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Abstract In spite of the large amount of work that has been done on coastal regions, a specific design for low-cost shore protection was serious need in small ports. Coastal protection structures are using to provide secured area for navigation and berthing of vessels. Using the breakwaters is the most popular method of coastal protection. Floating breakwaters are also using to provide partly calm water area by less cost in comparison with other breakwaters. Mentioned breakwater type is usually performed in small craft harbors that need impermanent calmness with low costs. Floating pontoons are one of the most usual floating breakwater types that composed of connected pontoons. Reports show that pontoon type floating breakwaters have appropriate performance in order to coastal protection. Floating breakwaters used to create tranquility in port. The floating breakwater use plenty in marine works, military operations, fishery activities, and recreational ports are abundant. In many ports, output ports are significantly affected by influx of waves, hence the existence of a floating breakwater seems to be necessary to increase efficiency and provide a safe environment for the ship. Breakwaters made in practical work, trying to reduce wave’s height in a specific location. The first record about a floating structure as a breakwater dates back to the early19th century. In 1811 General Bentham, the Civil Architect and Principle Surveyor of the Royal Navy of Great Britain, proposed a breakwater model for the British fleet at Plymouth. The Breakwater would consist of triangular sections of floating wooded frames moored with iron chain. The project cost was about one-tenth the cost of the rubble and granite mound structure which was finally adopted. The idea was rejected due to the concerns about its effectiveness during severe storms. In present research that conducted at hydraulic and marine structures laboratory of Tarbiat Modares University, two separated pontoons with variable distances, have been used as floating breakwater system. Considering the wave transmission coefficient as a main performance parameter, performance of the floating breakwater studied under the effect of the variation of distances between pontoons. In order to this, 160 tests lead in the wave flume and their results were compared with each other. Frequency domain method was used to analysis the raw data to decrease errors that may occur in time domain analysis method. In present research effects of distance between pontoons have been examined and the optimum distance of pontoons that leads to minimum transmission coefficient have studied. Results show that wave transmission coefficient of dual floating breakwaters is minimum at the range of the relative distance about 0.8 – 0.9 and its quantity will be maximum when relative distance of dual floating pontoons is at the range of 1.3-1.4. Keywords: irregular wave, dual pontoon floating breakwater, wave transmission coefficient, distance between pontoons.

Keywords: Irregular wave, dual pontoon floating breakwater, wave transmission coefficient, distance between pontoons

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