Performance of twin-pontoon floating breakwaters

• Main Author: Bhat, Shankar Subraya
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8624

A numerical and experimental assessment of the hydrodynamic performance of a moored twin-pontoon floating breakwater made up of either rectangular or circular section pontoons is presented. The performance is described in terms of transmission and reflection coefficients, breakwater motions, and mooring line tensions. The numerical model includes both hydrodynamic and mooring analyses. The hydrodynamic analysis is based on linear potential theory which utilizes Green's theorem. An available hydrodynamic model for single pontoon sections is extended so as to apply to a structure with two distinct portions below the water surface and so as to incorporate a mooring analysis. An iterative procedure involving consistency in the wave drift force is used to link the hydrodynamic and mooring analyses. A comparison of numerical results with and without the iterative procedure indicates its importance for situations with highly nonlinear moorings. A corresponding experimental study involving two-dimensional laboratory tests of a twin-pontoon moored floating is described. The experiments have been conducted in the wave flume of the Hydraulics Laboratory of the Department of Civil Engineering at the University of British Columbia. In the experiments, the breakwater performance is assessed using measured wave records at selected upwave and downwave locations in the flume, measured time histories of mooring line forces, and a video recording of breakwater motions. Tests with model breakwaters have been conducted for various pontoon spacings, pontoon drafts and mooring conditions, and for various wave conditions. A comparison of these results with the corresponding theoretical predictions is given. Numerical results of reflection coefficients K[sub r], for the case of a fixed breakwater indicate a minimum at relative wave frequency parameter ka, ranging from 0.6 to 1.0, which is attributed to the interference effect between the two pontoons. For a moored breakwater, the numerical results indicate the occurrence of negative added mass in heave and an associated sharp peak in the damping coefficient which may also be attributed to the spacing between two pontoons. Experimental results confirmed that the size of the pontoon in relation to the incident wave length, (i.e. ka) is a primary parameter governing the wave transmission past the breakwater. It is found that the twin-pontoon breakwater's overall beam should be at least three-quarter the span of an incident wave length (i.e. B > 3L/4), and the spacing equal to the width of individual pontoon in order for the breakwater to be effective. A comparison of results for a rectangular section with that of circular section shows that the performance of the two sections is very similar. The numerical model is found to provide reasonably good estimates of transmission coefficients, except in the vicinity of resonance. For conditions close to resonance, the experimental results of transmission and reflection coefficients, response amplitude operators and mooring line tensions at the anchor all show considerable scatter.