Degradation of axial friction resistance on buried district heating pipes

The axial soil resistance is a crucial component within the design process of buried district heating pipelines and is known to degrade during the service life time due to axial movement of the pipeline under cyclic temperature loading. In current design methods, a large bandwidth of the coefficient of friction μ between pipe and soil is considered to account for this effect. However, a more accurate consideration of cyclic effects on the axial resistance could lead to a more economic design of district heating pipelines. Experimental investigations were carried out to identify the main factors influencing the friction degradation. It was found that beside stress and relative density of the surrounding soil, the amount of resistance degradation and also its rate are strongly affected by the magnitude of cyclic pipe movement and the number of load cycles. It could also be observed that the degradation can partly be healed dependent of the order of load packages, i.e. subsequent cycles with constant displacement amplitude. These findings necessitate a new consideration of soil resistance within the service life prediction. A calculation concept is introduced, which describes the axial friction resistance as a function of overburden height, pipeline diameter and soil’s relative density, but in addition also dependent on the actual relative movement between pipeline and soil as well as on the history of cyclic displacements. Thereby it becomes possible to calculate more realistic friction resistances for arbitrary points of time, distinct for every location within the network.