Experimental And Numerical Investigation On The Hydrodynamic Response Characteristics Of The Immersed Tunnel Element Suspended From Twin-barge In Waves

During construction,the safe and controlled lowering of tunnel elements is one of the processes,which is most difficult to implement,and as a result requires a high level of technology to reduce risk of damage.However,during the installation processes complexed ocean environmental conditions(wind,wave and current)could result into excessive dynamic responses of tunnel elements that could jeopardize the safety and accuracy of installation procedures used during positioning and lowering procedures of the tunnel element.To study the hydrodynamic characteristics of the tunnel-barge system and the suspension cable tensions during immersion process,which provides a potential benefit to control the steady immersion installation and safety lowering during subsea tunnel construction.Considerably less study has been directed at installation procedures and the hydrodynamic characteristics of the immersed tunnel element and its deployment barge still need to be investigated further.It is therefore prudent that the detailed experimental and numerical analysis of the moored tunnel and twin-barge system are carried out in this thesis.To investigate the coupled motion characteristics of the tunnel element,experimental measurements of a moored tunnel element suspended from a twin-barge were conducted in a wave flume at a geometric scale of 1:50.Using the '6-D Measurement System'(6D-UMS)to obtain six degree of motions for both the tunnel and twin-barge.Response characteristics are obtained for multiple regular wave conditions and three different immersion depths.The experimental results indicate that the sway and heave motion amplitudes of the moored tunnel element suspended by a twin-barge are clearly reduced and the tunnel motions increase in the roll direction compared with the condition of platform submergence.The motion response of the tunnel element suspended by the twin-barge decreases with the increase of B/L and immersion depth,and increases with increasing wave height.The mooring lines play a role in reducing the motions of the tunnel and the twin-barge,particularly when subjected to large amplitude,long period waves.Experimental tests were conducted to investigate the coupled motion characteristics of a moored tunnel element suspended from a twin-barge under irregular wave actions.The motion response of the tunnel element and mooring behavior were discussed,the spectral analysis of the dynamic response of both tunnel element and twin-barge were used to study the effect of different significant wave heights,wave peak periods and immersion depths on tunnel motions.The motion characteristics of the tunnel element suspended from twin-barge were compared with that suspended from fixed platform,which to investigate the effect of twin-barge's movement on the tunnel motions.Multiple linear wave conditions and three immersion depths were tested.The results indicate that the motion response of the tunnel element decreases with increasing immersion depth,and the natural periods of the tunnel,barge and combined tunnel-barge system play key roles in the influence of wave conditions on the motions of the tunnel.It was found that the low-frequency motion of the tunnel element is large in small wave periods.The mooring system under such conditions needs to be considered carefully during system design in order to safely control the motions of the tunnel-barge system in energetic ocean environments.In this thesis,the corresponding numerical model of tunnel element suspended from a twin-barge as well as that from a fixed platform were developed to simulate the dynamic response of the tunnel-barge system in realistic sea conditions.The time-domain integral equation is established based on the Green function,the motion equations of the tunnel element are proposed by using Newton second's law.Using a mesh method and Morison equation for the quasi-static and dynamic analysis of the mooring system.The results are arranged to inform about the motion characteristics for tunnel-barge system in decay tests,regular waves and irregular waves.The results of irregular wave tests agree well with the numerical simulations of the tunnel-barge system in realistic sea states.Based on the numerical model,considering a number of scenarios which represent a range of energetic ocean conditions were used to simulate the dynamic response of the tunnel element during immersion stage.Three typical sea states were chosen to simulate the motion response characteristics of the tunnel element in realistic sea environments,the mechanical characteristics and spectral analysis for the tunnel-barge system subjected to irregular sea states were discussed.The simulation results based on the numerical model further show the agreement with the experimental results.