Drift-off warning limits for dynamically positioned FPSO and Deepwater Artificial Seabed (DAS) coupling system

Abstract The new Deepwater Artificial Seabed (DAS) system is developed for dynamically positioned Floating Production, Storage and Offloading (FPSO) vessel, with a focus on important issues of well access and riser design. Nonetheless, FPSO may drift off due to dynamic positioning (DP) system failures and ocean environment loads, and thus loses the capability to maintain position, which poses significant threats to the structural integrity of the DAS system. This study is committed to establishing criteria of drift-off warning limits for dynamically positioned FPSO and DAS coupling system. The fully-coupled dynamic model of the FPSO and DAS coupling system is established. On this basis, a drift-off analysis method is proposed in the determination of drift-off warning limits of FPSO. The effectiveness of the method is verified in the case study of the coupled FPSO-DAS system. Further, the correlative influence factors, consisting of the wave, wind and current conditions, different flexible jumper lengths, FPSO offsets as well as artificial seabed locations, are systematically investigated. The results demonstrate the effectiveness of fully-coupled analysis of the FPSO and DAS system for warning limits determination. The warning limits, consisting of point of disconnection (POD), preparation and initiation point of emergency disconnection sequence (EDS), are mainly located at the near end and far end relative to the artificial seabed due to different failure sites of flexible jumper. The safe area of FPSO is in a “butterfly” shape due to the design characteristics of the DAS system. It is noteworthy that three main aspects of warning limits can be affected by the corelative influence factors, i.e. the angle and radius coordinates of the warning limits as well as the time to failure. The wave height, environmental load direction and flexible jumper length have the most significant effects on the time to failure, the angle and radius coordinates of the warning limits respectively. This study provides a useful tool for the decision-making of DP operations in a drift-off scenario, and contributes to the safety control of the DP operation in the production phase.