Many LOPA scenarios have an independent protection layer (IPL) credit for operator intervention to prevent or mitigate the scenario. It is often difficult for the LOPA team or other safety teams to assess if the appropriate operator response credit is being taken. To allow a more consistent approach to this operator response credit, an assessment method was developed for safety team use.
A human reliability assessment (HRA) method for operator response to alarm(s) was developed as an assessment tool for safety teams. This tool uses methods and factors from several different human reliability and task analysis techniques available in the literature. The analysis tool consists of the following steps:
The key alarm response activities are identified and categorized. The time required for successful response is identified. The time for operator decision making is identified. The time required for operator action is identified. The performance shaping factors (PSF) are identified. The appropriate performance shaping factors (PSF) are assigned.
The human unreliability is then calculated based on above input.
Additional information on the quality of the alarm and the alarm actions are used to complete the operator alarm response evaluation. The tool is designed to show if one LOPA credit (probability of 0.1) is viable or not and if there is a possibility for more than one LOPA credit. However, two LOPA credits is beyond the scope of this tool.
The paper discusses the categorization of alarm response activities, the basis for determining the required alarm response time, the basis for estimating operator decision time, and the time for the operator to do the corrective action. The performance shaping factors used and their weighting is also discussed. The other parts of the operator response to alarm protection layer are identified and documented so that the probability of failure on demand (PFD) of the entire protection layer can be calculated and evaluated.
The calculation of the several sample alarm response analysis cases are presented to demonstrate how the HRA alarm response tool works. The results of this calculation are displayed in a tabulated form such that the impact of each operation and each PSF on the final result can be assessed. The tabulated data is then used to document the analysis results and to target the areas for operator response to alarm improvement. The design intention for this evaluation tool is to allow an operator alarm response PFD evaluation within 15 minutes.
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