| Summary: | 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.
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