Non-Resonant In-Line VIV and Its Implications on Model Tests

Recently, small-scale experiments were conducted by [1] to study in-line VIV in pipe spans. The experiments were performed with six different pipes of varying stiffness and mass ratio, but with the same length-to-diameter ratio. The response of the pipe with the lowest mass and stiffness, made out of Acrylonitrile Butadiene Styrene (ABS), was surprising. The in-line RMS A/D response of the ABS pipe was larger and over a much wider reduced velocity range than shown in design codes like DnV F105. Since these codes are commonly used to design real pipelines, the authors were interested in understanding these observations. In the past, observations of VIV response over a wide reduced velocity range have been explained using added mass. This paper shows that though added mass could play an important role, observations of the in-line and cross-flow response mode and frequency content suggests that there could be other reasons for the response observed in the experiments. In particular, this paper investigates the observed large response away from the region of resonant VIV and proposes that this non-resonant in-line response could be different from what researchers typically call VIV. The paper also investigates when such a mechanism could contribute to substantial in-line VIV motion. The implications of this work could be significant, not just for pipe-span design but also for scaling pipes for in-line VIV model tests.