part the frequency of the response is higher than the frequency of the input. 

 It does appear that two amplitudes are in the response: one at about 35 cm 

 and the other at about 15 cm. In addition, it appears that the oscillation 

 pattern repeats on a period of about ten times the apparent natural period 

 (coincidence?) . 



Taken together, these three figures are indicative of the typical response 

 of a nonlinear oscillator as represented in Figure 5. They also show some of 

 the difficulties in using numerical calculations of transient responses to 

 correlate with theoretical steady state responses. 



Static Excursion of a Moored Ship 



A diagram of the DD692 Destroyer in a four point moor is shown in 

 Figure 6. The lines are essentially catenaries in the quiescent state, and 

 substantial lengths of line lie on the bottom. Figure 7 shows the combined 

 effects of a 2 kt. surface current and a 30 kt. wind versus the heading relative 

 to the ship. Figure 8 shows two calculations of the excursion the e.g. of the 

 ship takes as the heading of the wind and current is varied through 180 

 relative to the original quiescent position of the ship. The effect of 

 neglecting the bottom interaction with the lines is clearly shown. The moor 

 appears much stiffer without the bottom interaction. The differences in 

 stiffness as well as the change in ship position could have significant influence 

 on dynamic response calculations. See Reference 1 for more details. 



Frequency Domain Dynamic Response Calculations for Moored Ships 



Following the approach represented by Equations (4) for some basic mooring 



configurations offers some insights. Reference 2 gives more detail and presents 



the figures which will be commented on briefly here. 



33 



