Advantages of the rain-flow method at the post-processing stage in comparison with the spectral approach

In some massive constructions, like off-shore platforms for example, it is sometimes difficult or impossible to register the loading process during many years of exploitation. Knowing the input random processes characteristics, it sometimes seems promising to estimate the output characteristics using the structural dynamic equation. Following this approach, the spectral densities are employed into longevity estimation. Many methods for estimation of the fatigue damage thought the spectral densities were developed. The question arises: is it practical to use the spectral approach in case of post-processing when the process realization already exists? The aim of this paper is to show some limitations of the spectral approach. First, the short review with a brief analysis of the accuracy of the spectral method is presented. It is worth mentioning, that the ultimate goal of the authors of those methods is to achieve the results, similar to those, which were obtained from the rain-flow method. Next, the real loading process is analyzed. To show the fact, that some of the assumptions of spectral methods are doubtful, the method of the digitally modelled process is introduced. Pro and contras of the spectral methods are discussed.