Floating Body in Irregular Waves 



The stiffness of each spring amounted to 5 tons/m. and is 

 independent of the elongation. The main particulars of the vessel 

 are: 



Length - beam ratio =5,3 



Beam - draft ratio =3.65 



Block coefficient = 0,750 



Midship section coefficient = 0.997 



Waterplane coefficient = 0,896 



Wave spectra were produced similar in shape to those 

 analysed by Piers on and Moskowitz for fully developed seas. These 

 spectra can be described by: 



SAtji) = -^e 



Both the produced and the hypothetical spectra are given in the 

 Figs, i through 5. 



The significant wave height is defined as: 



The average wave period is 



f = 27r^ 

 m, 



where 



•^0 



Sk(o}) dco 



The distribution of the wave elevations confirm well to the normal 

 probability distribution. 



The motions of the vessel and the forces in the horizontal 

 springs were measured in three irregular head seas and three 

 irregular beam seas each with different characteristics. The 

 duration of each test run corresponded to approximately half an hour 

 for the full scale vessel. This time period is considered to be 

 sxofficiently long for a reliable statistical treatment of the recorded 

 quantities, A typical recording of a horizontal motion of the moored 

 vessel is given in Fig, 6. A low-frequency motion in the natural 

 period of the ship-spring system is present upon which high-fre- 

 quency motions are superimposed. The spectral density of the, surge 

 and sway motions are given in Figs, 7 through 12, The distribution 

 of the forces and of the motions deviates from the normal distribution 

 though in many cases the deviation is not large. 



959 



