Assessing System Risk

Human error, poor maintenance, and interactions of products or systems with the environment produce consequences that should not be overlooked. Although useful for determining human health and safety effects, system risk assessment applies to all other categories of impact. For example, breakdowns or accidents waste resources and produce pollution that can lead to ecological damage. Large, catastrophic releases have a different impact than continual, smaller releases of pollutants.

In risk assessment, predicting how something can be misused is often as important as determining how it is supposed to function. Methods of risk assessment can be either relatively simple or quite complex. The most rigorous methods are usually employed to predict the potential for high-risk events in complex systems. Risk assessment models can be used in design to achieve inherently safe products. Inherently safe designs result from identifying and removing potential dangers rather than just reducing possible risks (Greenberg and Cramer 1991). A brief outline of popular risk assessment methods follows.

Simple Risk Assessment Procedures

These procedures include the following:

• Preliminary hazard analysis

• Checklists

• What-if analysis

A preliminary hazard analysis is suited for the earliest phases of design. This procedure identifies possible hazardous processes or substances during the conceptual stage of design and seeks to eliminate them, thereby avoiding the costly and time-consuming delays caused by later design changes.

Checklists ensure that the requirements addressing risks have not been overlooked or neglected. Design verification should be performed by a multidisciplinary team with expertise in appropriate areas.

A what-if analysis predicts the likelihood of possible events and determines their consequences through simple, qualitative means. Members of the development team prepare a list of questions that are answered and summarized in a table (Doerr 1991).

Mid-Level Risk Assessment Procedures

These procedures include the following:

• Failure mode and effects analysis (FEMA)

The FEMA is also a qualitative method. It is usually applied to individual components to assess the effect of their failure on the system. The level of detail is greater than in a what-if analysis (O'Mara 1991). HAZOPs systematically examine designs to determine where potential hazards exist and assign priorities. HAZOPs usually focus on process design.

Relatively Complex Risk Assessment Procedures

These procedures include the following:

• Faulty tree analysis (FTA)

• Human reliability analysis (HRA)

FTA is a structured, logical modeling tool that examines risks and hazards to precisely determine unwanted consequences. FTA graphically represents the actions leading to each event. Analysis is generally confined to a single system and produces a single number for the probability of that system's failure. FTA does not have to be used to generate numbers; it can also be used qualitatively to improve the understanding of how a system works and fails (Stoop 1990).

ETA studies the interaction of multiple systems or multiple events. ETA is frequently used with FTA to provide quantitative risk assessment. Event trees are also used to assess the probability of human errors occurring in a system.

HRA can be a key factor in determining risks and hazards and in evaluating the ergonomics of a design. HRA can take a variety of forms to provide proactive design recommendations.

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