Forces on Marine Risers in Coexisting Environment

Abstract

Experimental results for hydrodynamic loading from the combined effect of oscillatory motion and current on two productionriser configurations are presented. The riser systems tested consist of a single cylinder and a large center pipe surrounded by twelve satellites. The current is superimposed on the oscillatory motion in three directions: parallel; perpendicular; and oblique with a 45° angle. The experimental study showed a significant reduction in the drag coefficients when the oscillatory motion and the added current were parallel. The magnitude of this reduction depends primarily on the ratio of the maximum velocity of oscillatory motion to the steady current speed. The prediction of forces on marine risers using Morison's formula in a coexisting environment at low Keulegan‐Carpenter numbers yields unsatisfactory results due to vortex shedding. However, the use of this equation at high Keulegan‐Carpenter numbers in a coexisting environment was demonstrated to be safe and appropriate. The data and results of this study have shown that current strongly influerces the fluid loading of risers in the drag‐inertia dominated regime. Current has a profound effect on the drag and inertia coefficients used in the Morison equation.