Coupled Control for Motion Compensated Offshore Operations

The use of motion compensated equipment has become more frequent in modern DP operations, especially the ones related to the installation, maintenance and decommissioning of offshore wind turbines. In these operations, while low-frequency motions are compensated by DP systems, wave-frequency motions are compensated by gripper frames, heave compensators, hexapods, gangways and other type of motion compensated equipment. A study was performed to investigate if the coupling of a dynamic positioning system with a motion compensation device could improve the accuracy and efficiency of offshore motion compensated operations. The reference case consisted of the positioning of a payload, hanging on a ship-mounted crane, by means of a single horizontal active tugger line and the ship's DP system. Four controllers – consisting of two levels of awareness and two tuning conditions – were designed and implemented in a numerical model. The four strategies were then evaluated in terms of position offsets and actuator usage under a number of sea-states. Results demonstrated that coupled controllers are able to decrease the position deviations of the motion compensated object and decrease the usage of the motion compensation device. The paper describes the control strategies, numerical models and results of the simulations.