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In the present study, vortex-induced vibrations in both In-Line and Cross-Flow directions of long flexible cylinders within a uniform flow are predicted by a modified wake oscillator model. In detail, the Van der Pol's wake oscillator model with the drag and lift coefficients being modelled as random variables. The statistics of the force coefficient, i.e. mean value and standard deviation, are extracted from the literature reporting experimental measurements and numerical simulation results of the drag and lift forces experienced by the cylinder-type structure. With the statistics, a series of pseudo stochastic variables are generated to calculate the drag and lift forces. In Euler-Bernoulli equation governing the motion of the cylinder, tension is modelled as a function of flow velocity. Through comparing the predictions from the model with and without randomness to the experimental results, it is found that the scatter in vibration amplitudes, multi-frequency effect (broadband stochastic process) and frequency multiplication effect are successfully reproduced by the model with randomly drawn drag and lift coefficients. However, the overestimate of the oscillation frequency for the In-Line vibrations implies that the In-Line coupling is still a topic that deserves a further investigation.