The numerical study of viscous drag force influence on low-frequency surge motion of a semi-submersible in storm sea states

Abstract This paper investigates the low frequency (LF) surge motion of a semi-submersible platform in storm sea states without current, where the influence of viscous drag forces on the LF surge motions is focused. Four hydrodynamic models including one potential flow panel model and three panel and Morison's hybrid models were established. LF surge motion was calculated in time domain using AQWA. Five sea conditions were considered, including two relatively small sea states, one moderate sea state and two storm sea states. Under each sea condition, four sets of drag coefficient in the Morison's model were used for calculation. By comparing mean offset, extreme offset and standard deviation of LF surge motion between different hydrodynamic models in different sea states, the influence of Morison's elements above still water level (SWL) or below SWL, drag coefficient and sea states were investigated. By comparing numerical results with model test results, it is concluded that the drag forces on Morison's elements below SWL is the major source of platform viscous damping forces, while the drag forces on Morison's elements above SWL is the major source of viscous drift forces, which provide an important contribution to the mean and extreme offset of the platform LF surge motion under storm sea conditions.