Analytical analysis of pipeline–soil interaction during J-lay on a plastic seabed with bearing resistance proportional to depth

The pipe–soil interaction at the touchdown zone (TDZ) during pipeline installation is of great importance as the pipe embedment has significant influence on various aspects of the performance of the pipeline. Most of the published works focus on the pipe–soil interaction at a pipe segment of unit length ignoring the lay effects, or simplify the seabed as a series of springs. Deepwater deposits are usually very soft, exhibiting low strength and plasticity. This paper aims to reveal the pipe–soil interaction mechanism on plastic seabed during J-lay through an analytical model. The seabed deforms plastically during loading with certain stiffness, but the stiffness during unloading is infinitely large so that the deformation of the seabed is irrecoverable. This model is established based on a system of 15 nonlinear equations and the continuity of displacement, inclination angle, bending moment and shear force along the pipeline is all guaranteed. On the basis of this model, the influences of static and dynamic laying effects on both soil resistance and pipe embedment are quantitatively studied through analytical analysis. In addition, a comparison with a model with a traditional elastic seabed is made, which reveals that the seabed model has great influence on both the inner load distribution and soil resistance along the pipeline.

► We established an alternative method to estimate the pipe–soil interaction on plastic seabed during J-lay.
► We investigated the pipe embedment and soil resistance distribution.
► Soil softening increases pipe embedment and decreases peak soil resistance.
► Traditional “touchdown factor” and ‘dynamic factor’ are not precise for predicting pipe embedment.