Unstable Motion of Free Spar Buoys in Waves 



was measured . 



2, 22 - The heave Z (t) of the buoy was determined by 

 measuring with an accelerometer set within the model the accelera- 

 tion - parallel to the axis of revolution of the buoy . Rather than to 

 integrate twice the acceleration a (t) it was assumed that the motion 

 was sinusoidal . The heave is then related to the acceleration by the 

 relationship : 



a(t) 



Z (t) = 



o- 2 



This relationship is valid only if a (t) is sinusoidal and 

 if pitching has a small influence on the acceleration . In order to 

 satisfy as well as possible these two conditions , it was decided to 

 simulate only waves with wavelength not too large as compared to 

 tank depth (see shallow water waves theory) and with small steep 

 ness ( 7 = »'w/ A «£ 2 / 100) . The two purposes of this latter con- 

 dition are first to reduce the number of harmonics present in the 

 waves , second and more important to avoid large motions of the 

 buoy in pitching (influence on the acceleration a (t) as well as in hea- 

 ving : experience shows that when the wave steepness is large the 

 buoy performs very irregular vertical motions , which are difficult 

 to interpret (see figure 5) . 



A (t) 



Figure 5 



1013 



