Implementation of smart hawser and active control to suppress fishtailing instability and snap load of FPSO-Shuttle Tanker during tandem offloading

Fishtailing instability in Shuttle Tankers (ST) during tandem offloading from spread-moored Floating Production Storage and Offloading (FPSO) units is a critical phenomenon characterized by large-scale, long-period cyclic sway and yaw motions. This study investigates the stability of the FPSO-ST coupled system by incorporating current forces into the equations of motion. A linearized model was first employed to assess stability and identify controllable parameters, specifically hawser tension and length. Subsequent nonlinear stability analysis, utilizing phase portraits and fixed-point evaluation, revealed that the ST origin behaves as a marginal center-type fixed point, indicating high sensitivity to perturbations. Numerical simulations were performed using CHARM3D program under representative operational sea state. Results demonstrate that out-of-phase relative surge motions during system build-up generate significant high tension peak/snap loading. Also, large hawser angles occur indicating fishtailing instability. The large tension peaks and hawser angles subside as mean hawser tension increases, validating the analytical predictions. Two smart hawser systems - Magnetic-Rheological Elastomer (MRE) and Rigidity-Tuning Composite Elastomer (RCE) tensioners - were implemented. Utilizing PI and Skyhook control logic to modulate stiffness via electric current, these smart hawser systems successfully maintained high tension peak and tensioner strokes within safety limits, and reduce fishtailing motion.