Dynamic drift-off warning limits for dynamically positioned MODU with Deepwater Artificial Seabed (DAS) system coupling

A new Deepwater Artificial Seabed (DAS) drilling system, which enables the shallow-water rated drilling equipment to operate in deep and ultra-deep water, has been developed for dynamically positioned Mobile Offshore Drilling Unit (MODU), with a focus on key issues of well access and riser design. Nevertheless, the MODU may drift-off due to the critical failures of the dynamic positioning (DP) system and thus the well integrity will be gravely threatened. This study is committed to establishing the quantitative criteria of drift-off warning limits, which consist of the yellow limit, red limit and physical limit, for dynamically positioned MODU with the coupled DAS drilling system. To achieve this aim, a three-phase methodology is proposed on the basis of a fully coupled model of the MODU-DAS drilling system. Furthermore, a hybrid method integrating the orthogonal experimental design, back propagation neural network, and Garson's algorithm is developed to systematically investigate the importance of the correlative influencing factors on the drift-off warning limits in consideration of both the efficiency and accuracy of the numerical calculations. The validity of the proposed methodology is demonstrated by a case study. The results indicate that the dynamic drift-off warning limits are able to assist in decision-making of safe DP operations on the MODU and the operating depth of the artificial seabed is the most critical factor affecting the envelope of drift-off warning limits. These findings and recommendations could improve the safety of DP drilling operations.