A research to decrease the duration of the lifting phase for the Jumbo J-class, focussing on heeling

Abstract

For offshore installation projects Jumbo uses her J-class heavy lift crane vessels. The objects that need to be installed offshore are carried at deck or in the ship's hull. At location the object is lifted by the on-board heavy lift crane and slewed to the side of the ship. From the final overboard position the object can be lowered to the seabed. When the load is displaces by the slewing crane, the equilibrium of moments is disrupted and the ship will roll or heel. To reduce this moment change the ship pumps ballast water from one side to the other and causes a counter moment. This simple system has limited capacities and it was assumed that an improvement could reduce the duration of the lifting phase. This thesis is about a research to decrease the duration of the lifting phase for the Jumbo J-class, focussing on anti-heeling. To understand the context of the problem, research has been done on the company and the market she is operating in. Then the phenomenon of anti-heeling, the current ballast system and the operational limits were analysed. With the information obtained from this research a list of requirements and the load cases were selected. The analysis showed that the current system capacity is based on three manual controlled centrifugal pumps that can either been switch on or off. This system cannot easily vary the moment change and was believed to be limiting for lift operations. To find a new solution design methods are used to generate a variety of concepts. For five concept calculation has been done to predict their capacity and dimensions. The analysis showed that a new solution is unlikely to be financially attractive for the market Jumbo is operating in. Therefore the decision has been made to research the effects of the most economical potential solution. Variably frequency drives were applied to the pump system to see if the variations of the rotational speed of the pumps and therefore varying the flow rate would make easier to control the roll motions. Or even shorten the lift phase. To examine the ballast system and the sensitivities a model has been made. As this research focused on roll and heeling motions only, a 2D model is used to predict heave, sway and roll motions. The model is based on the equilibrium of moments where the ship is exposed to wave induced moments and the moments caused by the crane and ballast system. The crane and ballast system are effected by the ship motions and therefore coupled to these motions. The model allows to automate the control of the crane motions and the pump rotations. This allowed the model to test different lift scenarios under different wave conditions. The results showed that the variable frequency drivers can result in smaller differences of the heeling angles. But the results also showed that the ballast system was not the most important limiting factor. It was the moment change caused by the crane. When the crane tip with hanging load was displaced with a certain speed, it caused an impulse making the ship oscillate around the static heeling angle. The amplitudes of these oscillations and the static heeling angle could be reduced by the ballast system. But the best results were shown when the stability properties of the vessel were improved. Increasing the effective metacentric height from 2.25 to 3.25 showed significantly reduce roll excitations. Other conclusions are that previously very conservative roll angles of 1o were used, while the mast crane proved to handle roll angles of 4 o and larger depending on the load. Also the lay out of the current pump system leaves a lot of room of improvement. These improvements can be found in the recommendations as well as suggestions how to discover potential time savings during offshore operations.