The effects of penetration rate and strain softening on the vertical penetration resistance of seabed pipelines

Offshore pipelines in deep water are generally laid directly on the seabed, without any additional stabilisation measures. Design parameters that determine the soil resistance to lateral and axial motion of the pipeline are a function of the amount of vertical embedment. However, this latter quantity is difficult to estimate, partly because of the effects of soil heave around the pipeline as it penetrates, and partly because the soil shear strength depends on the strain rate and the degree of softening as the soil is sheared and remoulded. In this paper, a large deformation finite-element approach was adopted to study pipe–soil interaction during vertical embedment of pipelines on the seabed. The simple Tresca soil model was modified to incorporate the combined effects of strain rate and softening. The present large deformation finite-element method was validated by comparing the results with data from centrifuge model tests. A parametric study was then performed, varying the strain rate and softening parameters to explore their effects on penetration resistance. Simple expressions for penetration resistance, incorporating the effects of strain rate and softening, have been developed. The effects of soil strength vertical heterogeneity and buoyancy have also been explored.