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

Abstract

Understanding the level of conservatism in a riser system design for vortex-induced vibration (VIV) fatigue is an important issue for operators. This study represents a demonstration of the calibration methodology to derive consistent values for the Factor of Safety (FoS). The exercise is performed here based on medium scale VIV data and utilizing the most commonly used VIV prediction software by industry. The results emphasize the need for (i) a coherent approach to estimate the FoS to be used and (ii) monitoring/measurement of software improvements as this may increase risk of failure if the influence of such improvements on the FoS is not quantified.

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.…”

Modelling Risers With Partial Strake Coverage

“…The test matrix included bare, partially straked and fully straked risers in sheared and uniform currents. [1] Even though SHEAR7 V4.5 was predominantly conservative and only underestimated the damage rates at a few data points, the predictions for the partially straked riser in uniform current were particularly poor in the straked regions, over-predicting by a large amount. This is shown in Figure 1 which plots the predicted and measured damage rates for 22 different current velocities.…”

Using Model Test Data to Assess VIV Factor of Safety for SCR and TTR in GOM

“…The same approach would be valid in the advent of full-scale production riser VIV fatigue data. Previous attempts to calibrate VIV fatigue FOS have been made ( [9], [10], [11]). The current method is developed from the limit state function formulation in [11], with the main differences being the inclusion of the riser design life and the S-N uncertainty of the riser material.…”

“…Previous work for DeepStar VIV FoS investigations [11] used NDP 38m riser model test data, as well as a basic limit state function. The new FoS results presented here are drawn from three experimental campaigns to characterize VIV response.…”

“…The new FoS results presented here are drawn from three experimental campaigns to characterize VIV response. Software has been updated since [11], and the new results are based on the most recent versions of all VIV prediction software, using default parameters. Additionally, FoS are calculated using AMOG's multiple-event limit state function, including damage from multiple currents and uncertainties in the S-N behavior of the riser material.…”