Experiment on behaviours of long and flexible pipe in water.

For the purpose of studying the dynamical behaviour of long and elastic pipes hanging down from a floating platform into the water, forced motion tests of pipe models were done in the model basin. Fluid dynamic forces on the pipe models hanging and oscillating in water were estimated with measured forces required to give that oscillation to the models. The magnitude of the fluid dynamic forces thus measured, seems to be not so far from that expected on a cylinder oscillating in a rigid body mode. However an unexpected behaviour, lateral vibration induced by heaving motion, was observed of the pipe at some special frequencies of the oscillation. A mathematical model widely used of lateral vibration of elastic pipe in water was found to be capable of describing this behaviour as a parametric excitation of Mathieu type and a theoretical method was proposed to predict the magnitude of this excitation. Theoretical predictions, however, do not agree well with the measured magnitude on the pipe models in the water, while they agree in the air. A mathematical model, on whose rational basis further study is required, was proposed to improve the quantitative accuracy of predictions. (A)