## 1 Introduction  
COVID-19 and the Russia-Ukraine conflict are changing the energy landscape. Many countries are forced to accelerate their processes of the energy transition. Developing local sustainable and renewable energies has long shown strategic value. It is known that intermittent and stochastic renewable energies challenge the grid security and stability. This highlights the need for energy storage, particularly flexible-scale long-duration energy storage (LDES) [1]. Currently, PHS (Pumped Hydro Storage) is the most mature and prolific form of LDES, holding more than 95% of the worldwide market. In the absence of disruptive breakthroughs, this is unlikely to change for the foreseeable future. While dominant, PHS is not without its detractions of geographical restrictions, potential ecological and environmental disturbances, and high initial investment [2]. Compressed air energy storage (CAES), battery energy storage (BES), and hydrogen energy storage (HES) are regarded as promising alternatives to PHS and continue to evolve in market and government planning. Many demonstration and commercial projects have been deployed in recent years [3, 4, 5]. BES possesses obvious advantages in terms of flexibility and fast response. However, reliability, service life, and environmental concerns still require attention. Although BES is presently the most widely utilized and studied energy storage technology, it is still not competitive in terms of large-scale long-duration energy storage. CAES technology presently is favored in terms of projected service life reliability and environmental footprint. CAES challenges include relatively low round-trip energy efficiency and energy density. CAES economics are still rather variable, depending on the specific application. Generally, the cost of CAES is lower than BES and higher than PHS in terms of large-scale storage [6; 7]. There are many different types of CAES technology, including traditional diabatic CAES, adiabatic CAES, isothermal CAES, and LAES (liquid air energy storage). According to the storage modes of air, CAES can be divided into underground CAES with salt caverns and rock caves, above-ground CAES with artificial pressure vessels, and underwater CAES (UWCAES) with subsea storage caverns and artificial storage accumulators. HES is trailing behind due to various challenges in hydrogen production, storage, transportation, and utilization. Nevertheless, hydrogen energy pathways are receiving growing attention as more pressure is put on the availability of natural gas [8]. The rapid development of onshore renewable energies drives the booming of onshore energy storage technologies. The ocean, which occupies 71% of the surface of this planet, provides a vast source of renewable energies. Accordingly, offshore renewable energies are predicted to drive the development of corresponding offshore energy storage technologies. Offshore energy storage technologies can often leverage onshore technology counterparts. However, the harsh marine environment poses additional unique challenges [9]. In recent years, many novel offshore energy storage concepts have been proposed and investigated, such as UWCAES [10; 11], subsea PHS [12], subsea HES [13; 14], buoyancy energy storage [15; 16], floating energy storage [17], hydropneumatics energy storage [18], etc. Storing underwater/subsea is a significant feature of most offshore energy storage concepts. Compared with floating storage, underwater storage sustains less harsh environment loads from wave, wind, and current. UWCAES derives from onshore CAES and is one of the earliest developed offshore energy storage technologies. Compared with onshore CAES, the unique property of UWCAES is that the compressed air is stored and transmitted underwater. This brings both advantages and disadvantages.