Shear stress amplification around subsea pipelines: Part 3, 3D study of spanning pipelines

© 2015 Taylor & Francis Group, London. While the subject of scour, erosion and sedimentation around subsea pipelines has been studied for many decades, a reviewof the literature shows a number of areaswhere relatively little existing publishedwork has been presented, including the variation in seabed shear stresses around the shoulders of subsea pipeline spans. This paper presents the results of 3D CFD modelling using ANSYS FLUENT, which has been undertaken for a parametric range of 24 cases, including 6 different pipe/span geometries, as well as a range of steady current and wave cases at a range of incident angles. The effect of span presence is described using the previously published concept of a seabed shear stress amplification factor α at the span shoulder which extends a distance of βD around each shoulder, together with a related amplification factor γ for the mid-span region. Analysis of the results focusses on evaluation of seabed shear stress amplification factors compared to far-field ambient values. The results have relevance to the prediction of span growth rates and complement previously published experimental work under live-bed conditions. The present results have the potential to provide insights into span growth under clear-water conditions which have significant potential to result in seabed profile changes due to slow-processes induced by persistent ambient metocean conditions.