Low-frequency Viscous Forces Acting on Members of an Offshore Structure

Abstract

In order to predict low-frequency viscous forces acting on a semisubmersible moving with large amplitude and low frequency, it is necessary to know the viscous forces acting on cylindrical members of it. To get fundamental information on the viscous forces, force measurements of a lowerhull cylinder and a circular cylinder slowly oscillating in regular waves, and a circular cylinder oscillating with two different frequencies are carried out. The results show that the low-frequency hydrodynamic coefficients significantly depend on two parameters, K_c=U_LT_W/D and U_W/U_L, where U,T and D are maximum fluid velocity, period and representative diameter of the cylinders respectively. The subscripts W and L denate wave-and low-frequency components respectively.