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Frequencies of floating production, storage and offloading's (FPSO's) horizontal motions are much smaller than the natural frequencies of waves and thus 2nd-order difference-frequency wave drift forces could excite large response. In this paper, the 2nd-order wave drift forces and low-frequency (LF) horizontal motions of a multi-point moored FPSO are computed in bi-directional swell and wind-sea conditions offshore West Africa (WOA). Newman's approximation has been applied as the computation theory. Numerical approximation compares reasonably well with measured results of model tests. Under environmental conditions in WOA, it is concluded that swell induces larger drift loads than wind-sea at the surge/sway natural frequency, and they both are indispensable to the LF motion. However, the coupling interacting drift loads between different incident angles could be neglected due to the large peak period deviation between swell and wind-sea. Simplified approximation will be effective by superposition of drift force contributions from uni-directional long-crested swell and wind-sea. Newman's approximation provides a tool to accurately simulate drift loads on FPSO in the bi-directional waves in WOA. An important reference has been obtained for both LF motion prediction and mooring system design in bi-directional swell and wind-sea WOA. [ABSTRACT FROM AUTHOR]