Transient response of an innovative Deepwater Artificial Seabed (DAS) system under tendon failure conditions

Abstract

An innovative Deepwater Artificial Seabed (DAS) drilling system is developed to overcome the demanding technical and economic limitations for offshore drilling activities in deep and ultra-deep waters, with a focus on important issues of well access. However, tendon failure poses significant threats to the well integrity of the DAS drilling system. This study is committed to investigating the transient effects of tendon failure on the response of the fully coupled DAS drilling system. A nonlinear model, which couples the Mobile Offshore Drilling Unit (MODU), artificial seabed, riser system, intact and fractured tendons, is established firstly. Based on this, the transient response characteristics are systematically investigated in consideration of tendon breakage scenarios, covering the artificial seabed motion, the tendon tension, the strength of the drilling riser and tieback casing systems. The results indicate that the transient motion responses of the artificial seabed behave as large translational and rotational movements nonlinearly under tendon failure conditions. However, the heave motion of the inner artificial seabed, by contrast, is quite small due to the decoupling effect between the outer and inner artificial seabed in the vertical direction. Besides, the maximum tensions in neighboring tendons increase sharply caused by the transient effects of tendon failures, which may lead to cascading failure effect, even a total loss of the DAS drilling system. These findings can contribute to the safe design and operation of the DAS drilling system.