Adaptive backstepping control based on structural reliability for a turret-moored FPSO vessel

For floating production storage and offloading (FPSO) vessels, a positioning controller is necessary because only a mooring system alone cannot always keep the ship within a predefined region. This paper develops a new adaptive backstepping controller based on structural reliability index to deal with the model uncertainties of the FPSO vessel. Model uncertainties can be estimated by the adaptive law derived from the Lyapunov stability theory. The capacity of the mooring system can be fully utilized to position the FPSO vessel by adjusting the structural reliability index on the premise of ensuring the safety of mooring lines, and hence less control effort is needed for the positioning controller. The robustness of the proposed control algorithm is demonstrated through simulations using three sets of different system parameters. In addition, a comparison to the PID controller without the structural reliability shows that less control effort is needed using the proposed controller.