Experimental Investigation of the Dynamic Behaviour of a Water Intake Riser

Abstract

This paper considers the dynamic stability of free-hanging water intake risers. Suspended from a barge, these risers convey a great volume of cooling water, which is needed for offshore liquefaction process of natural gas. There is a contradiction between theoretical predictions and experiments for cantilever pipes pumping up water. Reported small-scale experiments did not show any instability, while theory predicts instability beyond a critical fluid velocity. To investigate whether the previous experimental setups did not allow to observe the instability or the pipe aspirating water is unconditionally stable, a new test setup was built which could attain a higher internal fluid velocity than the predicted critical velocities. A cantilever pipe of about 5 m length was partly submerged in water. The experiments clearly showed that the cantilever pipe aspirating water becomes unstable by self-excited oscillatory motion (flutter) beyond a critical velocity of water convection through the pipe. Below this velocity the pipe is stable, whereas above it, the pipe shows a complex motion that consists of two alternating types of motion. The first type is a nearly periodic orbital motion with the amplitude of a few pipe diameters and the second one is a quasi-chaotic motion with very small amplitude. Translating these results to offshore water intake risers, shows that for realistic internal flow velocities the riser might become unstable.