Vortex-induced vibration characteristics of two tandem spring supported cylinders with helical strakes

Abstract

Vortex-induced vibration (VIV) response can occur when a structure interacts with a fluid flow, resulting in fatigue damage. The use of multiple cylinders during drilling operations worsens the situation due to the flow interference between the cylinders, and the response may become more complex than with a single-cylinder. The industry has been challenged to minimise the effect of flow interference on the structure, which is highly dependent on the separation distance between the cylinders. The purpose of this study is to investigate the VIV on two spring supported cylinders in the tandem configuration besides examining the performance of helical strakes in reducing the cylinders’ vibration. A series of experiments were conducted in the water flume using circular cylinders, where both cylinders were free to oscillate in the cross-flow direction only under subcritical Reynolds number. The flow interference test was performed with separation distances of 3.5, 4.0, and 4.5D, where D is the cylinder’s diameter, and the critical separation distance was identified. The experimental results including the displacement response, frequency response, and power spectral density, were compared to those of the cases involved with the single-cylinder. The influence of different helical strakes’ arrangements was examined using the same method as in bare cylinders. The experimental results showed that the amplitude response of cylinders in tandem grew continuously as the reduced velocity increased for all separation distances, indicating the presence of wake-induced vibration (WIV). The lower branch of amplitude response, which was usually present in a single-cylinder, was discovered to be absent. The results also proved that when multiple cylinders were used, the cylinders would vibrate stronger than when the single-cylinder was used. The trailing cylinder vibrated slower than the leading cylinder for all separation distances due to the shielding effect of the leading cylinder. Additionally, the critical separation distance was determined at 3.5D, where the bistable regime existed. During this regime, two flow patterns appeared intermittently, with a flow transition between vortex formation from the leading cylinder and reattachment of boundary layer, leading to a considerable oscillation amplitude. Meanwhile, the helical strakes successfully reduced the oscillation amplitude for both cylinders at the critical separation distance. However, the effectiveness of the helical strakes was highly dependent on the strakes’ arrangements. The present study found that the strakes had a significant suppressive effect when installed at the leading cylinder (arrangement of leading straked and trailing bare cylinders (LS+T)) or both cylinders (arrangement of two tandem straked cylinders (LS+TS)). The novelties of the study are: (1) the investigation of two cylinders’ responses that both can move in the CF direction, which is new in the literature and (2) the evaluation of the suppression performance of helical strakes in critical separation distances for tandem rigid cylinders at different arrangement of strakes. Since most of the risers in offshore industries are not in a fixed condition, investigating the VIV of the oscillating cylinders is feasible. The findings are valuable to offshore engineers to forecast the VIV phenomenon of the oscillating cylinders, especially in a critical condition.