Wave slam response of large high-speed catamarans

The rapid evolution of high-speed sea transportation has led to the development of large, fast, lightweight vessels. The structural design optimisation of such vessels requires knowledge of the effect of sea loads on their structure. Of particular importance for high-speed catamarans are severe wet-deck slam events, which can impart a large global load onto a vessel's structure and as a consequence cause significant structural damage. The dynamic whipping response of the structure may also be important by making a significant contribution to fatigue damage. Extensive full-scale hull stress, motion and wave measurements were conducted on two Incat high-speed catamaran ferries. A definition of a slam event, for these vessels, was proposed and used to identify slam events from the data records. The character and effects of these slamming events are investigated with respect to a number of factors. Slam events were found to produce bending moments up to 700% of the largest underlying global wave loads. The data from the full-scale slam events, including an extreme slam event that caused extensive structural damage, was used in conjunction with finite element modelling to develop a realistic quasi-static slam loading scenario for structural design purposes. This slam load case gives a maximum bending moment approximately 16% greater than that stipulated by classification society Det Norske Veritas, with a greater bias towards the bow of the vessel. A method for scaling the design load case for use with new designs is also proposed. A technique for predicting the mode shape and frequency of the whipping behaviour utilising finite element analysis including the fluid-structure interaction is presented. The hydrodynamic added mass of the surrounding fluid was calculated using a two-dimensional panel method for a range of speeds. The calculated whipping modes are then compared favourably with those found through the full-scale measurements and exciter experiments. The exciter experiments were conducted on two vessels with the anchors being dropped and instantaneously arrested to excite the vessels' first longitudinal mode of vibration. The level of damping of the whipping behaviour is investigated through the full-scale results and exciter experiments. Methods for evaluating the various hydrodynamic components which constitute the damping are also presented including wavemaking damping, viscous damping and acoustic damping. It is concluded that structural damping is the dominant damping component. Fatigue life estimates, utilising full-scale slam data and derived whipping behaviour information, were conducted which found that slamming and whipping behaviour have a large influence on fatigue life. Finally, knowledge of the dynamic slamming response was utilised to develop a dynamic extreme slam design load case. This dynamic load case more realistically simulates the dynamic structural response of the vessel to a slam. It is the core component of a new practical methodology for the structural design of large high-speed catamarans for slamming