Impact Failure Limit against Freak Wave and Development of the Collision-Avoidance System

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Freak waves are abnormal waves surprisingly appearing on the sea surface with exceptional wave height or abnormal shape. The devastating effect of freak waves on offshore structures as well as ships has led to many people's life losses, great economic losses, even environmental pollution from oil leakage. This research aims to understand the generation of the freak wave and study their influences. Model test of elastic ship model in towing tank is carried out. the generation mechanisms of the dispersive focusing waves and Benjamin-Feir unstable waves as well as their generation methods in laboratory wave tank are presented. Nonlinear time domain seakeeping code accounting for the nonlinearities such as bottom emergence, hull shape geometry and hydrodynamic impact, etc is adopted to simulate the vertical motion and structural responses of the elastic ship model. The experimental results of model tests in regular waves agree very well in frequency domain with the predicted ones calculate … More d by the hydroelastic code. In dispersive focusing waves, the experimental and predicted motion and structural responses are compared in time domain. The maximum vertical bending moments are predicted very well, while the encounter components are a little overestimated and the dynamic responses of ship structure due to slamming seem to be underestimated by the numerical predictions. Basically, the current hydroelastic code can be used to predict the extreme wave loads, although the improvements are necessary. Following this, the experimental results of the model tests in dispersive focusing waves as well as unstable waves are decomposed into wave encounter frequency components and high frequency components by wavelet transform technique. The influence of various parameters of freak waves on these components, the differences of behaviors the ship model exhibited in both supposed freak waves, etc are examined systematically. Finally, the safety margin of the longitudinal strength of the real ship under the current freak waves is conducted with the explicit dynamic finite element code incorporating the plastic property and strain rate effects of steel. The results show that the structural integrity can be assured under the attack of 28m freak wave in terms of longitudinal strength. The evaluation flowchart is suggested based on the beam element model so as to provide a fast check of longitudinal strength of ship hull in freak waves during the preliminary design. Moreover, the influence of the slamming duration is also investigated. It is found that the ship hull could carry more loads in dynamic sense than it could in static sense. In the end, from the viewpoint of energy, an evaluation approach in accidental limit state based on energy absorption capacity is discussed and proposed. Less