The Influence of Waves on the Low-Frequency Hydrodynamic Coefficients of Moored Vessels
Vessels moored at sea will undergo low-frequency motions due to wind, current and waves. Computer programs are being developed to predict theoretically the motions of such vessels, particularly the motions in the horizontal plane, which consist of a high-frequency part (oscillations with periods corresponding to those of waves) and a low-frequency part. In these theoretical approaches only potential damping is taken into account; viscous damping, which substantially influences the low-frequency phenomena, is not included. Moreover, the damping of moored structures in resonance conditions seems to be influenced by additional damping caused by the waves. Both viscous and wave damping have to be determined experimentally At the Netherlands Ship Model Basin model tests have been carried out to determine the low-frequency hydrodynamic coefficients of a VLCC and a LNG carrier in surge direction in calm water and in regular waves. The measured data are compared with theoretical results obtained using a computer program based on three-dimensional linear potential theory.