Fatigue Assessment for Steel Lazy-Wave Risers (SLWR)

Both in-place slug induced riser fatigue and VIV fatigue during pre-laid phases are investigated in this paper for steel lazy wave risers (SLWR) tied back to a deep draft semi-submersible in about 1100m water depth, as part of the full scale engineering study effort for lazy-wave riser design. These results are compared in parallel with fatigue contributions from WLF, VIM and VIV fatigue. Dynamic analysis of risers is conducted including the effect of slugging with the vessel in its nominal position to eliminate environmental and vessel motion dynamic effects. Slugs are modelled as variation of mass and inertia, and centrifugal and Coriolis forces are included in the analysis. Rainflow counting method is used to calculate fatigue damage, which is similar to that used for wave loading fatigue analysis. Slug flow conditions are described in terms of lengths of slugs, slug velocity, slug frequency and duration, density of liquid slugs and/or gas bubbles over the field life and including the turn-down and start-up phases and late service life. Analysis results show that the slug fatigue damage contribution for lazy-wave risers can be significantly higher than that for conventional SCRs and it even could play a governing contribution for the buoyancy section. In-plane VIV fatigue is investigated for SLWR in wet parked phase with long-term current bins used in the analysis, which is from in-field measurement data. It is found that the riser wet park VIV fatigue analysis can be significant depending on the current profiles, hog bend height and the lay tension.