Switching Control Strategy for Oscillating Water Columns Based on Response Amplitude Operators for Floating Offshore Wind Turbines Stabilization  
[PERSON]  
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[PERSON]  
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[PERSON]  
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motions reduce the aerodynamic efficiency, shorten the fatigue life of tower and increase stresses on blades, rotor shaft, yaw bearing and tower base [11]. Therefore, it is important to restrain the FOWT's platform motions within an adequate range [12]. In order to decrease the FONTs' motions in different modes, authors have applied various methods to deal with the problem [13]. The application of heave plates attached to the spar-type FOWT with the aim of motion stabilization was introduced by [PERSON] et al. [14]. [PERSON] et al. in [15] conducted an experimental study to reduce the heave and pitch amplitude using the added catenary mooring cables as well as installation of the Wave Energy Converters (WEC) on a FOWT. Several authors have proposed the usage of inerters in barge-type FOWT to control the tower displacements such as [PERSON] et al. in [16] and [PERSON] et al. in [17]. [PERSON] et al. in [18] proposed the use of two different liquid column dampers to control the vibration in a spar-type FONTs. With the use of the bidirectional tuned liquid column damper and the tuned mass damper, the barge is able to decrease the pitch and roll angles in the FOWT. Some papers have proposed hybrid FOWT-WEC for multi-functional aims of wind and wave supplies extractions. [PERSON] et al. in [19] performed an experimental work to validate a novel hybrid FOWT-OWC semi-submerside floater. This combined system maximizes the wave energy production by chamber opening. Another alternative to deal with oscillatory behavior of a class of nonlinear systems has been proposed by [PERSON] et al. in [20]. A few articles have extended the application of combined barge-based FOWT-OWCs with consideration of full nonlinear equations of motion. [PERSON] in [21] designed a square-shaped moonpool placed at the barge's center that provides the option of adding an OWC within the tower of the wind turbine, but this hybrid system has not designed for the purpose of stabilization. [PERSON] et al. in [22] analyzed the performance of the hybrid barge-type FOWT-OWCs under different sea states to increase the stability of the system. However, the introduced approaches have not proposed a strategy to control the valves of OWCs. In this research article, the nonlinear barge-type model of NREL 5-MW baseline was investigated. Four OWCs have been housed within the barge in order to decrease the oscillations, particularly in pitch and top tower fore-aft modes, in the whole hybrid system under various sea states. With the usage of Response Amplitude Operators (RAO), a switching control strategy have been proposed to transient between opening/closing OWCs' valves. The controlled OWC-based FOWT has been compared to the traditional barge platform to show the performance of the controlled hybrid system. The rest of the manuscript is structured as: Section 2 explains the structure of hybrid FOWT-OWCs with its equations of motion. For the structure design, three platforms are considered including Standard barge platform, 4 OWC-based barge platform with closed valves and 4 OWC-based barge platform with open valves. Section 3 presents the problem statement in order to introduce the RAOs used for the proposed switching control strategy. The RAOs are used to analyze the dynamic response of the system. Section 4 considers a study case to study the efficiency of the proposed switching control under various sea states.