Study On The Coupled Dynamic Responses Of Tunnel Immersion System Under Wave Actions

The immersion of the tunnel element is the most critical and risky process during the immersed tunnel construction.The severe weather conditions can induce excessive tunnel motion and even results in significant consequences.Thus,it is vital to gain a better understanding of the hydrodynamic responses of the tunnel-pontoon system considering a wide range of energetic sea states.However,previous research cannot satisfy the practical need and the dynamic responses of the coupled system can also contribute to the hydrodynamic research area of multi-body system.In this study,extensive 3D experiments were carried out to investigate the key issues of the tunnel during its immersion standby and immersion stage in seas,which have both significant theoretical and practical meanings.With regard to the immersion standby stage,the tunnel and two pontoons were assembled as one rigid body.Thus,researches were focused on the dynamic responses of the tunnel-pontoons assembly and the optimal mooring configuration under wave excitations.For the immersion stage,the tunnel and pontoons were flexibly connected by four suspension lines and built a coupled system.Then,the effect of the immersion depth and negative buoyancy on coupling dynamic responses of the system were discussed in detail for vaious wave conditions.Most importantly,the coupling mechanism of the system was also explored based on the combination of the experimental results and the modal analysis by numerical simulation.The key research contributions are:1)During immersion standby stage,the mean drift force significantly contributes to the mean sway of the floating bodies when T≤Troll.Meanwhile,the consequent mooring tensions can reach a great level as that of relatively long waves of T=2Troll,which should be considered carefully.Moreover,the wave-on-deck occurrence provokes a significant offset of the system’s mean heave.The resonance roll at T=Troll incurs the greatest mean heave in downward direction,which introduces high risk for the tunnel to collide with the seabed.2)During immersion stage,the spectra charateristics of the motions and dynamic responses of the tunnel-pontoon system are directly dependent on the coupled modes of the system and the incident wave spectra.The natural frequency of the coupled system has a close correspondence with the peak frequencies in the spectra of the motions,suspension and mooring tensions,which is illuminated for the first time.3)Among the 18 coupled modes,the coupled roll resonance is found to have the most crucial effect on the dynamic responses of the system in both frequency domain and time domain.This influence is most critical for relative immersion depth D/Dmax=0.80:the tunnel’s roll and relative roll for Tp=TRoll can be 1.8 times as those of Tp=1.25TRoll,and 1.6 times for the tunnel’s pitch and 1.2 times for the suspension line tensions.Moreover,the roll resonance at Tp=TRoll also introduces the largest smallest clearance between the tunnel and seabed and consequently the highest touch-bed risk of the tunnel.To avoid the effect of roll resonance,it is recommended that the incident wave period should be smaller that the coupled roll resonance period of the system for D/Dmax=0 during its immersion operation,which is Tp≤7s in this study.4)It is indicated that the surge,pitch and yaw of the tunnel exhibits a sudden increase for relative immersion depth D/Dmax≥0.88,which is regarded as "the near-bed effect".The tunnel’s yaw for D/Dmax=0.88 can be 5.1 times as that of D/Dmax=0.80,and 3.0 times for the tunnel’s surge and 1.7 times for the tunnel’s pitch.The near-bed effect varies with incident wave period and it is suggested that the positioning operation should be conducted when the wave peak period is smaller than half of the roll resonance period for D/Dmax=0.88,which is Tp≤7s in this study.5)It is demonstrated that the relative heave and relative roll between the tunnel and pontoon have a crucial influence on the suspension line tensions.The relative heave controls the total suspension tension and the relative roll determines the standard deviation values of the tensions on four suspension lines(tension distribution).For relatively small immersion depth,the relative heave plays a leading role in the extreme suspension tensions,while as the immersion depth increases,the relative roll makes a greater contribution.This was not highlighted in the past literatures but is identified in the present study through extensive investigation.6)During the element ballasting,the freeboard elimination(D=0m,η=0.0%)is most critical scenario.The relative heave and relative roll experience significant amplitudes as a result of the green water loadings on the tunnel,which leads to a severe interation among bodies and provokes frequenct slack and snap loadings on the suspension lines.7)The negative buoyancy has both positive and negative effects on the dynamic responses of the system.The positive effect is that the increase of negative buoyancy can suppress the roll resonance effect and diminish the relative motions between the tunnel and potoons as well as the horizontal and vertical offset.The negative effect locates in the increase of the initial tensions on the suspension lines.The best balance between the stablitiy of the coupled system and the suspension line tensions is achieved with negative buoyancy η=1.3%～1.5%.8)During the immersion stage,the tensions on the suspension lines and mooring lines have their own most critical immersion depth.The medium immersion depth gets the largest suspension tensions.While,the smallest immersion depth(D/Dmax=0.04)has the greatest mooring line tension on the tunnel and the near-seabed immersion depth(D/Dmax=0.88)is most unfavorable for the mooring line tension on the pontoon.