Nonlinear dynamic model of deep-water RTS

In deep-water test conditions, a riser-test pipe system (RTS) is subject to ocean current vertex induced vibration (VIV) effect, itself vertical-horizontal coupled effect and test pipe flow induced vibration (FIV) effect, RTS is very easy to have buckling deformation, fatigue fracture and friction perforation. Here, the nonlinear vibration model for a deep-water RTS was established by using the differential element method, the energy method and Hamilton variational principle. Based on the elastic-plastic body contact collision theory, the calculation method of nonlinear contact load of pipe string system was proposed. The cubic-hermit difference shape function and Newmark-β method were used to discretize and numerically solve the vibration model of the deep-water RTS. By means of onsite pipe string system parameters and the similarity principle, the nonlinear vibration simulation test bench of RTS was designed, and vibration responses of riser and test pipe were measured. Comparing the measured results with the calculation ones using the theoretical model and single pipe vibration model, the correctness and effectiveness of the nonlinear vibration model of deep-water RTS were verified. Finally, vibration characteristics of the pipe string system of an actual example well in South China Sea were analyzed. It was shown that effects of local high-frequency vibration can't be ignored in vibration fatigue analysis of test pipe; positions on test pipe where test pipe is easy to have strength failure are mainly in its middle-upper and lower parts; the study results lay a theoretical foundation for safety design of deep-water RTS. © 2022, Editorial Office of Journal of Vibration and Shock. All right reserved.