Coupled and Lumped Methods in Time Domain Dynamics Analysis of Floating Structure with Multi-Bodies and Connector Beams

Abstract

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.