Mitsubishi Heavy Industries Technical Review Vol. 46 No. 4 (Dec. 2009)
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Study on Seismic Safety of the Small-bore Piping
and Support System
KUNIHIKO SATO *1
MASATSUGU MONDE *2
DAISAKU HIRAYAMA *3
TAKUYA OGO *4
In Japan, according to the revised Regulatory Guide for Aseismic Design of Nuclear
Power Reactor Facilities, September 2006, criteria of design basis earthquakes of Nuclear
Power Reactor Facilities become more severe. Electric power companies were requested by the
government to recheck (back-check) the seismic design of their nuclear power plants. Since
seismic safety is one of the major key issues of nuclear power plant safety, it has been
demonstrated that the nuclear piping and support system possesses large safety margins by
various durability test reports for piping in ultimate conditions. Though knowledge of the safety
margin has been accumulated from these reports, there are few seismic margin tests that both
the piping and support structures show inelastic behavior in extremely high seismic excitation
levels. In order to obtain the influences of inelastic behavior of the support structures to the
whole piping system response when both piping and support structures show inelastic behavior,
we examined seismic proving tests using E-Defense, which is the largest shaking table in the
world and owned by the National Research Institute for Earth Science and Disaster Prevention.
This paper introduces major results of the seismic shaking tests of the piping and support
system.
|1. Introduction
Since the seismic design of the pipes and supports in nuclear power plants is based on the
design yielding points, it has been considered that the piping and support system capacity has a
large margin that has been demonstrated by old studies.
Yet, still some technical uncertainties remain concerning the phenomenon when both piping
and support structures show inelastic behavior in extremely high seismic excitation levels.
The study is to comprehend the vibration characteristics (including inelastic properties) of
the complete piping and support system under immense earthquake conditions, and verify the
seismic margin. In addition it is part of the recheck (back-check) process for nuclear plant pipes.
Pipes with a bore size of 4 and 2 inches, which are commonly used in nuclear power plants, and
their corresponding supports were used in the tests.
This article reports the vibration test results of the 4-inch pipes using E-Defense. Vibration
tests of 2-inch pipes were executed using the shaking table in our Takasago Research Laboratory1,
and it has been verified that they have sufficient seismic margin against design-basis earthquakes.
|2. Test
The support element test was conducted to obtain load-displacement characteristics, and
seismic proving tests to obtain the inelastic response of the piping and support system vibration test
aiming at examining the vibration characteristics of the relevant system by using a full-scale test
model containing all elements such as pipes, supports, and fixtures.
*1 Deputy General Manager, Nuclear Energy Systems Engineering Center, Nuclear Energy Systems Headquarters
*2 Takasago Research & Development Center, Technical Headquarters
*3 Nuclear Plant Maintenance Engineering Department, Kobe Shipyard & Machinery Works
*4 Nuclear Energy Systems Engineering Center, Nuclear Energy Systems Headquarters
Mitsubishi Heavy Industries Technical Review Vol. 46 No. 4 (Dec. 2009)
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2.1
Support element test
The support element tests were designed to obtain the relationship between force and
displacement at piping, when the seismic force loaded on piping support equipment consists of a
U-bolt, support element, base plate and anchorage on a concrete base by confirming the behavior of
the equipment until failure is attained.
Figure 1 shows the outline drawing of a typical test specimen used in the element test.
Figure 2 shows the simulation analysis model. Figure 3 shows load-displacement characteristics
of support element test and simulation analysis. With respect to linear and second rigidities, the
simulation analysis results are similar to the test results. The analysis model does not include
concrete fixtures, but the end of anchor bolt is fixed.
Figure 1 support test model
Figure 2 FEM model of piping support
The model imitates the welding part.
Figure 3 Load-displacement
characteristics
2.2
Piping and support system vibration test
(1) Test model
The model is a piping and support system with a piping bore of full-scale 100A, Sch40,
which has elbows, a tee. Table 1 shows specification of test model. Figure 4 shows the piping
and support system model.
