Abstract The high cost (material, service, and production loss) involved to substitute a condemned flexible pipe whose pressure sheath has reached its theoretical preconized service life has motivated this study. Therefore, the main objective is to propose a constitutive equation for in-service aged polyamide 11 (PA11) describing the creep behavior as a function of temperature, stress level, and Corrected Inherent Viscosity (CIV), this latter parameter representing the level of material degradation due to hydrolysis. The constitutive equation may be employed for gap spanning analysis and also to subsidize the decision to extend the operational life of flexible pipes that have experienced more severe conditions or have been used for a longer time than designed. The current models to assess the remaining life of the sheath are based only on a single property decay based on corrected intrinsic viscosity (CIV) curves obtained from laboratory tests. To compare the result from the life-prediction model in use and the material mechanical behavior, an experimental campaign was performed using polyamide 11 (PA 11) samples retrieved from a 6″ gas production flexible flowline, which theoretically had reached a full-damaged condition after nearly 3 years operating at higher than specified temperature (80 °C). Dog-bone geometry specimens were machined from the internal, intermediate, and external layers of the flexible flowline pressure sheath. Once polymers are excellent thermal insulators, it was assumed that the material operated under different temperatures within the thickness and, therefore, presents different degradation degrees. CIV, tensile, and creep analyses were performed, confirming that the behavior is different for each region within the thickness of the pressure sheath. Differential scanning calorimetry (DSC), thermogravimetry analyses (TGA), and dynamic thermomechanical analysis (DMA) were performed to comparatively characterize the degree of crystallinity, amount of extractables and morphology of each section. A creep behavior model considering the gradient difference in the material is proposed. It is concluded that aging is different across the liner thickness, and the PA11 creep behavior may be expressed as a function of the CIV, temperature, and stress.

中文翻译：

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在役柔性流线聚酰胺 11 的蠕变行为

摘要 更换压力护套已达到其理论预锥使用寿命的废弃柔性管所涉及的高成本（材料、服务和生产损失）促使了这项研究。因此，主要目标是提出一个用于在役老化聚酰胺 11 (PA11) 的本构方程，描述蠕变行为作为温度、应力水平和修正的固有粘度 (CIV) 的函数，后一个参数代表材料的水平水解引起的降解。本构方程可用于间隙跨度分析，也可用于补贴延长经历了更恶劣条件或使用时间比设计时间更长的柔性管的使用寿命的决定。当前用于评估护套剩余寿命的模型仅基于基于从实验室测试获得的校正特性粘度 (CIV) 曲线的单一属性衰减。为了比较使用中的寿命预测模型和材料力学行为的结果，使用从 6" 天然气生产柔性流线中回收的聚酰胺 11 (PA 11) 样品进行了一项实验活动，理论上该流线已达到完全损坏状态在高于规定温度 (80 °C) 的情况下运行近 3 年后。狗骨几何试样由柔性流线压力护套的内层、中间层和外层加工而成。一旦聚合物是优良的热绝缘体，就假设材料在厚度范围内的不同温度下运行，因此，呈现不同的退化程度。进行了 CIV、拉伸和蠕变分析，确认压力护套厚度内每个区域的行为不同。进行差示扫描量热法 (DSC)、热重分析 (TGA) 和动态热机械分析 (DMA) 以比较表征每个部分的结晶度、可提取物的量和形态。提出了一种考虑材料梯度差异的蠕变行为模型。得出的结论是，随着衬里厚度的不同，老化是不同的，PA11 蠕变行为可以表示为 CIV、温度和应力的函数。确认压力护套厚度内的每个区域的行为不同。进行差示扫描量热法 (DSC)、热重分析 (TGA) 和动态热机械分析 (DMA) 以比较表征每个部分的结晶度、可提取物的量和形态。提出了一种考虑材料梯度差异的蠕变行为模型。得出的结论是，随着衬里厚度的不同，老化是不同的，PA11 蠕变行为可以表示为 CIV、温度和应力的函数。确认压力护套厚度内的每个区域的行为不同。进行差示扫描量热法 (DSC)、热重分析 (TGA) 和动态热机械分析 (DMA) 以比较表征每个部分的结晶度、可提取物的量和形态。提出了一种考虑材料梯度差异的蠕变行为模型。得出的结论是，随着衬里厚度的不同，老化是不同的，PA11 蠕变行为可以表示为 CIV、温度和应力的函数。提出了一种考虑材料梯度差异的蠕变行为模型。得出的结论是，随着衬里厚度的不同，老化是不同的，PA11 蠕变行为可以表示为 CIV、温度和应力的函数。提出了一种考虑材料梯度差异的蠕变行为模型。得出的结论是，随着衬里厚度的不同，老化是不同的，PA11 蠕变行为可以表示为 CIV、温度和应力的函数。