The theoretical and experimental studies on the nonlinear vibrations of a long vertical cylinder were performed. Especially, the geometrical nonlinearity due to large deflections can cause instability problem of the system. Therefore, a proper dynamic analysis is required. In order to investigate the nonlinear effects of the cylinder subjected to gravity force, time varying boundary conditions and large displacement, such as offshore structures, the numerical analysis and forced oscillation tests were performed These results were also compared With each other, and found to be generally m good agreement.
As oil exploration and production facilities move into deeper water, the offshore industries use pipes of considerable length for a variety of purposes, such as marine risers, and supporting legs of tension leg platforms. These structures are very long slender pipes which are connected to the sea floor at the lower end and connected to the drilling vessel or platform at the upper end These long vertical cylindrical structures are subjected to ocean environmental forces and large motions of the vessel. Therefore, a nonlinear analysis rather than a linear analysis is required to predict their correct dynamic behaviours. The geometrical nonlinearity due to large deflections of the cylinder can cause the instability problem of the system and fluid nonlinear effects including vortex shedding phenomena can change the dynamic behavior significantly In order to investigate the geometrical non-linear effects, two example cases are studied. One is the numerical test of a long vertical cylinder subjected to gravity force and in-plane exciting force in air. The other is the numerical and the model test of the riser subjected to top tension and time varying boundary conditions at the upper end finally, in order to investigate the fluid nonlinear effects, the forced oscillation test in water was performed.