Ensuring Mooring Line Safety for Deepwater Turret-Moored FPSO in Extreme Seas

Wang, Y.; Dou, X.; Ding, F.; Tuo, Y., and Liu, Y., 2015. Ensuring mooring line safety for deepwater turret-moored FPSO in extreme seas.

This study addresses dynamic positioning technology of deepwater FPSO moored to the seabed via a turret based spread mooring system, referred to as Position Mooring (PM). In normal weather conditions, the mooring system constrains the FPSO to the predefined region. In extreme seas, however, the mooring system behaves in a limited fashion to constrain the position such that the thruster force is also needed in order to avoid damage to the mooring lines. A controller based on structural reliability is presented for turret-moored FPSO. First, a mathematical model of FPSO in terms of the kinematics and dynamics and the environmental disturbance is established. Second, a mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, the mathematical model for the whole turret mooring system is established. Third, a PID and a state feedback controller based on structural reliability are designed to ensure mooring lines safety. Finally, a series of simulation tests are carried out for a turret-moored FPSO. The simulation results show that the dynamic tension of the mooring line is effectively reduced due to introduction of a reliability index in terms of controller design.