**1. Introduction**

In recent years, the field of offshore wind power generation has developed rapidly and has become the fastest-growing energy source for marine renewable energy. As of the end of 2020, the global wind power capacity reached 744 GW, with 50% increase over 2019 [1]. With the rapid development of offshore wind technology, offshore wind turbines are increasing in scale and size, and the scale of wind farms is also increasing. The national renewable energy laboratory (NREL) 5-MW reference wind turbine [2] and DTU 10-MW reference wind turbine [3] have been widely used in comparative studies. The largest offshore wind turbine MySE 16.0-242 16 MW with rotor diameter of 242 m was launched by Ming Yang Smart Energy in August of 2021. To further exploit the offshore wind from deep water, various floating offshore wind concepts were proposed including mainly spar, semisubmersible, tension leg platform (TLP), and barge. Compared to TLP and spar, the

**Citation:** Li, J.; Shi, W.; Zhang, L.; Michailides, C.; Li, X. Wind–Wave Coupling Effect on the Dynamic Response of a Combined Wind–Wave Energy Converter. *J. Mar. Sci. Eng.* **2021**, *9*, 1101. https://doi.org/ 10.3390/jmse9101101

Academic Editor: Eugen Rusu

Received: 10 September 2021 Accepted: 1 October 2021 Published: 9 October 2021

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semisubmersible platform is more feasible in various water depths and has low installation costs of the mooring system. At present, the concepts of semisubmersible wind turbines mainly include WindFloat [4], Dutch Tri-floater [5], Windsea [6], Windflo [7], braceless [8], V-shaped [9], OC4-DeepCwind [10], and so on. Wave energy is also a large energy source, with a much higher power density than wind power, but this energy technology is not fully commercialized at present due to its high cost and reliability issue. WECs can generally be categorized as oscillating bodies, oscillating water columns, and overtopping devices [11].

By sharing space, supporting structures, cables, and other infrastructure, combining floating wind turbine system and WEC can not only reduce the cost of the device but also capture wind and wave energy and greatly improve the utilization of renewable energy of the ocean. At present, many studies on numerical simulations and model tests based on different floating offshore wind concepts (barge, TLP, spar, and semisubmersible) combined with WEC have been carried out. Aboutalebi et al. [12,13] proposed to install an oscillating water column WEC on the barge platform to reduce the fatigue movement of the platform. In Michailides et al. [14], the required wind–wave analysis for harsh environmental conditions has been examined. Ren et al. [15] carried out an experimental and numerical study of dynamic response of a new combined TLP wind turbine and wave energy converter. Bachynski et al. [16] proposed a TLP combined three-point absorber WECs system and studied the performance under operational and extreme sea cases. Russo et al. [17], Tomasicchio et al. [18], and Xu et al. [19] carried out experimental studies of the dynamic characteristics of Spar Buoy Wind Turbine and studied its dynamic behavior. Hu et al. [20] carried out the optimal design of WEC and dynamic characteristics analysis of a hybrid system combing a semisubmersible floating wind platform (WindFloat) and WECs. Sarmiento et al. [21] carried out a new floating semisubmersible structure combined with WECs (three oscillating water columns, OWC) in order to characterize the performance of the platform and OWCs. The MARINA Platform project funded by the European Union [22] proposed a combined system of semisubmersible platforms with point absorption and oscillating water column WEC. Peiffer et al. [23] and Aubault et al. [24] carried out coupled dynamic analysis through numerical models and experimental models. Muliawan et al. [25,26] proposed a comprehensive concept that combines a spar-type floating wind turbine with a torus WEC (named STC). Wan et al. [27–29] studied the dynamic response of an STC under typical operational conditions and extreme conditions based on numerical and experimental methods. Another concept that combines a floating semisubmersible wind turbine and a flap-type WEC (named SFC) was proposed. Michailides et al. [30–32] systematically studied the integrated operation of SFCs through numerical and experimental models. Ren et al. [33] carried out experimental and numerical studies on the hydrodynamic response of a new combined monopile wind turbine and a heaving-type WEC under typical operational conditions. In addition, Wang et al. [34] studied the hydrodynamic response of the combined system of the semisubmersible platform and heaving-type WEC.

With the development of the field of offshore wind turbines, the aero-hydro-servoelastic coupling tool has become the key to solve the equation of motion and calculate the dynamic response of floating wind turbines. Different simulation tools have been developed so far. DeepLines [35,36], DARwind [37], Bladed, HAWC2, and FAST are the most well-known tools for fully coupled analysis of wind turbines. Jonkman et al. [38,39] developed a hydrodynamic module to consider the diffraction and radiation effects of floating platforms. Due to its open-source nature, it has also been used to participate in the development of a fully coupled framework. Kvittem et al. [40] combined AeroDyn with the non-linear finite element software SIMO/REFLEX. Shim [41] developed an interface that combines FAST with the fluid dynamic analysis tool CHARM3D. Recently, Yang et al. [42] developed a new aero-hydro-servo-elastic coupling framework based on FAST and AQWA (F2A) for dynamic analysis of FOWTs, combining the advantages of aero-elastic-servo of FAST and hydrodynamic analysis of AQWA [43].

In this study, with use of the recently proposed coupled analysis tool F2A, a fully coupled analysis is performed for a combined system consisting of a semisubmersible platform and a heaving-type WEC. A numerical model of the combined structure that was capable of simulating its motion and dynamic responses under different typical operational conditions was developed and used with F2A. A second model with use of a different analysis tool is developed for performing fully coupled analysis. Different types of analysis and comparison were performed to examine the fully coupled responses of the combined structure but also for emphasizing the effect of different modeling methods/techniques on the response quantities of the combined system. The time-domain dynamic response characteristics, mooring characteristics, and PTO-produced power of the combined system are examined and presented in this study. Under the excitation of only waves, there was small difference between the two numerical models, while under irregular wave and turbulent wind conditions, the difference between two models was larger. The impact of aerodynamic loads as well as aerodynamic damping is significant. When the wind velocity is small, aerodynamic damping has a significant effect on reducing the resonance of surge and pitch, and there is an obvious positive relationship. When the wind velocity increases, wind thrust has a greater impact than aerodynamic damping. The rationality behind the efficient use of the two examined numerical tools for studying combined concepts has been presented. Finally, an extreme condition is studied to ensure the survivability of the combined system. The results show that the two models had significant differences in dynamic motion and mooring force prediction under irregular wave and turbulent wind condition due to the wind–wave coupling effect at low frequency range.
