Dynamics of a marine stationary platform under action of the seismic loading is investigated. The structure consists of a tube-like ferroconcrete rod of variable cross-section. To describe the rod vibrations the beam-like model is used. The system of partial differential equation is reduced to the system with 6 degrees of freedom. Some numerical examples are studied. It is established that at design it is necessary to pay the main attention to the concrete strength when the extension deformations appear.