Introduction: The human factor played major role in the accident causation. Statistics show that human error and human mistake are the primary cause for more than 80 percent of the accidents in petrochemical industry.The aim of this study wasidentification and assessment of human error due to unsuitable design which causeaccident and damage to the equipment in thesour water and SRP unit of arefinery plant.

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Material and Method: This research was qualitative case study to identify and assessment of human errors of worker in the control room of sour water within a oil refinery plant. The group of tasks with potential to causing accidents wereidentifiedby direct observation of activities and documents and individual interview. Hierarchical Task Analysis method (HTA) was used tojob analyze and the results were presented in HTA charts. All possible human errors in work stages were identified and assessed using SHERPA method.Action Errors, Checking Errors, Retrieval Errors, Communication Errors, Selection Errors were analyzed and their related data were entered in SHERPA work sheet.

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Result: The findings showed that from all of the 118 identified errors 50% were Retrieval, 22/22% wereAction Error22/22% Communication and 5.55% Selection Error.

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Conclusion: To prevent and control occurring each of the identified errors and to limit the consequences of them, appropriate counter measures such as proper control measures in the form of changes in design, including install the appropriate colored tag, digital indicator and warning lights which must be used base on the kind of system consequently, of this study showed that SHEPA can be an efficientmethod to study humanness in operational site.