Dynamic Response of a Submerged Floating Tunnel Subject to Hydraulic Loading

Numerical Modelling for Engineering Design

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The submerged floating tunnel (SFT), also called an Archimedes Bridge, is a new type of infrastructure for wide and deep sea-crossings, regarded as one of the alternatives to bored and immersed tunnels and bridges. It is afloat in water employing its buoyancy and a support system to balance its self-weight. However, no prototype SFT has yet been built anywhere due to the immaturity of scientific research and engineering technology. The dynamic response of the SFT subject to operating and extreme environmental conditions, which determines structural safety and reliability, is a crucial issue that needs to be better understood. In order to better comprehend the response of the SFT to hydrodynamic forces, key points including hydrodynamic loads acting on the SFT and the structural dynamic response for various structural configurations, as well as the relations and interactions among these factors must be quantified.

In this study, hydrodynamic loads on various SFT cross-sectional geometries were computed. The parametric cross-section shape described by a Bezier-PARSEC curve was optimized using a hybrid Artificial Neural Network (ANN) and Genetic Algorithm (GA). The practical range of aspect ratios of the SFT cross-section was determined by conducting a sensitivity analysis under tidal current conditions. It was found that an SFT cross-section with an aspect ratio of 0.47 using a leading-edge BP curve under the given clearance is optimal for a balanced consideration of hydrodynamic performance and construction cost. Furthermore, the machine learning method used is shown to be a reliable and effective tool for the SFT cross section optimization design.

The hydrodynamic loads acting on the optimal cross-section shape were compared with simpler shapes under various environmental conditions of currents and waves, including the extreme environmental conditions of internal waves, tsunamis, and super typhoons. An internal solitary wave (ISW), described by the modified Korteweg de Vries (mKdV) theory, was adopted for the hydrodynamic loading analysis of the SFT based on field observations and high-resolution satellite images. It was found that the ISW can remarkably alter the buoyancy - weight ratio (BWR) of the SFT and hence, cause a large vertical hydrodynamic load on the SFT, threatening the safety and reliability of the SFT system. A worst-case tsunami and a hindcast typhoon in the Qiongzhou Strait were selected for extreme event hydrodynamic forcing analysis. It was found that extreme event hazards in the Qiongzhou Strait are rare due to the sheltering effect of Hainan Island. In terms of hydrodynamic forcing, the selected typhoon scenario is more devastating than the tsunami case for an SFT. The proposed parametric cross-sectional shape for the SFT shows better hydrodynamic performance than simpler shapes under all applied environmental conditions and is therefore recommended for the engineering design.

After investigating different types of hydrodynamic loads acting on the SFT, the global dynamic response (including vibration) of the SFT was assessed. A numerical model of a prototype super-long coupled tube-mooring-joint SFT system based on Finite Element Method (FEM) was developed to better predict flow-induced vibration (FIV) and structural dynamic response. A pragmatic approach for structural dynamic response computation under realistic oceanic conditions was developed considering the spatial randomness of hydrodynamic loads. Multi-scale hydrodynamic models including a large-scale oceanographic model and a small-scale CFD model were developed for determination of hydrodynamic loads. It was found that the SFT tube is unlikely to experience severe resonance under steady current conditions, but the vibration of the SFT tube is dominated by wave conditions, where a single dominant mode excitation of the tube with a large wave height and period cause large amplitude motion. In order to give insight into structural dynamic response under extreme environmental conditions, internal forcing on the SFT and structural response of the SFT were computed subject to the ISW and super typhoon loads. This showed that the displacement and acceleration of the SFT under the ISW are far smaller than the structural serviceability requirements, and resonance of the tunnel tube becomes unlikely under the ISW condition due to its rather low intrinsic frequency. The dynamic response of the SFT subject to the typhoon scenario is much more severe than that of the ISW case, and the horizontal stiffness of the moored tube greatly affects its dynamic response. The maximum bending moment and torque on the SFT occur at its shore connections, where failure risk due to structural fatigue or buckling are substantial.

The final aspect of this thesis aims to optimize the SFT structural configuration for minimization of hydraulic resonant loading. The core concept is to investigate the sensitivity of structural response to the structural fundamental frequencies outside the hydrodynamic frequency. It was found that natural frequencies of the SFT system are mainly affected by BWR, tunnel tube length, mooring configuration and stiffness, and joint and shore connection properties. A dynamic process for the SFT configuration optimization subject to different hydrodynamic loads can be established by smartly tuning the fundamental frequencies to mitigate structural dynamic response.