A simplified method to predict fatigue damage of offshore riser subjected to vortex-induced vibration by adopting current index concept

In the present study, an innovative method for estimating fatigue performance of risers under vortex-induced vibration (VIV) is proposed. Generally, fatigue performance is affected by the surrounding environment such as wind, wave, and current. It is well known that current is the most influential load among all for offshore risers. In structural safety aspect, strength and fatigue are the most important factors for riser design. In addition, fatigue design is affinitive to the VIV phenomenon. For the analysis of fatigue performance of riser, SHEAR7 numerical simulation code which is commonly used in offshore industry is applied. In order to identify the relation between current load and fatigue performance of riser, Fatigue damage versus Current index (F-C) diagram has been proposed. F-C diagram may cover change of current profiles and help predict the fatigue damage under VIV. In case of current profile, a total of sixty cases of current scenarios are considered based on six representative sea-states. The obtained results from this study will be a useful guideline to predict the effect of current on the fatigue performance of riser.