Springing occurs obviously for ultra-large ships due to their relatively low stiffness. The conventional quasi-static spectral analysis method cannot solve the problem of ship fatigue damage with consideration of springing, as it ignores the coupling effect of the elastic characteristics of the hull and the hydrodynamic load. To address the above problems, the stress-mode superposition method is used to solve the problem of the fatigue strength assessment for ship hull considering the springing effect. The fatigue of an ultra-large ore ship is calculated and compared with the results of the quasi-static spectral analysis method. The results show that considering the springing effect, the fatigue stress response has double peaks or even multi-peaks in the high frequency band when the encounter frequency is close to the natural frequency of the hull. The springing effect has a greater impact on fatigue when the ship sails in the bow quartering sea, and the springing greatly reduces the fatigue life of partial hull structures. This method can provide references for the ship fatigue assessment under the consideration of springing effect.

The Rules for the Classification of Trimarans by Lloyd's Register of Shipping are mainly for steel trimarans and are not applicable to aluminum-alloy trimarans. The fatigue life of a 120m aluminum-alloy trimaran is calculated by using the direct calculation method (spectral analysis method) based on the Miner linear cumulative damage theory. At the same time, the fatigue assessment results are corrected by the rain flow correction with consideration of the difference between treating the alternating stress process as a narrow band and the actual sea state. The results show that fatigue damage is easy to occur at the bridge of the aluminum-alloy trimaran, and hot spots have the different contributions to the fatigue damage under the different wave angle. The results of the fatigue life calculation are conservative if the alternating stress process is regarded as a narrow-band model. The conclusions can provide reference for the fatigue assessment of the aluminum-alloy trimaran.