NONLINEAR RESPONSE OF A SPAR IN DEEP WATER: DIFFERENT HYDRODYNAMIC AND STRUCTURAL MODELS

The research described in this paper addresses the behaviour of a spar in deep water. The spar is modelled as a rigid body with three degrees of freedom. It is connected to the sea floor by mooring lines which are attached to the spar at the fairleads. The inertia forces are calculated using a constant inertia coefficient, CM, as in the standard form of Morison's equation or using a frequency dependent CM coefficient based on diffraction theory. The drag forces are computed using the nonlinear term of Morison's equation in both cases. The analysis is performed in the time domain. The results of the two methods are discussed and compared. The effects of modelling the mooring lines as nonlinear springs neglecting the hydrodynamic forces acting on them are also investigated. Results including several second order effects are presented and compared with experimental data.