Assessment of Fatigue Safety Factors for Deep-Water Risers in Relation to VIV

Safety factors required to control fatigue damage of deepwater metallic risers caused by vortex-induced vibration (VIV) are considered. Four different riser configurations are studied: Cases I and II: Vertical tensioned $12in.$ risers suspended from a spar buoy at water depths of 500 and $1500m$⁠. Cases III and IV: Steel catenary risers suspended from a spar buoy, both at $1000m$⁠. For Case III, the riser diameter is $12in.$⁠, while for Case IV it is $33in.$ For each riser configuration, relevant design and analysis parameters which are subject to uncertainty are identified. For these quantities, random variables are established also representing model uncertainties. Subsequently, repeated analyses of fatigue damage are performed by varying the input parameters within representative intervals. The results are applied to fit analytical expressions (i.e., so-called response surfaces) utilized to describe the limit state function and to develop the probabilistic model for reliability analysis of the risers. By combining the random variables for the input parameters with the results from the parameter variations, a relationship between the fatigue safety factor and the failure probability is established for each riser configuration.