Structural life assessment of oil and gas risers under vortex-induced vibration

Estimating the fatigue damage due to vortex-induced vibration (VIV) is an important issue when considering oil and gas risers used in the offshore industry. In severe current profiles, especially in very deep waters, the VIV-induced fatigue damage could result in a relatively short life for risers. As water depth increases, the riser requires thicker wall to keep the stresses within the allowable limit of the material. Moreover, risers may interfere with each other in deep waters, which is not acceptable to the current design guidelines. Therefore, the current design philosophy prevents the riser clashing, which results in very expensive systems. As an alternative approach, such as the one recommended by the new provisions of DNV, interference and collision of risers are allowed. However, such an approach is only recommended for a riser that has been designed based on full consideration of VIV and a state-of-the-art fatigue analysis. In this research, the field data on multimodal vibration of a long flexible riser is used to conduct a series of experimental tests for fatigue life estimation of risers. The resulting VIV imposes a variable amplitude loading (VAL) on the riser, which makes the evaluation of its fatigue life difficult. Moreover, the influence of load interaction effects is also investigated through experimental tests. Lastly, influence of different frequency harmonics in the VIV-induced time history is also studied. The experimental results from this study indicate that the higher harmonics cause significant fatigue damage and cannot be ignored.