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

Abstract

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.

VIV fatigue reliability analysis of marine risers with uncertainties in the wake oscillator model

“…Khan and Ahmad [9] conducted a VIV fatigue reliability analysis of a deep water riser; nevertheless, the authors only considered random variables related to the fracture mechanics model. Leira et al [10] also performed reliability analyses, which incorporated various random parameters such as added mass, damping parameters, Strouhal number and lift coefficient. The impreciseness of the VIV model is captured by a single random variable termed the "model uncertainty factor", which is normally distributed with assumed mean and variance.…”

Reliability Based Factors of Safety for VIV Fatigue Using NDP Riser High Mode VIV Tests

“…Previous attempts to calibrate VIV fatigue FoS have been made using uncertainty distributions derived from engineering judgment and laboratory experiments, see for example [7]. It is important to note here that the uncertainties observed in controlled laboratory experiments are often significantly lower than those observed in the field.…”

Recent Development of the Empirical Basis for Prediction of Vortex Induced Vibrations

“…A typical requirement from design guidelines is that fatigue damage during the expected lifetime of a structure should not exceed the capacity with a certain safety margin. Studies on the safety factor have been published by Leira, et al [25] and Fontaine, et al [26], but none of these methods have been accepted as standards by authorities or industry.…”