Effect of clay content on the coupled seabed response and local scour around a free-spanning pipeline under combined waves and currents

Abstract

Scour around submarine pipelines on a sand–clay mixed seabed under combined wave and current loading conditions involves complex interdisciplinary interactions among hydrodynamics, seabed mechanics and sediment transport processes. Although previous studies have addressed the scour or dynamic response of the seabed in isolation, their relationship remains unclear. This study performs laboratory experiments to clarify the integrated processes between local scour and seabed response around pipelines laid on a sand–clay mixed seabed with clay contents ranging from 5% to 20%. The results show that increasing the clay content enhances the dissipation of wave energy and reduces the dynamic pressure under high wave steepness, while simultaneously promoting tunnel scour beneath the pipeline due to cumulative pore pressure build-up. The phenomenon reduces the velocity and local pressure below a pipeline. Furthermore, the presence of clay shifts the maximum oscillatory pore pressures to intermediate depths and alters the residual pressure distributions depending on the extent of the scour. Although upward seepage induced by residual pore pressure in clay-rich seabeds has limited influence on wave-induced shear stress, it plays a critical role in reducing the threshold for sediment initiation, accelerating early-stage scour and increasing suspended sediment concentration (SSC), particularly under large wave steepness (that is, H / L 0 = 0.052). At smaller wave steepness (that is, H / L 0 = 0.018), an addition of 5% clay enhances SSC and promotes scour, while a higher clay content suppresses sediment mobility, where seepage induced by oscillatory pore pressure may play a significant role. An empirical formula incorporating clay content, the Keulegan–Carpenter (KC) number, and the Shields parameter is proposed to support engineering design in mixed sand–clay seabed conditions. • Increasing clay content enhances wave attenuation but promotes tunnel erosion under high wave steepness. • Clay alters pore pressure profiles by reducing drainage and modifying scour-induced boundary conditions. • Upward residual seepage reduces sediment mobilisation thresholds, intensifying early-stage scour and SSC under large wave steepness. • An empirical scour depth formula that incorporates the CC, KC and Shields parameter shows strong predictive precision ( R 2 = 0.75).

Topics & Concepts