Comparative study of hydrodynamic performances of breakwaters with double-layered perforated walls attached to ring-shaped very large floating structures

A very large ring-shaped structure composed of spar-type modules is proposed as an intermediate base for supporting deepwater oil exploitation. One of the features of the structure is the attachment of a double-layered perforated-wall breakwater, which reduces the wave energy inside the harbour. To establish the hydrodynamics of the complex structure, the characteristics of the breakwater were experimentally investigated using six different configurations. The transfer functions of the waves inside the harbour were found to have multiple peaks, which were produced by the interaction of the transmitted waves with the diffracted waves. The incident wave amplitude was also observed to significantly affect the wave energy dissipation of the breakwater for short waves, whereas the effect was small for long waves. The wave loss coefficients, wave run-up, mooring force, and surge motion were all observed to increase significantly with decreasing porosity. However, the vertical motions were quite small owing to the low natural frequencies, and they were negligibly affected by the porosity. By quantitative estimation of the effects of the porosity, it was found that the low-frequency horizontal motion, mooring force, and wave attenuation were the critical factors to be considered in the design of a breakwater attached to floating structures.

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