Validation of Numerical Models for the Drift-Off Study in Drilling Vessels Using Full Scale Measurements

Abstract

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.