Determine actual root cause of incidents related to lost circulation and tight hole
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The topic of the presented masters' thesis has been Root Cause Analysis and identifying non-productive time that occur as a consequence of wellbore problems during drilling operations. Finding the root cause will make repairs more efficient and therefore limit the non-productive time. The main objective has been to describe and investigate common wellbore problems that might occur during drilling operations and to make a numerical model that can be used to find root cause to the most common drilling problems in order to determine the best corrective action that should be taken when an incident occurs. The model is mainly using a theoretical approach, but the result has been tested on real-life cases with some success. The model is presented as a table where the column represent different symptom. Along the rows each symptom is linked to relevant concepts of different strength. All of the identified symptoms leading to the same root cause are then added. The root cause with the highest total root cause value is the most plausible root cause. The entire thesis is built upon the Pareto principle (also known as the 80-20 rule) and only the most common root causes are presented as they are the ones that generates the most non-productive time. Only the most common symptoms for each root cause are mapped because these symptoms are most likely to identify the correct root cause in the majority of the cases. Lesser symptoms are not included as they have little to no effect when compared to the most common symptoms. Root Cause Analysis has proven itself as a powerful tool when analyzing a problem. Even with limited case histories, and problems as complex as offshore drilling, this approach seems to be quite accurate. As long as the input is good the model identifies the most plausible root cause and with improvements it is certainly an approach that can be used to help reducing non-productive time.