UNSTABLE MOTION OF FREE SPAR BUOYS IN WAVES 



J-C. Dern 

 Bassin d'Essais des Carenes 

 Paris j Franae 



ABSTRACT 



The stability of the motions in waves of a particular 

 type of free spar buoys is studied both theoretically 

 and experimentally. In a first step the motions are 

 determined from linearized equations. The coeffi- 

 cients appearing in these equations are obtained from 

 model tests. The obtained results are compared with 

 J.N.Newman's theory. 



(i) Tests show that the buoy may perform different 

 stable motions depending on its mass, the position of 

 its center of gravity and its inertia. 



In particular, when the buoy has a large static stabi- 

 lity in pitching, these motions consist of heaving and 

 pitching, their combination being stable and unique- 

 ly determined. 



On the contrary for low value of the static stability 

 in pitching, there exist two stable combinations of 

 heaving and pitching motions, they differ from each 

 other by the amplitude of the heave/wave ratio which 

 is moderate in the case of one combination and much 

 higher in the case of the second one. 



All the above motions are periodic and their period 

 is that of the waves. The existence of two stable com- 

 binations indicates that there also exists a third but 

 unstable combination. 



1003 



