Submerged floating tunnels (SFTs) are considered as one of the relevant concepts to replace ferry connection in the Norwegian Public Road Administration's plan to upgrade the E39 coastal highway. Current load including lift and drag force is one the most important factors challenging its safety reliability and fatigue lifetime. In this study, the lift force and drag force on a tunnel section model was studied through a stationary towing test of a twin-rigid-cylinder in tandem arrangement with various spacing ratios. The torsional moment induced by the difference of lift force on the two cylinders was also analyzed. A dynamic analysis of the SFT (with a fixed spacing ratio of 3) considering the current induced lift, drag forces and torsional moment was then conducted in SIMO/RIFLEX. The numerical simulations reveal that the lift force contributes significantly to the heave motion and the vertical bending moment My of the SFT. The drag forces on the twin cylinders, which act in opposite directions, strongly affect the sway motion and horizontal bending moment Mz compared with the results based on the drag forces acting in the same direction. The case studies provide a basic understanding of the dynamic behavior of an SFT under current load.
The Norwegian Public Road Administration (NPRA) is operating the highway E39 ferry free project, in which several wide and deep Norwegian fjords are considered to be connected by floating bridges or submerged floating tunnels (SFTs), instead of ferries (Norwegian Public Road Administration 2012). In this study, the focus is on SFT. The large dimension of the SFT, i.e. about 4km in length consists of two identical concrete tubes in a tandem configuration and is designed to have a submergence of 25-30 m. The current induced periodic lift force in crossflow (CF) direction and constant drag force in inline (IL) direction acting on the twin-tube system, as one of the most important factors in the safety design, are mainly focused in the paper.