West Africa Sea, which is located in the tropical area, has special environmental conditions. Relatively mild sea state and long period swell as the main component of waves lead to significant high energy in low frequency region. Frequently there will be a special sea condition which contains long period swell and local storm wave. Research in this paper mainly focuses on motion and force characteristics of offloading system (contains offloading line, CALM buoy and its mooring system) under the special West Africa environmental conditions. Radiation/diffraction theory is adopted to calculate potential force acting on CALM buoy. The viscous drag force is considered by special disk-type Morison elements which are arranged according to certain rules. Physical model tests of truncated offloading system are carried out to verify the numerical model. The motion and force characters of offloading system under West Africa swell and wind sea are studied in both numerical method and model tests method. CALM buoy motion characteristics in specific environmental condition is analyzed. Relationships between CALM buoy motions and flexible structure (mooring line or offloading lines) tension forces are also researched.

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