It is important that a floating drilling unit is able to disconnect from the wellhead in a timely manner during a loss of position to minimize risk to personnel, the environment, and equipment. For most dynamically positioned (DP) vessels, the concept of watch circles has been used for that purpose. The watch circles define when to initiate disconnecting action. Typical watch circles are referred to as green, yellow and red circles. When a vessel is in the yellow circle, the operation should be halted, while in the red circle, the Emergency Disconnect System (EDS) should be initiated.

Drift-off and drive-off assessment can be utilized in the determination of the green, yellow and red watch circles. Drift-off is a scenario of the vessel drifting away after a loss of thruster power, while drive-off is of the vessel being pushed away by excessive thruster force.

Drift-off and drive-off are functions of a vessel's characteristics and the prevailing environmental conditions. This paper presents the study on the drift-off and drive-off scenarios of a DP drillship under different environmental conditions. The vessel's excursion as a function of time is simulated using a time-domain simulation program aNySIM. The comparison of drift-off and drive-off speed under different environmental conditions will be made. The effect of environmental conditions on the determination of watch circles is assessed.

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