This paper deals with the observer-based dynamic optimal recoil controller design of a deepwater drilling riser system subject to friction force of fluid discharge and platform heave motion. First, by using exponential and polynomial functions conjunctively, a novel nonlinear computational model of the friction force is presented. Then, a linear system model is developed to describe dynamic characteristics of the friction force. Third, by using feedforward mechanisms and internal model principle jointly, a dynamic optimal recoil control scheme is proposed for the riser, where the feedforward mechanism is designed to compensate the friction force of fluid discharge, and the internal model principle is introduced to reject platform heave motion. The conditions of existence and uniqueness of a dynamic optimal recoil controller of the riser are developed. Fourth, a friction force observer is designed to solve the physically realizable problem of the recoil controller. Simulation results show that under the observer-based dynamic optimal recoil controller, the recoil of the riser can be attenuated significantly, and zero steady state errors of the riser can be guaranteed. In addition, from the perspective of achieving steady performance of the riser, the designed dynamic optimal recoil control scheme outperforms existing linear quadratic optimal control schemes.
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