An Investigation Into Wave Induced Drift Forces and Motions of Very Large Floating Structures

Abstract

This chapter investigates the drift forces and motions of very large floating structures (VLFS) in head waves through numerical simulations and model experiments. Three dimensional numerical method based on direct integration of pressure was applied to estimate the steady wave drift forces taking the elastic modes into account. Numerical simulations and experiments were performed with respect to two different types of VLFS, pontoon unit type, and semi-submersible unit type. Consequently, the different tendencies of motion and wave drift force of these two types were shown. Near field theory can predict the steady drift forces on VLFS properly by accounting its elastic deformation modes. In that, three-dimensional source method is effective in calculation of hydrodynamic forces. The rigidity affects both deflection and drift forces considerably, however, the flexibility brings conflicted effects toward deflection and drift force; thus, it is necessary to apply an optimizing technique from viewpoint of reduction of both hydroelastic response and wave drift force. The relative motion of structure affects the drift force dominantly for a beam like VLFS in head sea wave condition; thus, it is vitally important to move away the natural frequency of deflection from the frequency range of incident waves.