Global Dynamic Fluid-Structure-Interaction Analysis for a Submerged Floating Tunnel

Abstract

This thesis aims to gain insight into the global dynamic fluid-structure interaction response of a submerged floating tunnel (SFT) under the wave and current loading to enhance the design. To this end, the tunnel tube is modelled as a Euler-Bernoulli beam deformed in three directions (horizontal displacement, vertical displacement, rotational angle), the discrete anchoring system as a continuous elastic foundation considering geometrical nonlinearity. Then, the Morison equation is used to model the combined current and wave loading and the oblique wave loading. A simplified wake oscillator and a non-simplified wake oscillator are used to model the vortex-induced vibration (VIV) under current loading. Subsequently, the modal superposition method and Runge-Kutta method are applied to obtain the response of the SFT in the time domain. The Fourier transform and the wavelet transform are applied to the time domain signal to perform a frequency domain analysis. Finally, a test on a scaled SFT model is used for a case study. A parametric study is carried out based on the results to study the influence of the geometrical and structural design parameters. The results show that the motions in the horizontal direction and the rotational direction are significantly coupled together. Geometrical nonlinearity introduces the second-order effect to the system leading to a complicated vertical motion with a considerably larger displacement compared with the linear case. The amplitude of the VIV on the tunnel tube of the scaled model is very small based on the non-simplified wake oscillator. Moreover, it is found that, basically, increase of the BWR, increase of the stiffness in the cables, increase of the distribution of the net buoyancy in inclined cable, decrease of the tunnel length or decrease of the inclination of the inclined cables can reduce the maximum response of the SFT. Based on the above findings, it is concluded that a global dynamic analysis is suggested when a SFT is expected to be subjected to oblique wave loadings. Geometrical nonlinearity is necessary to be considered for an accurate analysis especially for the response in the vertical direction. It is also necessary to be considered when the influence of the BWR is of interest. The VIV on the tunnel tube is negligible base on the scaled model.