Strain In - Situ Experiment And Thermo - Mechanical Coupling Finite Element Analysis Of Multi - Layer Direct - Buried Hot Water Heating Pipeline

With displacement constraint by the surrounding soil, the directly buried heating pipeline releases less heat expansion induced by temperature rise and bears high stress. Fatigue failures of pipelines occur under excessive stress state and then threaten the safety of municipal facilities. This thesis focuses on the complicated pipeline-soil interaction and studies on the stress characteristics of directly buried multilayer hot-water heating pipelines. Firstly, based on stress principle of directly buried hot-water heating pipeline with large diameters, an in-situ stress measurement was performed. Meanwhile, the mechanical properties of polyurethane insulation materials were analyzed by uniaxial compression tests. Secondly, thermal-mechanical coupled finite element models were established for directly buried multilayer pipeline and single layer pipeline using general FEM software ANSYS. Finally, the thesis provides a comprehensive stress analysis on straight pipe, elbow, tee joint and folded corner. The main research findings are summarized as follows:1. An in-situ long-term stress experiment, whose measured data were transmitted wirelessly, was performed by mounting metal foil strain gauges on surfaces of directly buried multilayer hot-water pipelines which were operated for the first time. Based on the experimental results, stress distribution characteristics of trench-buried pipelines, as well as the variation of friction coefficient, were obtained. In-situ experiment indicated that the stress variation of steel pipe surface close to the trench wall differs with that far away from the trench wall. Therefore, different points of the same cross section bear different stresses. The friction coefficient depends on temperature which results in soil pressure variation. The equation of equivalent friction coefficient is derived.2. The existing strength analysis method ignores the impact of polyurethane on the performance of directly buried multilayer heating pipelines. Uniaxial compression test on polyurethane was given in the thesis. The mechanical properties and parameters were obtained. Results of uniaxial compression test show that the process of polyurethane compression consists of elastic stage, plastic stage and stress strengthening stage. The collapse region, which is influenced by the density of polyurethane, is wide.3. Thermal-mechanical coupled finite element models were established by using general FEM software ANSYS. Stress variations of straight pipe, elbow, tee joint and folded corner were analyzed. Results of numerical simulation indicate that the friction in the pipe-soil contact model agrees well with in-situ test, while the friction in soil spring model is lower. Displacement and equivalent stress of steel pipe in directly buried multilayer hot-water heating pipeline are obviously inconsistent with those of single layer steel pipe even though the geometry and boundary conditions are exactly the same, which indicates that the impact of polyurethane on the performance of directly buried multilayer heating pipelines cannot be ignored. The primary stress of elbow varies with internal pressure, pipe wall thickness and curve radius. The secondary stress of elbow varies with temperature rise, pipe wall thickness, curve radius and end displacement. Internal pressure has little effect on the secondary stress of elbow. The primary stress of tee joint varies with internal pressure, pipe wall thickness and branch diameter. The secondary stress of tee joint varies with temperature rise, pipe wall thickness, branch diameter and branch length. Internal pressure has little effect on the secondary stress of tee joint. Measures should be taken to reinforce the tee joint which locates in the fully restrained section and bears peak stress with extremely high value. Replacing the folded corner of a pipeline with elbow can reduce the maximum equivalent stress. Increase in buried depth and ground load can reduce the secondary stress of tee joints.4. Based on thermal-mechanical coupled finite element models, specific stress checking calculation of directly buried hot-water heating pipeline is given. The lengths of straight pipes which connect to pipe fittings, such as elbow, tee joint and folded corner, can be reduced in the finite element model based on pipeline-soil interaction analysis. The primary stress of elbow differs to that of straight pipe; therefore, stress checking calculation should be taken into account. The secondary stress of elbow, the primary and secondary stress of tee joint should be checked using thermal-mechanical coupled finite element models. The effect of bend arm on elbow is equivalent to imposing displacement constraint to the elbow. The effect of branch on tee joint is equivalent to imposing force constraint to the tee joint.