Transient response of a floating bridge due to sudden failure of a mooring line under seabed earthquakes

In this study, we investigate transient responses of a floating bridge due to the sudden failure of a mooring line under seismic excitations. A straight 4,600-m-long discrete-pontoon floating bridge designed for Bjornafjord on West Coast of Norway and moored in a water depth of 300 m is employed. A mid-fidelity time-domain-numericalsimulation coupled dynamics model was built by using a commercial software OrcaFlex. Pre-generated site-specific multi-directional ground motion time-series were imposed at all anchors of mooring lines assuming simultaneous movement. Subsequent dynamic responses of pontoons, structural responses of girders, and fairlead mooring tensions are examined for three different scenarios (i) intact, (ii) 1-line pre-failure (iii) 1-line sudden-failure cases. From the numerical results and pertinent analyses, it has been observed that transient responses of the pontoons and girders in the sudden-failure case can be significant and the resulting dynamic quantities become larger than those of intact and pre-failure cases. In the case of a pre-failed line, the transient effects are absent, and it will lead to underestimation in girder's dynamic bending moments and maximum mooring tensions. The effects of the transient motions become appreciable especially for girder's transverse bending moments and adjacent mooring dynamic tensions. The maximum mooring tension is observed in the sudden-failure scenario at the neighboring lines of the failed line as a result of the transient effects.