Research on Ship Structural Fatigue Damage Under Nonlinear Wave Bending Moment

Loads acting on ship structures are complex and randomly over time and the nonlinear effect caused by wave loading is one of the research focus. The linear and nonlinear vertical wave bending moment (VBM) in different speeds and sea states and their effects on ship structural fatigue strength were investigated for a flat container with high ratio of width to depth. The VBM under the linear regular waves and irregular waves were calculated based on the three dimension (3D) potential theory. The considered nonlinear wave loading was caused by sea pressure near the mean free surface as well as the geometric nonlinearity. Hydrodynamic calculations in regular wave were presented to figure out the frequency response function (FRF) of VBM in the mid-ship section. Irregular waves were verified to obtain the VBM history in 4 sea states. What’s more, VBMs from a segmented elastic model test were obtained to investigate the influence of nonlinearity. On the basis of the wave loadings obtained from simulation and test, the hotspot stress histories under irregular waves were deduced in time domain by using the beam theory. Fatigue cumulative damage per hour under several random sea states were obtained on the basis of the rain-flow counting and S-N curve. Based on the fatigue damage from the numerical analysis and model test, it is believed that speeds and significant wave height have a positive correlation with the fatigue damage of ship structures. A good agreement was obtained between the numerical analysis values and the low frequency part of the test and the nonlinear analysis in the simulation could offer reasonable prediction for the fatigue damage caused by the wave frequency response. Also shown as the test result, fully nonlinearities have a great contribution to the fatigue damage.