Local Scour Around the Touchdown Zone of a Pipeline-Riser System

This paper presents a series of physical model tests for investigating local scour around the touchdown zone of a pipeline-riser system used for liquefied natural gas (LNG) offloading. The local scour around a J-shape pipeline-riser system is influenced by several parameters, including the curvature radius to pipeline-riser diameter ratio (R/D), pipe embedment ratio (e/D), flow intensity (defined by the ratio of the Shields parameter to the critical Shields parameter, θ/θcr), and flow direction (α). The model tests were conducted using a model pipe with a diameter of D = 0.07 m and R/D = 5. Eight different flow directions (α = 0° to 180°) and six pipe embedment ratios (e/D = 0.5, 0.7, 0.9, 1.1, 2 and 3) were examined. The results indicate that flow direction α affects the equilibrium scour depth significantly for e/D = 3.0. When α ranges between 0° and 122.5°, the curvature of the model pipe appears to intensify the local scouring process. For larger α values, the scour depth is less than that observed around a vertical pile. For e/D = 3.0, local scour is seen to be a constrained process, and the horizontal pipeline remains buried. The equilibrium scour depth diminishes as e/D reduces from 3.0 to 0.7, and no scour propagation is observed along the pipeline on the seabed for this range. For e/D = 0.5, local scour continuously expands in the spanwise direction of the pipeline, leading to the formation of free spans. Critical e/D value for the development of free spans lies between 0.5 and 0.7.