The Role of 2D Seepage on Sediment Incipient Motion around a Pipeline

Pipelines have been used as one of the main transportation methods for the offshore industry, with increasing activities in marine resources recently. Prediction of seabed instability is one of key factors that must be taken into consideration for an offshore pipeline project. As the first step of the scour process, sediment incipient motion has been intensively studied in the past. Most previous investigations didn't consider the wave-induced seepage in the elevation of sediment motion. In this paper, two-dimensional seepage was considered to modify the conventional Shields number and its associated impact on sediment incipient motion around the trenched pipeline was investigated. Both flat and sloping seabeds are considered. The numerical results indicated that a peak or valley of the modified Shields number was formed below the pipeline and horizontal seepage flow tremendously impact the sediment motion in the vicinity of the pipeline. Parametric analysis concludes: the influence of the seepage around the pipeline becomes more significant in a large wave, shallow water in a seabed with large shear modulus and permeability, and larger pipeline diameter and smaller flow gap ratio. This will make soil particles be more easily dragged away from the seabed.

Automated summary

This study examines the key factor of seabed instability - sediment incipient motion in offshore pipeline projects, including a modification of the conventional Shields number with consideration of two-dimensional seepage. The results show that horizontal seepage flow around the pipeline has a significant impact on sediment motion in both flat and sloping seabeds, especially in cases of large waves, shallow water, seabeds with large shear modulus and permeability, and pipelines with larger diameter and smaller flow gap ratios.