Floating structures with multi-bodies and connector beams are tried in ocean engineering fields. Floating solar plant system is an example of those structures. The first step to analyze the system is to calculate wave forces, added mass and hydrodynamic damping in frequency domain. BEM (Boundary Element method) or HOBEM (Higher Order Boundary Element Method) could be applied for the frequency domain analysis. In the next step, time domain conversion is done from the frequency domain results. In general, convolution method and FEM (Finite Element Method) are applied to floating bodies and connectors, respectively, for time domain dynamic multi-body analysis. The body equation takes the fully coupled matrices in added mass and hydrodynamic damping. Such coupled method needs many computing times in dynamic analysis of floating structure if it has many floating bodies. Simplified lumped method were applied by some previous studies to calculate multi-bodies quickly. In the lumped method, only diagonal terms of added mass and damping are considered and they are attached to connector equation. However, the agreement between those two methods was not rigorously investigated yet in case of many floating bodies with connectors. This study compares the coupled and lumped methods in time domain dynamic analysis of multiple floating bodies with connectors. An example structure with 25 floating bodies connected with steel beams was analyzed. Regular and irregular waves in various heading directions were considered in the numerical example. Floating body motions and connector beam stresses were calculated by coupled and lumped methods and their results were compared.

INTRODUCTION

Multiple floating bodies connected with beams are frequently studied recently. Floating photovoltaic system with multi-bodies and beam connectors is an example of those multi-body type structures. Coupled equations for floating bodies and connectors need to be solved to analyze the wave-induced responses of the multi-body system. It may need many computing times because added mass and hydrodynamic damping matrices are very large if the system has many floating bodies. A simple method to consider only diagonal terms of added mass or damping was tried in some studies (Kim et al, 2018; Hong et al, 2018). However, the coupled method and lumped method are not compared rigorously for the multi-body cases. The objective of this study is to compare coupled and lumped methods in hydrodynamic analysis of multiple floating bodies with beam connectors in waves. Twenty five floating bodies connected with steel H-beams were analyzed in this study. Floating body motions and connector stresses are calculated for regular and irregular waves in various heading angles. And, the results were compared for the coupled and lumped methods to investigate agreements or differences of the methods.

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