In general, a DNV F2 curve is used for the weld joints and a DNV B curve for the riser connectors (coupling). Two stress concentration factors (SCFs) are used in fatigue analysis, one is 1.2 for the girth welds, and the other is roughly 2.0 for riser connectors, depending on the type of risers. In recent years, fatigue qualification testing has been performed to determine the actual S-N curve data. An engineering criticality assessment (ECA) analysis is conducted to derive defect acceptance criteria for inspection. For the drilling riser, the safety factor on fatigue life is 3 because the drilling joints can be inspected. The fatigue calculations are to account for all relevant load effects, including wave, VIV, and installation-induced fatigue. In some parts, such as the first joints nearest the lower flexjoint (LFJ), the fatigue life could be less, in which case the fatigue life will determine the inspection interval.

Component Capacities

For strength checks, various component capacities need to be defined such as:

  • Wellhead connector;
  • LMRP connector;
  • LFJ;
  • Riser coupling and main pipe;
  • Peripheral lines;
  • Telescopic joint;
  • Tensioner/ring;
  • Active heave draw works;
  • Hard hang-off joint;
  • Soft hang-off joint;
  • Spider-gimbal;
  • Riser running tool.


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