Wave drift damping on floating bodies performing slow motion in all the three horizontal modes surge, sway and yaw is considered. The coupled radiation-diffraction problem is formulated in a relative frame of reference, and the 3 x 3 monochromatic wave drift damping matrix is developed consistently to second order in the wave amplitude. The bodies may be of general shape, and the depth arbitrary. Explicit formulae suitable for numerical calculation are obtained and implemented into a low order panel program. We here apply the model to four different offshore structures, and the results show that the wave drift damping force has an important contribution to the mean second order wave force.
The present study is motivated by the needs in the offshore industry to design floating production systems in deep water. Production ships and oil platforms may experience slow drift motions with large amplitudes in the horizontal plane (surge, sway and yaw). These motions are excited by nonlinear loads due to wind, current and waves. Evaluation of the drift exciting force is of great importance, especially since it may be a slowly varying periodic force, with a frequency close to the natural frequency of a moored system. The motions are limited by hydrodynamic damping forces. The damping components include: Wave drift damping, viscous loads on the hull, drag forces on the mooring lines and risers, friction of the mooring lines on the sea-floor and damping due to the variation of the wind loads with the velocity of the structure. Here we consider only the first of these damping components, the wave drift damping. Wave drift damping can be defined as the first order correction in terms of slow drift velocity of the mean wave drift force.