Coiled Tubing Drilling from Riserless Light Well Intervention Vessel
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Riserless light well intervention (RLWI) vessels currently have a proven capability of drilling offshore wells with coiled tubing (CT) to approximately 1000 meter measured depth (MD). The method has been used to drill subsea core samples in Boknafjorden in Norway, and a shallow gas pilot at the Butch field in the North Sea. However, a feasibility study has shown a potential for RLWI vessels to drill offshore, multi-section wells with pressure integrity to depths of 2500 meters MD. The result of the study is a fit-for-purpose enhanced RLWI drilling unit able to drill multi-section, pressure-controlled exploration wells accommodating the regulatory drilling requirements on the Norwegian continental shelf (NCS). The enhanced RLWI drilling unit is developed based on known CTD technologies and methodology, and subsea well barrier equipment. The drilling unit is in the development stage and has never been deployed. Uncertainty in the operational performance and the economic viability of the drilling concept thus exists. CTD from a RLWI vessel poses operational limitations compared to conventional offshore drilling units, which must be taken into considerations when screening for well candidates. The thesis gives an overview of the equipment and methodology involved and evaluates the drilling concept by introducing a case study comparing the RLWI CTD unit with a semisubmersible rig with top drive drilling. A slim well design accommodating the drilling concept and CTD was developed, and simulations of wellbore hydraulics was performed to further validate the drilling concept. An economic analysis was conducted, comparing the RLWI vessel and the semisubmersible rig. The thesis concludes that the drilling concept may represent a game changer in exploration well drilling on the NCS if the drilling performance of the unit meets expectation. Cost savings upwards of 40% is forecasted compared to the conventional drilling rig in the case study. The drilling concept should therefore be of interest for oil companies operating on the NCS.