FLOATING BREAKWATERS: STATE-OF-THE-ART LITERATURE REVIEW 



by 

 Lyndell Z. Hales 



I. INTRODUCTION 



1. Background. 



The basic purpose of any breakwater is to protect a part of shoreline, a 

 structure, a harbor, or moored vessels from excessive incident wave energy. 

 Breakwaters may be broadly classified as either fixed structures (rubble- 

 mound, precast units, sheet-steel modules, etc.), transportable structures 

 (sunk-in-place barges, caisson units, etc.), tethered structures (buoyant 

 floats), or free-floating structures (pontoons, flexible membranes, porous- 

 walled units, scrap-tire modules, etc.). Most breakwaters, fixed or floating, 

 are passive systems; i.e., no energy is produced by the device to achieve wave 

 attenuation. The incident wave energy is either reflected, dissipated, trans- 

 mitted, or subjected to a combination of these mechanisms. Conversion of wave 

 energy into oscillatory motion through which power is generated is the concept 

 by which power can be extracted from ocean waves. This is essentially another 

 specific type of floating breakwater. 



From an historical standpoint, the application of a floating structure for 

 the attenuation of surface gravity waves was first considered by Joly (1905). 

 Only minimal efforts were expended on the concept until the necessity for 

 ensuring the offloading of men and materials during the Normandy invasion of 

 World War II, at which time two different types of wave barriers were devel- 

 oped by Great Britain. One of these developments was a portable barge-type 

 unit which was floated into position and sunk at a specific location by fill- 

 ing with seawater. This "phoenix" structure (204 feet long by 62 feet wide by 

 60 feet high) effectively intercepted the preponderance of wave energy to 

 which it was subjected. The second type of wave barrier was a true floating 

 breakwater which had a cruciform cross section (200 feet long by 25 feet wide 

 by 25 feet deep). This "Bombardon" was designed to withstand a wave 10 feet 

 high and 150 feet long, and was successful during the invasion. However, the 

 structure collapsed during an unexpected storm when the seas grew to 15 feet 

 in height with lengths of 300 feet, thus generating stresses more than eight 

 times those for which the structure had been designed. 



Interest in the open-ocean application of floating breakwaters then 

 declined until 1957, when the U.S. Navy Civil Engineering Laboratory (NCEL) 

 began a concerted exploration of the existing state of knowledge of transport- 

 able or floating units which could be adapted to protect small, moored craft 

 and work platforms used in cargo transfer operations. Attention was also 

 directed to military uses under rather severe criteria of incident wave 

 heights up to 10 feet and wave periods up to 7 seconds. The U.S. Army Corps 

 of Engineers (COE) has a potential requirement for a floating breakwater 

 system to provide partial protection to dredges and work boats involved with 

 the construction of coastal engineering features in the nearshore zone and on 

 exposed coastlines with similar wave climates (in addition to more sheltered 

 bay and estuary locations with somewhat less severe wave climates). Coordi- 

 nated research efforts in this area of mutual interests are currently in 

 progress. 



23 



