## 4 Challenges
### Technical Aspects
#### 4.1.1 Underwater Gas Storage
Gravity ballasting is the simplest and most reliable ballasting method, which can balance the huge buoyancy of gas by increasing the self-weight of the accumulator itself. The infrastructure to fix the gas storage accumulator on the seabed depends on the local seabed conditions. For soft and super soft seabed conditions (clay/sand), the foundation structure of the suction anchor or suction caisson can be used [85]. At present, suction caissons are widely used as the foundation structure of offshore wind turbines because of their technical feasibility, convenient transportation and installation, and cost-effectiveness [86]. Many scholars have conducted detailed research on offshore wind turbine foundations which can serve as a reference when designing underwater accumulators [87; 88; 89; 90; 91; 92; 93]. The loads on the foundation structure of underwater gas storage accumulators are similar to those of the offshore wind turbine foundations, but there are significant distinctions. A load diagram is shown in Figure 4. The foundation of an underwater gas storage accumulator is mainly subject to the coupling effects of vertical cyclic loading caused by self-weight and cyclic charging/discharging and horizontal loading caused by ocean current and overturning moment. If the gas storage accumulator is in deep-water soft soil, there is concomitant soil rheological effect [94]. Whether it is anchored by its own gravity ballast or by external forces, the stability of the foundation has to endure the trials of soil creep, relaxation, and local scour over time. Therefore, the design of the foundation structure is critically important. A suitable foundation structure can not only reduce the investment cost but also increase the ability of the accumulator to resist vertical, horizontal, and overturning effects and improve the corrosion and erosion resistance around the foundation, thereby ensuring the safe and stable operation of the gas storage accumulator over the long term. Separation vortices generated by the ocean current around the gas storage accumulator will erode the soil near the foundation, thereby affecting its stability. Therefore, it is important to understand the local scouring characteristics and predict the scouring depth around the foundation [95,96]. Protection measures for controlling local scour should be investigated after clarifying the mechanism of local scour around the foundation. Currently, there are two commonly used anti-scour measures; one is to use hard marine engineering materials to make physical anti-scour plates, and another is to change the flow field around the subsea structures to make the scour area away from the foundations [95]. Studies have shown that the anti-sinking plate support structure with a skirt can effectively alleviate the local erosion effect [97]. There are complex interactions between the foundation of the accumulator, the ocean current, and the seabed rock/soil. Thus, it is necessary to conduct a coupling analysis of sea-soil-structure interactions. In addition, the biofouling of marine organisms must also be considered, especially in several important seawater vents on the accumulator. Currently, materials such as titanium alloy and copper nickel alloy can be considered to prevent the adhesion and accumulation of marine organisms from blocking the channel [98]. Generally, the service life of the offshore platform is between 20 and 30 years by comprehensively considering the coupling effects of wind, wave, current, and corrosion. However, the operating environment of the underwater energy storage accumulators is much more moderate than that of surface platforms. Therefore, a longer lifetime between 25 and 35 years can be expected. When out of service, these accumulators do not need to be demolished and can be used as artificial reefs for marine organisms and anchors for offshore wind turbines.