## 3 Model Development of the Semisubmersible-Type FWTs
### Mooring System Models  
The station keeping of the platform is provided by three slack catenary chain moorings. In the multi-point mooring system, the fairleads of the mooring lines are located at the top of the base columns at a depth of 14.0 m below the mean sea level and at a radius of 40.87 m from the platform centerline. In the single-point mooring system, the mooring lines are allocated to a single fairlead which is located at the bottom center of the central column at a depth of 20.0 m below the mean sea level. To keep the same vertical line configuration for the fairness of comparison, in the SPM case, the water depth is adjusted to be also 6 m larger than the original water depth, i.e., 206 m, which almost has no effect on the wave dynamics since the water depth exceeds half wave lengths of most sea waves. In such configurations, all the lines can have the same line length and geometrical shape. The only factor that differs in affecting the system's behavior will then be the collection of the separated fairleads. Sketch of the two mooring system configurations from both the top view and side view is shown in Figure 2. Details of the multi-point mooring system and the single-point mooring system are listed in Tables 3-5.  
\begin{table}
\begin{tabular}{c c} \hline
**Platform Properties** & **Value** \\ \hline Platform draft & 20.0 m \\ Centerline spacing between offset columns & 50.0 m \\ Length of upper columns & 26.0 m \\ Length of base columns & 6.0 m \\ Diameter of main column & 6.5 m \\ Diameter of offset (upper) columns & 12.0 m \\ Diameter of base columns & 24.0 m \\ Diameter of pontoons and cross braces & 1.6 m \\ \hline \end{tabular}
\end{table}
Table 1: Geometrical specifications of the DeepCwind semisubmersible platform.  
\begin{table}
\begin{tabular}{c c} \hline
**Turbine Properties** & **Value** \\ \hline Rotor configuration & Upwind, 3 Blades \\ Cut-in wind speed & 3 m/s \\ Rated wind speed & 11.4 m/s \\ Cut-out wind speed & 25 m/s \\ Rotor mass & 110,000 kg \\ Nacelle mass & 240,000 kg \\ Tower mass & 347,460 kg \\ \hline \end{tabular}
\end{table}
Table 2: Selected properties of the NREL 5-MW baseline wind turbine.  
Figure 2: Sketch of the two mooring system configurations: (**a**) top view of the SPM (Single-Point Mooring) layout; (**b**) side view of the SPM layout; (**c**) top view of the MPM (Multiple-Point Mooring) layout; (**d**) side view of the MPM layout. Note that the aspect ratio of the floating platform is enlarged for the better illustration.  
The two mooring systems are modelled by the MAP++ and MoorDyn, which are both open-source available. MAP++ is a library designed for the purpose of an MSQS (Multi-Segmented, Quasi-Static) mooring line. The MSQS model is developed based on an extension of conventional single line static solutions [33; 34]. MoorDyn is based on a lumped-mass approach which is capable of modelling the mooring dynamics. Both of the two models consider mooring stiffness, inertia, weight, buoyancy and seabed friction. In addition, MoorDyn considers additionally the line damping and the line drag force, which have not yet been modelled in MAP++. The two sorts have been intended to support the modelling of marine renewable energy systems, e.g., floating offshore wind turbines, wave energy converters, ocean current turbines, etc. By adhering to the NREL's new modularization framework [19; 35], they are seamlessly integrated into the FAST software via a dynamically link mechanism.