Search

In drilling vessels, the dynamic positioning (DP) system has a great importance for the operation, since it ensures the station-keeping ability for the drilling operation. However, an emergency situation involves ungoverned drift due to problems associated with the DP system failures, such as thrusters, generators, powerbus, or control system. During this situation, the vessel drift is subjected to the influence of environmental conditions and the drift can lead to collisions with floating obstacles or submerged systems, wellhead emergency disconnection, damage to equipment and potentially causing major environmental disasters. It is then necessary to define a safety region for the drilling ship operation and to determine the limiting operation offset that the drilling vessel can disconnect from the wellhead without damage to any equipment. This offset limit is obtained through a riser analysis and drift-off study, important inputs for the Well Specific Operating Guidelines (WSOG). A validated time-domain simulator is required and able to predict the vessel drift trajectory after the DP failure under several environmental conditions. The aim of this work is to present a large set of model and full-scale drift tests and the validation of a time-domain numerical simulator (Dynasim), based on the main parameters of the drift tests: drift distance, heading variation, and trajectory. The comparisons between the numerical simulation results with full- and model-scale data demonstrated the accuracy of the numerical model, confirming that the simulator is a reliable tool to predict the motion of a drilling vessel after a blackout.

In drilling vessels, the dynamic positioning (DP) system has great importance for the operation, since it ensures the station keeping ability for carrying out the drilling operation. However, an emergency situation involves ungoverned drift due to problems associated with the DP system failures, such as thrusters, generators, powerbus or control system. During this emergency situation, the vessel drift is subjected to the influence of environmental conditions and the drift can lead to collisions with floating obstacles or submerged systems, wellhead emergency disconnection, damage to equipments and potentially causing major environmental disasters. Due to the possible occurrence of this emergency condition, it is necessary to define a safety region for the drilling ship operation and to determine the limiting operation offset that the drilling vessel can disconnect from the wellhead without damage to any equipment. This limit offset is obtained through a riser analysis and drift-off study, being an important input for WSOG (Well Specific Operating Guidelines). The study will provide vessel drift trajectory after the DP failure under several environmental conditions. A validated time domain simulator must be used for obtaining such data. In the present work, the Dynasim software, developed from the partnership between Petrobras and Brazilian universities, is applied. Thus, the aim of this work is to validate the Dynasim numerical model using drift data from full scale tests performed on vessels operated by Petrobras and under several environmental conditions. For the validation and calibration of the numerical model, three main parameters were compared: distance time-series, heading variation and trajectory during the drift of the vessel. The comparisons between full scale data and numerical simulation results demonstrated the accuracy of the numerical model. Therefore, the application of this tool provides a valuable contribution to the reliability of WSOG analysis, planning of the operations, allowing the determination of the drift path in relation to the wind, current and waves, and the definition of drift maps and establishing safety zones for the drilling vessel operation, minimizing the risk of collisions.

Dynamic Positioning (DP) systems are becoming more and more used in the Oil and Gas Industry. Over the last decades, the escalation in deep-water exploration and development resulted in an increased number of dynamic positioned rigs. But despite the increasing improvements in technology and capability, there is always a risk of a loss of position due to equipment failures or human errors. In consequence of such event, the DP drilling unit must be able to rapidly shut in the well and disconnect the BOP. Failure during an emergency disconnect procedure may result in damages to the rig, riser, wellhead, and, in worst case, an uncontrolled subsea blowout. Emergency disconnection may occurs at any time in dynamic positioning drilling rigs, as a result of a sudden blackout or by a quick drive-off, for instance. Because of that, all emergency procedures should be well understood by both DP and drill floor personnel. It is imperative that the risks of a position loss are previously assessed in order to reduce the potential damage caused by a disconnection. The two most relevant tools for reducing the impact related to the DP System failure are the Well Specific Operating Guidelines (WSOG) and the Emergency Disconnection Guidelines. The WSOG defines four diferent DP Status associated to a specificrig executing a specific operation and using specific equipments and its performance limits - "Normal Status," "Advisory Status," "Yellow Status" and "Red Status". The Emergency Disconnection Guidelines establish the actions in case of reaching each of these status. Emergency Disconnection Guidelines were established for various drilling, completion and workover operations, to be studied and discussed previously activities with BOP / Christmas Tree connected to the wellhead. The main goal of this paper is to present how the WSOG and the Emergency Disconnection Guidelines work together, bringing statistics and lessons learned of loss of position and emergency disconnections. Additionally, this work suggests emergency disconnect sequence (EDS) options for different operations and BOP configurations (number/type of shear rams available).