Transient numerical reconstruction and multi-parameter safety regulation of liquid accumulation–slug flow coupling behavior in deep-sea flexible risers for underwater compressed gas energy storage

Underwater compressed gas energy storage (UWCGES) is an emerging marine energy storage technology, yet liquid accumulation and slug flow in flexible risers pose critical threats to operational safety and stability. This study establishes a full-size numerical model of an American Petroleum Institute (API)-compliant 18-layer flexible riser for 500 m water depth UWCGES systems and reproduces realistic subsea operating conditions using the Offshore Lagrangian Gas and multiphase Analysis multiphase flow platform. A series of graded gas flow rates is adopted to conduct 36-h continuous simulations, systematically revealing the spatiotemporal evolution of liquid accumulation, the dynamic characteristics of slug flow, and the coupled pressure response mechanism. Results show that liquid accumulation initiates at the elbow and gradually spreads to the inclined section, with liquid holdup negatively correlated with gas flow rate but limited by a critical threshold. Slug flow exhibits a four-stage periodic motion: accumulation, outflow, ejection, and backflow, which induces severe periodic pressure fluctuations and reverses the pressure distribution between the inlet and elbow. This work clarifies the liquid accumulation–slug flow–pressure coupling mechanism, quantifies the regulatory effect of gas flow rate, and determines the critical flow rate for flow assurance. The findings enrich the gas–liquid two-phase flow theory for marine energy storage equipment and provide a theoretical basis and technical support for the structural optimization, pressure fluctuation control, and safe operation of flexible risers in deepwater UWCGES systems.