A considerable challenge in deep water field developments is the possibility of Vortex Induced Vibrations (VIV) of flexible risers due to presence of high currents. The most common remedy is the use of VIV suppression devices such as helical strakes. The background for this study is the lack of guidelines for estimation of hydrodynamic force coefficients for stroked risers. In particular there seems to be no publicly available experimental data on tangential force coefficients for stroked risers. Analyses indicate that tangential drag may be important for compliant steel risers, e.g. SCRs and steel lazy-waves risers on high-motion platforms in harsh environments. For large vertical motion of the platform there will be a substantial tangential relative velocity along the riser. Adding strokes to the riser will then effectively increase the tangential drag. Simplified formulations for hydrodynamic coefficients for stroked risers have been derived, including added mass and drag coefficients for both normal and tangential flow. The simplified formulations are derived from basic hydrodynamic theory without experimental calibration except for the drag coefficients for normal flow where the proposed drag coefficients are empirical based on experiments reported in the literature. An example of a global analysis of straked risers in deep water is presented to demonstrate the effect of tangential drag. Derived formulas for hydrodynamic coefficients to be used in global analysis of straked risers will be implemented in DNV GL Recommended Practices.
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