Hydrodynamic Forces and Coefficients on Flexible Risers Undergoing Vortex-Induced Vibrations in Uniform Flow

In this study, the distribution and variation of the hydrodynamic forces and hydrodynamic coefficients in both inline (IL) and cross-flow (CF) directions on flexible risers experiencing vortex-induced vibration (VIV) in a uniform flow were investigated. The hydrodynamic forces in the CF and IL directions were theoretically calculated by the Euler-Bernoulli beam vibration equation. With the least-squares method, the hydrodynamic coefficients in both CF and IL directions were further investigated. The results indicate that the hydrodynamic force and vibration displacement almost simultaneously reach their maximums. The excitation coefficients obtained in this paper do not always agree with those obtained by the forced-oscillation tests: some excitation coefficients are even negative within the usual defined exciting nondimensional frequency regime and are related with not only the nondimensional frequency and amplitude but also phase angles of the CF and IL displacements. The added-mass coefficient along a flexible riser has a significant variation compared with that of a rigid cylinder oscillating in a pure CF or IL direction. Moreover, compared with the excitation force, added-mass force is found to be the dominant component in the overall hydrodynamic force for the flexible riser undergoing VIV.