In order to predict low-frequency viscous forces 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 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, Kc*.= ULTW/D and UW/UL, where U, T and 0 are maximum fluld ve"focity, perlod and representative diameter of the cylinders respectively. The subscripts Wand L denote wave- and low-frequency components respectively. Low-frequency viscous forces acting on a semisubmersible are predicted by summing up two dimensional viscous forces acting on the cylindrical members. Force measurements of a semisubmersible model oscillating with low frequency in sway and surge modes are also carried out in regular waves. Viscous effects on the low-frequency hydrodynamic forces acting on the semisubmersible are discussed on the basis of the comparison between the predicted and experimental results.
Low-frequency and large-amplitude horizontal motions of moored floating structures are induced at resonant frequencies of the mooring system by low frequency second-order wave drift forces. To predict the low-frequency motion the low-frequency damping forces should be accurately estimated. Wichers et al. (1982a, 1982b, 1984) reported that the low-frequency hydrodynamic damping force on a moored vessels increases in waves as compared with that in still water, and this influence of waves on the low-frequency damp1ng force was called "wave damping". Hereafter, Saito et al. (1984), and Faltinsen et al. (1986) called it "wave drift damping" in order to distinguish 1t from the linear radiation wave damping with incident wave frequency.
