FAILURE INVESTIGATION OF UNDERGROUND DISTANT HEATING

PIPELINETibor Köves, Szabolcs Szávai, Gyöngyvér B. Lenkey,

Péter Rózsahegyi

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Bay Zoltán Foundation for Applied ResearchInstitute for Logistics and Production Systems

2nd Hungarian-Ukrainian Joint ConferenceOn SAFETY-RELIABILITY AND RISK OF ENGINEERING PLANTS AND COMPONENTS

Kiev19-21. 09. 2007.

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Buckling

Cracking

Bottom of pipe

Top of pipe

3. crack2. crack 1. crack

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The damaged sleeve pipe

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The crackBAY-LOGI

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Effect of heat expansion of the pipeline

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Description of failure

Buckling and cracking were found around a welded joint of a vacuum insulated underground distant heating pipe in May 2006.

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Twatermax = ~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.

Background information

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BAY-LOGIQuestions to be answered

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?

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Material tests

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)

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CalculationsBAY-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.

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Estimation of temperature changing of outer pipe BAY-LOGI

Possible minimum and maximum temperature values of outer pipe in different cases:

Tk_1b 44°C - 138°C water temp. without vacuum

Tk_2b 15.9°C - 50°C water temp. without vacuum

Tk_1v 35.2°C - 138°C water temp. with good vacuum

Tk_2v 12.8°C - 50°C water temp. with good vacuum

The theoretically maximum possible outer

pipe temperature difference:

Tmax = 31.2°C

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Calculation of pipe end dilatation, not considering the friction BAY-LOGI

In case of Tmax

Soil friction is not considered Pipeline was supposed to be straight

The individual pipeline lengths:Heating plant - 1. waterhouse L1 = 538.5m1. waterhouse - 2. waterhouse L2 = 382.7m2. waterhouse - 3. waterhouse L3 = 331.3m

Calculated dilatations of pipeline sections:

L1max = 20.2cm L2max = 14.3cm L3max = 12.4cm

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Calculation of critical friction coefficient BAY-LOGI

Relationship between critical friction coefficient and outer pipe temperature changing (T):

Dhg

dDTE

soilcrit ****4

*** 22

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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)

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Boundary conditions 1BAY-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

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Boundary conditions 2BAY-LOGI

Boundary condition of constrain from pipeline surrounding soil

Boundary condition of outer pipe temperature changing

(T=31.2°C)

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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

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2. model: The dilatation from 2. pipeline section increasing with thermal expansion. The breakpoints of the pipeline path can not

dissipate the dilatation

Results of modeling the pipe dilatation 2

Lmax = 22 cm !!!

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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.

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Determination of loadcase that could cause the failure

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Boundary conditions

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Determination of compression stress value that could cause buckling

450MPa compression stress

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50 mm

Determination of magnitude of soil motion that could cause deformation

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Coupled effect of soil motion and outer pipe temperature changing of stress state and

deformation

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Results of material testingBAY-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.

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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.

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Conclusions

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Thank you for your kind Thank you for your kind attention!attention!