## 1 Introduction  
Wind energy, as the clean energy with the most vigorous development in recent years, has attracted the attention of all countries in the world. China's investment in the wind power industry has also greatly increased. After the proposal to achieve carbon neutrality before 2060, the development of the wind power industry has entered a golden age. Compared with land-based fixed wind turbines, offshore wind power not only has more abundant wind resources, but also solves the problem of fixed wind power. The vision and noise of wind turbines and other issues, the wind power industry from land to sea must be the general trend of future development. The increase in the water depth of the continental shelf in China's waters is small, and the water depth in offshore waters is basically less than 60 m [1, 2, 3]. Therefore, it is necessary to study the development of floating wind turbines in shallow water conditions, and how to design and optimize them to adapt to small water depths and withstand harsh sea conditions. It is also one of the key issues in the current development of floating wind turbines. The first use of buoys and clump weights to optimize mooring systems dates back to 1976, [PERSON] et al. [4] proposed \"The guyed tower\", when the platform encounters extreme sea conditions, the response of the platform is too large, and the mooring system lifts the clump weights from the seabed, which can be regarded as the extension of the catenary to reduce the motion response of the partial platform.Morrison and [PERSON] [5] further proved the significant resistance of the guyed tower to horizontal loads due to the influence of clump weights. [PERSON] et al. [6; 7] investigated the optimization effect of buoys on the mooring system and determined that the mooring system can be optimized when the size, quantity and location of the buoys are selected reasonably. By 1997 [PERSON] and [PERSON] [8] did the analysis in the frequency domain as well as the time domain. [PERSON] et al. [9] applied buoys to the wave energy converters at 50 m water depth, which proved that the use of buoys on mooring lines can significantly reduce the weight of mooring lines and have a significant impact on the motion response of marine structures. [PERSON] et al. [10] proposed a novel hybrid mooring system suitable for deep-water semi-submersible platforms with clump weights and buoys. Through the time-domain coupling analysis, it is found that the hybrid mooring system can improve the ship's deflection problem and reduce the tension of the mooring line. [PERSON] et al. [11] took a NREL 5 MW floating wind turbine as the research object, and proposed a mooring system using pure anchor chain, clump weight anchor chain, elastic cable and nylon cable as mooring materials. The dynamic response characteristics of mooring system are studied by displacement, mooring tension and engine room acceleration. [PERSON] et al. [12] proposed seven mooring schemes using different materials, mooring accessories, and anchors based on a 5 MW semi-submersible floating wind turbine at a water depth of 50 m. The geometric and elastic mooring stiffness was fully utilized to compare the design options, and the advantages and disadvantages of the seven schemes were analyzed by static, attenuation, dynamic, and cost analyses. The rational combination of mooring accessories such as buoys, clump weights and mooring lines can become one of the breakthroughs in how to design and optimize the mooring system so that it can adapt to small water depths and resist harsh sea conditions. In this paper, the 5 MW-OC4 semi-submersible floating wind turbine is used as the research object, and SESAM software is used to calculate the frequency domain and time domain of the floating wind turbine at a depth of 40 m in the Bohai Sea. According to the motion response and tension of the wind turbine, choose a mooring method with better performance. The combined use of buoys, clump weights and mooring lines can reasonably distribute mooring stiffness and reduce mooring tension.