Oscillations of Cantilevered Tubular Steel Piles in Current Flow

Abstract

A laboratory experiment was conducted in a 20 m flume tank at Plymouth University to investigate the movement of, and forces applied to, a pair of in-line cantilevered piles. 2-minute tests were run with a 64 mm diameter upstream pile and downstream piles of 64 mm, 40 mm and 30 mm. Current velocities ranging from 0.1 to 0.9 m/s and spacings between the piles of 3 to 10 diameters were varied along with the downstream pile diameter. Initially, tests with all 3 piles on their own were conducted and the results showed a close comparison with the theoretical cross-flow oscillation frequency and the measured cross-flow oscillation frequency of the solitary pile. Similarities between the theoretical and measured steady drag forces were not identified but a significant relationship between steady drag force on the downstream pile and spacing was discovered. The results indicated that when the spacing was small, approximately 3 to 4 diameters, the downstream pile experienced more drag force than the upstream. The force steadily decreased until it appeared to level out around 9 to 10 diameters away. Analysis of the cross-flow oscillations of the downstream pile revealed that the 64 mm pile oscillated with a frequency close to the theoretical at all spacings and flow velocities. Whereas, the 40 mm and 30 mm piles oscillated with frequencies less than the theoretical, at all spacings and flow velocities. Despite the solitary 40 mm and 30 mm piles oscillating at the theoretical frequency. This demonstrated that the oscillation frequency of the downstream pile was governed by the size and therefore the vortex shedding frequency of the upstream pile.