Coupled versus Decoupled Analysis for Floating Body and Mooring Lines

Abstract

Coupled analysis is common in time domain analysis of a floating body with mooring lines. But, the coupled method may be time consuming because numerical model for both body and lines is usually big. If the body and lines are solved separately, the model may be simpler. For example, the body motion is solved in advance with spring model for mooring lines. Then, fairlead motion due to body motion is imposed to the lines to solve line dynamics. The decoupled method is simpler and faster. However, the result may be approximate. This paper compared computing time and accuracy of the coupled and decoupled methods in time domain analysis of moored body. A barge ship with twelve spread catenary mooring lines was analyzed as a numerical example. HOBEM (Higher Order Boundary Element Method) and FEM (Finite Element Method) were applied to formulating ship and line equations, respectively. The equations were solved by coupled and decoupled analyses and the ship motions such as surge, sway, heave, roll, pitch and yaw were compared for the two methods. Mooring line tensions and cpu time were also compared.