## 1 Introduction  
Traditional energy sources such as oil and coal are not renewable, thus cause environmental pollution. Meanwhile, wind power is a clean and renewable source of energy. In recent decades, this type of energy has developed rapidly. It is estimated that by 2030, wind energy will provide about 20% of the world's electricity demand. In particular, wind power is mainly concentrated on land, and onshore wind turbines are relatively mature. To obtain wind power with higher density, offshore wind power has become a major focus of the world's research, especially in the field of wind energy. Compared with onshore wind, offshore wind has high wind speeds, low wind shear, low turbulence, and other significant advantages. Also, the cost involved in tapping this energy is relatively higher; the trends of wind farm construction--from land to sea, from shallow sea to deep sea, and from fixed foundation to floating platform--are obviously due to the higher wind energy output. There are many kinds of floating wind turbine foundation platforms. They include spar [1; 2; 3; 4; 5; 6; 7], Tension Leg Platform (TLP) [8; 9], Semi-Submersible Platform [10; 11; 12; 13; 14; 15], and so on. Among them, the spar platform has the advantages of a low center of gravity, high flexibility, and low cost. Still, there are some problems in the practical application. That is, the spar must withstand the wave and wind loads experienced during extreme storm conditions. In addition, the expected higher costs of construction and maintenance must be addressed [16]. A lot of research has been done to reduce the motions of the spar type platform [16; 17; 18; 19; 20; 21; 22; 23], among which the study by [PERSON] and [PERSON] [24] applied several codes for the calculation of the motion of the OC3 Hywind spar platform, such as NREL FAST, ADAMS, HAWC2, etc. Most of the approaches seek to modify the platform structure [25; 26; 27; 28]. For instance, [PERSON] et al. [29] modified the spar-type platform by adding a heave plate. Their results showed that, compared with the original OC3-Hywind spar platform, the heave plate can effectively suppress the heave response of the spar platform. They also found that the viscous damping contributed by the heave plate is a major component of the total damping. Meanwhile, [PERSON] et al. [30] presented a spar-type substructure integrated with the heave plates moored by the optimized mooring system. The dynamic response of the platform under different load conditions were performed using AQWA. The results indicated that both the dynamic response and the mooring tension are significantly improved relative to that of the original OC3 spar. At the same time, [PERSON] et al. [31] presented a modified spar-type platform consisting of a main cylinder and four wider cylinders and with the mass, displacement, and pitch inertia similar to those of the OC3 Hywind with reasonable differences. The numerical results indicated that the motions of the modified spar-type floating offshore wind turbine (FOWT) in all of the modes were smaller than those of the OC3 Hywind FOWT, except in heave mode over 0.3 rad/s. [PERSON] et al. [32] suggested a state-of-the-art concept integrating a deep-water floating offshore wind turbine with a steel fish-farming cage (FOWT-SFFC). Aero-hydro-servo-elastic modeling and time domain simulations were carried out by FAST to examine the dynamic response of FOWT-SFFC for several environmental conditions. The results confirmed that hydrodynamic performance of FOWT-SFFC is much better than OC3-Hywind spar. [PERSON] et al. [33] further proposed a cell spar platform based on the concept of OC3-Hywind spar platform and calculated the dynamic response under wind-wave loads by AQWA. On the basis of the cell spar platform, a floating wind turbine with additional heave plate was built to study the influence. The results indicated that the cell spar platform has better motion performance than the original OC3-Hywind spar platform. Meanwhile, [PERSON] et al.