Tc 67/sc 4 n date: 2005-03-9 iso/wd XXXXXX ISO tc 67/sc 4/wg 6 Secretariat: Design of dynamic risers for offshore production systems Élément introductif — Élément central — Élément complémentaire Warning
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Transfer functionsTransfer functions can be obtained directly from frequency domain analysis programs. Time-domainRegular (design) wave simulation. These results are relatively simple to interpret for finding maximum responses to that particular wave, although relating that condition to a design seastate is problematical. Such an approach cannot be recommended when there is substantial dynamic response to frequencies near the design waves. The use of spectral analysis (based on transfer functions appropriate to the seastate to analyze) described in the following, is therefore preferred. Random-wave simulation. Time series from random-wave simulations must be processed to obtain extreme values. One approach is to fit a distribution, such as the Weibull 3-parameter distribution, to the peak values and to compute the extreme value for the storm duration of interest. This method is particularly attractive for long simulations up to the length of the storm’s duration. While computationally intensive, such an approach is well within today’s hardware capabilities. Statistical uncertainty can be reduced by increasing length of simulation time. Transfer functionsWhen developing transfer functions from regular wave analysis, such results may contain unwanted higher harmonics that must be removed by filtering. Random wave simulation results in time series that can be post-processed to develop transfer functions via Fast Fourier Transforms (FFT). One commonly used definition is:  ...(49)
where Sw(¦) = wave spectrum at ¦, RAO(¦) = response amplitude operator, response per unit wave amplitude at ¦.
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