ABSTRACT

A coupling hydrodynamic model of SWATH and reef topography was established based on the conventional potential flow theory and the incident wave force, diffraction force, radiation hydrodynamic coefficients motions of SWATH were calculated. The model was compared with the same SWATH in the uniform depth of water without the effect of topography. Furthermore a flexible mooring system with three mooring lines is also designed to further strengthen the stability of the SWATH followed by the time domain analysis to assess the hydrodynamic forces motion responses and mooring system performance in various environmental conditions including waves and current. The method provides a useful and efficient tool for the evaluation of the performances of SWATH coupled with reef topography and waves and current.

INTRODUCTION

Exploitation and utilization of marine resources including islands and reefs has become an important trend of economic and technological development. The surrounding islands and reefs are surrounded by reef plates with a width of hundreds of meters to thousands of meters and shallow water depth. These reef plates not only have abundant fishery and tourism resources, but also play an important role in protecting the island from waves and preventing seawater from eroding the island itself. The application of floating structures as the front station of the development near the islands and reefs can provide a foundation for the construction and resource development of islands and reefs without affecting the ecological environment. The Small Water-Plane Area Twin Hull(SWATH) is a new and high-tech ship design scheme for seakeeping. Most of the drainage volume of the ship is placed below the waterline. On the premise of meeting the requirements of hydrodynamics and structure, the waterline area is reduced as much as possible, and the wave force and motion response of the ship are reduced. The design concept of catamaran can provide a wide deck area for ships. Scientific exploration in the sea area near the island reef is characterized by rapid deployment and flexible use. Traditionally, the method of finite uniform water depth or infinite water depth is usually used to calculate the motion response of floating bodies. For deep sea areas or sea areas with little topographic change, the calculation results can already meet the needs of engineering applications. However, with the demand of island reef construction and development, the topography changes in the horizontal scale of catamaran along the waterline in the area near the island reef, which can no longer be expressed by uniform water depth. The change of topography affects the wave propagation, thus affecting the response of the floating body under waves. Therefore, it is necessary to consider the influence of terrain change on hydrodynamic performance when studying the hydrodynamic response of floating bodies near islands and reefs. However, the existing research results show that there is little research on this aspect, and it is difficult to consider the terrain change. Scholars have carried out some research on hydrodynamic performance of SWATH and hydrodynamic interference between shallow water topography and floating body. Lee et al. (1977) conducted three types of experiments which includes calm-water towing, forced oscillation and towing to determine the hydrodynamic coefficients. They found that the trend of the hydrodynamic coefficient with respect to frequency and speed. Djatmiko (1987) presented experimental data of SWATH motions, and compared the experimental data with the theoretical predictions based on existing two-dimensional strip theory and three-dimensional sink-source technique. Hart and Kiesow (1988) Conducted vertical plane oscillation experiments on several SWATH designs in development by the U.S. Navy. The results provided useful information in the development and validation of SWATH seakeeping prediction. Hart (1989) provides experimental results defining the stability boundary for the roll motion of a SWATH drifting in beam seas using a nonlinear math model of the coupled heave roll response. Buchner (2006) studied two body diffraction analysis and found that wave height increased on slope as the total wave energy remains constant while progressing into shallow water. Ferreira and Newman (2009) compared the diffraction effects of different sloping bottom configurations. They found that differences in depth at the bow and stern do have a substantial effect on the cross-coupling coefficients, especially the abrupt change of the side and rear of the slope will produce refraction and reflection effects. Eloot and Vantorre (2011) studied ship bottom hydrodynamic interaction in shallow water through experimental fluid dynamics. Vernengo and Bruzzone (2016) presented the hydrodynamic analysis of a SWATH suitable for various applications such as small and medium size passenger ferries which aims to perform a preliminary assessment of the hydrodynamic performance of a hull with such a complex shape in terms of seakeeping capability in regular head waves. Feng et al. (2016, 2017) adopted a continuous rankine source method to investigate water wave problems in an environment with a flat or sloping rigid seabed. The influences of uneven seabeds on the hydrodynamic characteristics associated with wave–body interaction problems are discussed. The influence of these topographies on the responses in all three degrees of freedom (heave, sway and roll) of a rigid floating body are investigated and discussed accounting for wave radiation and diffraction problems. Wang et al. (2018) investigated bottom topography influence on moored very large floating structures in coastal regions effects by a series of sensitivity studies. Bekhit and Popescu (2021) investigated a ship sailing in shallow water which affected by the interaction between the hull and the seabed in different forms. The result shows that it may result in the increase of sinkage and trim. Zare et al. (2022) employed Star-CCM+ to study the hydrodynamic performance of an advanced semi-SWATH model. The vertical motion responses were estimated at different wave encounter frequencies.

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