## 2 Platform Configuration and Mooring Layout
### OC3-Hywind Spar/UMaine-Hywind Spar  
The OC3-Hywind spar is a floating platform that was preliminarily developed in the Offshore Code Comparison Collaboration (OC3). By adopting the concept of the spar buoy \"Hywind\" from StatoilHydro of Norway, it includes adaptations suitable for supporting the NREL 5-MW machinery and appropriate for public application. In order to distinguish it from the original Hywind concept, it is referred to as the OC3-Hywind spar. The configuration of the OC3-Hywind spar features a deeply drafted, slender spar buoy at a depth of 320 m with three catenary mooring lines. The mooring lines are attached to the floating platform by means of a delta connection to increase the yaw stiffness of the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **OC3-Hywind** & **UMaine-Hywind** & **MIT/NREL TLP** & **UMaine TLP** \\  & **Spar** & **Spar** & **MIT/NREL TLP** & **UMaine TLP** \\ \hline Water Depth (m) & 320 & 200 & 200 & 200 \\ Diameter (m) & 6.5–9.4 & 6.5–9.4 & 18 & 6.5 (upper column)/ \\ Draft (m) & 120 & 120 & 47.89 & 24 \\ Mass, including ballast (kg) & \(7.466\times 10^{6}\) & \(7.466\times 10^{6}\) & \(8.6\times 10^{6}\) & \(7.7494\times 10^{5}\) \\ CM location of the platform below SWL (m) & \(89.92\) & \(89.92\) & \(40.61\) & \(19.72\) \\ Roll inertia about CM (kg-m2) & \(4.229\times 10^{6}\) & \(4.229\times 10^{9}\) & \(5.716\times 10^{8}\) & \(1.5078\times 10^{8}\) \\ Pitch inertia about CM (kg-m2) & \(4.229\times 10^{6}\) & \(4.229\times 10^{6}\) & \(5.716\times 10^{8}\) & \(1.5078\times 10^{8}\) \\ Yaw inertia about CM (kg-m2) & \(1.642\times 10^{8}\) & \(1.642\times 10^{8}\) & \(3.614\times 10^{8}\) & \(9.885\times 10^{7}\) \\ Mooring System & Catenary & Catenary & TauT-Leg & TauT-Leg \\ Number of mooring lines & 3 & 3 & 8 (4 pairs) & 3 \\ Un-stretched line length (m) & 902.2 & 468 & 151.7 & 171.4 \\ Line diameter (m) & 0.09 & 0.09 & 0.127 & 0.222 \\ Line mass density (kg/m) & 77.71 & 145 & 116 & 302.89 \\ Line extensional stiffness (N) & \(3.842\times 10^{8}\) & \(3.842\times 10^{8}\) & 1,500,000,000 & 7,720,000,000 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main particulars of floating platforms and mooring systems (SWL: seawater level).  
moorings. The dimensions of the OC3-Hywind spar and the layout of the catenary mooring system are illustrated in Figure 1.  
Similar to the configuration of the OC3-Hywind spar, the UMaine-Hywind spar is drafted at a depth of 200 m rather than 320 m. Therefore, slight modifications of the mooring system and the modes of the tower are required. The difference of water depth between the OC3-Hywind spar and the UMaine-Hywind spar is due to the fact that the University of Maine demonstration project seeks to examine the response of its three generic designs at the same water depth.  
The catenary configuration is one of the most common mooring systems with long steel chains whose weight and curved shape hold the floating platform in place. It has long mooring lines partly resting on the seabed to reduce horizontal loads on the anchors. Although the weight of mooring lines limits motion responses of the floating platform, it has greater freedom of movement than the other configurations. The installation procedure of the catenary mooring system is relatively simple. Lower section of catenary mooring lines would result in more disruption of the seabed.