Design formats for flexible pipes are briefly reviewed. A procedure for case-by-case reliability assessment of specified failure modes for flexible riser systems is outlined. Application of this analysis tool is illustrated for three different combinations of surface vessel/riser configuration. The ultimate limit states considered are:
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top end tensile failure excessive curvature at the hog bend
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axial compression at the sag bend For these potential failure modes, the various sources of uncertainty are identified and quantified. Based on this input, relations between safety factors and failure probabilities are determined in each case.
At present, design of flexible r1sers are typically based on a permissible stress format utilizing a single safety factor. Due to the wide application range for flexibles, the reliability level implied by such a format will be significantly different for various types of riser systems. A possible improvement can be achieved by introducing design formats based on partial load and resistance coefficients that are individual for each type of load effect. In particular, formats aiming at uniform utilization of individual pipe layers will be optimal. This applies to separate as well as combined load conditions (e.g. tension and torsion). Regardless which design format is being used, there is an urgent need for "assessment of the level of safety corresponding to a given set of safety factors or partial coefficients. Obviously, the main interest is in situ conditions rather than idealized laboratory conditions. Nondestructive testing (e.g. pressure testing) may provide some information in this respect. Full-scale measurements can serve to verify and improve computational models and procedures. However, a framework representing all relevant uncertainties will be required in order to make a systematic assessment possible.
