The dynamic response of the Palmerah Tidal Bridge

Abstract

The objective of this master thesis is to gain knowledge regarding the dynamic behaviour of The Palmerah Tidal Bridge to hydraulic loads. The Palmerah Tidal Bridge, an enterprise of the construction company BAM, will become future's largest tidal power plant and a floating bridge between two islands in the Flores region, Indonesia. A combination of the two functions requires a dynamic design that allows movements and a design that is able to withstand severe loads. A pre-feasibility design is proposed, however the technical feasibility is not proven yet. Safety and stability are two key concepts to substantiate the technical feasibility. In addition, an estimation of the probable motions and accelerations may indicate whether traffic is able to cross the bridge safely. The aim of the project is to create insight in the rotations and accelerations of the coupled floating bridge structure. In addition, obtaining information regarding the most sensitive structural parameters of the system may suggest design changes that result in an increase in stability. First, data is acquired that is required to create a virtual bridge model. The project location and present bridge design are analysed with available data, literature and calculations. Serviceability limits are estimated that allow traffic safely across the bridge. Governing loads that act on the bridge are determined and the structural properties of the bridge are calculated. The natural frequencies and hydrostatic properties of a single freely floating floater are calculated. The hydrostatic stiffness is used to calculate a first estimation of the roll-rotation as a result of current-induced pressures. The software programs used for the data computations are Matlab, Matrixframe, MathCad and Excel. Secondly, the data is converted into a virtual bridge model in the software package Ansys Aqwa. Ansys Aqwa is globally used to indicate the dynamic response of offshore maritime structures to wave induced pressures. The software has limitations regarding the current velocity implementation. The drag force should be defined manually and acts at the centre of gravity of the structure. As a result, the force 'moves' with the structure and the application point of the drag force can be located above the waterline. The virtual bridge model seems realistic for waves and positive flow speeds. However, the response to negative flow speeds is unrealistic, indicating that the Ansys Aqwa model is inaccurate. The model can not be calibrated as knowledge about the likely motion of the bridge is unknown, but the roll-rotation during a positive current is of the same magnitude as the stability calculation. The dynamic response to governing wave and flow speed combinations is computed. With an iterative process, the sensitivity to certain parameters is found. The observations and results that are obtained during the process resulted in various alternative design proposals that may decrease the magnitude of rotation with 70%. However, even with the improved design suggestions, the estimated serviceability limits are still exceeded. The dynamic response is based upon two components, a gradual main rotation that originates by the drag force and a fluctuating line over that curve that represents wave induced motion. In the end, wave induced motion will result in sincere discomfort for traffic.