Figure 14 and Figure 15 is the time history of 1^st and 2^nd order wave forces used in the simulation. Figure 16 is the variation of tension due to the irregular waves. Here, we can see the 2^nd order wave force is the main cause of the variation of the tension of mooring line.

　

　

 Figure 17 shows the difference between the exact calculation of 2^nd order wave forces by convolution integral of impulse response functions and by mean value assumption. Here, we can see the convolution integral of wave forces leads to the more dynamic evaluation of the tensile force of the mooring line.

　

 Time domain assessment of 1^st and 2^nd order wave forces on a ship in irregular waves is carried out, to simulate the various motions of a single point moored ship. It is shown that the typical slew motion in irregular waves are successfully simulated. Besides, the relation between the 2^nd order wave forces and the variation of the tension in mooring line become clear. More over, it is found that the drift force is the main cause of ship's slew motion but the 1^st order wave force also affects the ship's motion slightly. But, the validity and usefulness of the present calculation must be pursued by comparing the results with the model experiment in irregular waves.

　

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 Seung Keon Lee is Professor at the department of naval architecture and ocean engineering of Pusan National University in Korea. He graduated from University of Tokyo in 1988, received doctorate with dissertation titled "On the mathematical maneuvering model for a twin screw twin rudder ship". He is now manager of the Sea Grant Project of Korean Government in Pusan National University.

 Hyo Jae Jo is Associate professor at the department of naval architecture of Korea Maritime University. He graduated from University of Tokyo in 1991. The title of dissertation was "Research on the effects of multi-directional waves to the behavior of a moored offshore structure".

 Dong Hoon Kang is graduate student at the department of naval architecture and ocean engineering of Pusan National University. He received a master's degree in naval architecture from the Pusan National University in 2003.