The axial sliding resistance between a surface-laid pipeline and the seabed is an important design parameter that influences the pipeline end expansions, as well as lateral buckling and cyclic walking responses. This paper describes a new framework for quantifying axial pipe-soil interaction and reports data from interface shear box testing of three carbonate soils in support of this approach. The framework uses concepts from critical state soil mechanics, and encompasses the influences of stress level, pipeline roughness and shearing rate - including drainage and viscous effects. Each of these effects is independently quantifiable and relevant to design. Typical pipeline expansion rates span drained and undrained behaviour on deepwater mud, and typical pipeline coatings are often in the transitional range between fully rough and fully smooth. In addition, at the stress levels relevant to pipelines, the effective stress failure envelope is non-linear. The laboratory data comprises interface shear box tests on a range of carbonate sediments spanning from sand to fine silt. Novel procedures are used to replicate the history of loading and consolidation experienced by soil beneath a pipeline, revealing the consequent changes in the interface strength. The new framework provides a basis for integrating characterisation data from soil element tests and model tests of pipe-soil interaction. This has led to an improvement in design practice, through the use of recommendations that are more robust.

1. Introduction

The axial resistance between a surface-laid pipeline and the seabed affects the in-service end expansions and cyclic axial walking, as well as the initiation and growth of lateral buckles. This paper describes a new theoretical framework that captures the various mechanisms that control axial resistance. Results from interface shear box tests on three different carbonate soils are used to illustrate some of the mechanisms that control axial pipe-soil resistance.

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