Jib Rest Design for Crane Barge Type CB6324 - A New Design Approach

Abstract

The Damen Crane Barge type CB6324 is a transhipment barge, designed to operate in harbours, inshore and in near coastal waters worldwide, mainly for the on- and offloading of bulk carriers. In the second generation of this crane barge, optimization of the equipment and overall design is sought, increasing usability of the barge and reducing overall costs. The same is sought in the design of a jib rest, as it is found to be heavy and cost reduction can be realized by reducing weight. Additionally, it is suspected that the jib rest can be hit by the crane grab during operation. The new optimized design is restrained by the need to be able to withstand the load of the crane jib in travel conditions overseas and during its complete operational lifetime. As the barge needs to be able to operate worldwide, identification of Ultimate Limit Loads is key. Lloyds Register provides a conservative method to determine the loads acting on the floating crane barge and its equipment, based on extreme weather conditions. As specific data on the CB6324 is available, a different calculation method is proposed, using motion responses, sea state data and design criteria as an input. The method is based on probability of encountering a sea state, the probability of non-exceedance of a wave in such a sea state, and the operability of the barge, to predict the maximum probable accelerations encountered. Finally, analysis of two first generation crane barges built, show some fatigue crack forming on the jib rests during transport to the operational locations. This signifies an interest of a fatigue analysis and estimation, adding up to three design objectives: A) Finding load cases using modeled motion responses and sea state probability B) Finding a new jib rest design that complies to the criteria set by Damen and with the load cases found with design objective A C) Finding an estimation on the fatigue lifetime on both the first and new generation jib rest For the determination of the accelerations, a tool is developed, which automates the calculations for the maximum expected loads in worldwide near shore operation and transport. The barge motion responses in regular waves are determined and used as the primary input. Secondary input is composed of wave data, direction and spreading. Wind speed, roll, pitch, flooding angles and (bending) stresses in the crane pedestal are used as additional criteria. As an output, it gives an overview of operability over the world, along with a selection of the maximum expected accelerations in the critical directions. Parallel to the load calculations and development of the tool, the design of the new jib rest is made. Starting with a concept and building up detail as more information is brought to light in every calculation step. The resulting design is composed of a 15 ton, hinged, steel A-frame with a weight reduction of approx. 5 ton compared to the first generation. Fatigue prediction shows a significant improvement on the fatigue lifetime, as a transport load case estimates the new design spending approx. 10% of its fatigue lifetime. More weight reduction could be achieved by further research on whether the fr is a reasonable restriction for this barge. The stiffness of the jib rest is increased significantly to comply to this criterium in dropped down position. This weight reduction can only be realised if a less conservative load case calculation method is used, like the one proposed in this thesis.