Effects of broad-banded higher harmonics on fatigue damage of risers due to vortex-induced vibrations

Abstract

Recent works have discussed "chaotic" or "Type-II" riser motion and suggested that it is a general feature of VIV riser response. Chaotic riser response contains broad-banded harmonics and a combination of standing and traveling waves, unlike periodic or " Type-I" motion which contains only standing waves and exhibits very narrow harmonics. The extent to which regions of chaotic response increase the damage on the riser had not previously been explored in detail. To facilitate the exploration, a method for separating the effects of the fundamental frequency, higher harmonics, and the chaotic content was developed and applied to four cases from the NDP 38m data set. For test of a bare riser in uniform flow, the damage was increased by a factor of 2 when the higher harmonics of the signal were included and another factor of 2 when the broad-banded harmonics were included. Similar results were obtained for tests of the bare riser in sheared flow as well as a 50% straked riser in uniform and sheared flow. After the results have been supported by theoretical estimations from first principles, it is concluded that the increase in fatigue damage resulting from the chaotic behavior of the riser is as important as that of the higher harmonics of the signal.