Comments Regarding the CEL Seminar Discussion on 

 Mooring Dynamics - C.J. Garrison 



The discussion was primarily directed toward the various mathe- 

 matical models and assumptions which might be appropriate to the 

 mooring of ships or barges in random seas. Very quickly the 

 discussion converged on the question of the modelling of slowly 

 varying drift-forces and resulting motion. 



Provided one dealt with both the high-frequency motion and slowly 

 varying drift forces within the realm of the frequency-domain, 

 no difficulties should arise. However, while the mooring lines can 

 generally be adequately treated by use of a linear approximation 

 for relatively small oscillations about a mean position, this is 

 not the case for the large displacement oscillations resulting from 

 the slowly varying drift-forces. Thus, it seems obvious that a 

 time-domain analysis should be used to treat the slowly varying 

 motion while a frequency-domain analysis could be used for the 

 high-frequency motion. The difficulty in this approach is that 

 considerable interaction between the high-frequency motion and the 

 slowly varying motion exists. The frequency response of the 

 vessel is dependent on the mooring line tension which in turn is 

 dependent on the slowly varying drift. On the other hand the slowly 

 varying drift-force is strongly dependent on the high-frequency 

 response. It appeared that by the conclusion of the meeting this 

 difficulty remained and no obvious solution seemed to be forth- 

 coming. 



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