Analysis of Drift-off During Drilling on Deep Water
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- Institutt for marin teknikk 
For drilling operation in the deepwater, dynamic positioning system is used for platform to main the position. In case drift off (positioning system don t function), the drilling system might be damaged before an emergency disconnection can be performed. The time window between the beginning of the drift off and critical riser damage depends on the many facors. The purpose of the thesis is to identify the influence of coupling effects between the riser and platform on the time windown and identify a reliable model for this type of analysis. The dynamic nonlinear analysis is carried out in Sima for both coupled and uncoupled conditions. The critical scenario is defined that wind, current, and wave acting in the same direction. 300 m, 1000 m, 3000 m water depths are investigated. Different current speed is also researched in the parameter analysis. Tension level is mainly dependent on the offset. The coupling effects on the time before tensioning stroke running out is very limited but it tension don t increase as fast as the uncoupled analysis. Different currents are found only important for the drift off speed. Since the limited coupling effects, a simple analytical solution is proposed. The results proved to be way conservative considering the tension.