Active Pendulation Damping of Payload on Offshore Vessels

Abstract

Ship-mounted boom cranes play an important role in the offshore cargo transport process. In recent years, there has been significant need to increase stability of the payload during the cargo transport process for both safety and efficiency reasons. However, the stability of the payload during the transport process directly correlates to the ship's pitch and roll motion that in turn relates to the current particular sea-state. For some vessels the motion is so severe that operation in sea state 3 can be jeopardised, while cranes designed to have very short free hoist cable may be able to operate in sea state up to 5. This is obviously detrimental to the overall operability of offshore systems, and a simple and practical solution to solve this problem will be enormously beneficial in improving the efficiency of offshore lifting operations. In this thesis existing work, solutions and patents were studied. There is interesting work done in the field of payload pendulation control and some of them are patented. However these patents does not impose limitations in developing a new and improved solution for this problem. A model for the crane system including dynamic, kinematic and differential kinematics for the crane system is derived. This was done in a general way such that it is easy to adapt other cranes. A three dimensional model for the pendulating payload including movable fulcrum is derived using Lagrange's equations of motion. The system proposed does not include crane actuators in the control system. Dedicated actuators in a gantry frame arrangement at the crane tip is used to induce corrective actions. The objective for the system is to track the motion of the vessel and reduce overshooting pendulations induced by waves and operator. Two controllers were proposed for the system based on a PD controller and nonlinear feedback linearization. These had limited effect on the system. A possible solution to the insufficiency of the controllers might be to delay the corrective action. This approach is called a bang-bang method, and is made of two pieces, each separated by a certain time period depending on the pendulum swing period. This way the corrective action is added after the bob is in motion, the actuators will `catch' up with the bob and stop it's motion. If a better controller is designed it is plausible to develop a system that is able to reduce unwanted payload pendulations.