Abstract No.C171127-068
Author name(s): Zhen LIU1,2, Zhaodan XIA1, Sheming FAN1, Yuefeng WEI1, Peiyuan FENG1
Company: 1. Marine Design and Research Institute of China, China; 2. Shanghai Jiao Tong University, China
A ship is generally supposed to be far from large amplitude rolling motions when the ship is moving into head waves without direct transverse excitation. In reality, however, large amplitude rolling motions of ships arise frequently even for ships sailing in head waves due to nonlinear dynamic effects, such as parametric rolling. To date, most of the studies of parametric rolling focus on ships under the condition of principal parametric roll in regular head waves, i.e. the natural roll period is equal to or around twice the wave period of encounter. Little literature studies the likelihood of the large amplitude rolling of a ship under the condition of fundamental parametric roll in regular head waves, i.e. the natural roll period is equal to or around the wave period of encounter. In order to investigate the large amplitude rolling motion of a vessel in head waves especially under the condition of fundamental parametric roll, deterministic wave sequences were generated experimentally in the towing tank at Marine Design and Research Institute of China (MARIC) to carry out model tests of the vessel at zero speed. Regular waves, bi-chromatic waves and tri-chromatic waves with different combinations of wave periods, including fundamental and principal parametric roll conditions, are considered in the experiments. It is found that the regular wave train, whose period is equal to the natural roll period of the vessel, leads to striking rolling motions of the vessel. Also, the tri-chromatic wave train, whose prime period is equal to the natural roll period of the vessel and complemented by two primary waves of slightly greater and smaller periods of reduced wave amplitude, is found to trigger more pronounced large amplitude rolling motions compared to that caused by other types of tri-chromatic wave groups. It is indicated that the large amplitude rolling motions of the vessel should be attributed to the general resonance which was induced by some transverse excitations as the model ship could not accurately retain head wave condition during the tests. The practice in this study is supposed to be helpful to assess the safety of ships in actual sea states.
KEY WORDS: large amplitude rolling; parametric resonance; regular wave; wave group
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