Errors will be caused in calculating the fatigue damages of details in liquid cargo tanks by using the traditional spectral analysis method which is based on linear system, for the nonlinear relationship between the dynamic stress and the ship acceleration. An improved spectral analysis method for the assessment of the fatigue damage in detail of a liquid cargo tank is proposed in this paper. Based on assumptions that the wave process can be simulated by summing the sinusoidal waves in different frequencies and the stress process can be simulated by summing the stress processes induced by these sinusoidal waves, the stress power spectral density (PSD) is calculated by expanding the stress processes induced by the sinusoidal waves into Fourier series and adding the amplitudes of each harmonic component with the same frequency. This analysis method can take the nonlinear relationship into consideration and the fatigue damage is then calculated based on the PSD of stress. Take an independent tank in an LNG carrier for example, the accuracy of the improved spectral analysis method is proved much better than that of the traditional spectral analysis method by comparing the calculated damage results with the results calculated by the time domain method. The proposed spectral analysis method is more accurate in calculating the fatigue damages in detail of ship liquid cargo tanks.

中文翻译：

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液货舱内部件疲劳损伤评估的改进谱分析方法

对于动态应力与船舶加速度之间的非线性关系，采用传统的基于线性系统的频谱分析方法，在计算液货舱细节疲劳损伤时会产生误差。提出了一种改进的光谱分析方法，用于详细评估液货舱的疲劳损伤。基于这样的假设：可以通过对不同频率的正弦波求和来模拟波过程，并且可以通过对这些正弦波所引起的应力过程求和来模拟应力过程，通过扩展应力来计算应力功率谱密度（PSD）正弦波感应的过程变成傅里叶级数，并以相同的频率将每个谐波分量的幅度相加。这种分析方法可以考虑非线性关系，然后基于应力的PSD计算疲劳损伤。以液化天然气运输船上的一个独立罐为例，通过将计算的损伤结果与时域方法计算的结果进行比较，证明了改进的光谱分析方法的准确性比传统的光谱分析方法要好得多。所提出的频谱分析方法可以更精确地计算船用液货舱的疲劳损伤。通过将计算出的损伤结果与时域方法计算出的结果进行比较，可以证明改进后的光谱分析方法的准确性比传统的光谱分析方法要好得多。所提出的频谱分析方法可以更精确地计算船用液货舱的疲劳损伤。通过将计算的损伤结果与时域方法计算的结果进行比较，证明了改进的光谱分析方法的准确性比传统的光谱分析方法要好得多。所提出的频谱分析方法可以更精确地计算船用液货舱的疲劳损伤。