Volumetric Hardening in Axial Pipe Soil Interaction

Abstract

The Norwegian Geotechnical Institute (NGI) and The University of Western Australia (UWA) has been working on understanding pipe soil interaction for a long time. NGI has performed a series of large-scale model tests investigating the axial pipe soil interaction in soft clays. Variables investigated are the weight of the pipe, velocity of movement, roughness and diameter. One aspect of axial pipe soil interaction that needs further investigation is the increased axial resistance that is observed with cycling. This is attributed to volumetric hardening. However, there are also observations that this effect is erased by cycles at a higher velocity. In order to use the effect of volumetric hardening in design of pipelines, further investigation of the phenomenon is needed. The paper will describe the test equipment used in the NGI laboratory. It will investigate the increased axial friction with axial cycling of the pipe for a variation of the pipe weight at an undrained velocity. It will attempt to show, through measurements of water content of the clay around the pipe, that the clay is contracting during undrained cycling. The paper will also look into the mechanisms contributing to erasing this effect and the influence of pipe weight and velocity. Deep water fields are often developed using pipelines, both infield and for transportation. Through its life, pipelines go through a series of cycles of production and closedown that changes the pressure and temperature and thus changes the length of the pipe. The axial resistance of the soil on the pipe is one of the parameters governing the buckling and walking behavior resulting from the repeated lengthening and contraction of the pipe.