This present paper deals with mathematical modelling of the crane's mechanical, hydraulic, and control systems as well as the numerical simulation of offshore cranes performing sealift operations. The simulations are performed by FEDEM - a general non-linear dynamic analysis program for flexible multibody systems. The pedestal, the revolving crane superstructure called crane king, and the boom are flexible links modelled by shell finite elements and connected together by different joints. A mathematical model of the hydraulic system and a control system is implemented in the mechanism model, making it possible to realistically control the operation of the crane. The results from the dynamic simulations verify that the virtual crane gives a very good picture of the dynamic behaviour of the real crane in offshore environment. This study shows how dynamic amplification can be reduced by proper operation and design of the hydraulic and control system. Simulation examples are shown for sealift from supply vessel when both the supply vessel and the FPSO (Floating Production, Storage and Offloading vessel) are subjected to wave induced forces.
During the last decade, the focus in the North Sea offshore activities has changed from bottom-supported platforms to permanent vessels and floating installations, for example FPSO's. With regard to offshore crane operations, this complicates the situation both for internal load handling within the installation, and sealifts from or to a supply vessel. The reason for this is, of course, that the FPSO experiences wave-induced motions. It is obvious that a crane onboard a floating installation, and especially the boom tip of the crane will have wave induced motion components in both horizontal and vertical direction. The magnitudes of these motions are very dependent on the sea state, wave heading and the position of the boom tip.