Mechanics Analysis Of The Suspended Span Submarine Pipeline And Its Cause Of Seabed Liquefaction Research

With further development of marine engineering, submarine pipeline with important task of transporting oil and gas is getting more and more attention of people, and because the destruction of submarine pipeline affected by natural factors brings safety accident. Submarine pipeline is tubular device which is laid under the surface of water, total or part is spanning on the sea bed or buried in the seabed soil.Submarine pipeline and seabed contact with each other, affect and effect mutually. We analyze the coupling action between them to get the mechanical properties of submarine pipeline.The research object of this paper is submarine pipeline in deep sea and we reference to engineering mechanics, vibration theory, fluid mechanic and seepage mechanic and other disciplines related to the theory of knowledge, mainly apply finite element software(FLUENT,ABAQUS) to simulate submarine pipeline.The problem is generally a high Reynolds number problem in deep sea conditions. It needs more fine grid size and suitable turbulent model. Firstly we use FLUENT software to simulate high Reynolds number flow around the pipeline and gain vortex shedding and alternating lift curve of fixed pipe under the action of flow, verify the reliability of simulation using software and provide a reliable basis for further research. We study the influence of soil on flow. Then we change the distance between soil and pipeline, give flow field structure of submarine pipeline in different clearance and analyze the influence clearance ratio on flow drag and lift. Those provide load boundary data for further study of stress and deformation.We see seabed as elastic body establish contact between submarine pipeline and seabed and calculate coupling vibration submarine pipeline and seabed. We obtain its natural frequency and vibration mode and provide referenced basis for avoiding the intense vibration of pipe provided by resonance. Then we conduct transient dynamic analysis, load calculated lift on pipe, find out dangerous position and determine whether pipeline is safety. In addition, we analyze the influence of soil stiffness and span length on dynamic response of pipeline.Finally, we study the liquefaction problem of non-cohesive saturated seabed, using instantaneous liquefaction criterion which is put forward by Okusa based on the effective stress concept:when vertical effective stress of seabed at certain depth of is greater than the upper soil overburden pressure, it will happen soil liquefaction. This paper we choose elastic porous medium model to reflect the sand constitutive model, calculate vertical effective stress of seabed at certain depth and judge whether seabed happen liquefaction.In conclusion, this paper base on free spanning submarine pipeline mechanical properties and discuss problems of flow at high Reynolds number, flow around a circular cylinder, coupling vibration between free spanning submarine pipeline and seabed and liquefaction of seabed. The ultimate goal is to get the mechanical properties of submarine pipeline, judge whether the pipeline is safe or not, to ensure the normal operation of subsea oil and gas transportion.