Tidal Bridge Dynamics

Optimising the dynamic response due to wind, waves and current

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Currently, there is no infrastructure between the two Indonesian islands Flores and Adonara. The islands are separated by the Larantuka Strait, which has a width varying between 600 and 1000 meters. The local government would like a bridge between the two islands. However, as the water in the 18 meter deep strait is heavily subjected to tidal forces - creating a tidal amplitude of about 1.5 m and tidal flow velocities ranging up to 4.5 m/s - structural design is challenging. As traditional bridges were deemed too expensive, a new type of bridge was introduced: the `Tidal Bridge'. The pendulum founded floating bridge, which is proposed to span the deepest 400 meters of the cross-section, is designed with tidal turbines attached to the bottom of the structure. The energy production mitigates the financial burden that the 225 million US dollar Palmerah Tidal Bridge will bring. At the time of writing, a pre-feasibility study has been performed by Antea, proposing initial structural dimensions. BAM took over the design process, which lead to questions regarding the dynamic stability of the design. The objective of this report is to answer the following two research questions: 1. How can the dynamic response due to two-dimensional forcing of a Tidal Bridge be determined? 2. What design choices can further optimise the dynamic behaviour of a Tidal Bridge? A numeric tool has been created to predict the dynamics of the proposed design as function of an input of wind, waves, and current. Based on (experimental) literature, hydrodynamic coefficients determining the fluid-structure interaction were determined. The complex shape of the floaters did not allow appropriate validation of the accompanying added mass and radiation damping coefficients. Comparison with a model constructed in Ansys Aqwa showed values of similar magnitude, but precise magnitudes could not be determined. In order to find these important coefficients, a set of experiments has been performed to determine the added mass (moment of inertia) and radiation damping (moment of inertia) for heave and roll motion. The experiments showed that the added mass equations for heave were well defined, while the added mass moment of inertia equations for roll motion deviated up to 250 per cent. The acquired data was used to find better relations between the added mass (moment of inertia) and the floater dimensions. Please note, the empirical relations are based on limited data and with little mathematical background. Hence, the relations should be used with care. The radiation damping coefficients that were also extracted from the experiments showed no clear trend, but did confirm that the hand-calculations were of correct magnitude. Using the constructed model, forcing characteristics of the Tidal Bridges are investigated. In these computations, the Palmerah Tidal Bridge dimensions are used as a case study. It was noted that the extreme non-linearity of different elements of the problem (changing pendulum angle, hydrodynamic pressure field, and particle velocity/acceleration field) do not allow for linear approximation. While a linear mass-spring system predicts that the natural period is about 6.7 seconds, the maximum dynamic amplitude is found for wave periods of 9 seconds. This coincides with the largest expected waves for the Palmerah Tidal Bridge location. Reducing the natural period of the design is recommended. Additionally, research was done into the contribution of the various types of forcing, where it was found that traffic weight has negligible effect on the dynamics. Wind forces add only a few percent to the pendulum forces, but do have a significant contribution to the displacements. Furthermore, research into the impact of an approaching wave field showed that accelerations during first impact may overshoot the maximum steady state acceleration by more than 200 per cent. A parametric study on the dynamics of Tidal Bridges was performed, which did research into the the forcing combinations that lead to most amplification of the dynamics. It shows that different loading combinations are governing for accelerations in the three different degrees of freedom present in a two-dimensional system. In here, difference was found for the dynamic behaviour induced by waves from the two different wave directions, leading to two sets of three forcing combinations. This data was used to investigate the effect of three design parameters: the angle of the pendulum, the hinge location of the pendulum and the depth of the strait. Moreover, a sensitivity study is performed on a set of parameters defining the Tidal Bridge. It shows that the mass of the segment, the added mass, the floater length, the design turbine force, and the pendulum angle are most important if it comes to design optimisation. Based on the parametric study and the forcing characteristics, conclusions and recommendations are made for improvements of Tidal Bridge designs in general and more specifically: the Palmerah Tidal Bridge.