FPSO moored by STP mooring system plays a dominate role in oil and gas production in South China Sea. To study its hydrodynamic characters, a joint industry project on full scale measurement for Fenjin FPSO, headed by Shanghai Jiao Tong University and sponsored by CNOOC, was initiated in October 2007 and lasted for 25 months. During the project, a special phenomenon, named as SWING, was noticed. When a SWING happened, the FPSO’s heading angle would change greatly within a relatively short period, beyond the normal range of the weather-vane effect. Due to the large low-frequency motion, the tension forces of mooring system would increase greatly. Also, it may lead to positioning failure during the tandem offloading operation. So, the SWING motion is a great threat to the safety of FPSOs. A preliminary prediction for its cause as well as a fundamental analysis of its hydrodynamic mechanism is completed in this paper.

According to the statistical results of the SWING based on the Fenjin’s motion data during the full scale measurement as well as the corresponding wind wave and current data in the same period, a plausible prediction for its cause is provided. It is pointed out that the change of direction and velocity of the current within a short time might be the cause of SWING.

The research for the mechanism of the SWING is carried out through both the methods of analytical analysis and the model test. A hydrodynamic model is built to simulate the low-frequency motions of FPSO in time domain. And an in-house MATLAB program is written on the basis of the hydrodynamic model, to do the calculation. Analysis results are compared with the full-scale measured data, to prove the cause prediction.

Furthermore, on purpose of validating the program, a model test was conducted in Deepwater Offshore Basin in Shanghai Jiao Tong University. Several different current velocities were chosen in the test. The program results show a good agreement with both the data of full scale measurement and the model test results, which indicates that the hydrodynamic model and the code are both credible.

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