Dynamic Response Research Of Sea-crossing Deep-water Bridge Cofferdam Under Three-dimensional Wave Action

As the constructions of ocean engineering and offshore engineering develop, more and more sea-crossing bridges are built or are going to be built in the national "Middle-term and Long-term Railway Network plan" and "Expressway Network Planning". Recently, most of the sea-crossing bridges built belong to coastal bridges, and they are built in the water with a depth between twenty and fourty meters, such as Hangzhou Bay sea-crossing Bridge, Hong Kong-Zhuhai-Macau sea-crossing Bridge. However, the water depth of the strait passages in plan, such as Bohai Bay Passage, Qiongzhou Channel Passage and Taiwan Strait Passage, is perhaps more than sixty meters. Construction of sea-crossing bridges in deep-water faces great challenges, especially in the construction technology of deep-water sea-crossing bridge foundation.Since the sea environment of sea-crossing deep-water bridge are relative serious and complicated, the bridge foundation engineering, the design and the constructions of bridge cofferdam will be confronted with great chanllenges. Wave forces acting on the large cofferdam and the dynamic responses induced by them, have become an important issue urgent to be studied for the design and construction of the sea-crossing bridge. The design values of wave forces may be the factor that decides how to design well the bridge cofferdam and how to construct it safely. The security and reliability of the design or construction of the cofferdam can be guaranteed if the wave forces acting on it can be evaluated correctly. Moreover, the study into the interaction between the cofferdam and wave as well as into the dynamic response induced by the wave loads is of great engineering signifinance for the floating transportation and precision of the cofferdam positioning.The thesis focuses on the difficulty of the construction and positioning of cofferdam for sea-crossing deep-water bridge under wave effect and aims to study the wave forces, dynamic response of the cofferdam induced by wave effect. Firstly, to trace the time-dependent and domain-varying wave free surface correctly, the Multilayer Sigma Coordinate Transformation method is used to transform the actual irregular physical domain to the irregular cube domain. Secondly, the Immersed Boundary Method is adopted to deal with the irregular structure surface. Thus, the three-dimensional mathematical model which is used to simulate wave-structure interaction is established. Finally, the transient wave forces acting on the cofferdam for the sea-crossing deep-water bridge are obtained by applying the established three-dimensional mathematical model. On this basis, the dynamic response problem of anchoring cofferdam for sea-crossing deep-water bridge under wave effect is studied in combination with finite element method. How the wave parameters and the cable parameters will affect the vibration of the anchoring bridge cofferdam is studied in detail. And also it can provide necessary theoretical basis and technical supporting for the positioning of the anchoring bridge cofferdam. The main contents studied in the thesis are listed as follow:1. The mathematical model is established to simulate wave-structure interaction. To trace the time-dependent and domain-varying three-dimensional free surface, the Multilayer Sigma Coordinate Transformation method is used to transform the actual irregular physical domain to the irregular cube domain, so that it becomes easier to imposite the boundary conditions during numerical solution. How to treat irregular structure surface is another key point for the three-dimensional mathematical model to simulate wave interaction with structures. To deal with the irregular structure surface accurately, the Immersed Boundary Method is added in the model. To achieve the numerical implementation of the model above, the governing equations are splitted into three sub steps using the splitting operator method, the advection step, the diffusion step and the virtual boundary force solving step. All the three steps are discretized and solved with the use of different finite difference schemes.2. The verification for the established mathematical model is then performed. In this verification, the interactions between wave and circular cylinders with different submergences are simulated by the established established mathematical model. Then it is verified from both displacement computation of free surface and the wave forces computation. By comparing the present numerical simulation results to the analytical solutions and experimental results as well as other numerical simulation’s result, it shows that the established mathematical model can effectively capture the wave run-up around the structure and accurately calculate wave loads acting on the structure as well.3. The application research of the established mathematical model is done next. The effects of some parameters, such as incident wave angles, structure draught and incident wave periods, on the wave diffraction pattern and wave loads what the structure suffered are analyzed. Firstly, wave of different incident angles interaction with a dumbbell-shaped cofferdam is simulated. The result shows that with the increasing of incident angle, the longitudinal wave force on the structure increases, while the transverse wave force first increases and then decreases with the increase of angles. Secondly, wave interaction with a round-ended cofferdam of different draught is simulated. The result indicates that as structure draught increases, the longitudinal wave force acting on the cofferdam grows as the projected area increases, while the vertical wave force acting on the bottom of the cofferdam decreases as the wave particle velocity decreases. Finally, the interaction between circular caisson and three-dimensional wave under different incident periods is simulated. It shows that as incident wave periods decrease, the longitudinal wave forces and moments acting on the cofferdam increase firstly and then decrease, while the vertical wave forces on its bottom decrease.4. To overcome the challenges of the construction and positioning of the cofferdam in sea-crossing deep-water bridge under wave effect, the dynamic response research of the cofferdam induced by three-dimensional wave effect is performed, taking the cofferdam which is intended to be used for the Z03# pier of Guyumen rail-cum-bridge on Pingtan Stait Bridge as an object. In this research, the suffered transient wave force of the cofferdam for the sea-crossing deep-water bridge is obtained by applying the established three-dimensional mathematical model. On this basis, the vibration response and time-varying cable force of anchoring cofferdam for sea-crossing deep-water bridge under wave effect is studied with the use of finite element method. The effects of wave parameters (wave periods, wave height) and cable parameters (cable pretension, cross-sectional area of cable, the cable length) on the dynamic response of the anchoring cofferdam are analyzed. The result shows that as incident wave period decreases, the dynamic response of the anchoring cofferdam first increases and then decreases. While wave height increases, the dynamic response of the anchoring cofferdam increases. With the increasing of cable pretention and cable cross-sectional area, or decreasing suitably the cable length of the anchoring cofferdam can reduce its dynamic response effectively.Finally, the summary of all the research work done in the thesis is given, and prospect of the further research problems are indicated.