Experimental and numerical investigation of the effect of mooring stiffness on the behaviour of p-type floating breakwaters

Floating breakwaters (FBs) are suited to protect small marinas in mild sea conditions (wave periods up to 4 s and wave heights smaller than 1.5 m). Several companies in the world provide a very effective typology, a concrete rectangular caisson with two vertical plates protruding downwards from the sides. As these shapes resemble a P, they are referred to as P-type FBs. In a previous paper, the Authors proposed a new formula for the transmission coefficient based on experiments on six different geometries of this type of FBs, with mass varying from 16 to 76 kg (representing prototypes at different scale, up to 130 tons), all anchored with chains. The formula identifies a key non-dimensional parameter as essential to describe the transmission phenomenon. The aims of this paper are to investigate on the effect of the mooring stiffness, since the degree of restraining on the floating bodies is expected to alter significantly the FB's efficiency. For this purpose, new experiments were recently carried out in a wave flume on FB anchored with piles and tethered with elastic lines, and an exploratory analysis is performed by means of simplified numerical simulations. Based on these investigation, a second non dimensional parameter (d/h, i.e. draft over water depth) is identified as essential to predict whether lower transmission is obtained with a larger or a lower mooring stiffness. Only qualitative predictions are anyway proposed. Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE).