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.
In design of moored ship, time domain coupled analysis is common for body and lines and the coupled analysis was applied by many researchers (Hong and Hong, 1997; Kim et al, 1999; Garrett, 2005; Bae et al, 2011; Kim et al, 2013; 2016). Coupled analysis provides accurate results because it includes coupled effect between floating body and mooring lines. But, it may need much calculation time because it does many computations to get the coupled solution. Simpler analysis could be done if the body and lines are analyzed separately. For example, the body equation is solved in advance with simplified spring for mooring lines. Then, boundary condition at fairleads due to the body motion is imposed to the line equation to get the line tension. The decoupled analysis will be simpler and faster. However, the accuracy is not sure because the coupled effect is not totally included.