Minimum Safe Burial Depth of Submarine Pipelines in Well Graded and High Hydraulic Conductivity Sand

Abstract

An important unknown while burying a submarine pipeline is “What is it's minimum safe burial depth?”. It is a function of wave climate, water depth, engineering and hydraulic properties of the seabed soil, etc. To solve this problem, physical model investigations were carried out in a wave flume to assess the change in the horizontal and vertical hydrodynamic forces on a submarine pipeline for different depth of burial. The marine sand is well graded and has hydraulic conductivity of 1.84 mm/s. It is found that the horizontal force reduces non-linearly with increase in depth of burial. 75% of the magnitude of the horizontal force reduces by just burying the pipe, when compared to the one placed on the seabed. The vertical hydrodynamic force is smaller for half buried pipe and increases for just fully buried case due to the significant change in the magnitude as well as the phase difference between the pore water dynamic pressures. For any burial depth and for a constant wave height, increase in wave period has noticeable increase in the vertical wave force than the horizontal wave force. The horizontal wave force dictates the stability of the submarine pipeline, when it is placed on the sea floor. For buried pipeline, the upward wave force dictates. Workout example is presented as a ready reference.