A computational method for predicting motion dynamic wave load and sling tension of a crane vessel during heavy lifting operations is presented. The coupling motion effect between the vessel and the weight being lifted and the viscous damping have been taken into account. Solutions for a barge shaped and a semi-submersible crane vessel have been calculated as numerical examples and the results agree well with the available experimental data. In addition, a method to evaluate the workability of a crane vessel in given ocean environment is presented.
In the past, many theoretical methods have been used to predict the motions and wave loads of mono hull and semi-submersible ships1,2,3, but few papers deal with crane vessels during heavy lift operations taking into account the effects of the weight being lifted and the viscosity. The experiments show the effect of the weight is so great that the motion responses of the vessel are quite different from the those without this effect. The viscous effects also influence the motion responses significantly when the effect of the weight is included (especially for semi-submersible type of vessels).
In this paper, the 3D diffraction theory was used to calculate the hydrodynamic forces on the submerged hull of vessels and the viscous effect was taken into account. The motion equation has been modified to include the effect of the weight being lifted which has 8 degrees of freedom. The motion responses of the vessel and the sling tension during the heavy lift operation were analysed for both ship shaped and semi-submersible types of vessels and the results agree well with available experiments.
A method to predict wave loads on cross deck of semi-submersibles is also presented. The results show that the effects of standing waves in the vicinity of the twin hulls may produce the maximum bending moment on the cross-deck structure and that this kind of wave loads can be predicted by the present method. The motion resonant frequencies of entire system were shown to be different from those without the weight effect. Finally the workability of the crane vessel in given ocean environment was calculated in terms of regular wave results and ocean environmental data.
In order to describe the motion of the vessel and the weight being lifted, it is appropriate to divide the entire system into two parts, the vessel and the weight alone. Both of them satisfy Newton's second law. Small wave and motion amplitudes are assumed in order to apply linear theory. A right handed Cartesian coordinate system o-xyz with x-y plane m still water surface and z positive upward through the centre of the gravity is used.