VIV-Mitigating Buoyancy Module Performance Characterization Using Computational Fluid Dynamics

摘要

A new design of drill riser buoyancy is introduced with tri-helical grooves formed into the module. The primary purpose is to mitigate vortex-induced vibration (VIV) of the riser string which is a significant factor in causing riser cyclic stress and fatigue damage. Computational fluid dynamics (CFD) is employed to simulate the buoyancy module hydrodynamic behavior. A range of representative offshore environments is considered using a Reynolds-averaged (k-epsilon) turbulence model in 3-dimensional space. The hydrodynamic performance is mainly characterized in terms of the in-line and cross-flow drag coefficients (C_(dk), C_(dy)) and lift coefficients (C;). The CFD results show comparable drag coefficients with tow-tank testing data performed at SINTEF in Trondheim. Norway in July 2017. C_(dx) is shown to be consistent at approximately 0.60 - 0.65 for the Re = 105 - 107 range whereas C_(dy) and C_l approximates zero and is negligible. This, evaluated from a global riser perspective, means the riser has a predictably consistent drag performance with little variation in cross-flow and axial motion. The helical grooves form channels that divert flow axially away from the free-field flow directions which functions to disrupt regular flow order. This prevents the formation of a von Karman vortex street which is a requirement for vortex-induced vibration. The omni-directional nature of the helical grooves ensures this mechanism for breaking regular flow is achievable regardless of the environmental current direction acting on the riser. VIV is an increasingly significant problem with deep-water exploration in harsh weather environments. The new buoyancy design provides passive vortex shedding mitigation to top-tensioned risers which would reduce the propensity of VIV occurrence.