Effect of directional spreading angles on the wave hydrodynamic coefficients for vertical cylinder

Abstract

In the prediction of accurate wave force, the consideration of short-crested wave is important in representing the actual ocean condition. Owing that, nonlinear wave characteristics are expected to change the flow properties when the wave passes the structures, which showing these characteristics are crucial to be accounted. However, literature reported that the overestimation on the design of marine structures has shown a lower reliability in the wave force prediction considering long-crested wave. Further in detail, the effect of wave directionality and directional spreading angle from the actual sea conditions, i.e. the short-crested waves, have been neglected in a way. To prove the validity of this statement, an experimental investigation was conducted to quantify the effects of wave directionality on the wave forces and to propose the hydrodynamic coefficients incorporated the effect of directional spreading angles. The wave directionality in term of directional spreading angles were considered, ranging from 5 to 45°. Then, the measured wave surface elevation and wave forces were processed using least square method to compute the hydrodynamic coefficients and to evaluate the wave forces. To quantify the effect, force ratio factor was adopted. Based on the finding, short-crested waves led to 20 % force reduction as compared to the long-crested. On top of that, a reduction of 1.12 % of wave force has also been observed for every one-degree incremental of the directional spreading angle. As conclusion, the wave directionality was found contributing to the reduction of wave force which may provide new improvement on the accuracy of wave force formulation for the design of lean marine structures.