## 1 Introduction  
The submerged floating tunnel (SFT) is an innovative solution used to cross deep waterways [1; 2]. The SFT consists mainly of a tunnel for vehicle transportation and mooring lines for station-keeping. The tunnel is usually positioned at a certain submergence depth, typically greater than 20 m, with positive net buoyancy that is balanced by mooring lines anchored in the seabed [3; 4]. Considering that wave/current/wind effects are greatly reduced, the cost is almost constant along the length [5], and ship passage is not obstructed by the structure, the SFT has been regarded as a competitive alternative to floating bridges and immersed tunnels. In this regard, since Norway's first patent in 1923 [6], many proposals and case studies have been published worldwide, which includes Hogsfjord/Bjornafjord in Norway [7; 8; 9], the Strait of Messina in Italy [10], Funka Bay in Japan [11; 12], Qandao Lake in China [13; 14], and the Mokpo-Jeju SFT in Korea [15]. Even though there is no actually installed structure in the world despite extensive research [16], the first construction of the SFT is being considered by Norwegian Public Road Administration (NPRA) with global interest [17]. To provide sufficient confidence for the concept, feasibility studies under diverse catastrophic environmental conditions, such as extreme waves and earthquakes, must be extensively studied in advance. Along this line, numerous researches have been carried out to verify structural safety in wave and seismic excitations on the SFT. Regarding wave-excitation effects, [PERSON] et al. [18] evaluated the effect of mooring-line configurations on SFT dynamic responses including possible snap loading. [PERSON] et al. [11] and [PERSON] et al. [19] focused on slack mooring phenomena at various buoyancy-weight ratios (BWRs) of the SFT and inclination angles of mooring lines. [PERSON] et al. [3] conducted parametric studies to investigate the effects of the BWR and mooring-line stiffness. Dynamic motions at varying BWRs and the corresponding comfort index were investigated by [PERSON] et al. [20]. [PERSON] et al. [21] compared experimental results with simplified numerical approach for a segment of the SFT. [PERSON] et al. [22] evaluated the influence of VIV (vortex induced vibration) of mooring lines on the SFT dynamic responses using a simplified numerical model. In addition, with regard to seismic-excitation effects, [PERSON] et al. [4] carried out a coupled dynamic analysis to investigate the effect of wave and seismic excitations. [PERSON] et al. [13] suggested detailed procedures to generate artificial seismic excitations and performed the corresponding structural analysis. Dynamic responses at various shore connections under transverse earthquake were investigated by [PERSON] and [PERSON] [23]. [PERSON] et al. [24] and [PERSON] et al. [25] focused on hydrodynamic fluid-structure interaction induced by vertical fluid fluctuations known as the seaquake. [PERSON] et al. [26] derived simplified analytical solutions for 2D and 3D cases and computed SFT dynamic responses in diverse stiffness conditions. [PERSON] et al. [6] compared the dynamic effects induced by wave and seismic excitations. During the past decade, various SFT-related studies have been carried out in the second author's research lab. [PERSON] et al. [27] compared the dynamics of a moored-SFT segment in regular waves for various BWRs and mooring types between experimental results and numerical simulations. For the numerical simulations, both commercial program (OrcaFlex) and in-house program CHARM3D (Coupled Hull And Riser Mooring 3D) were used for cross-checking. [PERSON] et al. [16] further investigated the dynamics of the short tunnel segment under irregular waves and random seabed earthquakes. Then, the initial studies of hydro-elastic responses of a long SFT with many mooring lines by random waves and seabed earthquakes were conducted by [PERSON] and [PERSON] [28] and [PERSON] et al. [29] by using commercial software, OrcaFlex.