W = 20 in 



b olio st 



S^ZZZZZZ22222 2ZZZZZZZZZZ2 



d = 5.875 in 



' //>///// 



w??? 



~ 7/////;;/;//s////s//;//// /s<>//Jss s~7~ 



Figure 20. Single-pontoon floating breakwater (after Ofuya, 1968). 



concept was based primarily on an attempt to provide a floating breakwater 

 whose wave damping property is derived mainly from the criterion of large 

 mass, which determines the depth of submergence. The radius of gyration 

 remains nearly constant with depth of submergence, but the metacentric height 

 (and hence stability) varies considerably with depth of submergence. It is 

 intuitively logical to believe that the performance characteristics of the 

 single-pontoon floating breakwater will depend not only on the initial wave 

 steepness, H-/L, and relative breakwater width, W/L, but also on the ratio 

 of natural period of oscillation of the structure, T , to incident wave 

 period, T. The natural period of heaving motion depends on the mass and 

 elasticity. An increase in mass causes a proportional increase in draft, and 

 the elasticity of the system remains nearly constant; hence, an increase in 

 mass produces an increase in the natural period of oscillation. 



Ofuya (1968) determined that for a 100-percent increase of mass (all other 

 parameters remaining constant), the natural periods of heave and roll increase 

 by 40 and 33 percent, respectively. However, he found that the metacentric 

 height, which determines the stability of the floating body, decreased by 

 about 45 percent. This decrease of metacentric height with increase of mass 

 imposes a limitation on the extent to which the natural period of oscillation 

 of a single pontoon may be increased. 



a. Two-Dimensional Wave Attenuation Tests . Ofuya (1968) conducted exper- 

 imental studies of a two-dimensional nature to determine the attenuation 

 effectiveness of the single-pontoon floating breakwater. The results are 

 shown in Figure 21 as transmission coefficient, C t , versus relative water 

 depth, L/d, for three different depths of submergence. The structure was 

 found to be effective in damping waves over a wide range of values of L/d; 

 the difference in effectiveness between the pontoons of different submergence 

 is small in the lower range of values of L/d but tends to increase for large 

 values of L/d. A nonunif ormity in the data display occurred because of har- 

 monic components in the response characteristics of the breakwater. 



Carver (1979) conducted two-dimensional (and three-dimensional) wave 

 attenuation tests for the proposed floating breakwater at East Bay Marina, 

 Olympia Harbor, Washington. This harbor is located in the extreme southern 



52 



