Steady current-induced drag on piggyback risers

A key design concern of marine risers for deep water applications is the drag force due to the underwater currents. This project has considered two piggyback riser models with the respective cylinder diameters at 90×160mm and 90×250mm. A series of runs were carried out in a towing tank to determine the drag coefficients of the individual cylinders in the speed range of 0.75 to 2.8 m/s. The effects of hydrodynamic interference between the cylinders were investigated by systematically varying the position of the smaller cylinder with respect to the larger one and the current. It was found that the diameter ratio of the two cylinders and the circumferential position of the smaller cylinder with respect to the larger one had significant effects upon the drag loading while the effects of Reynolds number and wall-to-wall gap within the ranges examined were less significant. For the two models examined here, the typical drag coefficient of the larger cylinder is around 0.7, whereas the drag coeffici ent of the smaller cylinder varies from a negative value of -0.8 to a positive value of 2.7.