Axial Pipe-soil Interaction: A Suggested Framework

Abstract

Abstract This paper reviews the main geotechnical mechanics for a pipe embedded in soft clay and displaced axially. A particular focus is the undrained case where fundamental equations for shear strength provide a basis for relating the mobilized shear strength to the pipe velocity. The link between the maximum stress imposed by a vertically penetrating pipe and the yield stress of the underlying soft clay suggests that the ratio between the maximum pipe load (during penetration) and the submerged weight of the pipe could be a significant parameter. A non-dimensional group is suggested to expresses the pipe velocity in terms of a length of contact and the coefficient of consolidation of the soil to identify drained and undrained response. A chart for pipe friction factor expressed as a function of pipe velocity is then derived for both peak and residual conditions. The work is a contribution to the SAFEBUCK JIP that has collected the data that will permit further details for the model to be developed. Introduction The paper by White et al (2011) sets out the work by the SAFEBUCK JIP on the mechanics, behavior and performance of pipelines undergoing axial displacement. Such pipe displacement occurs over time and involves a range of velocity, repeated loading, reversals in direction of movement and variations in vertical pipe load. Axial pipe-soil interaction was initially considered relatively simple and attention was focused on lateral resistance. However, what appeared simple, particularly in the absence of precise data, turned out not to be so once detailed data was obtained. White et al (2011) and associated SAFEBUCK papers at this conference provide further background. The authors joined SAFEBUCK at the end 2010. Their background on axial pipe-soil interaction stems from the development, field testing and data interpretation for the SMARTPIPE® equipment that permits full scale testing of pipe-soil interaction on the seabed (Hill and Jacob, 2008; Denis and de Brier, 2010). The merit of SMARTPIPE® is that it tests pipe-soil interaction in situ. Direct observation can be made of full-scale pipe installed directly at the seabed and subjected to cycles of axial or lateral loading. Two important results from such tests to date have been the acquisition of pore pressure data around the pipe circumference and the direct measurement of interaction between the pipe and the in situ soil reproducing the seabed conditions for full-scale pipelines. In general, laboratory testing at 1g or in the centrifuge uses reconstituted soil. The purpose of this paper is to describe a framework for the behavior of pipelines in axial displacement, and to describe the required parameters and laboratory tests needed to measure those. The aim of presenting the framework now is to permit further development within SAFEBUCK based on the data available within the JIP. Ideally, in a manner similar to White et al (2011), the findings from this future work will be published with sufficient data to provide more specific guidance for design involving axial pipe-soil interaction. Background on axial pipe-soil behaviour It is perhaps not surprising that axial pipe-soil interaction has proved challenging. Many complex phenomena in soil mechanics are involved, and a number of these are discussed below. Soft clay at the seafloor in deep water exists at very low effective stress. The soil has high water content (liquidity index > 1) and is difficult to sample undisturbed. Such soil conditions are not easy to replicate in laboratory tests developed for much stronger soil and higher stress level.