The slamming and whipping vibratory response of a hydroelastic segmented catamaran model

A 2.5m hydroelastic segmented catamaran model was developed based on the 112m INCAT wave-piercer catamaran to simulate the vibration response for the measurement of dynamic slam loads during head sea tests at the Australian Maritime College towing tank. Numerical computations and results of dynamic finite element methods were used as a basis for predicting the motions and loads acting on the hydroelastic catamaran model. A load measurement system and data acquisition system was developed to measure the dynamic wave loads and motions of the catamaran model. A two degree of freedom spring-mass theoretical model was developed to predict the first longitudinal modal frequency response of an NPL6A hydroelastic model and was applied to the development of a three degree of freedom spring-mass system used to predict the wet and dry whipping frequency response of the catamaran model. Vibration tests were performed on the catamaran model in air and still water to investigate the effects of stiffness and mass on the first longitudinal modal frequency response. The whipping frequency of the catamaran model was validated using full-scale slamming and whipping accelerations obtained on a 112m INCAT catamaran vessel during a delivery voyage to Japan. Towing tank tests were performed in regular seas to measure the dynamic slam loads acting on the centre bow and vertical bending moments acting in the demihulls of the catamaran model as a function of speed, wave frequency and wave height to establish the operational loads acting on the full-scale 112m INCAT catamaran vessel. An analysis was performed on the catamaran model slamming kinematics in regular seas to form a basis for developing numerical techniques used to predict slam occurrence and slam severity. The effects of variations of wave height, model test speed and wave encounter frequency were combined so as to express slam severity in terms of the maximum relative bow velocity. Peak slam forces measured on the bow of the model were found to approach the weight of the model this being similar to the findings of full-scale vessel trials. Slam impulses when scaled found greater slam durations at full-scale while scaling of the peak strain energy showed that the hydroelastic model provides a good basis for simulating the energy imparted by the slam into whipping. Thorough testing over a range of encounter frequencies and wave heights exposed the catamaran model to extremely severe slamming without evidence of complete bow immersion indicating that the INCAT 112 m wave piercer design is inherently seaworthy and relatively immune from deck diving in seas up to 5.4m.