In this paper, the indirect time domain method is adopted to analyze the dynamic responses of a mooring crane ship. The motion equations of its lifting load system are derived from Lagrange's equations and the features of the coupling motion between the crane ship and its lifting load system are presented. The hydrodynamic coefficients of the crane ship are calculated on the base of 3D potential flow theory. The lumped-mass model is used to analyze the dynamic features of its mooring lines. The nonlinear features of the motion equations being taken into account, the displacements of the crane ship and the lifted cargo are calculated by using the time domain analysis. In particular, the angular displacements of the lifted cargo are calculated and the traces of the lifted cargo in space are represented when it is lifted or lowered at the speed of 0.6m/s. The response spectrums of the lifted cargo in sea state II are obtained by using the frequency domain analysis in irregular waves. They show that the range of angular displacement frequency in resonance state is wider than that in non-resonance state. In resonance state, the energy of the lifting load system is more concentrated and the peak values of the response spectrums of the lifted cargo are more obvious. The curves of the response spectrums present obvious nonlinear features. The analysis methods and calculated results of this paper are of reference value for offshore lifting engineers and designers.

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