Lateral instability of subsea pipelines on soft clay: shear failure versus interface slippage

Subsea pipelines on a soft clayey seabed in deep waters are susceptible to lateral instability under steady currents. Two distinct mechanisms govern such instability behaviours: shear failure of the underlying soil (mechanism I) or slippage along the pipe–soil interface (mechanism II). This study presents a unified analytical model, based on limit equilibrium analysis and passive earth pressure theory, to quantify the ultimate lateral resistance while explicitly capturing the interplay between these competing mechanisms and the influence of the soil berm. The model is validated against experimental data and benchmarked against existing empirical solutions. Parametric analyses demonstrate that the critical embedment ratio for lateral stability decreases with increases in either soil undrained shear strength or pipe submerged weight. Furthermore, the governing mechanism exhibits contrasting trends: stiffer soils promote interface slippage (mechanism II), whereas greater pipe weight or higher interface adhesion favours soil shear failure (mechanism I). From these insights, an explicit correlation is established at the transition point, relating the critical dimensionless net vertical force to the embedment ratio and interface adhesion factor. This relationship identifies the dominant instability mode, offering a practical, mechanics-based tool for subsea pipeline on-bottom stability design.