ABSTRACT:

Although there are many studies dedicated to the problem of vortexinduced vibration (VIV) of submarine pipeline spans, VIV experiments considering the effect of soil at the span shoulders are very rare. To investigate the effect of the soil on the VIV of submarine pipeline spans, a pipe model, 16mm in diameter, 2.6m in length and with mass ratios (mass/displaced mass) of 4.30 was tested in a current tank. The pipe was laid horizontally on the soil, and had a free span in length of 2.138m. The tests in still water and in a current were conducted. The flow velocity was in the range of 0-0.57m/s. The response of the model was measured using fiber optic strain gauges. The frequency responses and the time-domain tracing of cross-flow strain responses are presented and analyzed. The experimental results exhibit several valuable features: the natural frequency of the model has a fuzzy property, but the response frequency in a current has not and increases with the increase of flow velocity; the strain amplitude for the sandy soil case is larger than that for the clay soil case.

INTRODUCTION

Vortex shedding from a submarine pipeline span may occur when a fluid flows past the pipeline, and induces fluctuating hydrodynamic forces. When the frequency of vortex shedding approaches the natural frequency of the structure, the span will be excited into a resonant state and undergo a large amplitude vibration. Vortex-induced vibration (VIV) is of practical importance for the pipeline spans. Large responses give rise to oscillatory stress, which can cause unacceptable fatigue in pipeline. The VIV of submarine pipeline span is a complicated problem involving both the fluid structure interaction and the pipe-seabedinteraction. The VIV of circular cylinders had been investigated in the last three decades by many researchers.

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