Research on modeling and control of thruster-assisted position mooring system for deepwater turret-moored FPSO

It 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 harsh weather, however, the mooring system behaves so limited to constrain the position that the thruster force is also needed in order to avoid the damage to the mooring lines. According to the kinematics and dynamics, the mathematical model of FPSO with the thruster assisted position mooring system is derived and established based on the idea of the Manoeuvring Model Group (MMG). Environmental disturbance model are established, in which the disturbance forces of the wind, wave and current are considered. The mathematical equation of turret-moored mooring system is established based on the catenary theory. The PM controller is designed by using integrator backstepping method. In moderate and extreme conditions, simulation tests have been conducted for the designed thruster assisted position mooring control system. The simulation results show that the dynamic tension of mooring line is effectively reduced due to the introduction of thruster assisted positioning mooring control measure. The positioning accuracy is improved obviously.