Temperature effect on the axial behaviour of a pipeline buried in soft clay

Axial resistance is a key component to resist thermal buckling of an offshore oil and gas pipeline operating under a high temperature. The previous studies on the axial resistance of the pipeline were mostly performed under an isothermal condition, ignoring the temperature effect that would alter the strength of the seabed and modify the axial resistance of the pipeline. For this reason, a novel temperature-controlled experimental setup is developed in this study. Model tests were then carried out for simulating the axial interaction between clayey seabed and pipelines, which were heated to two typical temperatures (i.e., 15 and 55 degrees C) under the drained condition and then axially loaded under the undrained condition till failure. Additionally, the mobilized shear strain (and therefore mobilized shear stress) at the soil-pipe interfaces under the two temperatures are also investigated, based on a newly developed temperature-controlled interface shearing apparatus that has a transparent window for image recording and analysis. It is found that the pipeline with high temperature (55 degrees C) exhibits 20% larger initial axial soil resistance stiffness than the low-temperature pipeline (15 degrees C). This suggests the thermal consolidation has increased the soil stiffness around the high-temperature pipeline. Despite the increased undrained shear strength (s(u)) of the thermally consolidated clay around the high-temperature pipe, its peak axial resistance is 10% lower than that of the low-temperature pipe, the maximal normalized parameter of axial resistance alpha(f) (max) decreases from 0.90 to 0.70, when the temperature rises from 15 degrees C to 55 degrees C. This is attributed to the thermally induced suppression of the stick-slip (referred as "thermo-lubricity") at the soil-pipe interface, which encourages the interface slippage prior to a full mobilization of s(u) around the pipes. Consequently, a temperature elevation from 15 to 55 degrees C has led to a reduction 36% (from 70% to 34%) of s(u) mobilization at the soil-pipe interface, which is observed in the interface shearing tests.