ABSTRACT

A full time-domain two-dimensional (2D) numerical model is developed for the coupled dynamic analysis of submerged floating tunnel (SFT) structures. For the hydrodynamic loads, a higher-order boundary element method (HOBEM) based on the potential flow theory is developed to calculate the velocity potential at each time step. The mooring-line dynamics are based on the rod theory and the finite element method (FEM) with the governing equations is described in a global coordinate system. In the coupled dynamic analysis, the motion equation for the hull and the dynamic equations for mooring-lines are solved simultaneously using the Newmark-β method. The numerical model is utilized for an SFT motion response simulation at different buoyancy weight ratios (BWRs) in waves of different frequencies. Numerical results including motion response and tension at the top of mooring lines are presented, and some significant conclusions are derived.

INTRODUCTION

Submerged Floating Tunnel (SFT) is a new concept of traffic structure that crosses the strait, bay and lake. The SFT has a large internal space, which is sufficient to meet the requirements of roads and even railways. There exist harsh natural conditions in some fjords, and traditional spanning methods (such as: cross-sea bridges, immersed tunnels) are not feasible due to the environmental conditions and technical constraints, however, SFT offers the possibility of crossing.

SFT is continuously exposed to wave effects from the free surface because the submerged depth is not deep enough to neglect the free surface wave. Mai (2005) also applied Morison equation to solve the fluid force acting on the SFT, and established a static / dynamic finite element numerical model for the SFT structure, in which the analysis of static / dynamic response could be carried out, respectively. Wang (2008) further considered the role of nonlinear lift force and simulated the overall motion response using finite element software subsequently. Long (2009) used fifth-order stokes wave theory and Morison formula to calculate the dynamic response of SFT under different BWRs, and optimized range of BWR were proposed for the SFT prototype. Lu et al. (2011) investigated SFT dynamics when going through tether slacking and the related snap force under wave condition, and provided an alternative philosophy for SFT structural design on concerning preventing the occurrence of tether slacking and snap force. Seo et al. (2015) proposed a simplified method for estimation of the behavior of an SFT in waves and the tests with physical models in a wave flume were carried out for variation.

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