ABSTRACT

This paper presents an experimental study on wave loads and hydro-elastic responses of a very large container ship in irregular waves. A segmented ship model with variable cross-sections backbone is newly designed to meet similarity conditions of vertical, horizontal and torsional stiffness at the same time, and stiffness can be varied lengthwise. Both head wave and oblique wave are adopted in irregular wave tests. High-frequency wave loads, which have a significant effect on the springing phenomenon, are found in both head wave and oblique wave test conditions through frequency spectrum analysis and account for the larger proportion in oblique waves. The non-linear wave-induced sag/hog ratio of vertical bending moments may also result from the high-frequency component. Bow flare slamming is most important to the 10000 TEU container ship in full load condition, while the stern and stem slamming become more significant when the draft gets smaller. Stiffness seems to have little impact on the frequency of slamming occurrence. However, the mean slamming pressure and loads caused by slamming is much more severe when the stiffness decreases.

INTRODUCTION

With the enlargement tendency of container ships in recent years, the relative stiffness of container ships has become weaker, resulting in more significant wave-induced vibration and non-linear springing/whipping phenomena, which should be paid more attention to. As stiffness is reduced, the natural frequency has become closer to the encounter frequency, resulting in linear springing. Meanwhile, nonlinear springing happens when the encounter frequency is several times as natural frequency. When slamming happens, the transient pressure on the ship hull may lead to local and global effects. According to the definition by Faltinsen (2005), the global effect is so-called whipping. In a word, springing is regarded as steady resonant motion, while whipping is a 2-node vibration caused by transient slamming pressure.

So far, many efficient experimental and numerical methods have been developed to simulate hydro-elastic phenomena. Among them, the segmented model test is considered a valid way to study wave loads and impact forces. In the aspect of testing ship models (ITTC, 2017; ITTC, 2021), both segmented and non-segmented testing models are kept developing in recent years. Segmented models with backbones were described by Maron and Kapsenberg (2014), Jiao et al. (2016), Houtani et al. (2018) described a non-segmented flexible container ship model without backbones where the vertical bending and torsional vibration modes are replicated. Jiao et al. (2017) developed a tank testing system to simulate ship motions and loads in a real sea state. As to springing and whipping responses, the previous research mainly paid attention to the vertical bending in head waves (Ding et al. 2015, etc.). In recent years, there is also an increased focus on oblique wave impact.

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