Standard analysis models applied for motions of moored floaters are based on potential flow perturbation methods with wave frequency response governed by first order wave forces and low-frequency response governed by second-order difference frequency wave forces. These models have been shown to have limitations in extreme sea states where nonlinear wave excitation and viscous drag forces above still water level may dominate. This effect is particularly visible for the low frequency excitation since the potential flow contribution goes to zero for long waves. In the present study non-linear wave excitation and viscous drag contributions on a semi-submersible is modelled by Morison’s load formula since the columns and pontoons are slender elements. A numerical simulation model is developed using SIMO [6], in which viscous forces and damping are included by the drag term of Morison equation and with drag coefficients recommended from DNV-RP-C205 [1]. Low frequency surge responses calculated by the combined potential flow drift forces and viscous drag from Morison load model are compared with model tests for waves only and for combined wave and current conditions.

A simplified formula for current and viscous effects on wave drift force, generalized to non-collinear conditions is presented and compared with model test results.

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