A study of the interaction between depression internal solitary waves and submerged floating tunnels in stratified fluids

Internal wave is a wave phenomenon that traverses almost the depth of the ocean, posing safety challenges and threats to any underwater structure, including the submerged floating tunnel (SFT). Based on the Navier-Stokes (NS) equations and the density transport equation, a numerical model for analyzing the nonlinear interaction between depression internal solitary waves (ISWs) and SFTs in stratified fluids is proposed. After verification with theoretical and experimental results, the hydrodynamic model is utilized to simulate the prototype ISW-SFT interaction under three conditions, i.e., fixed SFT, unconstrained SFT, and constrained SFT. The variation of wave and velocity fields around the SFT in the interaction is simulated, which leads to the wave forces on the SFT as well as the induced motion responses. The simulated results show the wave forces, including horizontal force, vertical force, and moment, are closely correlated with the submergence, wave amplitude and density ratio. When the SFT is unconstrained, the sway and heave displacements are extremely large, while the roll angle is relatively small. When the SFT is constrained by mooring line, it is found the motion displacements are much smaller than the unconstrained condition, inferring the high efficiency of mooring line constraint. However, as motion displacements in the constrained condition are close to the allowable deflection limit of floating bridges for reliability and serviceability, the motion responses should be carefully considered, especially when the SFT encounters internal soliton trains, which may lead to the probability of fatigue failure.