Hydrodynamic Coefficients From In-Line VIV Experiments

Abstract

For subsea pipelines installed in areas with uneven seabed free spans may occur and fatigue failure due to vortex induced vibrations (VIV) is one of the main concerns related to these spans. In order to install pipelines in such areas the safety against fatigue failure from in-line (IL) and cross-flow (CF) VIV must be documented. Although maximum oscillation amplitudes in the IL direction are considerably smaller than the maximum amplitudes in the CF direction, the IL fatigue damage normally prevails and may limit the allowable span length. The reason for this is that the IL vibrations initiate at a lower current velocity (i.e., reduced velocity) than the CF vibrations and would hence be excited for a longer period of time. Prediction tools for VIV may be split into parametric Response Models such as described in DNV-RP-F105 and methods based on empirical coefficients such as SHEAR7 and VIVANA. Methods based on force coefficient have until recently been limited to CF VIV due to lack of hydrodynamic coefficients for IL response. This paper presents results from forced IL oscillation experiments of a smooth, rigid cylinder in uniform flow. The results are presented as dynamic in-line coefficients for the pure IL regime, i.e. reduced velocity between 1 and 4, at Reynolds number 24.000. The results are compared with IL results from free oscillation experiments found in the literature.