X. FLEXIBLE-MEMBRANE FLOATING BREAKWATERS 



For short waves in the upper layers of the water column, a deep-draft 

 floating breakwater is not needed; however, for long waves, deep draft may 

 be desirable but difficult because of such large mooring forces. Hence, an 

 optimization is required between wave attenuation aspects and mooring loads. 

 Soft floating systems have certain operational and logistic advantages over 

 rigid structures. Damage and possible loss from collisions in rough water 

 should not be a problem for flexible floating breakwaters; therefore, the 

 mooring arrangement is less critical. Although the strength of the material 

 in a flexible barrier may be a limiting factor, a properly designed flexible 

 breakwater will probably not be subject to the resonances which increase the 

 peak mooring loads in rigid breakwater systems. Because of the collapsible 

 nature of a flexible breakwater, from the military standpoint, a transport 

 vessel could carry more linear feet of this breakwater than other types. 

 Several varieties of surface floating membranes have been investigated, as 

 well as component structures constructed of fluid-filled bags from such 

 flexible-membrane materials. 



Among the concepts tested as flexible wave barriers, the blanket layer 

 types are preferred to bag types for operational and logistic reasons. 

 Puncturing problems and the equipment needed for filling bags complicate the 

 construction of a bag system. The major disadvantage of all blanket layer 

 types is that great widths relative to the wavelength are required to obtain 

 satisfactory wave attenuation. For ocean waves, this implies great absolute 

 lengths, since wave height reduction depends largely on the viscosity of the 

 water, and this property is relatively ineffective in the absence of great 

 turbulence. 



A significant number of laboratory experiments have been conducted with 

 various types of floating membrane and fluid-filled bags. Although the 

 studies are valuable in visually studying breakwater action and comparing 

 data, the studies may not provide definitive data for assessing the poten- 

 tial of flexible floating breakwaters. The data cannot be scaled directly 

 for prototype predictions because the role of the properties of the materi- 

 als was not explicitly defined (Jones, 1974). 



1. Membrane Layers. 



The wave damping properties of several floating plastic materials were 

 investigated by Keulegan and Kulin (1958), and plastic mats were experimen- 

 tally studied by Schwarts and Watts (1959), and Watts (1960), as a result of 

 the 1958 tests. Specific tests of certain patented designs of floating mem- 

 brane layers were conducted by Frederiksen and Wetzel (1959), and by Ripken 

 (1960a). 



a. Wave Trap. The U.S. Rubber Company developed the Wave Trap, a 

 proprietary design of a wave absorber, in the late 1950' s. Ripken (1960a) 

 conducted experimental studies of the Wave Trap which was then investigated 

 analytically by U.S. Rubber Company (1961) and Miller (1961a, 1961b, 

 1961c). The system consists of a large, thin, rather impermeable floating 

 sheet fitted with numerous attachment cords supporting a large, thin valve 

 sheet (horizontally weighted to trap short-period waves, vertically weighted 

 to trap long-period waves) (Fig. 160). The valve sheets are shown in 



219 



