LOW-FREQUENCY RESPONSES OF NONLINEARLY MOORED VESSELS IN RANDOM WAVES- DEVELOPMENT OF A 2-SCALE PERTURBATION METHOD

A method of determining hydrodynamic responses of a non-linearly moore d floating vessel in random waves is presented based on potential flow theory. The moored vessel is modelled as a weakly non-linear Duffing' s oscillator. A two-scale perturbation method is utilised to solve the water wave boundary value problem characterising small wave-slope and non-linear mooring stiffness parameters. As an example of the approac h, a semi-analytical solution is obtained for a truncated cylinder in finite water depth. It is shown that the nonlinearity in the mooring s tiffness can contribute significantly to a non-linear hydrodynamic ter m in the perturbation expansion, and hence to the wave drift response of the floating vessel, depending on the parameters of the vessel and the characteristics of the wave spectrum. (C) 1998 Elsevier Science Lt d. All rights reserved.