5. Economic Feasibility Studies . 



Economic evaluations of potential tethered-f loat breakwater installations 

 have been prepared for site-specific locations by both private consulting 

 firms and by the U.S. Navy. In general, it was found that such feasibility 

 varied widely with the wave exposure and bottom depth characteristics at the 

 various sites. The tethered-f loat breakwater shows potential use where con- 

 ventional structural breakwaters cannot be built because of extreme water 

 depth, where protection is needed only for a short period of time, and where 

 the protected area must shift with a transient maritime operation. For 

 shallow-water short-fetch sites, the feasibility of the tethered-float break- 

 water looks very promising. For shallow-water, long-fetch sites, current 

 research on bottom-resting ballast frames and higher specific gravity floats 

 may produce systems which will serve specific needs better than present alter- 

 natives. For deepwater, long-fetch sites, the tethered-float breakwater 

 system appears feasible only for naval or military operations (Moffatt and 

 Nichol Engineers, Ogden Beeman, and International Maritime Associates, 1977). 



a. NCEL Evaluation. Two of the three designs of the open-ocean tethered 

 float breakwater considered by Jones (1978) were evaluated to estimate their 

 feasibility from the cost-effectiveness standpoint. 



(1) Concrete Barge-Type (Floating) Ballast Tethered-Float Breakwater . 

 For estimating cost, the float was assumed to be a 400-pound 5-foot-diameter 

 float costing about $450 each in some quantity. One tether with special ter- 

 mination was expected to cost $175. The concrete ballast module for these 

 floats weighs about 5,600 pounds and costs between $300 and $500 per cubic 

 yard (1978 dollars). The transportation system and work boats required to 

 assemble the structure are not included in the comparative cost estimates. 

 The cost of materials for a 7-second breakwater, with moorings designed for 

 less than a 100-foot depth, was estimated to be $9,600,000 for a breakwater 

 with 15 moored elements. 



(2) Concrete Articulated-Frame (Floating) Ballast Tethered-Float 

 Breakwater. Data for floats and tethers were as assumed in the concrete 



barge-type estimate, with the ballast cost estimated to be $160 per float. 

 The flexible intermodule connectors were a development item and were assumed 

 to cost $1,500 each. Lines and fittings for connecting moored elements will 

 add 5 to 10 percent to the other costs; moorings will add 5 to 10 percent, 

 depending on the water depth and the length of the breakwater. The cost of 

 materials for a 7-second breakwater with moorings designed for less than a 

 100-foot depth was estimated to be $7,600,000 for a breakwater with 15 moored 

 elements. 



(3) Steel-Frame (Bottom-Resting) Ballast Tethered-Float Breakwater . 

 This design consists of -128 cylindrical floats approximately 2 feet in diame- 

 ter and 4 feet high, assembled from automobile tires and attached to a steel- 

 frame ballast module, 30 by 60 feet. Proposals to supply floats for an ocean 

 experiment ranged between $50 and $500 per float, with an average cost of $125 

 for each float, tether, and termination assembly. The ballast system added an 

 additional estimated $225 per float. A cost comparison of the bottom-resting 

 structure with the two floating structures was not made by Jones (1978) 

 because such a comparison would be meaningful only if the breakwaters produced 

 the same degree of wave attenuation. For the two floating structures, the 

 number of rows of floats is known from existing performance prediction tech- 

 niques for low-density, spherical floats in deep water. For the bottom- 

 resting system using high-density, pseudo-cylindrical floats in shallow water, 

 no performance prediction technique has been advanced. 



182 



