Experimental study of the motion characteristics of a twin-circular submerged floating tunnel under wave actions

Abstract

The experimental investigations into the motion characteristics of a twin-circular submerged floating tunnel under wave actions were conducted. Both the motions and the mooring line forces were measured. The dynamic response characteristics of the structure were analyzed based on the experimental data using the theory of Mathieu instability. It was found that when the incident wave frequency was close to twice the natural frequency of the structure, the resonant sway motion with half of the incident wave frequency and a large amplitude would be excited. The half-wave frequency motion also affected the mooring line forces, causing the force difference between the weather side and lee side to oscillate at the half-wave frequency. This resonant sway motion is attributed to the Mathieu instability caused by the time-varying effective stiffness of the structure system. The structure exhibited clearly a horizontally asymmetric motion, due to the superposition of the resonant motion of the half-wave frequency and the forced motion of the wave frequency. The exciting forces and their phase relationship with the motion were further discussed. It should be noted in the practical engineering design that the large resonant motion will threaten the traffic safety of the submerged floating tunnel.