DRAINED CLAY-PIPE INTERFACE RESISTANCE AT LOW NORMAL STRESSES AND ELEVATED TEMPERATURES

Abstract

Offshore pipelines transport hydrocarbons under high pressure and high temperature conditions in order to improve/control their flowability. These pipelines are usually thermally insulated to maintain an elevated temperature and prevent any heat loss to the surroundings. However, the temperatures at the outer-wall of the pipes may still be elevated and therefore potentially altering the interface resistance between the pipeline and the seabed. This study aims at experimentally investigating the drained shear resistance at the soil-pipe interface at elevated temperatures and low normal stress conditions typical of field conditions. A series of direct shear tests are performed using a modified-for purpose direct shear apparatus that allows for very low confinement stresses and the control of interface temperatures. Low and high plasticity clays are consolidated from a slurry and sheared against smooth and rough interfaces to characterize the peak and residual interface shear response under drained loading conditions. The modified shear test apparatus is equipped with a heating circulator that allowed for the sustained control of temperatures at the soil-pipe interface to the desired levels: 22℃ and 60℃ for the temperatures used in this study. Results indicated that the effect of elevated temperature on the interface resistance is highly complex and dependent on the roughness of the pipe coating, the plasticity of the clay, and the magnitude of the applied normal stress. For low plasticity clay, the smooth coating showed increases in peak and residual interface strength at elevated temperatures while the tests conducted with rough coatings indicated reductions in interface strength at elevated temperatures. For high plasticity clays, an opposite trend was observed with elevated temperatures causing a decrease in the peak strength for the smooth interface and a slight increase for the rough interface. The residual strength was however reduced in both types of interfaces at elevated temperature. These results are important and necessitate further interface testing at elevated temperatures in the low pressure range to cover the wide spectrum of combinations of clay plasticity and pipe roughness that may exist in practice.