## 5 Conclusions  
Global performance analysis of the SFT was carried out for survival random wave and seismic excitations. To solve tunnel-mooring coupled hydro-elastic responses, an in-house time-domain-simulation computer program was developed. The hydro-elastic equation of motion for the tunnel and mooring was based on rod-theory-based finite element formulation with Galerkin method. The dummy-connection-mass method was devised to conveniently connect multiple segmented objects and mooring lines with linear and rotational springs. Considering the slender shape of the structure, hydrodynamic forces were computed by the modified Morison equation. The numerical results produced by the developed program were in good agreement with those by the commercial program  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Parameter** & **Numerical Model** & **Maximum** & **Minimum** & **Standard Deviation** & **Unit** \\ \hline Horizontal displacement & Eq & 0.070 & \(-\)0.073 & 0.023 & \\  & Eq + Sq & 0.070 & \(-\)0.072 & 0.023 & m \\ Vertical displacement & Eq & 0.039 & \(-\)0.042 & 0.013 & m \\  & Eq + Sq & 0.019 & \(-\)0.019 & 0.006 & \\ \hline \multirow{2}{*}{Mooring tension (line \#1)} & Eq & 9872.22 & 3783.56 & 801.58 & \\  & Eq + Sq & 8728.01 & 4631.84 & 649.80 & \\ \multirow{2}{*}{Mooring tension (line \#2)} & Eq & 9722.09 & 3291.79 & 829.07 & \\  & Eq + Sq & 8778.38 & 4491.79 & 649.33 & \\ \multirow{2}{*}{Mooring tension (line \#3)} & Eq & 12,295.18 & 4918.66 & 958.46 & kN \\  & Eq + Sq & 11,009.40 & 6691.91 & 649.58 & \\ \multirow{2}{*}{Mooring tension (line \#4)} & Eq & 12,512.53 & 5633.35 & 925.67 & \\  & Eq + Sq & 11,001.66 & 6772.99 & 652.55 & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Statistics of the SFT motions and mooring tensions at the middle location under irregular seismic excitations (Eq: earthquake, Sq: seaquake).  
Figure 14: Time histories (without seaquake) (**a**) and spectra (**b**) of mooring tension #4 at the middle location under seismic excitations  
Figure 15: Time histories of vertical responses of the tunnel (**a**) and mooring tension #4 (**b**) in the middle location under seismic excitations with and without seaquake effect.  
OrcaFlex based on lumped-mass method. The extreme wave excitations caused the maximum SFT dynamic motions of 24 cm and 6 cm in the horizontal and vertical directions and the corresponding mooring tensions below allowable level. Snap motions and loadings of mooring lines were not observed. Under regular seismic excitations, large resonant responses of the tunnel were observed at 1 st and 3 rd natural frequencies. In the case of seabed earthquake, the seabed motions are transferred to SFT through mooring lines and through seawater fluctuations called seaquake. When the latter is further considered, horizontal responses were not affected but vertical responses become significantly reduced especially at its lowest natural frequency. After analyzing the behaviors of the two contributions, it was found that the reduction was caused by the phase-cancellation effect. However, in other cases, the phases could enhance each other to increase the total responses of the SFT. Under extreme irregular seismic excitations, the maximum SFT dynamic motions of 7 cm and 2 cm were generated and the corresponding mooring tensions were about 30% smaller compared to the extreme wave case. However, when the frequencies of seismic excitations are closer to SFT natural frequencies, larger dynamic amplifications are expected.  
All authors have equally contributed to publish this article related to design of target model, validation of numerical modeling, simulations, analysis, and writing.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1A5A1014883).  
The authors declare no conflict of interest.