The ballast module is a rectangular, welded-steel framework made from 

 scrap railroad rails about 60 feet long, 30 feet wide, and 4 feet high. Each 

 module, which weighs about 115,000 pounds, has four cylindrical tanks and 128 

 scrap-tire floats mounted within the framework. Important features of the 

 ballast design are the economical use of scrap metal and the retrieval use of 

 buoyancy tanks. When the tanks are empty, the ballast module is suspended by 

 the 128 floats. When the tanks are fully flooded, the ballast module rests on 

 the bottom with a foundation reaction on one module of about 50,000 pounds. 

 Empty tanks provide a net upward force of about 50,000 pounds to raise the 

 ballast off the bottom. This module can be towed with ballast submerged, or 

 floated by add-on buoyancy, and can be stored on the sea floor. 



4. Tether Termination Assembly. 



The function of the tether is to maintain the floats at a fixed distance 

 above the ballast, allowing them to oscillate freely in all planes about an 

 axis formed by a line through the base of the tether and perpendicular to the 

 ballast. The tether length is set according to the chosen float; the two 

 should form a pendulum with a natural frequency equal to that of the most 

 significant waves which the tethered-f loat breakwater is designed to atten- 

 uate. Tether tensil strength is determined by the buoyancy of the float and 

 appropriate safety factors. The material must be able to withstand long-term 

 ocean use and corrosion, biological fouling, and shock loads. 



There are essentially two scales of tethered-f loat breakwaters: the 

 marina scale and the open-ocean scale. The primary difference between the two 

 scales is the size and buoyancy of the floats to be used. Marina tethered- 

 float breakwaters are designed to use floats of about 1-foot diameter; the 

 open-ocean scale uses floats up to 5-foot diameter. The marina floats provide 

 about 27 pounds of buoyancy each, and the 5-foot-diameter ocean float provides 

 up to 4,000 pounds of buoyancy each, depending on construction. Suitable 

 tether termination assemblies have been developed and used on marina tethered- 

 float breakwaters; however, because of manufacturing difficulties, these 

 smaller assemblies cannot be scaled for the larger systems. 



a. Design Considerations. The Naval Undersea Center, San Diego, 

 California, developed a tether termination assembly suitable for open-ocean 

 installation on a tethered-f loat breakwater. Flexure life was judged to be 

 the most critical consideration in the initial design. Float movement through 

 the water causes the tether to flex randomly in all planes about the tether 

 axis. Best estimates indicate that, over a 5-year period, the tether will 

 flex ±11° about the tether axis for 18 million cycles and ±17° for an addi- 

 tional 1 million cycles. No severe shock loading on the tether is anticipated 

 after installation, since the ballast is designed so that even under storm 

 conditions the floats will not surface enough to significantly reduce the 

 tensil load on the tethers. However, experience indicates that shock loading 

 may occur during assembly and installation; hence, it is essential to use 

 risers which are nontorquing. 



Uniformity of stretch and creep in a tethered-f loat breakwater is also 

 an important design consideration. If stretch and creep are not uniform 

 throughout the array of tethers, some of the floats will eventually surface to 

 the point where their tethers become slack. Thus, it is necessary to select 

 a riser which has no significant stretch or creep, or one which can be con- 

 trolled by proper design and manufacture. Designs must also include environ- 

 mental considerations such as biological fouling or corrosion, which will 



