Time Simulations of Ocean-Going Structures in Extreme Waves

A summary is presented on calculation procedures to determine extreme responses of ocean-going structures using nonlinear time domain simulations. As such nonlinear simulations are time consuming full scatter diagram assessments are not feasible. Advanced techniques are therefore required to assist in the selection of response critical sea states. Of particular importance is to what extend a few or even a single design sea state can be used to determine a lifetime design value. An overview of methods, which can be used to determine design sea states, is given and discussed. Secondly, if a design sea state is specified different approaches are possible to assess a response in that sea state. Regular, random irregular and conditioned irregular waves can be modeled, which affect both the accuracy and the computation time. Alternative extreme wave conditions are considered, in perspective to existing techniques, and the application of freak waves for response simulation is discussed. An example simulation is shown of a cruise-ship in a freak wave. In this example different phases are identified and the associated physical phenomena are discussed. Based on this study and the calculations, requirements for future research are formulated in order to use extreme/freak waves in response analyses and secondly to consider the use of freak waves in design procedures.