PREHEP: human error probability based process unit selection

The paper describes a methodology to use human error probabilities (HEPs) as a basis for selecting functional process units in the design phase. The method helps us to understand the influence of human error (HE) on functional robustness of the units in earlier design phases, e.g., at the functional analysis level. This methodology can be used to detect the need for human operator support. The method consists of several steps. First alternative configurations of functional process units with different complexities are developed. For each configuration, a fault tree is developed to find the initiating events (failures of equipment) which lead to a chosen top event. This top event is an undesired event such as an overflowing tank. The initiating events are used to create event trees (ET) with special emphasis on operator actions, such as monitoring the process and fault diagnosis. A diagnosis diagram is used to simulate the fault diagnosis process and to identify the initiating failures. The probability of a top event due to human error can then be found by using existing HEP-data and by normalizing the failure probabilities of the equipment. The methodology is demonstrated for two examples of functional process units, each with two levels of complexity.