For better understanding the mechanism of the occurrence of pipeline span for a pipeline with initial embedment, physical and numerical methods are adopted in this study. Experimental observations show that there often exist three characteristic phases in the process of the partially embedded pipeline being suspended:
local scour around pipe;
onset of soil erosion beneath pipe; and
complete suspension of pipe.
The effects of local scour on the onset of soil erosion beneath the pipe are much less than those of soil seepage failure induced by the pressure drop. Based on the above observations and analyses, the mechanism of the occurrence of pipeline spanning is analyzed numerically in view of soil seepage failure. In the numerical analyses, the current-induced pressure along the soil surface in the vicinity of the pipe (i.e. the pressure drop) is firstly obtained by solving the N-S equations, thereafter the seepage flow in the soil is calculated with the obtained pressure drop as the boundary conditions along the soil surface. Numerical results indicate that the seepage failure (or piping) may occur at the exit of the seepage path when the pressure gradient gets larger than the critical value. The numerical treatment provides a practical tool for evaluating the potentials for the occurrence of pipe span due to the soil seepage failure.
When a submarine pipeline is laid upon seafloor, there always exists some embedment into the soil. In severe ocean environments, the soil beneath the pipeline may be scoured, and the pipeline will thereby be suspended above the seafloor. The occurrence of pipeline span is proven to bring much potential for vortex-induced vibrations of pipelines. Therefore, to efficiently avoid the occurrence of pipeline span is highly desired in the pipeline engineering.
