Experimental Study on Flow-Induced Motion of an Array of Four Cylinders with Different Spacing Ratio

Experiments regarding the FIM - flow-induced motion on an array of four floating cylinders with low aspect ratio, H/L=1.5, and three different distances between column centers, or spacing ratio, S/L=2,3 and 4, were carried out in a towing tank. The array of four cylinders was elastically supported by a set of four linear springs to provide low structural damping to the system. Three different section geometries were tested, namely circular, square and diamond. Two different current incident angles were tested, 0 and 45 degrees. These configurations of the four-column arrays were selected to cover the range of the main SS - semi-submersible and TLP - tension leg platforms. The aims were to understand the FIM of the four-column systems and to compare the results of the single column and array of three cylinders. The range of Reynolds number covered . Concerning the geometry of the columns sections; the amplitude results showed larger amplitudes for the four-cylinder array than the single cylinder case for circular and diamond cases, in which cases the phenomenon was similar to VIM - vortex-induced motions. In turn, the amplitudes for the single square case were higher than for the array of square cylinders; in this case, the galloping phenomenon was predominant. Concerning the distance between column centers, the amplitude results for the four-cylinder array with S/L=3 and 4, were very similar. The four-cylinder array with S/L=2 acted as a single cylinder because the proximity of the cylinders changed the wakes around them. Finally, the presence of pontoons had a significant effect on the motions in the transverse direction, and this presence was responsible for decreasing in the amplitudes.