Fatigue damage of a steel catenary riser from vortex-induced vibration caused by vessel motions

• We confirmed that out-of-plane VIV had occurred to a SCR under vessel motion even at a minimum local KC number as low as 40.
• We discussed phenomena like instantaneous shedding frequency, TDP variation, tension variation and traveling waves.
• We found out that middle part of the test riser was the ‘power-in’ region for out-of-plane VIV under vessel motions.
• We found out that, TDP, upper sag-bend and top of the SCR, were 3 hot spots for maximum fatigue damage.
• We found out that travelling waves and TDP variation made maximum damage locations shift from ‘power-in’ region to both ends.

A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the vortex-induced vibration (VIV) and its fatigue damage under pure top vessel motion. The top end of the test model was forced to oscillate at given vessel motion trajectories. Fiber Bragg grating (FBG) strain sensors were used to measure both in-plane and out-of-plane responses. Four different factors have been discussed to understand the VIV responses and fatigue damage results: instantaneous shedding frequency, touch down point (TDP) variation, tension variation and traveling waves. Out-of-plane VIV associated with strong time-varying features was confirmed to have occurred under pure vessel motion. Both KC number and maximum shedding frequency were investigated and indicated that the middle part of the truncated model riser was the ‘power-in’ region for out-of-plane VIV. Meanwhile, fatigue damage caused by out-of-plane VIV was found to be strongly dependent on both top motion amplitude and period. The probability distribution of the maximum damage exhibits 3 critical locations in the test model: TDP, upper sag-bend and top of the SCR. Strong traveling waves, TDP variation and end wave reflection have been proven to cause the maximum damage locations to shift from the ‘power-in’ region to these three positions. Finally, a maximum fatigue damage diagram with top motion amplitude, period and maximum shedding frequency was constructed.