Abstract

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 °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 °C) exhibits 20% larger initial axial soil resistance stiffness than the low-temperature pipeline (15 °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 α_{f max} decreases from 0.90 to 0.70, when the temperature rises from 15 °C to 55 °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 °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.

摘要

轴向阻力是高温下运行的海上油气管道抵抗热屈曲的关键部件。以往对管道轴向阻力的研究大多在等温条件下进行，忽略了温度效应会改变海底强度并改变管道的轴向阻力。因此，本研究开发了一种新颖的温度控制实验装置。然后进行模型试验，模拟粘土海底与管道之间的轴向相互作用，在排水条件下将管道加热到两个典型温度（即15和55℃），然后在不排水条件下轴向加载直至失效。此外，基于新开发的温控界面剪切装置，该装置具有用于图像记录和分析的透明窗口，还研究了两种温度下土管界面处的动员剪切应变（以及因此动员剪切应力）。研究发现，高温（55℃）管道的初始轴向土抗刚度比低温管道（15℃）大20%。这表明热固结增加了高温管道周围土壤的硬度。尽管不排水剪切强度增加（研究发现，高温（55℃）管道的初始轴向土抗刚度比低温管道（15℃）大20%。这表明热固结增加了高温管道周围土壤的硬度。尽管不排水剪切强度增加（研究发现，高温（55℃）管道的初始轴向土抗刚度比低温管道（15℃）大20%。这表明热固结增加了高温管道周围土壤的硬度。尽管不排水剪切强度增加（高温管道周围热固结粘土的s _{u )，其峰值轴向阻力比低温管道低10%，轴向阻力最大归一化参数}α _{f max}从0.90减小到0.70，当温度从 15 °C 升至 55 °C。_{这是由于热致抑制了土壤}-管道界面处的粘滑运动（称为“热润滑性”），这在管道周围的硫完全流动之前促进了界面滑移。因此，温度从 15°C 升高到 55°C 会导致s _u减少 36%（从 70% 到 34%）。在界面剪切试验中观察到土壤-管道界面处的流动。