Evaluation of Effects of Wave-induced Vibrations on Fatigue Damage of Large Ships

Abstract

Low frequency response to normal wave loads and high frequency response due to wave-induced vibrations coexist in the ship's structural loads. This paper addresses the effect of the wave-induced vibrations on long-term fatigue damage in various types of ships. A bulk carrier, a VLCC, and a container carrier are employed as subject ships. A weakly nonlinear method proposed by one of the authors is employed for evaluating the load effects. Slamming loads are separately implemented by using momentum theory.The whole ship is represented by using shell FE model. The calculations are performed for various short-term sea states. Stress cycles are counted by using Rainflow counting method. Then, long-term fatigue damage is evaluated by using the Palmgren-Miner rule. A series of sensitivity analysis is also performed to clarify the characteristics of the fatigue damage by the wave-induced vibrations in oblique seas as well as in head seas. It is shown that the amount of the increase in fatigue damage depends on the wave load characteristics of the ships in waves as well as the structural properties such as natural frequencies of flexible mode.