Underwater compressed air energy storage was developed from its terrestrial counterpart. It has also evolved to underwater compressed natural gas and hydrogen energy storage in recent years. UWCGES is a promising energy storage technology for the marine environment and subsequently of recent significant interest attention. However, it is still immature. In this study, the latest progress in both academic and industrial fields is summarized. Additionally, challenges facing this emerging technology are analyzed. The pros and cons of UWCGES are provided and are differentiated from the terrestrial variant. Technical, economic, environmental, and policy challenges are examined. In particular, the critical issues for developing artificial large and ultra-large underwater gas storage accumulators and effective underwater gas transportation are comprehensively analyzed. Finally, the demand for marine energy storage technology is briefly summarized, and the potential application scenarios and application modes of underwater compressed gas energy storage technology are prospected. This study aims to highlight the current state of the UWCGES sector and provide some guidance and reference for theoretical research and industrial development.

As an emerging flexible-scale energy storage technology, underwater compressed gas energy storage (UW-CGES) is regarded as a promising energy storage option for offshore platforms, offshore renewable energy farms, islands, coastal cities, etc. Liquid accumulation often occurs in underwater gas transmission pipelines, which is a challenge to overcome. In this study, an experimental investigation is carried out on the pressure distribution characteristics of liquid accumulation flow in hilly terrain under the condition of Zero Net Liquid Flow. A slug flow pressure model with different inclination angles at four times is established and verified, and its error range is within ±20%. Analysis revealed that reduction and growth in pressure difference are related to the outflow of slug in an inclined pipe. A high-speed camera is used to capture the movement of liquid accumulation under Zero Net Liquid Flow (ZNLF) and record the associated dynamic parameters. By imaging the motion of liquid accumulation and detecting the pressure changes in the pipeline at various times, the pressure fluctuation in the pipeline at the slug flow cause is studied. Outcomes from this work can be leveraged to help further the development of underwater compressed gas energy storage technology.

Underwater compressed gas energy storage (UW-CGES) holds significant promise as a nascent and viable energy storage solution for a diverse range of coastal and offshore facilities. However, liquid accumulation in underwater gas pipelines poses a significant challenge, as it can lead to pipeline blockages and energy transmission interruptions and adversely impact pipeline operation. In this paper, experimental and Oil and Gas Assays (OLGA) simulation studies have been conducted on the formation process of slug flow in pipelines. Firstly, experiments are conducted to capture high-speed camera images of slug flow under various liquid accumulation volumes and inclination angles. Subsequently, an OLGA model is developed to verify the experimentally observed flow regime, pressure, and slugging speed. Therefore, the flow regime verification results exhibit substantial consistency, and pressure variations display uniform trends, with an average slugging velocity error of 6.42%. The results indicate that the formation of slug flow involves three distinct stages: slug flow growth, ejection, and backflow. By analyzing slug flow, it can gain insights into the relationship between pressure and slug flow formation, exposing the sensitivity of this phenomenon to pressure fluctuations. These results further enhance recognition of the operational status of UW-CGES pipelines and provide support for safe operation.

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.