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Pipe–soil interaction on free-span shoulder subject to vortex-induced vibration

Free span refers to a subsea pipeline section that is suspended over the seabed. Such a pipe section is vulnerable to vortex-induced vibration leading to possible fatigue pipe damage. Existing studies on pipe–soil interaction on a free-span shoulder are limited and the methods provided in the commonly used design guidelines appear to rely on either plausible assumptions or simplified conditions that warrant further investigations. In this study, a centrifuge model study has been carried out to evaluate pipe–soil interaction of an embedded pipe subject to cyclic motions. Parametric studies involving effects of pipeline motion amplitude and initial embedment on pipe–soil interaction are conducted. The test results reveal that the degradation in soil stiffness can be correlated with pipeline motion cycle and amplitude. Simplified methods are then proposed for the preliminary evaluation of equivalent soil stiffness.