineered systems. Every engineered system will have one or more functional intents, and an engineered system is considered to have failed if it fails to fulfill its functional intent. Losses occur only when engineered systems fail. Engineered systems fail because of the gaps between the functional and design intents, and due to deficiencies. All engineered systems are susceptible to failure, but the failure modes and the failure likelihoods vary significantly. The failure modes of interest change with the loss scenarios. Scenario paths leading to the loss events are comprised of scenario events, which are failures and successes of engineered systems, in general. Deficient engineered systems fail with higher likelihoods, and the effect of deficient engineered systems on risk is discussed. As an example, a simplified approach that is built on engineered systems is provided to evaluate the fire and explosion related risk. Finally, the efficacy of engineered systems based approaches to perform risk assessments for a variety of requirements is emphasized.