Study of Behaviours of a Long and Flexible Pipe in the Water

Abstract

Dynamics is studied of long and flexible line structures, such as a riser pipe and a cold water pipe for OTEC hanging down from a floating platform on the sea surface into the water. A mathematical model widely used is shown to be capable of describing general features of the behaviours observed at forced motion tests of a pipe model in model basin. Among them lateral motion induced by axial forced motion is a little unusual. This is caused by parametric excitation of Mathieu type. We propose a method to predict magnitude of this limit cycle type of motion.Theoretical prediction of this parametrically excited motion by the analytical model is not completely satisfactory. Numerical analysis admitting finite amplitude effect of motion is shown to predict this motion with complete accuracy. Other interesting behaviours of 3D motion of line structures in the water predicted by the numerical analysis are presented and the finite amplitude effect is discussed.