Softening and consolidation around seabed pipelines: centrifuge modelling

Solutions for lateral breakout and axial response of submarine pipelines are well established if the undrained shear strength conditions of the soil are known and defined simply (such as uniform or increasing proportionally with depth). In reality, the geometry of the free surface and the distribution of undrained shear strength around a submarine pipeline post-lay are affected by the lay process. This is because of soil berms that form adjacent to the pipe, and remoulding and subsequent reconsolidation of the seabed. The effect of post-lay consolidation on the subsequent lateral and axial response of submarine pipelines has not been previously investigated through physical model testing. This paper presents results from centrifuge model tests describing lateral breakout behaviour of a pipe on soft clay as a function of (a) pipe installation conditions, (b) post-lay pipe weight and (c) consolidation prior to breakout. In addition, the effect of post-lay consolidation on axial pipe response is studied. The experimental results are compared with available numerical and analytical predictions. The results quantify the influence of the installation process, pipe weight and post-installation consolidation on the lateral breakout resistance and trajectory of the pipe and also the axial pipe response, and show how existing prediction methods can capture these effects.