The main principle forrisk control during offshore well activities is to always maintain two independent and tested well barriers towards any potential source of inflow. The short lifespan and dynamic nature of well drilling operations makes this a challenge. Experiences from several industry accidents the last decade reveal that two well barriers were not properly maintained by the drilling personnel during the operation and thus that safety was compromised. Probabilistic risk assessments are considered key for risk management of low probability and high consequence activities such as offshore oil and gas well drilling. The objective of this article is to present a method that can be used to address human factors modelling as an integral part of a well drilling operation risk assessment. The method represents an adoption and extension made to the human reliability analysis part of an existing method denoted 'Risk OMT'. Risk OMT is a risk influence modelling method with a modelling principle that includes human factors assessment. Risk OMT has been demonstrated for purpose of analysis of leak scenarios related to planning or execution and control of offshore process maintenance activities. (C) 2016 Elsevier Ltd. All rights reserved.