Vessel and cargo motions

Abstract

When lifting operations are performed by a heavy lifting vessel, the motions of the vessel and cargo are predicted by conventional six degrees of freedom calculations. It is assumed that the cargo is attached to the crane top in order to decrease the stability of the vessel and perform accurate calculations. While these conventional calculations can be used to calculate the vessel motions and themotions of the crane top, the cargo motions are not taken into account. In conventional calculations the dynamical aspects of the cargo are not taken into account and the forces due to cargo motions are neglected. In several recent projects carried out by BigLift Shipping swell waves have presented BigLift Shipping with challenges. Due to these swell waves the vessel and cargo where excited which caused problems during these lifting operations. Therefore BigLift Shipping is intrested in a method to calculate the vesselmotions in combination with the cargo motions. There are two objectives of the research presented in this report. The first objective is to increase knowledge of the dynamical effects of the cargo on the vessel motions and vice versa. The second objective of this research is to investigate if it is possible to reduce the cargomotions during lifting operations by changing variables in a lifting plan or by controlling the cargo with cargo control systems. To fulfill both objectives a tool is created. The tool that is created is a mathematical model of the vessel and cargo body. In this model the vessel and cargo are modelled as a multi-body system and is based on the frequency domain. A multi-body frequency domain model is able to calculate the responses of both the vessel and cargo motions quickly. For the input of the model the vessel constraints of the vessel and cargo body with each other and the outside world has to be described in a mathematical way. The constraints can be divided over four different modules. The first module of constraints follow from the vessel hydrodynamics. The connection of the cargo to the vessel via the crane cable are described in the second module. The influences of mooring lines on the multi-body system are described in the third module. Lastly the influences of cargo control systems are described in the fourth module. With the different modules the influence of variables of a lifting operation on the vessel and cargo motions can be studied. The output of the tool consists of the vessel RAOs. Together with wave spectra calculations these RAOs can be translated to response spectra and most probable maximum values. The tool that is created is used to fulfill the two research objectives. First a set of calculations are done to determine the baseline responses of the multi-body system. Different variables of a lifting operation are varied and the influence of the cargo mass, crane cable length, crane top position, vessel draught and vessel metacentric height are studied. This study showed that these variables can have a large influence on the responses and natural frequencies of the multi-body system. Cargo control systems are also implemented in the multi-body model. Calculations that included cargo control systems showed that the increased stiffness of such systems can vary the natural frequencies of the different degrees of freedom in the multi-body model. When damping is applied via cargo control systems the most probable maximum values of the motions can be reduced with maximumof 55.3%.