Vortex-Induced Vibrations (VIV) are well-known and related to the majority of cylindrical structures subjected to strong winds or currents. The VIV limit the lifetime of the structure because they increase the forces and so the fatigue. When several structures of this kind are put together in close interaction, the wake effects (Wake Induced Oscillations - WIO) sometimes involve strong instabilities. If these structures are flexible or mobile, oscillations of several diameters can be observed and collisions can occur ([4] & [6]). Such structures are widespread in the oil industry where the extraction of oil in deep water can be done by means of risers. In some cases, risers are connected to a floating support called FPSO (Floating Production Storage Offloading) and held in tension by buoys (figure 2). These buoys are located at depth where waves do not have any significant influence. However, in these areas, the magnitude of currents can be sometimes important. Consequently, engineering companies have to find solutions to prevent hydrodynamic interactions between risers and buoys. For a better understanding and characterization of wake effects, an experimental study is carried out at the Ifremer (Institut Fran¸cais de Recherche pour l'Exploitation de la Mer) flume tank in Boulogne-sur-Mer, France. This work is completed within the framework of the project Clarom cepm co 3007/04, in partnership with Doris engineering, Saipem s.a., Institut Fran¸cais du P´etrole, Oceanide, Ecole Centrale Marseille & Total. After presenting the experimental setup, we will focus on the interaction effects between two cylinders in close proximity. In this study, we quantify wake effects and risks of collision between structures. Several orientations and spacings between the cylinders are considered and tested. A short numerical study performed with the CFD code Fluent is also presented, for which hydrodynamic coefficients and motions of a single cylinder in a flow are numerically evaluated.