Modelling the axial soil resistance on deep-water pipelines

Axial pipe-soil resistance is an important aspect of deep-water pipeline design, since it influences the longitudinal and lateral buckling responses under thermally induced expansion and contraction of the pipeline. Experimental evidence has shown that the axial resistance, expressed as a proportion of the submerged pipeline weight, can vary by an order of magnitude, depending on the rate of axial movement and cumulative time. This paper provides a theoretical framework for assessing the magnitude of axial friction. The framework is developed within a critical-state context using effective stresses, applicable to any degree of drainage in the soil, quantifying the magnitude and duration of excess pore pressures generated near the pipe/soil interface. Two other aspects of behaviour are added to match the observed velocity dependence of axial resistance: (a) a damage term, leading to contractive volumetric strain at the interface; and (b) strain-rate dependence of the mobilised soil strength. Analytical expressions are derived that capture the above features of the response. The resulting variations of normalised frictional resistance with time and velocity are then shown to match experimental data from interface shear-box tests, representing a planar idealisation of the same behaviour, and from model pipe tests.