The Study On The Nonlinear Vibration Behaviors Of Top-Tensioned Risers In Deep Water

The risers are key structures in the oil and gas exploration in the deep sea. Thenonlinear vibration characteristics are investigated for TTRs in deep water under VIV,parametric excitation and VIV-parametric excitation. The study has importanttheoretical and engineering meaning for the development of theory and technology ofTTRs in deep sea.The vibration equations are established by taking into account the top tensionand geometric nonlinearity of the risers, considering both the parametric excitationcaused by the heave motion of platform and VIV lift force. Regard the system as aweak nonlinear system, approximated analytical solution of the equations arecalculated using the method of multiple scales. Firstly, considering the externalexcitation include only the harmonic VIV lift force, the case of first-order andsecond-order primary resonance are selected. By solving the equations, the nonlinearVIV response characteristics of the deep sea risers are obtained and analyzed.Secondly, only considering the parametric excitation caused by the heave motion ofplatform, simplify the top boundary condition to a dynamic boundary condition whichcan be described as simple harmonic motion, the case of first-order and second-orderparametric resonance are selected and the nonlinear parametric responsecharacteristics of the risers are obtained and analyzed. Finally, both the parametricexcitation caused by the heave motion of platform and VIV lift force are taken intoaccount, the case of first-order parametric resonance are selected, the situation of VIVfrequency close to the risers’ lateral natural frequency and far away from that areconsidered respectively and the nonlinear vibration response characteristics of therisers are obtained and analyzed.The results show that when VIV is considered only, the major responsefrequency is equal to the excitation frequency, and when the first-order primaryresonance occurs, the response amplitude reaches its max value. When the parametricvibration is considered, the vibration response is mostly dominanted by the first modeand the amplitude response of the first-order parametric vibration is larger than that ofother cases. When both the parametric excitation and VIV lift force are taken intoaccount, the vibration response amplitude of the risers at that time are apparentlylarger than the former two situations and the frequency component are more complicated. The overall results show that in the design and application of deep searisers, the coupling between parameter excitation and VIV should be consideredadequately to avoid the two kinds of resonances to occur simultaneously and to ensurethe safty of deep sea risers.