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Simulation of Dynamic Behaviour of a FPSO Crane

Lifting operation on a FPSO (Floating Production, Storage and Offloading vessel) is much more demanding for the crane and the crane driver than similar operations on a fixed offshore platform. Due to the motion of the FPSO in heavy sea and strong wind, the crane is subjected to additional dynamic forces as well as swinging loads. The present paper deals with numerical simulation of the dynamic behaviour of the Norne offshore crane during lifting operation. The simulations are performed by FEDEM – a general nonlinear dynamic analysis program for flexible multibody systems. The pedestal, “king” and boom are flexible links modelled by shell finite elements and connected together by different joints. The hoisting rope and the hydraulic cylinders are modelled by linear and nonlinear spring and damper elements. A control system is implemented in the model making it possible to control the movement of the boom and the winch to compensate for the relative motion between the ship and the supply vessel and keep the load in rest relative to the vessel. Examples are shown of calculated natural frequencies and mode shapes as function of hoisting rope length and boom angle. Furthermore, maximum dynamic stresses in different sections/details are presented as function of how the crane is operated. Besides giving necessary stress data for design verification against overload and fatigue, the presented model can be used to optimise the operation procedure, determine the maximum allowable load for various sea states and to calculate necessary power to control the motion of the load. Introduction. During the last decade, the focus in the North Sea offshore activities has changed from bottom supported platforms to permanent located 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 installation on which the crane is located, experiences wave induced motions. The motions of a floating vessel can be completely described by six motion components, three translational and three rotational motions. These are called the degrees of freedom for the vessel, see Figure 1.