Analysis of tether anchored floating suspension bridge subjected to large ship collisions

Abstract

The Norwegian public road administration wants to replace eight ferry crossings on the main road in western Norway with fixed crossings. This requires novel technologies and new solutions as several of them are too extreme for today’s technology. One challenge the bridges face is ship impacts. The concept studied in this paper is a tether anchored floating suspension bridge. It has a length of 4000m and water depths of over 500m. In addition to the fixed shore supports, two floaters support the three main spans, each with a length of 1385m. A shipping lane goes between the two floaters, making them susceptible to impacts. To remedy this, it was proposed to install a floating structure around each floater, connected to the floater by tethers. In a collision, the barrier dissipates energy through increased submersion, drag forces and hydrodynamic added mass as it moves. This will reduce the amount of energy absorbed as elastic and plastic strain energy and kinetic energy in the bridge. A review of the collision risk studies show that the largest risk is associated with a container vessel of 17000tons. The paper studies the global response of the bridge subjected to large and extreme container vessel collisions both with and without the barrier. The analyses are time domain dynamic simulations in the computer program USFOS. All structural elements in the model are beam elements. The ship beam accounts for both pitch and translational inertia, including the effect of hydrodynamic added mass. The ship mass is given an initial velocity corresponding to the collision speed. A series of springs connect the ship to the bridge. One of the springs represents the non-linear force deformation characteristic of the vessel bow, calculated by finite element modelling and simulation with LS-DYNA software. These simulations included both impact against a rigid wall, representative of collisions against unprotected pylons, and impact against a rigid toroidal surface, representative of a collision against the barrier. Response quantities included in the paper are lateral displacements of the floaters, accelerations at the bridge girder and towers, and forces in main cables and tethers.