Enhanced Riserless Mud Recovery System with Hybrid Riser Configuration: Breaking Through Ultra-Deepwater Limits

Abstract The riserless mud recovery (RMR) system has achieved commercial success in the last two decades, yet its application has largely been confined to water depths under 2,000 ft due to structural limitations of its mud return line in deepwater environments. This study proposes an innovative adaptation of the RMR system based on a hybrid riser configuration. In this design, the hybrid riser replaces the conventional mud return line, serving as the primary circulation conduit. A flexible jumper connects the riser's top buoyancy modules to the floating drilling vessel, achieving mechanical decoupling. This configuration delivers three key functional advantages: (a) isolation from wave-induced platform motions and severe sea states, (b) stable axial support for multi-state subsea lift pumps, (c) a closed-loop dual-gradient drilling fluid circulations system. Simulation analysis demonstrates that the hybrid-riser-based RMR system exhibits strong adaptability to harsh sea states and greater water depths by effectively decoupling the subsea pipe string from the dynamic motions of the floating platform. The mud return line primarily deforms in the direction of the ocean current. A top tension ratio greater than 1.5 and a submerged depth between 200 and 250 m are identified as optimal for maintaining the near-vertical altitude of the mud return line. Owing to its superior adaptability to significant water depths, this hybrid-riser-based riserless mud circulation system holds broad application prospects in deepwater drilling and completion engineering. It provides a technically feasible solution for the safe and efficient development of deepwater natural gas hydrate resources.