On the coupling terms in the low-frequency viscous reaction forces of moored tankers in deep water

Due to the restricted water depth of experimental facilities one has to rely more on computational tools for the design of turret and spread moored tanker based FPSOs in deep water. These tools have to be as accurate as possible.
The mechanics to compute dynamically mooring lines and risers are understood. By means of potential theory the first order tanker motions and wave drift forces with and without current can be adequately computed nowadays, see for instance Ref. 1.
Besides the low-frequency (lf) wave drift forces also the lf viscous reaction forces (damping) exist in still water and current. The lf reaction forces/moment induced by the lf motions in the horizontal plane are mostly dominated by viscosity and hardly to compute.
By means of model tests the viscous forces in the separate surge, sway and yaw modes of motion were measured in still water and in a current field, see for instance Ref. 2. The importance of the coupling terms due to the combined modes of motions, however, is not known.
By means of a lf large stroke mechanical oscillator prescribing combined motions on the tanker hull the coupling terms were investigated in still water and in current. The results will be presented in the paper.

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