CONNECTING PIPE 



FLUID INLET 



4-5 DIA- 



-MOORING LUG 



MATERIAL : ALUMINUM 



OCEAN 



HARBOR 



Figure 102. Two-dimensional experimental investigation of twin- 

 cylinder floating breakwater (after Ofuya, 1968). 



on the ratio a /a 



also. These periods increased with decreasing values 

 of a /a (as indicated in Fig. 103) which was consistent with a decrease 

 of a /a causing an increase in the radius of gyration of the structure and 

 a decrease in its metacentric height. When both cylinders were filled with 

 water, the range of stability of the breakwater decreased considerably. Near 

 the limit of stability, a slight lateral displacement of the filling fluid 

 caused the structure to float with both cylinders vertically oriented, and 

 the performance of the breakwater was severely impaired. For optimum wave 

 attenuation with one cylinder filled, the submergence position had a value of 

 a l/ a 2 = 0.4 and appeared to be independent of the incident wavelength (Fig. 

 104). 



Ofuya (1968) found that wave damping by the twin-cylinder floating break- 

 water depended on several factors, such as wave reflection, interference 

 effects, and particularly at small values of L/d, turbulent action in the 

 gap between the cylinders. All these factors contributed to the existence of 

 a floating position for optimum wave damping. For large values of L/d, the 

 structure would not be effective as a breakwater since long waves pass through 

 with little damping, as indicated in Figure 105. However, for L/d < 1.3, the 

 twin-cylinder floating breakwater has a transmission coefficient, C t , less 

 than 0.5. As shown in Figure 105, an almost linear relationship exists 

 between C t and L/d for wave steepness in the range 0.045 to 0.065. This 

 relationship does not, perhaps, indicate the complex nonlinear interaction 

 processes occurring between the structure and the waves. For small values 

 of L/d, wave breaking in the gap between the two cylinders appeared to be 

 the dominant mechanism influencing wave damping. The heaving, swaying, and 

 rolling motions decreased considerably in the range of L/d, when the struc- 

 ture was effective in wave attenuation. 



154 



