| Summary: | Master's thesis in Petroleum engineering Global consumption of the fossil fuels is continuously growing. The oil and gas industry is extending the frontiers of exploration and production towards challenging areas such as deep-water zones and arctic regions. Moreover, the industry is forced to improve recovery from the older fields. Therefore, the number of deep-water and subsea wells is increasing, and at the same time, the number of operations required to maintain these wells is growing as well. Intervention and maintenance of subsea installations can be performed either with a rig or a vessel, depending on the complexity of the activity. However, rig interventions are far more expensive compared to the vessel based, and this supports the statement that marine operations should be performed mostly by vessels in the future and should be optimized by reducing the number of intervention days and development of new safe and cost-effective techniques. Based on the Norwegian Continental Shelf (NCS) history, company job reports and discussions with experts, the objective of the thesis is to determine which operational steps and processes of subsea activities can be improved, and how the downtime can be reduced. Additionally, the possibility of performing rigless plugging and abandonment (P&A) operations utilizing light well intervention vessel by implementing state-of-the-art techniques is evaluated. My conclusion from the study shows that time spent on some operations might be reduced by checking and updating the procedures and optimization of some steps within light well intervention (LWI) and inspection, maintenance, and repair (IMR) activities. Also, this thesis proposes how different P&A phases can be performed without utilizing a rig. Recommendations and results are grouped into separate sections and presented at the end of each major chapter. submittedVersion
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