Determination of Vortex Shedding Frequency Around Offshore Pipeline Using Unsteady Drag Force Model

Laying offshore pipelines on the uneven sea floor makes the flow turbulence and instability behind the pipe, which is termed as the vortex shedding. Vortex shedding causes a periodic change in the net hydrodynamic pressure around the pipe which in turn increases significantly the potential of fatigue damage of a pipeline at the free span section. This potential especially will rise when the vortex frequency is synchronized with one of the natural frequency of the pipeline span and/or resonance occurs. Thus, during the design process for avoiding resonance, it is crucially important to estimate the frequency of vortex shedding behind the pipeline to limit the length of pipeline on the free span section to its allowable value. Due to the essential role of obtaining vibration drag forces and as a consequence vortex shedding frequency as well as its induced time dependent pressure and velocity distribution, using numerical simulation model for simulation of hydrodynamic forces acting on the pipe become necessary. In the present study, the vortex shedding frequency around an offshore pipeline is calculated by employing the pattern of unsteady dynamic drag force. The unsteady pattern of dynamic drag force is obtained by the model, which is constructed based on the Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with a standard k-ε turbulence model for turbulence closure. As a results of employing this numerical model, vortex shedding frequency, caused by current, around the offshore pipelines is calculated along with its induced turbulent pressure and velocity field. Finally, the results of estimating vortex shedding frequency behind the pipeline is compared with those of experimental data, for different situations of laying offshore pipeline on the seabed and various environmental conditions. The results reveal that the employed numerical model is capable of evaluating vortex shedding frequency around an offshore pipeline.