This study aims to characterize the hydrodynamic behavior of top-tensioned drilling riser joints with complex cross-sectional profiles when exposed to offshore current environments, such as slick joints and conventional buoyant joints. Computational Fluid Dynamics (CFD) has been employed to analyze these component cross-sectional profiles and define the component drag coefficients (Cd) and lift coefficients (Cl). These coefficients determine the riser external loading, which in turn affects the riser internal stresses. The Strouhal number St is also calculated which characterizes vortex shedding.
Validation of the CFD model is performed based on 2-dimensional flow past a cylinder. Following this, 2-dimensional representations of the detailed drilling riser components are created. The flow speed range considered corresponds to prevalent offshore environments, with Reynolds number (Re) = 105 – 106. The average forces are determined by subjecting the riser to various flow angles of attack across the 360 degrees range, giving the Re-dependent Cd and Cl. Variations in vortex-shedding is also analyzed by calculating St to demonstrate the effects of different flow attack angles.
Validation of Cd and St for flow past a circular cylinder show good agreement with literature. Variation of the riser component outer profile and flow attack angle show variation in the drag and lift force experienced as well as the vortex shedding behavior. More accurate Re-dependent Cd was produced for the various cross-sections analyzed that could be used in subsequent global riser analysis to simulate the full connected system from vessel, riser system, wellhead, and conductor and casing (interacting with the seabed).
Application of these more accurate drag coefficients result in more representative riser motions and loads in the global riser analysis. The benefit is potentially increasing vessel and operational uptime, and accurately calculating the drilling riser system fatigue life. Ultimately, this enables drilling operators and their stakeholders to make better-informed decisions on their offshore operations.