Author(s)

Thibaut Van Zwijnsvoorde, Luca Donatini, Wim Van Hoydonck & Evert Lataire

Abstract

With the increasing size of container ships, accurate methods to model manoeuvring and mooring conditions are indispensable. especially in confined waters, where the ship speed is low or even zero, wind forces add a significant contribution to the force balance. the calculation of wind forces is typically done using wind coefficients based on wind tunnel tests. in these computations, a reference wind pressure must be used which is often based on the wind speed at 10 m height. When the wind blows over a rough surface however, the wind profiles become non-uniform, resulting in much higher wind speeds near the top of the ship, for the same wind speed at 10 m height. in case of differences between the wind profile used in the wind tunnel and the one expected in the reality, an appropriate reference pressure should be used. a method proposed by blendermann to calculate such reference pressure is applied in this paper to a wind force calculation for an ultra large container ship. it is shown that, depending on the roughness of the surface, the reference pressure can be a factor 2 to 3 higher than the one corresponding to 10 m height. this means that wind forces are potentially highly underestimated. the results of the method are compared with CFD simulations with a uniform and non-uniform inlet profile. the comparison shows a good agreement between blendermann’s method and computational fluid dynamics (CFD) results for the surge force and roll moment. on the other hand, blendermann’s method seems to overestimate the sway force, but more simulations are needed before a firm conclusion can be drawn.

Keywords

wind coefficients, ULCS, wind profile, reference pressure, CFD