Research On The Scouring Mechanism Around The Submarine Pipeline And Protection On The Silt Seabed

Marine oil and gas has become an important component of the energy industry in China, which is of significance on the national economic construction and the strategy about national security. Submarine pipeline has many merits, such as continuously deliver, large conveying capacity, convenience management and so on,and it has been used extensively in ocean engineering. However, the environment where the submarine pipelines are laid is very severe. The seabed near the pipeline is very prone to be washed away and eroded by local flow caused by wave and tide, finally, which may cause the pipeline to be suspended, lead to accidents, and result in huge losses. The problem, seabed scouring around the pipeline and pipe suspended, is an important research content of the pipeline project, and significant on the academic and engineering application. However, on the mechanism of scour around submarine pipeline under the complex dynamic environment, the existing research results is still not perfect, especially on the silt seabed.The main aim of this thesis is to search for the mechanism of scour around the pipe laid on the silt seabed through the physical module experiments.On the basis of scanning previous study results, this paper simulates the scour around bare and semi-buried pipe in four hydrodynamic environments including small waves, big waves, one-way current, wave-current through the great physical module experiment. In the experiment, the pressures on the pipe wall and surface soil near the pipeline are monitored.First, this paper details the scouring process of the bed near the pipeline in the experiments, and compares the difference of scour in different situation. On the basis, the paper concludes: (1) The scour around the pipe will not be everlasting and there is an equilibrium. (2) The scour is regular, and in each group test, the process is: the sediment particles began to move– the tunnel comes into being– rapid erosion– achieving balance. (3) These factors , bare pipe, bigger pipe diameter and increasing wave cycles and length, may make the critical erosion depth increased. (4) When wave and current are combined, the scour is very rapid and the scope is very large. But the velocity of current is rapid, the wave can only speed up the scour, and don't affect the depth, so the critical depth is less than the depth of the wave alone, near the one-way flow alone. In addition, the paper compares several formula of calculating the erosion depth, and find that the model of CHAO and HENNSSY is more applicable to the silt seabed.Second, the paper analyses the pressures on the pipe wall and surface soil near the pipeline when the wave and current are combined.Finally, the paper introduces several scour protection technologies now widely used both here and abroad. In addition, the paper test the bionic protection technology through the experiment.