## 4 Challenges
### Technical Aspects
#### 4.1.1 Underwater Gas Storage
The equipment is located on the seabed with relatively stable conditions, but it is still necessary to analyze the hydrodynamic stability of gas storage accumulators. Large gas storage accumulators have a large characteristic scale and large flow Reynolds number, which may lead to fatigue damage due to vortex-induced vibration caused by currents. [PERSON] et al. studied the hydrodynamic characteristics of an underwater gas storage accumulator with a gas storage volume of 1000 m\({}^{3}\) under different flow conditions through fluid-structure coupling and modal analysis [21]. However, the study considers the underwater gas storage accumulator as a rigid body. Only the one-way impact of ocean current on the gas storage accumulator is considered, while the two-way fluid-structure coupling between the ocean current and accumulator is not considered. In fact, the volume of commercially acceptable underwater gas storage accumulators should be much larger than 1000 m\({}^{3}\). Due to the large scale of large or super large underwater gas storage accumulators, these accumulators should be regarded as a non-rigid structures, and the fluid-structure coupling research should be carried out to study the fluid force and wake flow field under different water flows and accumulator structures [80]. Potential solutions to avoid the vortex-induced vibration phenomenon should be explored. In addition, when the gas storage accumulator is in different gas storage states of varying volume and pressure, its natural frequency will change. This should also be considered in further research. Because the gas is circularly charged and discharged into and out of the accumulators, huge buoyancy loads fluctuate dramatically. Although the underwater gas storage accumulator is not a pressure vessel, the cyclic buoyancy load may lead to fatigue damage to the accumulator structure. The concrete structure is likely to produce large cracks, especially under the action of tensile stress, which will further affect the safety and stability of the accumulator. Thus, it is necessary to analyze structural weaknesses and locally strengthen the accumulator structure. During the design of large underwater gas storage accumulators, the impacts of submarine geological disasters and dropped falling object impact on the safety of the accumulator should also be considered. The sliding of seabed sediment and the change in geotechnical characteristics caused by submarine geological disasters are likely to lead to the instability and even destruction of gas storage accumulators. Falling objects may cause direct damage to the accumulators. Thus, specific protective structures need to be designed. If an underwater gas storage accumulator is damaged, it can lead to a large, rapid escape of compressed gas. The volume of this compressed gas will expand rapidly during the acceleration process, which will pose a potential hazard to the safety of the surface and underwater vehicles and sea creatures [81; 82; 83]. It is necessary to conduct relevant research and evaluation to mitigate this potential. In addition to the requirements of structural durability, the large underwater gas storage accumulators have extremely high requirements for foundation stability. The marine environment and geological conditions in deep water are complex and there is a large density difference between gas and water. Therefore, underwater gas storage accumulators must be effectively ballasted and anchored. Accumulators must withstand the loads caused by the huge buoyancy generated by a large amount of compressed gas in the water, as well as the complex marine environmental load, and even force majeure load such as undersea earthquakes and tsunamis [84]. In general, reinforced concrete structures should take advantage of their gravity ballast, and steel structures should utilize gravity ballasts or foundation structure anchorage [9; 21].