Mechanical Evaluation of the Drill String in Deepwater CCUS Open-Loop Drilling

During deepwater carbon capture, utilization, and storage (CCUS) riserless (open-loop) drilling, the drill string is directly exposed to a complex marine environment, resulting in mechanical behavior and failure mechanisms that differ from those under conventional riser-supported drilling conditions. This study develops a full-depth dynamic model of a drill string in riserless drilling that covers both the seawater and formation sections. In the seawater section, the Morison equation and a wake oscillator model are introduced to represent wave–current loading and vortex-induced vibration (VIV), respectively. In the formation section, the equations of motion are derived from Hamilton’s principle, and nonlinear contact between the drill string and borehole wall is treated using the penalty function method. A deepwater CCUS drilling operation in the South China Sea is examined to systematically evaluate drill-string deformation, internal-force and stress distributions, heave transmission, and full-depth fatigue damage. The results reveal a dual-scale coupled response: large-displacement, low-frequency drift occurs in the in-line direction, whereas small-displacement, high-frequency oscillation occurs in the cross-flow direction. The region near the mudline is the principal mechanical weak point and fatigue-control location along the entire well depth. Since the drill string moves downward, every joint passes through this hotspot, making cumulative fatigue damage a primary concern for drill-string integrity management. Damage localization is primarily caused by the combined effects of axial-bending stress and high-frequency VIV. The effectiveness of the passive heave-compensation sub is strongly load dependent. It effectively suppresses the transmission of large-scale axial displacement but provides limited attenuation of high-frequency weight-on-bit (WOB) fluctuations. These findings provide a theoretical basis for drill-string design, integrity assessment, and operating-parameter optimization in deepwater CCUS drilling.