(2) Seismic wave
Since the input excitation wave should cover the major mode of typical buildings of
nuclear power plants in Japan. It was difficult to create the seismic wave of the wide-band
target spectrum. So, we designed three successive waves. Input level of the seismic wave was
controlled according to the purpose of test cases. Figures 5 and 6 show the basic excitation
waves and response spectra, respectively.
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(3) Test cases
Major test cases for each piping and support system are presented in Table 2. Test case 3,
weights were added to the pipes to input a big seismic force that exceeds the vibration capacity
of the shaking table. Test case 4, weights were added to the pipes and removal of five main
supports to make the pipes and support system more sensitive to vibration, and applied
vibration at the level to damage the pipes.
Table 1 specification of test model
Element Quantity
Material
Remarks
Total length: Approx. 24 m
Pipe
1 set
4B carbon steel
Elbow: 9 ea.
Tee: 1 ea.
Nozzle: 3 places
Internal
－
fluid
Support
Total
weight
Water
Internal pressure: 1.4 MPa
Cantilever type Expansion Anchor
16 ea.
(with U-bolts) (including embedded type)
Approx. 80 t
Base mat size
－
(10.4 m × 6.4 m)
Figure 4 schematic view of test model
Figure 5 excitation waves
Figure 6 Response spectra of the excitation waves
(horizontal direction)
Table 2 test case for piping and support system
Test case
Input level
Test model
Case 1
Ｓ2
Test model A
Case 2
９×Ｓ2
Test model A
Remarks
Applied vibration at the design earthquake level of the test model
Amplified the acceleration of the S2 wave by 9 times.
Amplified the response of the piping and support system with additional
Case 3
α×Ｓ2
Test model B
weights. The shaking waveform was adjusted (with the time axis) to the
natural frequency of the piping system.
Removed five main supports (supports 4,5,7,8,9) to make the pipes and
Case 4
β×Ｓ2
Test model C
support system more sensitive to vibration, and applied vibration at a
level to damage the pipes.
*Ｓ2: Design seismic wave of the test model
|3. Vibration Test Results for the Piping and Support System
(1) Outline of test results
Table 3 lists the natural frequencies of the each test model sets. Figure 7 shows the
vibration modes of these test model sets. The tee-branch portion is excited exclusively in the
shaking direction on the first mode. The upper location (support 15) is dominant in the vertical
direction on the second mode.
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2
Test case 1 is adjusted to the design acceleration (target: 1.47 m/s ) which is input. Test
case 2 is adjusted to a level about 9 times as large as the design acceleration (target: 13.8 m/s2),
which substantially exceeds the response acceleration (maximum: 6.8 m/s2) observed at the
upper end of the foundations of TEPCO’s Kashiwazaki-kariwa Nuclear Power Station during
the Niigata Chuetsu-Oki Earthquake. Test case 3 has the additional weights to enlarge pipe
displacement. Test case 4 has additional weights and support removal to generate larger pipe
displacement.
These test cases did not leak internal liquid from the piping, and the piping and the
support system held the seismic margin more than 9 times as large as the design seismic wave.
Table 3 Natural frequencies of respective piping and
support system
Natural frequency(Hz)
1st
Remarks
2nd
3rd
4th
Test model A 13.7
16.6
25.8
28.1 Original test model
Test model B
6.3
10.9
15.6
18.2 piping and support system
Test model C
3.5
9.6
13.5
15.2 Five supports were removed.
Amplified the response of the
with additional weights.
Figure 7 Vibration modes of piping
systems (test model A)
(2) Analysis and evaluation
The test model did not break as a result of the test, because of the damping ratio
generated higher than design damping ratio.
The damping ratio of the piping and support system increases at the shaking level
because of the increase mainly due to the wear of the U-bolt and the plastic deformation of
support members.
