Benchmarking of SHEAR7v4.5: Comparisons to Full-Scale Drilling Riser VIV Data and Legacy Analyses

SHEAR7 [4], industry’s most widely-used vortex-induced vibration (VIV) fatigue damage software analysis tool, has been recently revised to include a fundamental change in methodology based on observations from Deepstar’s high-aspect-ratio, slender pipe towing tests in the Gulf Stream near Miami, Fla. The key revision pertains to the way in which VIV excitation zones (or “power-in” zones) are calculated along the length of the riser. Namely, it is no longer assumed that multiple structural modes are excited simultaneously by a given current profile along the length of the riser in a manner known as multi-mode response. Rather, based on the towing tests, it is assumed that potentially excited modes participate in the response one-at-a-time in a time-sharing fashion. The fraction of the total event time accorded to each responding mode is proportional to the input power of its exciting force, or, may be set to be uniformly distributed among all responding modes. This fundamental change can have a significant effect on the fatigue damage calculated for a given riser and current profile and direct comparisons to previous analyses are not straightforward as the input parameters of the software have been altered along with the analysis methodology itself. In a recent paper [1], BP utilized measured data that it had collected from risers in the field during several of its worldwide drilling campaigns to calibrate SHEAR7v4.4 to yield an average safety factor of ten on fatigue damage when VIV occurs. Given the extent of the changes to the analysis method, it was important to revisit that study and ascertain whether the latest software version similarly captured the trends of the full-scale data and could be calibrated to maintain an appropriate factor of safety. Furthermore, where measured data were not available it was important for consistency to identify and rationalize differences between analyses with the current and penultimate versions of the software. This paper describes BP’s benchmarking of SHEAR7v4.5. Comparisons are made between predicted and measured VIV fatigue damage for several full-scale drilling risers to demonstrate the efficacy of a calibration for the latest version. In addition, comparisons are made between VIV fatigue damage predictions using SHEAR7 versions 4.4 and 4.5 for drilling risers as well as for a typical deepwater SCR in typical design Gulf-of-Mexico loop currents. The version-to-version differences are illustrated and explained. Finally, results of sensitivity studies conducted with respect to the new parameters in SHEAR7v4.5 are presented. A key finding is that while the predictions, on average, are similar from version to version; the scatter in predictions — which leads to requirements for large safety factors — is largely unimproved.