Technical Durability Analysis of District Heating Networks

Description:

Due to changing operational temperatures, relative movements between the pipeline and the surrounding soil occur. For the lifetime estimation of a district heating pipeline, the fatigue loads of every pipeline component (steel pipe, PUR-insulation, PE-HD coverage) at any place are of interest. As a consequence, a calculation model is needed, with whom displacements and forces of the pipeline can be evaluated, dependent on temperature and loading history. An appropriate system might be the spring-beam-model (c.f. figure 1). Herein, the axial and lateral springs reflect the behavior of the adjacent soil while the pipeline is idealized with beam elements.

The definition of spring stiffness is a challenging task. For the axial friction resistance, dependencies on temperature as well as load cycle number are known from the literature. The latter mentioned is also true for the lateral bedding resistance.

It is current practice to investigate the soil-pipeline interaction in axial and lateral direction independently of each other. In fact, there is an interdependency of friction forces and bedding resistance, dependent on the angle of oblique motion. Concerning these reaction forces under combined loading the knowledge is very limited. Therefore, for an economic and safe design of district heating networks, investigations including the previous mentioned topics are necessary.

Within this research topic, the calculation model shall be developed and implemented in the framework of the finite element method (FEM). The spring stiffnesses have to be defined with regard to geometric properties (pipeline diameter, overburden height etc.), soil condition (unit weight, relative density etc.) as well as relative displacement, temperature level and cycle number. Finally, the aim is to carry out parametric studies on the behavior of district heating networks under different operational concepts.