Using various cylindrical rigid riser models, top forced oscillation tests have been carried out to study, interactions between hung-off risers and an internal fluid. A theoretical model of a weakly-damped oscillator has been applied to reveal the mechanism of such interactions. It has been shown that the theoretical model can explain the experimental results adequately. Coefficients of friction on inner surfaces of risers have been quantitatively investigated. Applying the theory for oscillating flows in pipes. the expressions that represent relationships between friction coefficients and Reynolds numbers have been partly, confirmed with the experimental data.