Motion compensated offshore knuckle boom crane

A mechanical feasibility study

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To overcome the problem of wave-induced vessel motions on the crane tip, technologies have been developed to compensate for these motions. Most vessels nowadays, are equipped with dynamic positioning systems, these systems reduce the yaw, surge and sway movement of the vessel. This leaves, the motions roll, pitch and heave uncompensated. To reduce these motions, two main elements of motion compensation can be identified, consisting of heave compensation and anti load swing compensation. Existing technologies, like active and passive heave compensation tackle one of these elements. Other methods like the Stewart platform can compensate for both the heave motion and the load swing of the crane by stabilizing a platform on which the crane is mounted. In existing literature, not much work has been done to incorporate both the heave compensation and anti-swing control procedures in a combined control method. It would be much simpler if the crane could compensate the heave and payload swing by using only the crane's actuators.

This thesis, studies the mechanical feasibility of a motion compensated knuckle boom crane, compensating for the wave-induced vessel motions by controlling the available actuators in such a manner, that the crane tip stays stationary. A typical knuckle boom crane has three actuated degrees of freedom, consisting of the slewing, boom luffing and jib luffing angle. To study such a system, a numerical model is developed that combines the multi body dynamics of the knuckle boom crane with models for the hydraulic actuation of the crane and incorporates the full six degree of freedom vessel motions. The actuated degrees of freedom are controlled using three separate proportional integral controllers. The desired trajectories for the controllers are calculated using the geometric relations of the crane, based on the assumption that the desired crane tip coordinate in the global axis system is known.

Using the numerical model, insight is gained in the natural frequencies, forces/moments on the system, the necessary hydraulic properties and motion compensation performance. From the simulation results, it can be concluded that the cylinder forces and slewing torques increase when the motion compensation is turned on, compared to an uncompensated crane. Comparing the simulation results, to the Barge Master T40 crane, it can be concluded that cylinder forces, cylinder sizes, stroke velocities and power requirements are in line with requirements that can be achieved with existing technology. The main limitation, is expected to be the rated torque of the slewing motor, when the crane is positioned parallel to the vessel. In this position the main contributor to the motion compensation is the slewing motor. Using the proposed concept, motion compensation performance comparable to the existing Barge Master T40 is achieved. From the presented simulation scenarios, results indicate that the operational parameters and motion compensation performance of the system are feasible and competitive compared with existing technology.