An insight into the competition in the coupling response of an S-shaped flexible riser subjected to both internal and external flows

Abstract

This study experimentally examines the coupling response of an S-shaped flexible riser exposed to internal gas-liquid two-phase flow and external shear current using the optical non-intrusive measurement. The vibration tests were conducted with the internal fluid transported at a constant velocity of v_i = 0.9 m/s with three representative gas-liquid ratios (Q_g/Q_l) and the depth-averaged reduced velocity of external current (U_rm) ranging from 9.32 to 23.19. The riser top was hanged underneath an oscillating cylindrical platform. The out-of-plane response is enhanced in the presence of external current, while the in-plane response is suppressed due to the more even distribution of liquid slugs. The response competition among the riser, buoyancy module and platform is quantified in terms of dominant frequencies and interaction lengths, which are sensitive to Q_g/Q_l, U_rm. The shift of zero value among the coupling length, affecting length and affected length indicates the switching of dominant role. In general, the response competition in the in-plane direction is more intense than that in the out-of-plane direction.