Floating Body in Irregular Waves 



For a given moored system with approximately linear spring 

 stiffness and damping coefficient the same linear relationship must 

 be found between mean excursion and low frequency amplitude of 

 motion for various seaways. This appears not true. Especially In 

 heading waves large discrepancies can occur. From this observation 

 I am led to suppose that It will be not allowed to describe a practical 

 wave spectrum by a slowly modulated regular wave In order to 

 e^laln the obtained test results. 



Based on the mentioned observations I am led to suggest the 

 following hypothesis. 



Hypothesis; The wave forces on a moored body In Irregular 

 waves which are responsible for the excitation of the mass -spring 

 system In Its resonance frequency are the second order low-frequency 

 wave forces on the body In fixed condition, I.e. the drift force due to 

 the reflection of waves. 



The Influence of the ship motions can be neglected. 



One of the conclusions of this hypothesis is that the exciting 

 force for the long-periodic motion Is a function of wave character- 

 istics and shape of the body alone, and not dependent on the mooring 

 system or on the weight distribution of the moored body- 



The hypothesis Is supported by numerical motion calculation 

 for comparison with experiments , which will be shown later on. It 

 Is needed however to extend the number of comparisons In order to 

 obtain the restrictions of the proposition. 



The proposition can be made acceptable in the following way: 

 Suppose the Irregularity of the wave could be described by a more or 

 less regular wave pattern in which a few discrete steep waves are 

 present. Intuitively It can be stated that the occurrence of a few 

 high waves In an otherwise nearly regular wave pattern gives rise to 

 some violent ship motions. Through Inertia affects these motions 

 occur mostly after the corresponding high waves have passed the 

 vessel. 



Hence, Interaction between the high waves and the resulting 

 motion on the pressure distribution around the ship's hull is drasti- 

 cally reduced, by the mentioned retardation between exciting force 

 and resulting motion. Hence, the effect of such a single high wave 

 on the floating body Is consequently restricted to the Instantaneous 

 effect, l,e, the effect of the wave reflection, on a fixed body. The 

 corresponding exciting force due to reflection Is only dependent on 

 body form and wave characteristics. 



Conclusion; The mean drifting force on a floating body In 

 irregular waves Is dependent on the joint action between waves and 

 body motion. 



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