District heating (DH) systems constitute a smart and environmentally friendly solution for energy distribution in the heat sector in Europe. This technique is still expanding but already faces some issues such as status assessment of the current DH networks and the development of new generation networks for low-temperature DH. Therefore, it is essential to understand the ageing behaviour of pipes under operating conditions and to find the relevant parameters that control the degradation processes. Many factors affect the deterioration of DH pipes, especially the polyurethane foam, which makes it very complex to find a reliable prediction model. Models based on a linear Arrhenius relationship using results from high ageing temperatures seem to be incorrect. For this study, 10 pipes that have been in service for many years in Sweden and Norway were evaluated. The aim was to study the impact of natural ageing on the mechanical adhesion and chemical structure of the polyurethane foam, which affects the pipe's performance. A test method developed at the Research Institutes of Sweden (RISE), called RISE plug method, was used to study the mechanical adhesion strength. In addition, Fourier transform infrared spectroscopy was used to observe any change in the chemical structure. The results were compared with previous analyses of DH pipes exposed to accelerated ageing. This information helps to provide a better comprehension of the deterioration of the current generation of pre-insulated DH pipes and to improve the accelerated ageing methods used nowadays to predict the technical lifetime of DH pipes. Our results suggest that the lifetime of DH pipes has been underestimated when using artificial ageing at relatively high temperatures. The data collected from naturally aged pipes gave confirmatory information about their physical status compared with our laboratory tests. This study also suggests that infrared analyses could be used as an early indication of the degradation of the polyurethane foam at the interface with a steel pipe.

区域供热（DH）系统构成了欧洲供热部门中能源分配的智能，环保解决方案。该技术仍在扩展，但已经面临一些问题，例如当前DH网络的状态评估以及用于低温DH的新一代网络的开发。因此，必须了解管道在工作条件下的老化行为，并找到控制降解过程的相关参数。许多因素会影响DH管的劣化，尤其是聚氨酯泡沫，这使找到可靠的预测模型变得非常复杂。基于线性Arrhenius关系使用高老化温度结果的模型似乎是不正确的。对于这项研究，对瑞典和挪威已经使用多年的10条管道进行了评估。目的是研究自然老化对聚氨酯泡沫的机械粘合和化学结构的影响，该影响会影响管道的性能。瑞典研究所（RISE）开发的一种测试方法称为RISE塞方法，用于研究机械粘合强度。另外，使用傅里叶变换红外光谱法观察化学结构的任何变化。将结果与之前对加速老化的DH管的分析进行了比较。此信息有助于更好地理解当前预绝缘的DH管的劣化，并有助于改进当今用于预测DH管的技术寿命的加速老化方法。我们的结果表明，在相对较高的温度下使用人工时效时，DH管的使用寿命被低估了。与我们的实验室测试相比，从自然老化的管道收集的数据提供了有关其物理状态的确认信息。这项研究还表明，红外分析可以用作与钢管界面处聚氨酯泡沫降解的早期指示。