These vibration tests proved that the magnitude of the damping effect was an important
element in assuming the ultimate state of pipes and supports and therefore, we conducted
analyses on the relationship between response displacement and damping of the pipes based on
test results.
For test case 4 where the maximum response displacement was measured, we evaluated
the inelastic behaviors of the pipe elbow based on the simulation analysis.
(a) Response displacement and damping of pipes
Table 4 shows the maximum response displacements summarized for each test case.
Figure 8 shows the relationship between maximum displacement and damping. It has been
proved that the damping ratio is enlarged with an increase in the vibration level, and a
maximum damping ratio of about 9% is observed in test model A.
The damping ratio is estimated by the half power method from the transmission function,
which is derived from the horizontal acceleration to the vibration table, by the Auto Regression
method with the duration of the largest response level.
Table 4 Maximum response displacement of the piping system
(tee and elbow A are those shown in Figure 9)
Test case
Max. displacement of
Max.
acceleration of piping and support system
(at tee)
the wave
1
1.46 m/s2
1.7 mm
2
15.3 m/s2
16.2 mm
3
7.96 m/s2
51.7 mm
4
13.6 m/s2
239.8 mm
Support displacement
(ductility ratio)
Maximum Strain
Notes
No. 7
No. 8
No. 9 range of elbow A
support
support
support
1.6 mm
1.3 mm
1.6 mm
0.01%
Elastic range
(0.12)
(0.1)
(0.12)
15.9 mm 9.1 mm 15.4 mm
0.04%
(1.2)
(0.68)
(1.16)
50.3 mm 31.8 mm 46.6 mm
0.17%
(3.78)
(2.39)
(3.5)
－
－
－
1.37%
Mitsubishi Heavy Industries Technical Review Vol. 46 No. 4 (Dec. 2009)
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Figure 8 maximum response displacement and the damping ratio
(b) Inelastic behavior of the elbow
The relations between the strain amplitude at the flank of the elbow outer surface and
displacement amplitude of the piping system are obtained in Figure 9 for Elbow A by the
analysis. The maximum displacement of the tee at the test case 4 is about 240 mm, which was
measured by displacement sensor.
Figure 10 show the maximum circumferential strain range of Elbow A is about 1.3 %.
Figures 11 show the simulation result. The circumferential strain of the elbow generated under
240-mm displacement of the tee agrees with the test result of 1.3%. This means that we have
succeeded in accurately obtaining the status of fatigue of the pipe elbow.
Figure 10 Local strain waveform at elbow A (test case 4)
Figure 9 Pipe elbow simulation
analysis model
Figure 11 Relation between tee displacement and local
strain of elbow A
Mitsubishi Heavy Industries Technical Review Vol. 46 No. 4 (Dec. 2009)
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|4. Conclusion
The results of the study are summarized as follows:
(1) By loading a repetitive force to the support element, we succeeded in verifying the behaviors
and actual yield strength of the piping and support system during the period until the system
lost the support function.
(2) In the vibration test in which immense earthquake conditions were simulated, we succeeded in
verifying the seismic safety of the small-bore piping and support system.
(3) In the post-test simulation analysis and a later comparison between the test and simulation
results, the simulation model excellently reproduced the inelastic behaviors actually
observed. This result proves that the analytical method used is valid for simulating inelastic
behaviors.
Acknowledgment
This study was entrusted to us by the Kansai Electric Power Co., Inc., Kyushu Electric
Power Co., Inc., Shikoku Electric Power Co., Inc., Hokkaido Electric Power Co., Inc., and Japan
Atomic Power Company. We received various advice and assistance from these companies, for
which we would like to express our appreciation.
References
1.
2.
E. Shirai et al., “Inelastic test of the small bore piping and support system : Part 1”, ASME PVP, 2008
E. Shirai et al., “Inelastic test of the small bore piping and support system : Part 2”, ASME PVP, 2008