Semi-autonomous control of offshore cranes

Offshore lifting operations are often carried out in harsh and inhospitable environments. The prevailing sea conditions cause motions of the vessel which in turn induce motions of the suspended load. The crane operator is required to try and reduce these load oscillations through manual manipulations of the slewing and luffing actions of the boom. While the operator is occupied in reducing load oscillations the overall lifting operation is not proceeding. Even the most skilled operators may not achieve enough reduction in load motions to proceed with a lift. A control scheme is proposed in which the load position in space is controlled by a combination of feed-forward and frequency domain feedback control. The feedback controller is designed to deal with disturbances and remove any residual motions from the feed-forward controller. Superposition of the control signals produces the total control action.