The numerical simulations for actual size of a drilling riser, 0.5334m in diameter (D) and 800m in length (aspect ratio of 1500) with a top preload of 3000kN (1.5 times wet-mass) were carried out to address the VIV characteristics in this paper. The FSI simulations were implemented by solving the unsteady Reynolds Navier-Stokes equation and the k-ω turbulence model equation coupling with the dynamic response equation, which has been validated in the previous study. The even distribution of current velocity is set as 0.8m/s. Namely the Reynolds number is up to 426000, which belongs to the classical critical fluid flow region. The obtained results indicate that a large unsymmetrical and bending deformation appear to the riser, the riser vibrates with 9-order mode in in-line direction and 6-order mode in cross-flow direction. The maximum root-mean-square (RMS) vibration amplitudes are about 0.7D and 0.35D respectively in cross-flow and in in-line flow directions, which indicates that the VIV of the drilling riser is characterized by the high-amplitude and multi-mode response in cross-flow direction and high-frequency and multi-mode response in in-line direction.
The vortex-induced vibration (VIV) of slender cylinder structures is encountered in a great variety of engineering fields, especially in offshore oil and gas exploration. As a consequence of significant interactions of vortex shedding and structural dynamics, VIV has the potential to cause severe fatigue failure of these structures. As the key equipment of offshore drilling platform, drilling riser is encountering new challenges causing by higher aspect ratio (L/D~1000–3000, Chaplin et al, 2005) of risers and more complex ocean current environment. This leads to sustainable attentions on the prediction and suppression of VIV for the long flexible cylinders.
The understanding of VIV for a long flexible riser is mainly based on model test and numerical simulation. Model test is generally a favorable approach to provide design data and verifications. Over the past few decades, many typical model tests have been carried out on deep water riser with large aspect ratio (Vandiver, 1983; Vandiver and Marcollo, 1977; Chung et al, 1994; Trim et al, 2005; Chaplin et al, 2005; Lie and Kaasen, 2006; Huarte and Bearman, 2009; Huang et al, 2011b; Gu et al, 2013). These experiments give better insights into some important VIV properties, including response amplitude, dominant mode, vortex pattern and suppression efficiency, and provide some good benchmarks for verifying numerical prediction models.
