On the Low-Frequency Hydrodynamic Damping Forces Acting on Offshore Moored Vessels

Moored floating structures for drilling, production-storage-offloading or other purposes are being installed in ever increasing water depths and in areas where the environmental conditions are also more severe. Such structures, moored permanently in high seas, have to survive safely the most severe weather conditions. Therefore of importance to understand the mechanisms which govern the motions and the mooring forces of these facilities.
In deep water the mooring systems inevitably have soft elasticity characteristics. With the increase in the elasticity of the mooring, the low frequency horizontal motions induced by low frequency second order wave drift forces also become larger. The low frequency resonant motion components completely dominate the horizontal motions and, consequently, also the mooring forces. In order to predict the amplitudes of the low frequency resonant motions the magnitude of the second order wave drift forces and the values of the low frequency hydrodynamic damping must be known.
The low frequency hydrodynamic potential damping due to the radiated waves is negligibly small. In general the low frequency damping is determined by viscous effects and a damping caused by the presence of waves. The last mentioned damping is called the wave damping. Dependent on the wave spectra the wave damping can significantly dominate the viscous damping contributions.
In this paper results are given of a study of the origin of the wave damping. For this purpose the second order wave drift forces acting on the moored vessel in head waves have been expanded to the low frequency surge displacement and surge velocity. The wave damping can be defined by taking into account the dependence on velocity of the second order wave drift forces. To verify the results, model experiments were carried out in which the velocity dependent second order wave drift forces were determined.

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