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.
Marine riser has an important role to connect the oil and gas fields on the seabed to a floater and to transfer production materials. There are various types of marine risers according to the operating conditions such as the water depth and floater types. The straight rigid type is a representative and conventional model among various riser pipes due to its simple shape, and many of these are still operating in the oil and gas fields. However, as offshore floaters move to the deep water in the depths of above 1500 m, the need of the steel catenary riser (SCR) or steel lazy wave riser (SLWR) has been increasing in consideration of the economic and efficient operations. The SCR and SLWR, on the contrary to the straight rigid riser, have large curved shapes by the hog/sag design. For this reason, a careful approach is needed to estimate a fatigue life of the curved rise pipes in a different way to that of straight rigid risers. The floater motion and the vortex induced vibration (VIV) induced by an incoming wave and current are important factors in the estimation of riser's fatigue life since they lead to structural damages and reducing operational lifetime (Chakrabarti, 2005). Among them, the investigation of VIV is considered to be one of the challenging problems and ongoing research in the design of the marine riser, owing to the difficulty of the prediction. Although it is expected that the curved shape of the riser can cause the different VIV characteristics compared to those of the straight rigid riser, little attention has been given to the VIV and wake dynamics of the curved riser pipe, which have direct engineering significance.
