The dynamics of deepwater offloading buoys

Deepwater offloading buoys are being used in West Africa to allow efficient loading of spread-moored FPSOs. These buoys have a relatively small displacement when compared to other floating systems, with the majority of the displacement being used to support the mooring system and the oil offloading lines. This results in a floating system that has a very active response to the environment, coupled with feedback from the mooring and flowline systems. This wave frequency dominant motion behaviour has been shown to result in severe fatigue damage to the mooring and flowline components, and thus must be estimated accurately to ensure that the system is designed with sufficient fatigue life. The paper presents results from timeand frequency-domain simulations of the offloading buoy system to demonstrate its response characteristics. The results and comparisons demonstrate the importance of accounting for the coupling between the mooring and flowline system, and the buoy hull in predicting the system response. Results from sensitivity studies are also be presented to show the relative importance of the mooring and flowline system mass, stiffness and damping contributions to the system response, and thus the fatigue life of the various components. The response of the buoy with an offloading tanker connected is also be presented and compared to that of the buoy alone.