Modal study of a prototype of oil rig for frequencies responces functions and main vibrational modes analysis. The activity can be devided into two main steps. First characterization was performed throug numerical model solved according with FEM theory with CAE software. Starting from the 3D model, realized with Creo Parametrics, all single support oil rig sub structure, as well as the whole system, have been studied in order to analyze the singole sub structure influence on the whole model modes. The matematical model was validated with experimental tests exciting the structure through impact hummer. All the main stucture parts were excited in their main symmetric planes.