## 1 Introduction  
As the effects of global warming have become more apparent in recent years, major countries are working toward carbon neutrality. And there are high expectations for renewable energy with low CO\({}_{2}\) emissions in the power generation sector. Hydro, wind, solar, and biomass have taken the lead among renewable energy sources. Although two-thirds of the Earth's surface is covered by oceans, most ocean-based renewable energy, including tidal and ocean current power generation, is still at the demonstration stage, and it is difficult to say whether it will be put into practical use globally. Tidal power generation is a technology that uses tidal currents to generate electricity. Since tidal level changes are a periodic phenomenon caused by the gravity of the moon, it is also easy to predict the amount of power generated by tidal current power generation. In addition, ocean current power generation is a technology that uses ocean currents. Ocean currents are generated by solar heat, westerlies, the Earth's rotation, and the topography, and do not change significantly, so it is easy to predict the amount of power generated via ocean current power generation. Therefore, tidal current/ocean current power generation is easier to manage as part of an existing power supply system than wind and solar powers, which have already been introduced a lot. There are two types of tidal current/ocean current turbines: the bottom fixed type, which has its base fixed to the seabed, and the floating type, which is moored with one or more mooring lines. Each has pros and cons, but, to reduce the levelized cost of energy (LCOE), floating tidal/ocean current power generation is promising because of its ability to utilize faster currents close to the water surface, ease of construction, maintainability, and ability to follow changes in the current direction--in other words, no yaw control required. Especially, single-point moored floating tidal current/ocean current turbines have the potential to significantly minimize the mooring system and its construction costs, which account for a large proportion of the initial cost. To operate a single-point moored floating tidal current/ocean current turbine stably, the average torque acting on the mooring line must be reduced to zero. One of the technologies that makes this possible is the coaxial counter-rotating rotor (CCRR). This rotates the front rotor (FR) and rear rotor (RR) coaxially, and so they rotate in opposite directions. Several papers have reported that the power coefficient of the rotor increases slightly compared to a single rotor. However, with CCRRs, the influence of the mutual interference between the FR and RR is large, so an analysis method that takes this into consideration is required. CCRRs are used in tidal/ocean current turbines and wind turbines, which are the subject of this research, as well as propulsion systems for aircrafts, ships, and helicopters. The characteristics of tidal/ocean currents that are relevant to wind turbines are that they are incompressible flows with open channels; the loads on each rotor are, thus, generated in almost the same plane, and the blades have a relatively large aspect ratio. The existing analysis techniques are listed below in order of fidelity. The methods for analyzing counter-rotating wind turbines are relatively well-summarized in [PERSON] et al. [1]. Full-model computational fluid dynamics (CFD)  
The most common analysis method for CCRRs is full-model CFD, which models the rotors in one domain and analyzes it dynamically. This method has the advantage of being able to analyze rotor mutual interference and fluctuating loads even if there is misalignment between the flow direction and rotor axis or shear of the inflow. But it requires extremely huge amounts of computational resources and time. Furthermore, at least three cases of simulations are required to find the condition where the torques of the FR and RR are balanced, and so it is not realistic to apply in practical designs. [PERSON] et al.