4.2.3. Produced Wave Power

The wave energy was captured by PTO system. The relative heave velocity from both codes are similar to each other (Figure 7a). From Equation (19), the damping force can be observed in Figure 7b. Based on the relative heave velocity and damping force, the produced wave power could be identified, which is similar from F2A results and AQWA results (Figure 7c).

**Figure 7.** Time series of different responses of the combined structure under regular waves: (**a**) Relative Heave Velocity; (**b**) Damping Force; (**c**) Produced Power.

Under only wave conditions, F2A and AQWA had good consistency in simulations of the multibody dynamic response. This was because F2A integrates the AQWA hydrodynamic module.

#### *4.3. Irregular Wave and Turbulent Wind Conditions*

This section presents the motion response, mooring force, and produced power under irregular wave and turbulent wind conditions.

#### 4.3.1. Motion Response

Figure 8 shows the motion response in surge, pitch, and heave directions under normal operation conditions in LC2 and LC3. The responses of surge and pitch in both load cases from F2A (Figure 8a,b) simulation are smaller than those predicted by AQWA (Figure 8e,f). This is due to the fact that the wind load in F2A and AQWA is differently implemented. In F2A, the wind field is pre-generated and both wind aerodynamic load and damping are considered due to the rotating turbine rotor. However, in AQWA, the wind load is implemented as an external load using dll function and no interaction and aerodynamic damping is included. The difference for the relative heave motions simulated by two tools was limited for both LC2 and LC3 (Figure 8c,d).

**Figure 8.** Time domain motion response of the semisubmersible platform: (**a**) Surge in LC2; (**b**) Surge in LC3; (**c**) Relative Heave in LC2; (**d**) Relative Heave in LC3; (**e**) Pitch in LC2; (**f**) Pitch in LC3.

Figure 9 shows motion response in LC4, in which the wind speed is larger than rated wind speed. Similar as the load case LC2 and LC3, the motion responses in surge and pitch direction from F2A are lower than those motions from AQWA due to the aerodynamic damping. The relative heave motions predicted from two codes are similar to each other.

**Figure 9.** Motion responses of the semisubmersible platform in LC4: (**a**) Surge; (**b**) Relative Heave; (**c**) Pitch.

4.3.2. Mooring Line Force

The mooring line forces of ML1 and ML2 from LC2, LC3, and LC4 were compared for F2A results and AQWA in Figure 10. Basically, the mooring line force of ML2 in the upwind direction is larger than the mooring line force of ML1 in the downwind direction because wind and wave will drive the combined system to move in a downwind direction from its equilibrium position. Therefore, ML1 will get relaxed and ML2 will get tensioned. For the mooring line force of ML1, the reduction of the mooring force from its force at equilibrium position from F2A simulation is less than the reduction of the force from AQWA simulation due to the difference of the surge motions from two codes in all three load cases. Thus, the mooring line forces of ML1 from F2A are larger than those from AQWA (Figure 10a,c,e). However, for mooring line force of ML2, the increasing of the mooring force from its force at equilibrium position from F2A simulation is less than the increasing of force from AQWA simulation. Therefore, the mooring line forces of ML2 from F2A are less than those from AQWA (Figure 10b,d,f). Due to the pitch control above the rated wind speed (LC4), the wind thrust force is much smaller than that in LC2 (below rated wind speed) and LC3 (at rated wind speed). Therefore, the aerodynamic damping effect is much smaller. In this case (LC4), the contribution from wave load is much larger than wind load, and the discrepancy from two codes is minimal. Therefore, there is a slight difference in the mooring line force of ML2 for LC4.

**Figure 10.** Mooring force of: (**a**) ML1 in LC2, (**b**) ML2 in LC2, (**c**) ML1 in LC3, (**d**) ML2 in LC3, (**e**) ML1 in LC4, (**f**) ML2 in LC4.

4.3.3. Produced Wave Power

The relative heave velocities between the semisubmersible platform and the WEC, the damping force and produced power from WEC in LC2, LC3, and LC4, are presented in Figure 11. Due to less effect of aerodynamic damping on heave motion, no significant difference is identified for the relative heave velocities, damping force, and produced power. When the wave height increases from LC2 to LC4, the relative heave velocity (Figure 11a,c,e) also increases due to the large relative heave motion in a severe sea state, which is beneficial to capture wave energy. Figure 11b,d,f present the vertical damping force. The F2A and AQWA simulation results were similar. As the wave height increases, the vertical damping force increases significantly. Figure 11g,h,i show the produced energy power from WEC. With the sea state moving from mild to severe (LC2 to LC4), more power can be produced from the wave. Figure 11i is the produced power under severe sea conditions (LC4). It can be seen the maximum power in LC4 is even as large as 3.5 MW (not shown in Figure 11f), which is comparable to the power produced from the wind turbine. However, the mean produced power is less than 600 kW, and it shows obvious instability, while the produced power of WEC is much smaller. Therefore, the wind power production for NREL 5 MW WT will make the main contribution to the total power production of the combined system under the severe sea conditions.

**Figure 11.** Time series of different responses of the combined structure: (**a**) Relative Heave Velocity of LC2, (**b**) Damping Force of LC2, (**c**) Relative Heave Velocity of LC3, (**d**) Damping Force of LC3, (**e**) Relative Heave Velocity of LC4, (**f**) Damping Force of LC4, (**g**) Produced Power of LC2, (**h**) Produced Power of LC3, (**i**) Produced Power of LC4.

4.3.4. Statistical Analysis

Figure 12 displays the statistics of responses in operational conditions from both F2A simulation and AQWA simulation. The maximum and minimum values are determined as the highest crest and lowest trough of the corresponding time series. Figure 12a–c show that the simulation results of F2A and AQWA were different in the surge and pitch motion, and the heave motion results were slightly different, which was consistent with the previous results. When the wind velocity was the rated wind velocity of the wind turbine (12.4 m/s), the surge and pitch motion amplitudes of the platform were the largest, and the heave motion was greatly affected by the hydrodynamic load. When the wave height increases (LC2–LC4), the heave amplitude increases accordingly. Figure 12d shows that under the three operational conditions, the statistical values of the mooring force of ML2 simulated by F2A and AQWA were slightly different. Similarly, when the wind velocity was the rated wind velocity of 12.4 m/s (LC3), the mooring force of ML2 was the largest. Figure 12e shows that as the sea state becomes worse (LC2–LC4), the damping force and the produced power increase sharply. By comparing, aerodynamic loads were confirmed to have significant effects on surge and pitch motion and mooring forces, while hydrodynamic loads had significant effects on the vertical responses (heave, damping force, and produced power).

**Figure 12.** Comparison of the statistical results of in LC2, LC3, LC4: (**a**) Surge Motion; (**b**) Relative Heave Motion; (**c**) Pitch Motion; (**d**) Mooring Force of ML1 and ML2; (**e**) Damping Force (Df) and Produced Power (Pp).

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