Abstract: During a full-scale measurement project for Fenjin Floating production, storage and offloading (FPSO), an unusual phenomenon, herein named "swing", was discovered. Until now, little research on this phenomenon has been done; in this study, it was defined and its characteristics and fundamental mechanisms were analyzed, including the threat that it poses to the safety of the FPSO. Data collected during the period of the measurement project (October 2007 to October 2009) were statistically analyzed and several criteria for distinguishing swing were assessed. The number of occurrences of swing during the measurement period was obtained as well as the timing of the FPSOs yaw motion for each occurrence. Also, the tensile forces in the mooring lines during one swing motion were calculated. The effect of swing on the mooring system and the offloading operation process were also assessed. A preliminary speculation as to the cause of the swing is proposed, based on wave, wind and current data. A fundamental hydrodynamics mechanical model for the low-frequency motions of FPSO was constructed, and a MATLAB program was written in-house to calculate the yaw motion of the FPSO in the time domain to enable the swing mechanism to be studied. [Copyright &y& Elsevier]