LOADING ON A VERTICAL CYLINDER IN MULTIDIRECTIONAL WAVES

This paper presents laboratory measurements of local and total loading on an isolated vertical cylinder in irregular unidirectional and mult idirectional waves. Maximum Keulegan-Carpenter numbers in individual w aves were about 16, and maximum Reynolds numbers about 3 x 10(4). It i s shown that in these conditions, existing theoretical and numerical m odels underestimate the reduction in loading on a cylinder due to wave spreading. Besides the changes that are predicted when Morison's equa tion is used with constant coefficients, there are hydrodynamic influe nces that contribute further force reductions. Comparisons with Dean's (1977) hybrid approach suggest that in the present conditions these r eductions are in the region of 3 and 6 percent for a spreading functio n cos(2s) theta, with s = 8 and s = 2, respectively. Larger reductions can be expected at higher Keulegan-Carpenter numbers, though scale ef fects are likely to become more important in the drag-dominated regime .