Abstract
In the past few decades, there have been many numerical studies on the scour around offshore pipelines, most of which concern two-dimensional setups, with the pipeline infinitely long and the flow perpendicular to the pipeline. Based on the Ansys FLUENT flow solver, this study establishes a numerical tool to study the three-dimensional scour around pipelines of finite lengths. The user-defined functions are written to calculate the sediment transport rate, update the bed elevation, and adapt the computational mesh to the new boundary. The correctness of the model has been verified against the measurements of the conventional two-dimensional scour around a long pipe and the three-dimensional scour around a sphere. A series of computations are subsequently carried out to discover how the scour hole is dependent on the pipeline length. It is found that the equilibrium scour depth increases with the pipeline length until the pipeline length exceeds four times the pipe diameter.
Highlights
Pipelines are often regarded as the lifeline for the oil and gas industry
Underwater surveys on the wellbeing of on-bottom pipelines are often conducted after the passing of a tropical storm, and a risk assessment is held based on these empirical formulae, pipeline strength, and importance, etc. to decide whether any scour remedial measures are required before the arrival of the storm
The model emphasized the important role of streaming in unsteady sediment transport and was later applied to predict the scour around vibrating pipes, with the results summarized into nondimensional regression formulae to describe the relationship between the scour depth, vibration frequency, and amplitude [13]
Summary
Pipelines are often regarded as the lifeline for the oil and gas industry. Local scour around offshore pipelines may leave pipelines to be suspended above the sea floor, making them vulnerable when exposed to currents, waves, shipment, and fishery activities. Numerical studies are based on the potential flow theory, which can correctly predict the maximum scour depth and the shape of the upstream part of the scour hole. These previous models cannot explain the gentle slope of the scour hole downstream of the pipe. Hatton et al (2007) used Flow-3D software to numerically simulate the three-dimensional flow around partially buried submarine pipelines subject to combined actions of waves and currents, but the bed was fixed and only the scour potential was inferred [16].
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