Fatigue analysis of ship hulls under non-Gaussian wave loads

Abstract

For ships with fine lines it is well known from measurements that the wave-induced sagging bending moments can be considerably larger than the hogging bending moments. This difference cannot be predicted by a linear theory but requires a non-linear formulation for the hydrodynamic loads. Here, non-linear theories based on the strip theory concept can yield an adequate description of the problem, but a straightforward solution technique using a time simulation procedure is not acceptable for a fatigue damage estimate due to the large time interval required. Previously, a quadratic strip theory formulated in the frequency domain has been used to determine the statistical distribution of the peak values of the hull bending moment for a ship sailing in a stationary stochastic head sea. Subsequent comparisons with full-scale tests showed excellent agreements. In the present paper a formula for the fatigue damage of a structural component subjected to a quadratic random stress response is derived. A simple example is used to demonstrate the accuracy of the formula. Then based on the aforementioned quadratic strip theory, the influence of the non-linearities in the wave profile and in the hydrodynamic loads on the fatigue damage of the hull platings is given. The calculations are performed for a fast container ship and it is found that the non-linear effects will increase the fatigue damage by 50–100% in moderate sea states.