Slowly varying wave drifting force on a very large floating structure in short crested waves

Hydroelastic responses of very large floating structures (VLFS) have been studied activity in Japan. Most targets of them are hydroelastic behavior and an estimation method of linear hydrodynamic forces for the prediction of it. However a design of mooring system is very important to install a VLFS in a real sea area. Then, it is advisable that slowly varying wave drifting forces, which are the 2nd-order wave excitation, are predicted accurately. The authors have validated the theoretical method of the wave drifting force on the elastic floating bodies in long crested and crossing irregular waves by comparing with results of the model tests. It seems that irregularity and multi-directional quality greatly affect to the wave drifting force on a VLFS comparing to general scale floating structures. In this paper, characteristics of the slowly varying wave drifting forces on a VLFS are investigated systematically by numerical calculations with elastic behavior. A wave drifting yaw moment increases due to geometrical asymmetry of elastic behavior in short crested waves. Slowly varying wave drifting forces occurs due to interactions of wave frequencies in component waves. In case of the multi-directional irregular waves, there are effects due to the frequency's interaction of component waves in not only a same direction but also different direction. The terms of the latter effect are investigated in this paper.