Vortex-Induced Motion (VIM) is a highly non-linear and nonstationary dynamic phenomenon which is very important for offshore platforms, such as the MPSO (Monocolum Production, Storage and Offloading System). An important particularity of the VIM is the amplitude and period modulation signal, which makes it difficult, perhaps not accurate, to analyze the data by using the Fourier transform. An analysis procedure addressing this matter was developed based on the Hilbert-Huang transform method (HHT). A case study of VIM of a MPSO applying a previously developed procedure is presented. Then, the usual spectral analysis method, Fourier transform method, was compared with the HHT applied on experimental data carried out from VIM tests on MPSO. The results were compared to draw some additional conclusions. The HHT is applied to the VIM phenomenon, aiming to disclose some hidden dynamic characteristics, such as the time modulation and jumps of multi-branched response frequencies. The experimental results for the MPSO compared by means of the two analysis methods consider different headings, drafts, and external damping conditions. The comparison between the methods made clear the larger non-stationary behavior in the motions in the inline direction, in which the differences were up to 20% in some cases; on the other hand, the results for motions in the transverse direction showed to be similar.