Previous work [1] has given an approach to estimate vortex-induced vibration (VIV) factors of safety (FOS) for deepwater drilling risers. This work compared fatigue predictions from the industry-standard fatigue analysis software SHEAR7 with full-scale measured data. The field data have been collected over the past several years from five drilling risers without VIV suppression devices. These risers are on rigs under contract to BP worldwide, at sites susceptible to high currents. Collectively, the data correspond to 9,600 10-minute field measurements, equivalent to 0.18 years of continuous monitoring.

A worked example for a deepwater drilling riser in the Gulf of Mexico was used to show how the FOS methodology can be applied in the case of multiple design currents each with a different annual probability of occurrence. However, the paper cited future work that included development of a mathematical approach that would permit a designer to calculate a modified FOS that accounts for the number of events (currents or wave seastates) analyzed.

This paper documents a mathematical approach developed that can be used to calculate the riser design FOS based on multiple events of varying fatigue damage amplitude and probability. Note that the approach is not limited to VIV. It can be used for long-term wave fatigue life for example, assuming the designer has an understanding of the statistical differences between software predictions and actual field response.

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