Three-Dimensional Hydrodynamics And Bedform Evolution Around A Submarine Pipeline

Many failure cases of pipelines on seabed could be attributed to scouring under the long term effects of water flow and wave. Hydrodynamic characteristics and the seabed evolution mechanism are crucial for the safe operation of such pipelines, and they have been studied in this paper. The flow velocity around pipelines and the local topography are measured for water flow and wave of varied angles, with the (generalized) experiments on fixed bed and sand bed. Based on the N-S equation, the three dimensional hydrodynamic numerical model is developed, coupled with the DES turbulence model.Obvious temporal and spatial differences exist in the flow structures around the pipe when considering different hydrodynamic effects. Shown by the computational results, the periodic development and shading of the wake behind the pipe stems from the strong shear flow caused by flow separation in the frontal part of the pipe. Streamwise spiraling flow with small scale vortical structures are observed near the seabed both before and after the pipe, the turbulence is much more intense behind the pipe compared with that in front, and specially, the biggest turbulent intensity appears near the wall; the distributions of vertical-lines velocity and turbulent kinetic energy appear to be in layered architecture. The streamlines oscillate under the influence of the wave:in the first half period, the bottom velocity aligns with the direction of propagation, while they are in opposite direction in the second half period. When considering the couple effects of the wave and current, the vertical velocity varies the same way as the flow in front of the pipe, however, it’s totally different in the wake, where irregular periodic oscillations can be observed. In each phase, mixture phenomena can be observed between normal velocity and axial velocity.The analysis of the key roles which influence the hydrodynamic performances. With the increase of the incidence angle, the vortical region in front of the pipe squeezes gradually, while the wake moves gradually far more downstream; in the meanwhile, the scale of the vortices, the recirculation region, and the wake region extends gradually, the center of the wake departs from the pipe line, with a weaker influence from the axial flow. The generation and shading of the wake, together with the scale of the vortices are affected by the gap ratio: when 0<e/D<0.3, the shading frequency increases with the decrease of e/D, and the pattern of the wake does not have a clearly change when e/D> 0.3. With the influence of the wave, the KC number plays an important role, the flow rarely separate when KC number is small; with the increase of the KC number, the axial velocity and bottom velocity around the pipe also increase, relatively long vortex street are observed on both sides of the pipe together with the vortex shading phenomena.Erosion and deposition development of the seabed act similarly under different hydrodynamic conditions; they can be divided into three stages, i.e. scour onset, quickly extension and equilibrium. However, the partial scour morphologies around pipeline vary with different hydrodynamic conditions. Under the flow, erosion occurs beneath the pipeline and deposition occurs behind the pipeline; differently, under the wave, erosion and deposition appear symmetrically on both sides of the pipeline; furthermore, under the couple of flow and wave, asymmetric scour extends downstream beneath the pipeline, and the maximum depth is larger than the linear summation of flow and wave effects. With the increase of the incidence angle, the maximum position of equilibrium scour depth moves downstream and depth and width magnitude become larger.A comprehensive empirical formula of equilibrium scour depth under the flow and wave was established through the dimensional analysis of impact factors on local scour equilibrium depth around the pipeline, and were validated by experimental data. The relative scour equilibrium depth around the pipeline under the flow increases with the increase of incidence angle and Froude number, and decreases with the increase of gap ratio and sediment medium size. Under the wave, the relative scour equilibrium depth around the pipeline increases with the increase of the incidence angle and KC number, and decreases with the increase of the gap ratio e/D.