The search for efficient geometries to utilize as Drilling Riser Buoyancy Modules (DRBMs) for drag and VIV reduction has led to the concept of Inverted Helical Strakes (IHS). IHSs involve fluid flow around a generally circular cylinder with intermittent gaps as a result of internally protruding helical strake grooves. They are fundamentally different to external helical strakes, which achieve disruption of coherent vortex shedding due to an external sharp edge where flow separation is tripped. Inverted helical strakes first appeared in the literature in 2005 when options were being investigated for reducing the Vortex-Induced Vibration (VIV) and drag on oil and gas risers. In recent attempts to find applications for drilling riser buoyancy, smaller grooves which don’t reduce the available buoyancy as much as large grooves have been examined. This paper provides a critical review of the literature then presents the results of water channel tests performed by the authors on various embodiments of inverted helical strakes in both fixed and free vibration modes. The efficacy of inverted helical strakes to achieve drag and VIV reduction is summarized.
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