Effect of wind, current and non-linear second order drift forces on a moored multi-body system in an irregular sea

A study is made on the slowly varying oscillation of a moored multi-body system in wind, current and waves. Linear potential theory is used to describe the fluid motion and three dimensional source technique is applied to obtain the transfer function of drift forces in waves, which is used to obtain the slowly varying drift forces in horizontal plane for the multi-body floating system. Hydrodynamic forces are calculated including the interaction effect between the floating bodies. Davenport wind spectrum is adopted to calculate the fluctuating wind forces and OCIMF data are used for the estimation of wind and current forces. Interaction of currents between the vessels is not taken into account for the sake of simplicity. The non-linear time domain simulations are carried out in long crested irregular waves. Based on it, slowly varying motion responses is examined including the effects of the wind, current and non-linear second order drift forces. From the present study, it is found that the effect of second order drift forces is quite significant in comparison with effects of wind and current. The second order drift forces causes large slowly varying oscillation, and moored forces that relate for failure of the mooring system