Experimental Study And Numerical Simulation On Post Trenching Of Submarine Pipelines

Submarine pipelines are one of the most important manners for offshore oil and gas transportation, which play more and more important role in offshore engineering and economy development. In order to protect these pipelines, they need to be trenched and buried. The post trenching is the most common trenching technology, and jet trenching machines are the most common machines in the post trenching. The jet trenching machines equipped with jets or jetting swords use jetting system to inject water into seabed to create a temporary trench, which allows cable or pipeline to descend into the sea bed before being buried under the re-depositing sediment. In recent years, the the pipe diameter becomes bigger and bigger, so creating trench to bury pipelines becomes more and more difficult. Apparently, in order to apply jetting power to create trench in offshore engineering, to carry research on jet trenching cannot be evasive. This paper studies the factors that may affect the trenching performance of jet trenching machines by carrying experiment and numerical simulation.In the first part of this paper, similar researches have been summarized, and then the physical model is designed and the appropriate model sand is used in tests. The topographic deformation due to the scour and the sediment concentrations are investigated to study the effects of the jet velocity and horizontal translation velocity. The change process of a scour profile with time after impinging is analyzed and the similarity of the scour profile is investigated. Experimental results show that the increase in jet velocity hardly increases the maximum scour depth in the asymptotic state when the jet velocity has reached a certain value. If the horizontal translation velocity is controlled in a certain range, the factor can take a more obvious effect on the scour radius than the scour depth does. The change process of a scour profile with time after impinging can be divided into two stages, and there are two reasons leading to the changes. The scour profile was found to be self-similar when normalized by appropriate length scales.Finally, the paper uses Flow3D to simulate jet trenching. Firstly, the sensitivity to some parameters of the sediment transport model within FLOW-3D is studied to build a sediment transport model. Secondly, jet flow is simulated without sediment to study the hydrodynamic characteristics ot jet. Finally, the scour by moving jets and sationary jets is simulated, and the flowfield of jet is described. The suspended sediment content and the scour profile are analysised.