Solid Breakwater 

 Perforated Breakwater 



1.2 1.4 1.6 

 Model Period, T (s) 



Figure 129. Comparison of seaward mooring line forces 

 for porous-walled versus solid-walled 

 floating breakwater (after Marks, 1966). 



(c) minimum mooring forces, (d) mobility, and (e) ease of construction 

 (including availability of construction materials). In early 1960, A.T Ippen 

 of the Massachusetts Institute of Technology, Cambridge, Massachusetts, pro- 

 posed a variation of the porous-walled structure which would satisfy a number 

 of these requirements. He initiated a systematic study to analyze the dynamic 

 properties of this floating breakwater, which consisted of an array of open 

 tubes alined in the direction of wave propagation but extending over only the 

 upper part of the water depth, with a random distribution of tube lengths. 

 Barnett (1962) performed the initial experimental testing, with subsequent 

 investigations by Ippen and Bourodimos (1964), Bourodimos and Ippen (1968), 

 and Chatham (1971). 



The basic idea for the type of tubular floating breakwater proposed in 

 this conceptual model was to extend the reflecting and energy dissipating 

 action over a wider span of the wavelength and, if possible, randomize or 

 scatter the periodic motion .associated with regular waves, thereby dissipating 

 the energy into turbulence. Such action can be accomplished by an array of 

 tubes of random lengths arranged in several rows to various depths below the 

 sea surface and alined parallel to the direction of wave propagation. If the 

 oncoming wave crest is over one end of an open tube and the wave trough is 

 over the other end in such an array, a pressure differential exists over the 

 tube length. This pressure differential induces a flow in the tube which is 

 out of phase with the normal wave-induced velocities. Changes with time in 

 this pressure gradient are periodic and are transmitted over the tube length 

 with the speed of an elastic wave. Hence, periodically reversing flows are 

 generated through the tube which leave the tube ends as jets, the kinetic 

 energy of which must be dissipated into turbulence at either end. 



190 



