Operational Barrier Elements in Critical Drilling Operations: Understanding Failure Mechanisms in the Primary Well Barrier
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- Institutt for marin teknikk 
This thesis concerns the use of human reliability analysis in offshore quantitative risk analysis (QRA). A QRA is a risk analysis which produces numerical values for probabilities of, and consequences from, undesired events. It typically analyzes several entire accident sequences, each originating from various hazards that are present on an offshore installation. From a QRA, assessments regarding the level of risk can be made. Criticism has however been directed towards offshore QRAs, for their lack of focus on Human and Organizational Factors (HOFs) (Skogdalen and Vinnem, 2011). A feasible way of including HOFs in QRA is by using human reliability analysis (HRA). A HRA is a systematic identification and evaluation of the possible errors that may be made by operators, maintenance personnel, and other personnel in the system (Rausand, 2011). The three essential parts from a quantitative HRA is human error identification, quantification and reduction.The goal of the thesis is to apply HRA to a precursor of one of the hazards present on a drilling rig; the uncontrolled release of hydrocarbons to the installation and surroundings - a blowout. In order to do this, pressure control during drilling is discussed. A qualitative human reliability analysis of a operation performed in offshore drilling is subsequently performed using the discussed HRA methods. The operation is called trip out of hole and constitutes all actions required by rig personnel to retrieve the entire drillstring, bottom hole assembly and bit from the bottom of the well. Maintaining primary well control throughout the operation means ensuring there is a positive pressure differential between the column of mud in the well and the pressure of the fluid inside the pore spaces of the exposed formation. If overbalance is lost, formation fluid will rush into the well and the situation must be controlled by means of a blowout preventer. The thesis makes use of HRA methods to identify and causally represent potential human errors which may cause such an influx to occur. A hierarchical task analysis has been developed, upon which human error identification has been performed. Lastly, the relevant errors have been combined with technical faults in a fault tree for the top event primary well control failure occurs during trip out of hole . The fault tree logically depicts the basic events which by themselves, or in combination with other basic events, are sufficient to cause the top event to occur. The results from the analysis show that human error causing dynamic pressure changes in the mud column are the most critical for primary well control during the operation.