1. FAILURE INVESTIGATION OF UNDERGROUND DISTANT HEATING PIPELINE
Tibor Köves, Szabolcs Szávai, Gyöngyvér B. Lenkey,
Péter Rózsahegyi
BAY-LOGI
Bay Zoltán Foundation for Applied Research
Institute for Logistics and Production Systems
2nd Hungarian-Ukrainian Joint Conference
On SAFETY-RELIABILITY AND RISK OF ENGINEERING PLANTS AND COMPONENTS
Kiev

2. The damaged sleeve pipe
BAY-LOGI
Buckling
Cracking
Bottom of pipe
Top of pipe
3. crack
2. crack
1. crack
2nd Hungarian-Ukrainian Joint Conference

3. 2nd Hungarian-Ukrainian Joint Conference
The crack
BAY-LOGI
2nd Hungarian-Ukrainian Joint Conference

4. Effect of heat expansion of the pipeline
BAY-LOGI
2nd Hungarian-Ukrainian Joint Conference

5. Description of failure
BAY-LOGI
Buckling and cracking were found around a welded joint of a vacuum insulated underground distant heating pipe in May 2006.
2nd Hungarian-Ukrainian Joint Conference

6. Background information
BAY-LOGI
DTwatermax = ~70 °C in the inner pipe (before 2005).
Before the failure there was problems with vacuum more times.
During winter in 2005 the water temperature increased by 20°C (to T=138°C, Tdesign=150°C). At the same time, 20 cm axial dilatation of the pipe end was observed.
Operating temperature range: Tmax=138 °C, Tmin=50 °C.
Possible welding defects and shifting of pipeline due-to specific pipe-installation technology.
2nd Hungarian-Ukrainian Joint Conference

7. Questions to be answered
BAY-LOGI
Could any material or welding defect cause the cracking?
Could the heat expansion of the pipeline alone cause the buckling?
What is the effect of the friction between the pipeline and the soil?
What is the effect of lack of vacuum?
What type and how the high stress caused the buckling and cracking?
How can a similar failure be prevented in the future?
2nd Hungarian-Ukrainian Joint Conference

8. 2nd Hungarian-Ukrainian Joint Conference
Material tests
BAY-LOGI
Visual inspection of cracked surface
Stereomicroscopic inspection of fracture surface
Tensile test of base material
Tensile test and bending test of welded joint
Chemical composition test from base material for checking material quality
Macro-inspection of transverse polished section of welded joint (on cracked place, and where no cracks in welded joint)
2nd Hungarian-Ukrainian Joint Conference

9. 2nd Hungarian-Ukrainian Joint Conference
Calculations
BAY-LOGI
Estimation of temperature changing of outer pipe, in case of good and bad vacuum– analytical calculation
Calculation of pipe end dilatation depending on the outer pipe temperature, considering the friction – analytical calculation
Modeling the pipe dilatation by FEM (with beam elements)
Finite element calculation to estimate load magnitude and type of failure.
2nd Hungarian-Ukrainian Joint Conference

10. Estimation of temperature changing of outer pipe
BAY-LOGI
Possible minimum and maximum temperature values of outer pipe in different cases:
The theoretically maximum possible outer pipe temperature difference:
DTmax = 31.2°C
2nd Hungarian-Ukrainian Joint Conference

11. Calculation of pipe end dilatation, not considering the friction
BAY-LOGI
In case of DTmax
Soil friction is not considered
Pipeline was supposed to be straight
The individual pipeline lengths:
Heating plant - 1. waterhouse L1 = 538.5m
1. waterhouse - 2. waterhouse L2 = 382.7m
2. waterhouse - 3. waterhouse L3 = 331.3m
Calculated dilatations of pipeline sections:
DL1max = 20.2cm DL2max = 14.3cm DL3max = 12.4cm
2nd Hungarian-Ukrainian Joint Conference

12. Calculation of critical friction coefficient
BAY-LOGI
Relationship between critical friction coefficient and outer pipe temperature changing (DT):
2nd Hungarian-Ukrainian Joint Conference

13. Modeling the pipe dilatation by FEM
BAY-LOGI
Models:
1. All pipe nodes are radially linked to surrounding soil
2. No links at breakpoints of pipeline path
3. No links between pipeline and soil (no soil around the pipe)
2nd Hungarian-Ukrainian Joint Conference

14. 2nd Hungarian-Ukrainian Joint Conference
Boundary conditions 1
BAY-LOGI
Boundary condition of dilatation from 2. section of pipeline, and radial constrain from wall of the waterhouse at the end of 2. pipeline section
Boundary condition of radial constrain from wall of the waterhouse at the end of 3. pipeline section
2nd Hungarian-Ukrainian Joint Conference

15. Boundary conditions 2
BAY-LOGI
Boundary condition of outer pipe temperature changing (DT=31.2°C)
Boundary condition of constrain from pipeline surrounding soil
2nd Hungarian-Ukrainian Joint Conference

16. Results of modeling the pipe dilatation 1
BAY-LOGI
1. model: The dilatation from 2. pipeline section, and the thermal expansion dissipate at breakpoint of pipeline
3. model: Pipeline section shifting to another position
2nd Hungarian-Ukrainian Joint Conference

17. Results of modeling the pipe dilatation 2
BAY-LOGI
DLmax = 22 cm !!!
2. model: The dilatation from 2. pipeline section increasing with thermal expansion. The breakpoints of the pipeline path can not dissipate the dilatation
2nd Hungarian-Ukrainian Joint Conference

18. Determination of loadcase that could cause the failure
BAY-LOGI
Models:
1. Axial loading on pipe.
2. The pipe end shifting perpendicular to pipe axis, therefore the
loading at breakpoint is bending.
3. Axial loading and bending on pipe.
2nd Hungarian-Ukrainian Joint Conference

19. 2nd Hungarian-Ukrainian Joint Conference
Boundary conditions
BAY-LOGI
2nd Hungarian-Ukrainian Joint Conference

20. Determination of compression stress value that could cause buckling
BAY-LOGI
450MPa
compression stress
2nd Hungarian-Ukrainian Joint Conference

21. Determination of magnitude of soil motion that could cause deformation
BAY-LOGI
50 mm
2nd Hungarian-Ukrainian Joint Conference

22. 2nd Hungarian-Ukrainian Joint Conference
Coupled effect of soil motion and outer pipe temperature changing of stress state and deformation
BAY-LOGI
2nd Hungarian-Ukrainian Joint Conference

23. Results of material testing
BAY-LOGI
Marks of fatigue failure were not recognised on fracture surface
The fracture surface indicated a ductile mode of fracture but without significant plastic deformation
Fracture started from the heat-affected zone
Fracture started on one side of welding, then go through the welding and continues on other side, which means that torsional load must have been present.
Base material and weld properties were O.K. No any specific welding defect was found.
2nd Hungarian-Ukrainian Joint Conference

24. 2nd Hungarian-Ukrainian Joint Conference
Conclusions
BAY-LOGI
The effect of friction between soil and pipe on pipe end dilatation is negligible, the dilatation is rather restricted depending on pipeline geometry, geologic and laying relations.
The heat expansion alone is not enough to cause the recognized displacement, soil parameter change is additionally needed (e.g. higher level groundwater).
The restricted heat expansion alone could not cause the failure so it could caused by combined bending-torsion and tensile stress due to heat expansion and soil movement.
2nd Hungarian-Ukrainian Joint Conference

25. Thank you for your kind attention!
BAY-LOGI
Thank you for your kind attention!
2nd Hungarian-Ukrainian Joint Conference