Novel controller for the coupled surge-pitch problem by controlling the metacentric height of sandglass-type FDPSO

Compared with the conventional vessels and platforms, the water plane area of the sandglass-type FDPSO (floating, drilling, production, storage and offloading unit) under upright condition is smaller, which makes its longitudinal metacentric height lower and natural frequency of pitch motion smaller. As a result, coupled surge and pitch motion problem would be caused unexpectedly. In this paper, from the perspective of controlling the metacentric height, a novel dynamic positioning (DP) controller is proposed. The proposed controller can make the natural frequencies of surge and pitch motions away from each other. Consequently, both the surge and pitch motions satisfy the positioning accuracy. Besides, the coupled resonant and non-resonant zones with the control parameters are investigated. Considering the system stability, positioning accuracy and power consumption, a complete analysis process of optimal control parameter selection is proposed. By comparisons with two existing control laws, the novel controller is established with better performances on robustness, positioning accuracy and energy consumption. •The essential reason of coupled surge and pitch motion problem for the sandglass-type FDPSO was found.•By controlling the longitude metacentric height, a novel controller was proposed to solve the coupled resonance problem.•Considering the stability, ability and consumption, a new design method was proposed to obtain optimal control parameters.