Nonlinear analysis of waves induces motions and loads in large amplitude waves

• Main Author: Mortola, Giuseppe
• Published: University of Strathclyde 2013
• Subjects: 623.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.581927

An ocean going vessel, sailing in severe seas, experiences motions and loads that are largely affected by nonlinear phenomena. These effects deviate the responses from the linear prediction, modifying their magnitudes, symmetry and frequency characteristics. The change of the actual wetted geometry due to motions and large ambient waves elevation and the occurrence of impulsive phenomena, such as bottom impact, are some of the main sources of nonlinearities. Current state-of-art in seakeeping, applied to ship design, is based on the assumption of small amplitude motions and linearity between the excitation and the response. These techniques have been proved, during the years, to be reliable for small and moderate sea states, but they are not effective in large amplitude waves. The understanding and prediction of the behaviour of the vessel in rough seas is of crucial importance for its design, and therefore there is a need for better methods and practices. Application of nonlinear seakeeping methods in a every-day design situations is limited. The complexity and the computational cost of some methodologies, together with the absence of standardised procedures, are the main causes for the reduced use of such a methodologies. The work presented in this thesis aims to develop a practical nonlinear seakeeping approach that can be used in a design content to model wave induced motions and loads in large amplitude waves. The wave-body interaction problem is solved using a time domain nonlinear two dimensional approach, that considers the actual wetted hull portion and the relative velocity between the structure and the waves. The vessel is modelled as a flexible body to allow structural dynamics. The proposed formulation takes into account impulsive phenomena due to water impact, on both the bottom and the flare of the hull, using a combination of analytical and empirical techniques. The proposed methodology is applied to the S-175 and the Wils II 13,000 TEU container ships. The validation of the proposed method, conducted in both small and large amplitude regular waves, shows the capability of the technique to correctly predict the behaviour of the vessel also when linear methodologies fail. The analysis demonstrates the importance and the reliability of hydroelastic methods for the prediction of wave induced loads, especially when whipping is relevant. A procedure, which applies the proposed methodology for the evaluation of maximum expected values of wave induced motions and loads is presented. Long term analyses are conducted, using both linear and nonlinear method, to study the e ect of nonlinearities. The comparison between linear and nonlinear approaches shows an increase of maximum load responses when nonlinear hydroelasticity is applied. This study highlights also the dependency of the results on the selection of the return period and operational velocity profile of the vessel.