Effect of the curved shape on the vortex shedding phenomena of a riser

Abstract

The curved riser has locally different forces due to its large radius of the curvature, and the consideration of force variations along the curved shape is essential to understand the vortex-induced vibration (VIV) of the riser. Moreover, since the riser is exposed to the current in various directions, it is also crucial to identify the effect of the incident flow angle on the VIV. Therefore, this study, based on the numerical simulation, focuses on how the sectional force coefficients are coupled with wake structures according to the incident flow angle and the Reynolds number. The real Reynolds number of the riser can be varied with the water depth and is about in the range of 102 to 106. In these conditions, it is difficult to simulate the flow around the entire riser, which involves various flow features including laminar, transition and turbulence. Due to this reason, the simplified curved shape of the riser and the specific low Reynolds numbers of 100 and 500 were considered in the academic approach. Also, this approach has been successfully applied in other relevant studies. For the use of the simple riser model, the geometrical similarity is ignored since the riser has a significant extension from a floater to a seabed, which leads to the difficulty to the scaling of the riser in a computational domain. This research shows that the VIV characteristics of the curved riser are thoroughly different from those of the conventional straight riser due to the curved shape. Namely, there is a three-dimensional effect on the VIV and the sectional forces along the curved riser owing to the interaction between the wake and axial flow. Consequently, this study suggests that new VIV database including the three-dimensional effect should be established for the realistic estimation of riser’s fatigue life. ？ 2019 by the International Society of Offshore and Polar Engineers (ISOPE).