Fatigue assessment is a critical issue for marine structures which usually encounter a combination load of a low frequency and a high frequency component. Furthermore, two components are commonly analyzed separately. In a strict sense, each of two components cannot be considered as narrowband processes. Recently, the authors proposed a combination of fatigue damage method (Han-Ma method) for predicting fatigue damage on the assumption of two well-separated and narrowband Gaussian components. The method is extended to non-narrowband and frequency overlapped cases in this paper. In addition, the application of existing combined spectrum method with dual narrow-banded correction factor (such as Jiao-Moan method, Low's bimodal method, Low-2014 method) and combination of fatigue damage method (such as DNV method, Huang-Moan method) is also investigated. These methods are also compared with some general wide-band spectral methods such as Dirlik method, single-moment method, Benasciutti-Tovo method and Park method. The results show that Jiao-Moan method, Low's bimodal method, Low-2014 method and Han-Ma method can still provide very accurate fatigue damage estimation when two Gaussian processes are separately non-narrowband and have overlapped frequencies. In particular, the accuracy of Low-2014 method and Han-Ma method is very close to that from some general spectral methods in the case that two Gaussian components have overlapped frequencies.
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