SAFEBUCK JIP - Observations of Axial Pipe-soil Interaction from Testing on Soft Natural Clays

Abstract

Abstract This paper outlines recent research into axial pipe-soil interaction from the geotechnical elements of the SAFEBUCK Joint Industry Project. The operational axial pipe-soil friction strongly influences the initiation and cyclic development of lateral buckles, and also controls the magnitude of pipeline end expansions as well as rates of axial walking. Results from model tests performed at the University of Cambridge are presented in this paper, and provide new insights into the axial pipe-soil response on fine-grained clayey soils. A simple test arrangement was used to pull an 8 m long plastic pipe axially over a bed of soft natural clay collected from a deepwater location offshore West Africa. Many axial sweeps were performed, spanning a wide range of velocities (0.001 mm/s - 5 mm/s) and a wide range of intervening pause periods (up to several days). Both of these variables had a strong influence on the axial pipe-soil resistance - or ‘friction’. The peak values of equivalent friction factor were as high as 1.5 and the residual values were generally in the range 0.2 - 0.5, but fell to below 0.1 in some cases. Higher peak values are associated with longer waiting periods between axial sweeps. The lowest residual values are associated with the fastest rates of shearing. This wide range of axial resistance was observed in a single test using the same pipe resting on the same soil, which is disconcerting from a design perspective. To identify the origin of this variability, an interpretation based on the generation and dissipation of excess pore pressure is explored. This provides a reasonable explanation for the results, but some unexpected aspects of the behavior remain. The results show the important influence of pore pressure effects, consolidation, and the level of drainage during sliding. They also highlight the complexity of axial pipe-soil interaction. For these experimental results, conventional design calculations do not provide adequate predictions of the observed behavior except for during very slow drained movements. The undrained behavior is not captured by conventional design calculations, which provides a cautionary warning to designers. In particular, in the slow-draining natural clay used in this experiment, very low equivalent axial friction factors - as low as F/W' is ~ 0.1 - can be sustained for a long period of movement. The SMARTPIPE® is a recently-developed tool for performing pipe-soil interaction tests in situ offshore, using an instrumented model pipe mounted on a seabed frame. Selected results from a SMARTPIPE® cyclic axial pipe test performed at a deep water location are also presented and discussed. The results support the proposed interpretation based on the generation and dissipation of excess pore pressure. Some differences exist between the in situ and model test data but they can be explained by the smaller magnitude of axial velocity tested, the higher coefficient of consolidation of the in-situ soil and the absence of pause periods between sweeps. Minimal data from experiments on axial pipe-soil interaction is in the public domain, so the results provided here represent a significant contribution to the available knowledge. This research is continuing within the SAFEBUCK JIP, via additional model testing using a new facility that is described in this paper. The aim is to establish new and more robust design guidance for pipe-soil interaction, to support the reliable and efficient design of seabed pipelines.