## 2 The FOWT-SFFC Concept
### Fishing Cage as Floating Foundation  
The fishing cage acts as the floating foundation for the wind turbine. It includes nine columns (a vertical column in center and eight inclined side columns), 16 pontoons (eight radial and eight ring), 48 braces (eight top radial braces, eight top ring braces and 32 side braces). All columns and braces are cylindrical, while the pontoons' sections are rectangular. Figure 2 illustrates the side view of the fishing cage.  
Compared to HDPE fishing cages, the conic configuration of cage not only effectively helps to increase the contained water volume in fishing cage but also to lower the overall gravity center. Shown in Figure 1, the inner space of cage can be subdivided into eight sectors to raise a variety of fishes. The wind turbine tower and the living quarter sit right on the top of the central column. It is also worth mentioning that the radial pontoons further extend out of the bottom octagon ring of cage to provide greater buoyancy and restoring moment. In order to balance the gravity of FOWT-SFFC with the buoyancy, high density concrete is placed in the radial and ring pontoons for ballast. The cage has eight side nets and a bottom net to enclose the fishing space. Side nets are fixed at side frames,  
Figure 1: Illustration of the concept of floating offshore wind turbine with a steel fish-farming cage (FOWT-SFFC).  
while the bottom net is attached to lifting devices inside the cage such that it can be moved vertically from the bottom to water surface for capturing fish with ease. Nets are made of copper alloy to resist sea water corrosion and biofouling.  
The draft of fishing cage with mooring lines is 65 m at the undisplaced position. The confined water volume in fishing cage is 200,000 m\({}^{3}\). The top of the central column is 15 m above the still water level (SWL) to avoid water splash and slamming on the living quarter. As a result, the bottom of tower is 15 m above SWL. The dimensions of key structural members of fishing cage are given in Table 1. Table 2 gives the mass breakdown and total mass of fishing cage (including the living quarter). The roll and pitch inertias of the fishing cage about its center of gravity (CG) are both 12,217,724,634 kg\(\cdot\)m\({}^{2}\) due to symmetry, and the yaw inertia about its vertical centerline (axis \(Z\) in Figure 2) is 13,552,990,000 kg\(\cdot\)m\({}^{2}\). Note that CG is 9.29 m above the keel.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Structural Members** & **Sectional Diameter or Breadh \(\times\) Height (m)** & **Length (m)** & **Wall Thickness (m)** \\ \hline Central Column & 6.5 & 80 & 0.027 \\ Side Column & 3.8 & 76.21 & 0.02 \\ Radial Ponton * & 4 \(\times\) 1.8 & 41.29 & 0.02 \\ Ring Ponton * & 4 \(\times\) 1.8 & 27.77 & 0.02 \\ Top Radial Brace & 1.5 & 25.25 & 0.012 \\ Top Ring Brace & 1.5 & 19.47 & 0.012 \\ Upper Side Brace & 1.2 & 38.21 & 0.01 \\ Lower Side Brace & 1.2 & 41.60 & 0.01 \\ \hline \hline \end{tabular}  
* The shape of ponto is rectangular.  
\end{table}
Table 1: Dimensions of fishing cage structural members.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Key Components** & **Mass (kg)** & **CG * (m)** \\ \hline Steel Structure & 3,417,340 & \(-\)37.10 \\ Ballast Concrete & 9,142,116 & \(-\)64.49 \\ Living Quarter & 200,000 & 15 \\ Copper alloy Nets & 133,000 & \(-\)36.73 \\ \hline Total & 12,892,456 & \(-\)55.71 \\ \hline \hline \end{tabular}  
* Center of gravity (CG) is measured from SWL (Figure 2).  
\end{table}
Table 2: Mass breakdown of fishing cage.  
Figure 2: Side view of the fishing cage.