Van Oortmevssen 



an oscillating motion around a point which is situated in the steep part 

 of the load-excursion curve, as indicated in figure 12. The relevant 

 part of the curve may be regarded as linear with an inclination c. 

 Consequently, the resulting surge motion is given by the linear appro- 

 ximation of equation (22) : 



-i 6Jt , , 



x = x . e (23) 



a v ' 



in which 



x = the amplitude of the motion 



a, 



After substitution of (23) in (22), we find that the amplitude 

 of the surge motion will be : 



F 

 xa . 



X a = | IT < 24 > 



c - m CJ 



1 v ' 



The resulting maximum reaction force in the anchor system 

 becomes : 



F„ = 4, 730 + x . c (25) 



Rx max. a 



In figure 13 the maximum reaction force in x-direction is 

 given to a base of the spring constant. From this figure it becomes 

 obvious that it will be very hard in this case to design a proper an- 

 chor system. Resonance will occur if : 



c = m CJ (26) 



v 



and, since most of the wave energy is related to wave frequencies 

 between OJ = 0. 2 and CO = 1.0, values of c between 2, 400 and 60, 000 

 ton/m should be avoided. 



A value of c higher than 60, 000 ton/m means an almost ri- 

 gid connection to the sea bottom, which must be able to absorb a ho- 

 rizontal force of over 60, 000 ton; this does not seem to be a practi- 

 cal solution. On the other hand, if c is chosen to amount to less than 

 2, 400 ton/m, the risk exists that in irregular seas the slowly varying 

 drift force induces resonance phenomena. 



In reality the problem is much more complicated than was 

 assumed in this simple calculation : besides the surge motion, also 

 heave and pitch may be of importance, and due to the high waves, the 



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