A new design of offshore riser buoyancy with tri-helical grooves has been shown to provide consistently low drag and effective vortex-induced vibration (VIV) suppression across the offshore flow speed spectrum when considering a smooth surface finish. In real-world conditions, surface roughness increases with prolonged service wear and marine growth. The current work looks to consider surface roughness effects using 3-d CFD simulations. The new design still shows low drag and effective VIV suppression performance up to a higher level of surface roughness, which makes it suitable for long-term offshore condition applications.
Offshore riser buoyancy modules function to reduce overall weight. For a drilling riser system, this facilitates the reduction of riser tensioner system utilisation. From the production riser perspective, it enables more optimal riser shapes to be achieved with the limiting considerations of riser over-bending and hang-off loading. However, these buoyancy modules increase the riser external diameter resulting in larger hydrodynamic drag loading. In addition risers are principally cylindrical making them susceptible to VIV. A new design of buoyancy with tri-helical grooves as in Figure 1 has been shown to provide consistently low drag across the offshore environment range whilst maintaining effective VIV suppression (Lai, 2018; Gaskill, Wu and Yin, 2018).
Previous CFD simulations and physical tow-tank testing of this form have been performed using geometry or components with relatively smooth external surfaces. Practically after years in service, it is foreseeable that the surface finish would have roughened due to operational wear or external marine growth. The current work looks to elucidate the effects of buoyancy module surface roughness on hydrodynamic performance as seen during its service life.
Computational fluid dynamics (CFD) is employed to simulate the buoyant riser section to analyze hydrodynamic behaviour in the 3-d space. The 3-d representation of a periodic section of the tri-helically grooved cylinder geometry is shown in Figure 2. A range of current magnitudes representative of offshore environments is considered using a transient Reynolds-averaged Navier-Stokes (k-epsilon) turbulence model. A detailed analysis of the new design considering a comprehensive range of flow speeds and surface roughness is performed. Tow-tank testing results are used for comparison as the initial benchmarking point. The hydrodynamic performance is mainly characterised in terms of its drag coefficients Cd components. Recommended Cd values for typical riser components are available based on DNV-RP-C205 (2010) or API RP 16Q (2010).
