Pipe-soil interaction mechanism during pipeline upheaval buckling in loose saturated sand

Headnote

ABSTRACT: This paper presents the observed deformation and strain fields around model pipeline based upon results obtained from eight full-scale vertical pull-out tests on pipes immersed in saturated loose sand in the laboratory. It is found that the pipe-soil interaction mechanism during the uplift event can be divided into four stages: pre-peak, near-peak, post-peak transition, and post-peak steady-state. In addition, the evolutions of the "uplift zone" and "infilling zone" above the pipelines during these four stages are discussed, with considerations for a range of embedment ratios and pipe diameters.

1 INTRODUCTION

Pipeline Upheaval Buckling (UHB) refers to the global buckling tendency of buried pipelines in the vertical or near-vertical direction. UHB is particularly relevant to thick-walled high pressure/high temperature (HP/HT) pipelines and must be considered in design. Design against UHB in offshore engineering practice is often conducted in dedicated Finite Element Analysis (FEA) packages with the pipe modelled as one-dimensional beam elements and the surrounding soil medium modelled as non-linear springs. These soil springs are usually simplified as bi-linear or tri-linear (DNV-RP-F110, 2007), and characterised by two key parameters (Wang et al., 2012):

* Maximum available soil resistance, Rmax; and

* Mobilization distance, Sf, which is the required upward movement of the pipe to achieve Rmax

Prediction of Rmax in cohesionless soils has attracted substantial research effort (Trautmann et al. 1985, Schaminée et al. 1990, Dickin 1994, White et al. 2001, Bransby et al. 2001, Thusyanthan et al. 2010, Wang et al. 2010a,b, Wang et al. 2011, Thusyanthan et al. 2011). The majority of published research involves experimental studies such as full scale and/or geotechnical centrifuge testing. On the other hand, finite element analysis is not routinely used (White et al. 2008) possibly due to the difficulty of devising realistic input parameters to describe the dilantancy and plastic flow associated with large deformation in the surrounding granular soil medium adjacent to the pipeline.

In comparison, research on Sf has only recently picked up pace, and possible marked differences between tabulated values in design codes and experimental data have been reported (Wang et al. 2010a,b, Wang et al. 2012).

In principle, both Rmax and Sf represent a salient moment within the evolution of a multi-phase pipesoil interaction mechanism during an UHB event. This paper aims...