Frequency Domain Fatigue Analysis |
 
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Frequency Domain Fatigue Analysis |
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Flexcom possesses an ancillary module, LifeFrequency, which is a frequency domain fatigue postprocessor. The LifeFrequency procedure for calculating the fatigue life at a point on a riser is based on generating a spectrum of combined bending and axial stress at that point, for each combination of wave height, wave period and wave direction in the long term environmental data for the location in question.
LifeFrequency has two modes of operation as follows, differing in how these stress spectra are produced:
•Stress Spectra Mode. In this mode, your fatigue analysis is preceded by a series of Flexcom frequency domain random sea analysis runs that you perform directly in Flexcom, to find the dynamic response for each combination of wave period, wave height and wave direction in the scatter diagram. The input to LifeFrequency is then a list of Flexcom output files, from which LifeFrequency reads in turn the stress spectra required to complete the fatigue life estimation.
•RAOs Mode. This mode is similar to the Postprocessor with Stress Spectra mode, in that your fatigue analysis is preceded by a range of Flexcom frequency domain random sea analyses. The difference is that you run analyses for selected combinations of wave height, period and direction only, and you postprocess these to generate RAOs of effective tension and bending moment (or stress). These RAOs then become the LifeFrequency inputs, and the program uses these to transform wave spectra in the scatter diagram for which you did not do a dynamic analysis, into stress spectra.
Each LifeFrequency analysis generates a plot of the minimum fatigue life at each hot spot plotted against distance along the hot spot set. The figure below shows an example of such a fatigue life plot. The distance on the X or horizontal axis is measured from the first element of each hot spot set. The Y or vertical axis actually displays the fatigue life (note the logarithmic scale). If there are two or more the user-defined hot spot sets in the fatigue run, a curve is generated for each, and LifeFrequency automatically superimposes these curves.
Sample Fatigue Life Plot
Further information on this topic is contained in the following sections:
•Numerous keywords are contained in the $LIFEFREQUENCY section, which relates to frequency domain fatigue postprocessing.
If you would like to see an example of how these keywords are used in practice, refer to A01 - Deepwater Drilling Riser or B01 - Steel Catenary Riser.