With elevated interest in offshore drilling, transportation of production fluid to surface facilities are under heavy scrutiny. Particularly, design of marine risers either in standalone or in a piggyback configuration continues to dominate the focus of the research community in assessing the influence of hydrodynamic drag on its operation. In this study, full three dimensional Computational Fluid Dynamics (CFD) simulations were performed to estimate the flow induced drag on piggyback risers under steady current configurations. The turbulent structures generated in the flow domain are resolved using SST k-omega turbulence modeling approach.

Effect of flow velocities and circumferential position of the smaller cylinder with respect to the larger cylinder, were investigated on drag coefficient. Two different piggyback riser models, with varying cylinder diameters were considered. The predicted drag coefficients under a wide range of Reynolds number were in good agreement with the experimental data. The effect of diameter ratio of the two cylinders and the position of smaller cylinder were identified to play an important role in the generation of flow structures within the domain. Numerical simulations identify and capture key hydrodynamics interference such as drag reduction and vortex shedding patterns that are critical in the design of piggyback riser configurations. For example, the drag coefficient of the largest cylinder is lowest value when it is placed in the wake of the smaller cylinder and the drag on the small cylinder depends significantly on the diameter ratio of the cylinders and their relative position. Further, detailed discussions pertaining to vortex shedding patterns under various model configurations are elaborated. This study clearly demonstrated the applicability of numerical tools to gain insight into the hydrodynamic interaction between two cylinders placed under close proximity.

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