Numerical Simulation Of Local Scour Under And Force On A Submarine Pipeline

At present, the submarine pipeline transport manner has been becoming a quick and economic manner for the offshore oil and gas transportation. The direction of the oil exploration and development is turning to deep sea area, with the environment becoming increasingly bad, it would threaten the transportation devices. Once the pipelines broke and the oil came out to the sea, it would be a catastorphic damage that would be hard to recover. To clean up the spilled oil also would cause a secondary pollution. Therefore, the problem of the transportation device safety absolutely can not be ignored.In the complex marine environment, pipelines placed on the erosion seabed cause the change of local flow field, also the local seabed profiles lead the pipeline to suspending. At the same time, the flow influences the pipeline and causes corresponding response of the pipeline. Appropriate measures can be done to reduce the damage caused by scour by understanding the mechanism of the scour and pipeline force action. By using SIMPLEC algorithm of the finite volume method, solving unsteady flow equation of N-S with k-co turbulence model closure, simulating the even cohesiveless bed load transportation, a2D submarine pipeline scouring numerical model is established to obtain the local scour process of the pipeline from lying on the seabed to suspending. There are three main models, including flow model, bed load model and scour model (seabed changing model). The cut cell approach has been introduced to the scour model with the obstacles cutted out of the background Cartesian coordinate structured mesh and succeeded in avoiding traditional reconstruction process. Also, a pipeline force model is established by combining the flow model and force model and would be used to calculate the force of the pipeline which suspending on the scour profiles of any special moment of the scour process. By contrasting with the physical model experiment results of Mao(1986)[1] and the numerical model results of Lei and Cheng(2005)[15], the numerical scour model results match well with the case of the inlet flow velocity of0.35m/s. The pipeline force model results of two cases including the gap ratios of0.3and0.5also match well with that of Lei and Cheng(1999)[27]. The paper calculated the cases of0.35m/s and0.43m/s inlet flow velocity and found out that drag coefficient increasing when the scour hole’s depth to length ratio E/L and the gap ratio E/D go up; lift coefficient increasing when E/L and E/D go down in0.35m/s inlet flow velocity case and turn out opposite in anther case.