Floating bridge global responses to slowly varying wave loads

Abstract

The global responses of a 5530-m-long floating bridge under linear and slowly varying wave loads were studied. The bridge has modes from seconds to around 2 min. A comparative study was carried out applying full Quadratic Transfer Functions (QTFs) or Newman's approximation (Newman, 1974) for slowly varying loads. Four groups of QTFs were computed: one fixed pontoon, one pontoon with specified stiffnesses, three fixed pontoons, and three pontoons with stiffnesses for the mid-pontoon. The importance of damping to the global response was demonstrated by a sensitivity check on the viscous damping. The study showed the importance of hydrodynamic interaction in both first and second order wave loads and responses. Hydrodynamic interaction led to more ‘distributed’ QTF peak values away from the diagonal, which may reduce the correctness of Newman's approximation. Parametric study showed that the floating bridge responses were sensitive to wave direction, wave height and period. This study suggested that full QTFs based on three fixed pontoons provided the most conservative results for the base case. Newman's approximation provided reasonable levels of overall responses but underpredicted specific responses in this study. Though the full QTFs method is preferred in a detailed analysis, the efficient Newman's approximation is considered as a valuable tool for large-scale screening analysis during the early design phase of practical engineering.