## 4 Challenges
### Technical Aspects
#### 4.1.1 Underwater Gas Storage  
As aforementioned, there are mainly two types of underwater gas storage, underwater fabricated accumulator storage and subseabed geological storage. Although the research on  
Figure 3: Challenges for developing UWCGES. seabed geological structure gas storage has gradually evolved in recent years, the research is rather limited. Fabricated underwater accumulator storage remains the primary research topic in this field. Generally, artificial accumulators can be divided into two categories: flexible and rigid. Flexible gas storage accumulators based on flexible composite materials have been studied and applied in academia and industrial demonstration. The field testing of small-scale (1-100 m\({}^{3}\) scale) flexible gas storage accumulators reveals that the concept is feasible. Unfortunately, flexible gas storage accumulators are still not reliable in a harsh and complex marine environment. There is not yet evidence to support long-duration storage and critical maintenance is not often convenient or economic. Large-scale gas storage accumulators are required for large-scale underwater compressed gas energy storage. Subsequently, this heightens challenges to the reliability of large and super large flexible gas storage accumulators [9; 28]. Comparatively, rigid gas storage accumulators based on reinforced concrete or steel structures have higher marine reliability and feasibility. That said, because the gas storage accumulator must sustain current flow, high salinity, high pressure, scouring, and other complex effects, a series of problems such as concrete cracking and reinforcement corrosion may occur after long periods [68; 69; 70; 71]. These may lead to the structural failure of these underwater gas storage accumulators. Underwater gas storage accumulators must be highly reliable and potentially even maintenance-free. Therefore, it is necessary to investigate the structural durability, failure mechanism, and life prediction of gas storage accumulators [72]. In view of the durability of reinforced concrete structures used in ocean engineering, researchers have proposed the use of fiber-reinforced polymer, sustainable alkali-activated cementitious materials, and other ways to improve the durability of steel concrete structures [73; 74; 75]. The interaction mechanisms between rigid gas storage structures and the marine environment are complex, and accurate prediction is very difficult. Therefore, the durability of both reinforced concrete structures and steel structures is still facing severe challenges and uncertainties. In recent years, offshore engineering projects using steel concrete structures have emerged, such as concrete support structures for offshore wind turbines, sea crossing bridges, submarine immersed tunnels, and offshore floating platforms [76; 77; 78]. The success of these projects marks the increasingly widespread application of reinforced concrete structures in the field of offshore engineering. With the advancement of research, the underwater gas storage accumulator made of reinforced concrete will be feasible in the near future. In order to ensure the structural integrity of underwater gas storage accumulators, detailed structural design and structural dynamic analysis should be conducted. This includes the static, dynamic, and fatigue considerations related to cyclic charging and discharging loads, hydrodynamic loads, and gravitational and geotechnical loads [79]. Detailed structural dynamics analysis can also minimize the manufacturing, installation, and maintenance costs of gas storage accumulators. Achieving the balance between structural strength and cost is extremely important for the commercial application of underwater gas storage accumulators. Compared with surface structures, underwater storage accumulators can largely avoid the impacts of adverse environmental conditions such as wind, waves, currents, icebergs, ice flows, and ships on the sea surface.