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: • Case I and II: Vertical tensioned 12” risers suspended from a spar buoy at water depths of 500m and 1500m. • Case III and IV: Steel catenary risers suspended from a spar buoy, both at 1000m. For Case III, the riser diameter is 12”, while for Case IV it is 30”. For each riser configuration, relevant design and analysis parameters which are subject to uncertainty are identified. For these quantities, random variables are established including 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, the relationship between the fatigue safety factor and the failure probability is established for each riser configuration.