Session: 06-16-01 Wave Mechanics, Modeling and Wave Effects

Submission Number: 156328

On the Generation of Nonlinear Design Irregular Waves in a Wave Basin 

In this study, temporally evolving nonlinear design irregular waves (DIWs), also known as "response conditioned waves", were generated in a wave basin. DIWs are meant to represent the most probable wave episode that could excite a ship response with a given extreme value. Several approaches have been devised to determine DIWs using surrogate models of ship responses in waves, such as frequency response functions, sometimes in conjunction with nonlinear corrections, or weakly nonlinear seakeeping codes. Such DIWs are useful for efficiently evaluating the extreme values of ship responses at specific probability levels, by examining the ship responses in the DIWs using high-fidelity numerical models or model tests. For quantitative evaluation of the extreme ship responses in model tests, it is crucial to reproduce the DIWs in the tank with high accuracy. Some studies have reported that DIWs obtained within the linear wave framework cannot be accurately reproduced in a wave tank, thereby the wave maker signals must be corrected for accurate reproduction. The difficulty in accurately reproducing DIWs in wave tanks can be attributed to the fact that the nonlinear wave evolution, which naturally occurs in physical tank tests, is neglected in the DIW estimation and wave maker signal calculation stages. Therefore, this study aims to experimentally demonstrate that DIWs can be accurately reproduced in a wave basin by considering the nonlinear wave evolution. To this end, this study employs the "HOSM+FORM" approach, which combines the higher-order spectral method (HOSM) and the first-order reliability method (FORM) to obtain DIWs. The HOSM is used to simulate the nonlinear evolution of waves, while the FORM is used as a detector for a DIW associated with a given extreme ship response value. This study targets the vertical bending moment of a container ship. An experiment was conducted in a wave basin to generate the DIWs obtained by the HOSM+FORM. The experimental results demonstrated that the DIWs can be accurately reproduced in the wave basin without the necessity of correcting the wave maker signals, thanks to the incorporation of the nonlinear wave evolution effect in the DIW estimation and wave maker signal calculation phases.

Presenting Author: Hidetaka Houtani The University of Tokyo

Presenting Author Biography: Hidetaka Houtani is a Lecturer at the Department of Systems Innovation in the School of Engineering at the University of Tokyo, Japan. Prior to his appointment at the University of Tokyo in 2019, he was a researcher at the National Maritime Research Institute, Japan, from 2012 to 2019. His research interests include ocean waves (especially freak waves), ship seakeeping, and ship hydroelasticity. He received his Ph.D. in Environmental Studies from the University of Tokyo in 2015.