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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- Coupled vessel/riser analysis
- Design statistics and transfer functions
- Frequency domain
Special modeling considerationsEquivalent pipe modelGlobal analyses of multiple tubular riser systems are generally performed using an equivalent pipe model. The equivalent pipe models are generated using the assumption that the riser section properties can be calculated from a compound section. Using this assumption, which ignores non-isometric properties, the riser properties are obtained by adding the areas, moments of inertia, effective tensions and masses of all of the tubulars together along the length of the riser. This implies that the tubulars are in continuous contact with each other and that the displacements (and thus the curvatures) of all of the tubulars along the length of the riser are identical. In reality the tubulars are not in constant contact with each other, and therefore, the displacements of the tubulars are not identical. As a result, the global displacements describe the general or approximate displacements along the length of the riser tubulars. The closer the riser system configuration is to the compound section assumed for the global analysis, the closer the global displacements are to the actual displacements of the tubulars. Coupled vessel/riser analysisRiser simulations are usually conducted as uncoupled analyses using assumed quasi-static offsets and wave-frequency motions computed from RAO’s from separate moored-vessel analyses. Some programs permit solution of the total vessel/riser/moorings problem wherein the latter are represented by finite elements. This is called coupled analysis and it is usually done in the time-domain. It is useful when: risers can significantly influence the response of the vessel; a more complete representation of vessel motions is desired that includes, for example, slowly varying motions or more accurate setdown effects. Coupled analyses are more computationally intensive since many more degrees of freedom are being solved for at each time step. Design statistics and transfer functionsRiser analyzes are run either to: 1) determine maximum values or 2) develop transfer functions, usually for fatigue analyses or for spectral computation of extremes. Quite different techniques are required in each case. Frequency domainExtreme valuesFrequency-domain analysis results in root-mean-square, Xrms, and zero-crossing period, Tzr, for stresses and deflections for a particular seastate according to:  ...(45)
 ...(46)
 ...(47)
where mn = nth spectral moment, f = frequency, Hz, S(f) = power spectrum of the response. To predict the maximum values, these must be multiplied by an extreme factor computed from the distribution of peaks. Many analyses have assumed Gaussian responses so that the extreme value is obtained from: Xmax = Xmean + Xrms . F ...(48) 
where Pe = exceedence probability, Xmean = mean value of response, D = duration of condition (often 3 or 6 hours). While adequate for relatively mild conditions, such as for fatigue analysis, drag dominated responses are generally non-gaussian and are not well represented by this type of analysis in extreme conditions. This is particularly true in the splash zone, where the effects of intermittent wetting are important for the statistics.
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