The Effect of Crusts on the Undrained Vertical Penetration Resistance of Pipelines

Accurate prediction of the embedment depth of surface-laid pipelines is fundamental to determine in-service performance, which is closely related to the resistance the seabed provides during pipe-lay. The presence of a surficial crust overlying normally consolidated seabeds increases difficulties in accurately assessing the variation in penetration resistance with depth. In an attempt to address these difficulties, this paper presents results from a series of large-deformation finite-element analyses of a pipeline vertically penetrating crusty seabeds under undrained conditions. A detailed parametric study was carried out to examine the effects of crust strength geometry (strength ratio, crust thickness, and strength gradient) and softening properties (soil sensitivity and ductility) on the penetration response and the evolution of soil flow mechanisms. A surficial crust leads to higher penetration resistance relative to a normally consolidated seabed without a crust. The numerical analyses revealed the transition in failure mechanism as the pipe penetrates through the crust into the underlying clay. Separate mechanisms identified at various transition points allowed for a mechanism-based empirical approach that produces a depth profile of penetration resistance for crusts of varying strength characteristics and thicknesses. The accuracy of this simplified calculation method typically is within 10% of the numerical results, and is suitable for use in engineering practice providing an evidence-based approach for the calculation of pipeline penetration in seabeds with surficial crusts.