Comparison of Dynamical Responses of an Offshore Riser with Linear and Nonlinear Structural Characteristics Through Nonlinear Normal Modes

This work investigates the effect of structural non-linearity on the oscillations of an offshore riser subjected to vortex-induced vibration. The riser is modelled by a vertical pipe with steady internal flow which has no direct nonlinear influence on the motion of the coupled system of the structure and fluid surrounding it. The only excitation in the model comes from the pressure difference on the riser arising as a result of vortex-shedding behind its cylindrical body. Simulations of dynamical responses in modal space were performed for two similar models in order to show how a weak geometrical nonlinearity influences oscillations of the riser and consequently the lift coefficient governing forcing from external flow. The structure in the first model had purely linear characteristics, while a nonlinear axial strain relation was introduced in the second one. Comparing responses of the two models, it was found that effects the nonlinearity had on the oscillation were considerably greater when internal flow was included in the calculation as opposed to when the riser was an empty pipe.