Three-Dimensional Model of a Hanging Cold Water Intake Pipe With Internal Flow and Top-End Excitation

Abstract A hanging cold water pipe has been increasingly becoming an important component with a surging interest in floating OTEC. This paper reports a numerical model of the dynamics of a hanging water intake pipe subject to the top-end harmonic excitation and internal flow and its applicability compared with a tank experiment. The dynamic model based on a finite beam element is formulated by considering internal flow, inlet flow and hydrodynamic forces from external fluid. The experimental setup is configured to investigate the vibration shape of a 4-meter pipe with a high internal velocity. The experiment provides data on the modification of the natural frequency, damping ratio, and three-dimensional vibration shape by the changing internal flow velocity. By comparing the experimental data and the solutions of the dynamic model, it is confirmed that the present numerical model can sufficiently reproduce these changes. Unstable vibrations at frequencies 1.1 to 1.5 times the first natural frequency are observed in the experiment, which reveals the limitations of the numerical model. Furthermore, the model is used to investigate the internal flow effect on an actual scale CWP for a 100 MW OTEC plant.