Effect of Higher Stress Harmonics and Spectral Width on Fatigue Damage of Marine Risers

Abstract

Recently, experimental data from vortex induced vibrations (VIV) of long flexible cylinders have revealed the existence of (i) higher harmonic stress components, and (ii) chaotic response characteristics, in addition to the well-known first harmonic responses. To assess the effect of these phenomena on fatigue damage of long flexible structures, we generate alternative stress time series, with and then without higher harmonic components; as well as time series, which have either sharply peaked power spectral densities (PSD), or broad-banded PSDs, characteristic of chaotic response. We show that for stresses containing higher harmonics, the predicted fatigue damage to the riser can increase by a factor of 2.5 relative to the damage caused by stresses containing only the first harmonic and having equivalent total power. Likewise, for stresses with a significantly spread PSD, an indicator of chaotic response, the damage to the riser is increased by a factor of 1.8, relative to a stress with narrow-band PSD and the same total power. Thus, it is found that the increase in fatigue damage caused by chaotic response can be as significant as that caused by the stress higher harmonic components. Topics: Stress , Fatigue damage , Marine drilling risers