Drilling operations are taking place in ever deeper waters and harsher environments using dynamically positioned (DP) vessels. A DP control system maintains the vessel in a fixed position or pre-determined track exclusively by means of active thrusters based on measured position from satellites and hydroacoustics. In particular, two failure scenarios are critical for safe DP operations: drift-off with loss of thrust and drive-off with unstable thruster usage. For drilling operations; ensuring safe emergency disconnect (ESD) when needed and avoiding unnecessary disconnecting when not needed, both the drilling riser behavior and the vessel offsets are of main concern. Concerning the time required to correctly decide and execute ESD drive-off is considered as most critical. It is important to ensure safe operations, maximize operability and avoid disconnection of the riser as far as possible.
In this paper we will investigate riser and vessel dynamics during drive-off in order to increase the operation window that is normally defined from quasi-static considerations. Our analyses show that the traditional limits for vessel offset might be too conservative, and that more realistic criteria can be found by taking riser dynamics during transition into account.