The paper describes the present state-of-the-art in fatigue analysis for vortex-induced vibration (VIV) of ultra-deep water unfaired drilling risers (1000 m water depth and above) in steady currents. It complements an earlier work on the broader issues of deep-water riser design and management.
To date, strength and VIV fatigue life analyses have always been based on the enveloping current profile, i.e., the locus of predicted extreme speeds for the various current meter depths. For most offshore structures, this is a conservative approach, and it was believed that this was also true of deepwater risers. However, recent work for BP Norway has shown that this is not the case, and that analysis for the mean concurrent profile (i.e., the current at any given instant in time) gives predicted fatigue lives an order of magnitude lower. It is thought that this was not previously known to the industry.
Techniques are presented for the selection of concurrent design profiles, and the analysis methods and assumptions for VIV response prediction and fatigue damage calculation are described. Fatigue life results are presented for five deep-water wells, all in the Atlantic Margin area (West of Shetland, the Rockall Trough and the Voring Plateau). Riser safety management via monitoring of predicted damage is discussed, and cumulative damage results are given for a recent well.