Nonlinear dynamic response of suspended span pipe conveying high temperature and high pressure flow

Internal high temperature and high pressure flow of a subsea pipe will cause high axial force, which could induce global buckling of the pipe. The buckling pipe, which curves out of the seabed, forms a suspended span and vibrates under the action of cross currents. The dynamic response of a subsea pipe with initial vertical deformation caused by thermal buckling under the combined action of internal and external flow was analyzed. Firstly, the Euler Bernoulli beam model is used to simulate the suspended span pipe. Through stress analysis of the pipeline element and internal fluid elements, the kinematic equilibrium equation of the suspended span pipeline is established to determine the thermal buckling equilibrium state of the internal high-temperature fluid pipeline. Then, the wake oscillator model is introduced into the equation to establish the dynamic coupling equation of an internal high temperature and high pressure fluid pipeline under the action of internal and external flow. By using the Newmark--β method the discrete vortex induced vibration response equation is solved step by step, the vibration displacement, velocity and acceleration of the pipeline in time history are obtained, and the nonlinear vibration time domain prediction method of pipeline vibration with initial deformation is established. Firstly, the vibration modes of a pipeline with and without thermal buckling under different modes are compared, and the influence of the thermal buckling effect on the vibration mode of the pipeline under different modes is observed; By analyzing the pipeline vibration frequency change trend with time and combined with the change trend of the stiffness matrix in the modeling process, it is deduced that the periodic change of pipeline’s axial force will cause the periodic change of natural frequency. We changed the external fluid flow velocity of the pipeline, observed the changes of vibration frequency, mode and amplitude of the pipeline, and concluded that when the pipeline changes from low mode vibration to high-order mode vibration, the pipeline changes from periodic motion to chaotic motion, and then changes to periodic motion with the increase of external flow velocity. In this process, the phenomenon of multi-modal vibration will appear in the pipeline, and the multi-modal effect has a great influence on the vibration of low-order modes; We then changed the fluid temperature and pressure inside the pipeline, observed the changes of pipeline vibration frequency and vibration amplitude, and get that the increase of temperature and pressure in the pipeline will lead to a decrease of pipeline vibration frequency and an increase of pipeline vibration amplitude.