Nonlinear control of an active heave compensation system with time-delay

Crane systems used during offshore installations in harsh sea conditions must satisfy rigorous requirements in terms of safety and efficiency. The vertical vessel motion which is due to ocean waves excites dynamic forces which have an extensive effect on the overall crane structure and its lifetime. Furthermore, the operator gets handicapped during fine positioning of the payload. Thus, compensation systems for the vessel motion during offshore operations become more and more important in the near future. This paper presents such an active compensation system. The proposed system controls directly the hydraulic driven winch of an offshore crane to compensate the vertical vessel motion and to achieve trajectory tracking of the payload in an earth fixed coordinate frame. The control algorithm is divided into two parts. The first part is used to decouple the load's motion from the one of the vessel. The second part is designed for trajectory tracking and stabilization. It takes the time-delay between the winch and the payload, which is due to very long rope lengths used in deep water lifting operations, into account. The active heave compensation approach is evaluated with simulation results.