Examination of Hydrodynamic Forces Acting on a Group of Circular Cylinders at High Reynolds Numbers

A group of circular cylinders exists in many engineering practices, such as offshore drilling riser system. Due to the interference between the riser main tube and auxiliary lines, the hydrodynamic forces acting on the riser system is much different from those on a single circular cylinder. It is very rare in the publication and still not certain in the determination of the forces in the drilling riser design of the industry. Particularly, it is unclear of the hydrodynamic forces when the Reynolds number is very high which is quite common in the real ocean fields. In this paper, the stationary riser system consisting of a group of six circular cylinders with unequal diameters is considered. The hydrodynamic forces acting on the main cylinder in the Reynolds number ranging from 10⁵ to 2×10⁶ are numerically calculated by solving the Reynolds averaged Navier-Stokes (RANS) equations. The Spalart-Allmaras RANS model is employed to account for the turbulence effect. It is found that drag coefficients are close to 1 when the incoming flow is symmetrical with respect to the configuration of the cylinders and are dramatically reduced when the incoming flow is asymmetrical. No “drag crisis”, which is a well-known phenomenon in a single cylinder case, is found in this particular range of Reynolds numbers. A detailed analysis, including the flow field and pressure distribution around the main tube, is also presented in the present work. The numerical result of the hydrodynamic forces on the main line is very helpful for the engineers to determine the drag coefficients in the practice of drilling riser system design, under the guidance of API-RP-16Q.