Low-frequency second-order wave-drift forces and damping

The effects of free-surface waves on floating structures are of great importance in the offshore industry. Besides the first-order responses to the waves, second-order effects play a role, in particular in the situation where an anchored object may be excited in its frequency of resonance. This paper concerns the study of this phenomenon. Two different approaches are developed, each with its own advantage. Special attention is paid to excitation forces generating the motions; also a theory to determine the inviscid damping that limits the extreme excursions is discussed. Numerical results are presented for a sphere, and numerical results for two classes of tankers, namely for a VLCC and a LNG-carrier, and a semi-submersible are compared with experimental data obtained at the Maritime Research Institute in the Netherlands (MARIN).