ABSTRACT
The most critical failure modes in the DP system cause a drive-off or drift-off scenario, where the vessel is moving away from the well center. In such scenarios, the operator needs to activate emergency disconnect of the marine riser before the vessel reaches a site specific red limit. The red limit takes into account the time it takes for the emergency disconnection system to complete, in addition to the vessel speed and the physical bend limit for the marine riser. A probabilistic analysis of drive-off and drift-off scenarios and the red limit is developed for a semi-submersible drilling unit used for drilling operations at a specific site on the Norwegian continental shelf. A blowout frequency model is established using coupled event tree and fault tree models. The total blowout frequency and the red limit for drilling operations derived in this probabilistic manner are presented. This paper presents an alternative methodology to determine the red limit size based on probabilistic modeling of position loss scenarios. By using this methodology, the red limit can be set based on what is considered an acceptable blowout risk.
