CFD Simulations of Vortex-Induced Vibrations of a Subsea Pipeline Near a Horizontal Plane Wall

Abstract

Subsea pipelines, when exposed to free spans, can experience vortex-induced vibrations. This phenomenon was described as a resonance condition occurring when the vortex shedding frequency and the natural frequency of a structure approach common oscillation frequency. Fatigue life of the pipeline can be adversely affected by a high amplitude oscillations attributed to the vortex-induced vibrations. A numerical study has been performed on the effects of wall proximity on the vortex shedding of an elastically mounted circular cylinder. In addition, the study was extended to investigate the influence of a second cylinder with a smaller diameter rigidly coupled with the large cylinder. Such configurations can be regarded as a model of a subsea pipeline or, in case of coupled cylinders, a subsea piggyback pipeline in a free span situation. A series of two dimensional, numerical studies using open source CFD code OPENFOAM has been performed. Simulations were performed in two flow regimes, a laminar vortex street regime at Reynolds number Re = 200 and an upper transition regime at Re = 3.6×10e6. A range of reduced velocities covering a frequency lock-in phenomenon was investigated. Hydrodynamic forces and response amplitudes were mapped with respect to the reduced velocity. Furthermore, a study of the phase differences between the hydrodynamic forces and cylinder displacements was conducted. The motions of the cylinder were recorded and presented on the trajectory plots. The frequency components of hydrodynamic forces and displacements were analyzed with the FFT algorithm in the frequency domain. In order to gain insight into the effects of the shear layers interaction in the area around the oscillating cylinder, flow visualizations of the numerical simulations were analyzed.