The Soft Yoke Mooring and Offloading system (SYMO®) has been developed in SBM to allow the tandem mooring of an LNG carrier to the stern of an LNG FPSO or a Floating, Storage & Regasification Unit in harsh environments. The SYMO® system consists of two mooring legs supported by a crane. A yoke structure is suspended at the bottom of the two mooring legs by uni-joints. The tip of the yoke is connected to the bow of the LNG carrier via roll, pitch and yaw articulations. When connected the yoke structure is horizontal. Upon disconnection, the yoke settles at an equilibrium position characterized by a pitch angle of 35° to 40° with respect to the horizontal. Owing to the low friction articulations, the yoke oscillations decay very slowly and the yoke may come too close to the LNG carrier bow. A passive system has been developed to improve the SYMO® dynamics at disconnection whereby a small proportion of the overall yoke weight is replaced by water. This water is allowed to move in a suitably designed tank network thus shifting around the overall centre of gravity and yoke equilibrium angle as pitch oscillations take place. The above principle has first been investigated using a kinematics program before being model tested at a large scale of 1:16. A large matrix of tests has been performed to investigate the effect of the water ballast volume and the layout of the tank network. Once the principle was confirmed experimentally, CFD computations were performed in a blind fashion and compared to the experiments.

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