Abstract

     Fjord-crossing floating bridges are sophisticated structures subjected to complex environmental loadings including combined action of wave and current. However, the effect of wave-current action is often neglected in a conventional engineering practice. This paper presents a numerical study of the dynamic response of a floating bridge under the combined action of waves and current. The effect of wave-current interaction on the hydrodynamics associated with the bridge pontoons are first evaluated by using a three-dimensional potential flow solver. A model of the entire floating bridge is then established and analysed in the time domain. The accuracy of the model is verified by comparison with available experimental data for a 1 km long curved floating bridge. Parametric studies are subsequently carried out to investigate the effect of wave-current interaction on a 4.6 km long floating bridge model for crossing the Bjørnafjord. Results show that the wave-current interaction has a significant effect on a fjord-crossing floating bridge studied in this paper. Neglection of such an interaction could lead to substantial overestimation or underestimation of the structural responses depending on the environmental headings.

Introduction

     The Norwegian Public Roads Administration (NPRA) launched a ferry-free coastal highway project along the coastline of Norway. This project aims to reduce the total travel time from Kristiansand in the South to Trondheim in the North (see Fig. 1) by approximately one half using floating bridge technology to connect roads across fjords. Some of the fjords have long crossing spans up to about 5 km. This brings challenges to the design and construction of floating bridge structures. Various design options were proposed and studies were carried out to investigate their performance [1], [2], [3], [4], [5].

Conclusions

     This paper is concerned with a numerical study of the global structural responses of a 5 km long floating bridge under the combined action of waves and current considering the wave-current interaction. The accuracy of the proposed computational model is validated by comparison with available experimental data and numerical results for a curved floating bridge which is described in the literature. Next, a straight and side-anchored floating bridge model based on the Phase 3 design concept for the Bjørnafjord crossing is put forward to examine the stochastic structural responses. For the purpose of comparison, the bridge responses under different cases considering various combinations of wave and current, their interaction effects and two different return periods are investigated. Analysis results show that when waves travel with current in the same direction, the wave-current interaction effect can significantly amplify the bridge responses. The level of amplification increases with the speed of the current. Under 1-year load cases, the amplifications in the standard deviations, which reflect the dynamic components of the results, in the weak axis bending My, strong axis bending Mz and axial force Fx are found to be up to 52%, 36% and 30%, respectively. Under 100-year load cases, such amplifications in My and Fx are up to 122% and 67%, which are more than doubled as compared to those under 1-year load cases, while Mz experiences a 50% increase generally. However, when the interaction effect is ignored, the common design practice of superposing wave and current loads may lead to slightly lower bridge responses than the case neglecting current due to the viscous drag effect. Accordingly, a significant underestimation could arise if the wave-current interaction effect is ignored.