*4.1. Free Decay Test*

A free decay test was performed in the numerical simulation to determine the natural frequencies of the surge, heave, and pitch of the platform. The natural frequencies are listed in Table 4. Figure 4 was the time domain curve of free decay test of platform. The results showed that the natural frequencies of the surge (Figure 4a) and heave (Figure 4b) of the platform were the same from two codes, and there is a difference of about 11.7% of the natural frequency of the platform pitch from two codes.

**Table 4.** Natural frequencies (rad/s) of platform.


**Figure 4.** *Cont*.

**Figure 4.** Time domain curve of free decay test of the semisubmersible platform: (**a**) Surge; (**b**) Heave; (**c**) Pitch.

#### *4.2. Regular Wave Condition*

In this study, the simulation of typical regular wave conditions (LC1) was performed, and the characteristics of the motion response, mooring response, and produced power of multiple bodies (F2A and AQWA simulation) were compared.

#### 4.2.1. Motion Response

In this section, the motion response under a regular wave (*H* = 2.0 m, *T* = 9.0 s) was selected, and a motion response of three degrees of freedom (DOF) of the platform (surge, relative heave, pitch) is presented (Figure 5). The results were compared between the fully coupled framework F2A and the hydrodynamic software AQWA.

The surge motion of the platform is shown in Figure 5a. Compared with the multibody simulation results of F2A and AQWA, the amplitude was basically the same, but the simulation results of F2A at the wave peak were slightly larger. The relative heave motion responses simulated by F2A and AQWA were basically the same (Figure 5b). Slightly larger pitch response from F2A could be identified compared with AQWA results. Under only wave conditions, F2A and AQWA had good consistency in simulating multibody motion characteristics, especially in relative heave.

**Figure 5.** *Cont*.

**Figure 5.** Time domain motion response of the semisubmersible platform: (**a**) Surge; (**b**) Relative Heave; (**c**) Pitch.

### 4.2.2. Mooring Line Force

Figure 6 shows the time-domain response of the mooring force of Mooring Line 1 (ML1) and Mooring Line 2 (ML2). The mooring line force of ML1 (downwind direction) was smaller than that of ML2 (upwind direction) when subjected to waves in the heading direction. Results from F2A have slightly larger mooring line force than those from AQWA simulation.

**Figure 6.** Mooring force of: (**a**) ML1; (**b**) ML2.
