## 4 Challenges
### Technical Aspects
#### 4.1.2 Underwater Gas Transportation  
Subsea fluid transportation is relatively mature and widely utilized in the offshore oil and gas industry. However, the processes are generally rather complex, and the investment cost is very high. Comparatively, underwater gas transportation in UWCGES systems should be easier because there are fewer detrimental impurities and the operating conditions are more moderate.  
Multiphase flow is the main challenge in the gas transportation of UWCGES. Figure 5 shows a typical gas transportation process. Compressed gas enters the underwater gas transportation pipeline after pressure regulation and drying at gas stations. The temperature of seawater decreases with the increasing water depth. The heat exchange between the pipeline and seawater causes the temperature of the compressed gas in the pipeline  
Figure 4: An underwater gas storage accumulator founded on a suction caisson transmitting V-H-M loading at the top of the foundation. (H: horizontal load; V: vertical load; M: moment load).  
to decrease gradually. After reaching the dew point, the water vapor condenses and precipitates, thereby forming liquid accumulation in the low-lying part of the pipeline due to gravity. This may lead to a complex and harmful gas-liquid two-phase flow and even gas-liquid-hydrate three-phase flow. Due to the turbulent nature of fluid flow and the complex interaction of gas-liquid-solid multiphase flow, the complex changes of various flow patterns and fluid pressure fluctuations will present and further result in a series of hazards. First, liquid accumulation will inevitably lead to a reduction in pipeline flowrate, a decline in transportation efficiency, and an increase in pressure drop and energy consumption; secondly, the existence of water will aggravate the corrosion in the gas transportation pipeline and there is a risk of water hammer. At the same time, the fluctuation in pressure and flowrate will induce pipeline vibration and the surge of pressure-regulating equipment. In turn, pipeline vibration will affect the two-phase flow in the pipeline and further aggravate the fluctuation in pressure and flowrate. If the liquid accumulation is not treated, it will seriously affect the safe and efficient operation of the underwater gas transportation system and even cause accidents. For example, during the operations of the UWCAES demonstration system and the underwater testing experiments, the liquid accumulation issues persist without exception, which significantly interferes with system operation [28]. Therefore, the accumulated liquid in the gas transportation pipeline must be removed through pigging. The actual pigging operation mainly depends on experience and is regularly planned. Unplanned/unnecessary pigging operations not only increase the operation cost but also increase the frequency of shutdown. Therefore, the optimal method is to accurately predict the state of liquid accumulation and determine the pigging scheme before the liquid accumulation leads to the deterioration or failure of the system. Therefore, it is necessary to develop low-cost gas transportation support technologies, such as online monitoring and the removal of liquid accumulation in underwater pipelines.