05 



a> 0.2 0.4 0.6 8 1.0 



o 



u Ratio of Structure Width-to-Wavelength, W/L 



Figure 19. Calculated transmission coefficients for floating 

 structure with various mooring conditions (after 

 Yamamoto, Yoshida, and Ijitna, 1980). 



draft of the structure, the free-floating condition provides a rather poor 

 wave attenuation (it was effective only to waves smaller than 1.6 W/L). 



The open spring mooring with the diiuensionless spring constant, K = 0.1, 

 worsened the results due to the undesirable rolling motion generated by the 

 waves. The conventional slack moorings are usually approximated by open 

 spring moorings with relatively weak spring constants. Hence, the effect of a 

 slack mooring system (e.g., large tidal range) appears to be a worsening of 

 the wave attenuation of a floating breakwater. The cross-spring moorings with 

 K = 0.20 or higher significantly improved the wave attenuation. The small 

 wave transmission created by this mooring condition for W/L values between 

 0.4 and 0.9 is associated with small body motions. 



For this floating body, a cross-spring mooring provides a no-transmission 

 condition at low wave frequencies or long wavelength approximately W/L = 0.10. 

 These results suggest that in order to effectively use a shallow-drafted 

 floating structure as a breakwater, a cross-spring mooring with proper spring 

 constants can be optimized. If the condition is W/L = 0.10, the cross-spring 

 mooring would appear to be extremely effective for narrow-banded waves such as 

 ship waves adjacent to floating breakwaters, the problem investigated by 

 Stramandi (1974). Although numerical techniques are continually improved to 

 be adaptable to various structure and boundary conditions, the physical 

 hydraulic model remains a powerful tool to supplement and verify such compu- 

 tational procedures. 



49 



