Instability of a Simplified Model of a Free Hanging Riser Conveying Fluid

Studying stability of a vertically suspended, fully submerged pipe conveying water (free-hanging water intake riser), researchers have found that if a critical flow velocity is reached, these pipes may flutter. However, there are different predictions for the value of this critical velocity. Researchers have mentioned values changing from infinitely small fluid velocities up to velocities which are unachievable in practice. The nonlinear hydrodynamic damping caused by the surrounding water seems to be crucial for correct description of the stability of the submerged riser aspirating fluid. In this paper, using the Morison’s equation, the nonlinear drag is taken into account as a function of the relative velocity between the current and the velocity of the riser itself. The nonlinear system is studied employing the Galerkin method. Ten-mode discretization turns out to be necessary to obtain an accurate result. It is shown that the current has a strong stabilizing effect. If the internal fluid flow exceeds a critical velocity the riser performs self-sustained oscillations with the amplitude smaller than one diameter.