Numerical Analysis Of Stability Of Submarine Pipelines On A Sandy Seabed

Pipelines are economical and efficient means of transporting petroleum and natural gas and are widely used in ocean petroleum engineering. The instability of submarine pipelines due to liquefaction of the seabed is one of the major problems to be considered when the pipelines are designed. When gravitational waves propagate over the ocean, they cause fluctuating pressure upon the seabed, which will further induce excess pore pressure and effective stress within the seabed soil, and make the distribution of the stress in the seabed change. The pipelines laid on the seabed also have effects on excess pore pressure and effective stress in the vicinal seabed, and this makes the stress in the seabed more complex. With the developments of excess pore pressure and effective stress, parts of the seabed will become unstable or even liquefied. Therefore, it is of theoretic importance and applied value in engineering to research the coupled response of wave, pipelines and soil.A finite element analysis is made on the pipelines laid on a sandy seabed via ABAQUS, which is a sort of non-linear finite element analysis software. The developments of stress in the seabed under wave and pipelines gravity loading are researched, and then the study on the liquefaction result of the seabed soil near the pipe is made. These will provide the reference ideas for pipelines design.Firstly, the porous elastic model is used for the seabed to make a coupled analysis of wave-pipeline-soil system and the results of the distribution of excess pore pressure and effective stress in the seabed under the wave and the pipeline gravity loading are obtained. Seven parameters, permeability, wave depth, wave height, wave period, wave theory, pipeline parameter, soil elastic modulus, are discussed about their effects on excess pore pressure and effective stress in z-direction in the seabed.Subsequently, to make the simulation closer to real condition, Gibson soil is adopted. Variable permeability and soil shear modulus are considered, and then distribution of excess pore pressure and effective stress are obtained.At last, soil gravity is considered and based on the object-oriented program language Python, a secondary development is made on post processor of ABAQUS. Zen and Yamazaki's criterion of wave-induced liquefaction is used to analyze the seabed liquefaction. The liquefaction results of the seabed under wave and pipeline gravity loading are obtained and analyzed.