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 19-21. 09. 2007.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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