Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I)— Development of Pipe —by Tomoaki Takeuchi * , Akira Takahashi * , Hidenobu Nakai * , Kenichi Ishii * 2 ,Yoshiyuki Makino * 2 , Kozo Imai * 3 and Tetsuo Inoue * 3
A flexible pipe for gas transmission with composite structure has beendeveloped, which can be installed without joints for long distances alongmeandering open-cut routes or in underground conduits. The structure of this pipe is such thatmultiple stainless steel tapes are gap wrapped in spiral on a polyethylene pipe, whereby thesteel tape thickness is optimized to make the pipe applicable to a wide pressure range frommedium- to high-pressures, while maintaining flexibility by minimizing the pipe wall thickness.
Moreover, earthquake resistance is one of its advantageous features. In this report, the designof the steel tape-armored polyethylene pipe in conformity to the Technical Standard for Gas
Facilities of Japan is described, together with the results of various characteristic evaluation tests
on the prototype pipes.

ABSTRACT1.

INTRODUCTION

Polyethylene pipe for gas transmission 1) has a great
possibility of being widely used in quake-prone Japan,
since the pipe is flexible and earthquake resistant. Taking
advantage of its flexibility among others, the pipe can betransported in long continuous lengths wound on a small
reel to be installed for long distances without joints. With
respect to the applicable internal pressure, the use of thepipe had been limited to the low pressure range of 0.1
MPa for gas transmission, but the range was approvedin 2003 to extend to the medium pressure range B of
0.3 MPa. Although steel pipe and cast iron pipe are
generally used above that pressure range, the practicalapplications overseas raise the prospect of using
polyethylene pipe in higher pressure ranges.
A simple pipe structure using polyethylene pipe
only would require, it was thought, a considerable wall
thickness to make the pipe highly pressure resistant,thus greatly impairing the flexibility of the pipe, i.e. oneof its inherent advantages. Accordingly, we proceeded
to develop a composite structure for medium- to highpressure pipes for gas transmission that satisfy thevarious requirements including sufficient pressure
resistance, while maintaining the desirable flexibility. The
design concept of a steel tape-armored polyethylene pipe
(hereinafter called STAPP), together with the structure of
the prototype pipe, will be presented in this report. Alsopresented will be the contents and the results of various
characteristic evaluation tests using the prototype pipethat have been carried out in conformity to the Technical* Tokyo Electric Power Co.
* 2 Ecology & Energy Lab., R&D Div.
* 3 Electric Power Engineering Dept., Power Cables Div.

Standard for Gas Facilities of Japan.
The development of the mechanical fitting, an important
technological element constituting the pipe system, will
be reported in another paper 2).

2.

DESIGN OF STAPP FOR GAS
TRANSMISSION

2.1 Pressure Range Applicable to Polyethylene Pipe
Table 1 shows the pressure range applicable topolyethylene pipes for gas transmission.
Table 1Pressure range applicable to PE pipes for gas
transmission.

Internalpressure (MPa)Japan
Overseas~0.1

0.1~0.3

Applicable

To beapplicable
Applicable

0.3~0.7

0.7~1.0

1.0~

To be
applicable

Whereas the applicable pressure range is limited to
0.3 MPa (medium-range B) domestically, it has been
actually expanded to 0.7 MPa in overseas countries
including UK and Germany. An appropriate increase of
the wall thickness, which is necessary to make the pipe
applicable to higher internal pressures, would present
significant drawbacks in terms of flexibility and ease oflong distance installation. Possibilities of reviewing thesafety factor to expand the applicable pressures while
suppressing the pipe wall thickness are currently being
studied overseas, but such measures seems to be notvery successful, given the higher pressures probably
needed in future.

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —ISO 4437 (Buried PE Pipes for the Supply of GaseousFuels) specifies that the safety factor for designingpolyethylene pipe wall thickness should be 2.0 or more,so that overall safety factors of around 3.0 are actuallyadopted in designing the polyethylene pipes that areused in pressure ranges of not more than 0.7 MPa. If wedesign a polyethylene pipe with an inside diameter of
150 mm and an internal pressure of 1 MPa using a safety
factor of 3.0 in accordance with ISO 4437, the pipe would
be required to have a wall thickness of as much as 22mm. Such a thick-walled pipe would greatly degrade theflexibility making reel winding difficult, resulting in thenecessity of jointing a number of short straight pipes inconstructing gas transmission lines, and thereby loses
the advantage of using polyethylene pipe.
2.2 Structure and Design of STAPPThe STAPP developed here has, as shown in Figure
1, a composite structure consisting of, from the inside:
inner pipe of polyethylene, steel tape wrapping layer
and plastic jacket. The steel tapes of stainless steel arewrapped in spiral with a constant gap in the pipe axis
direction forming two layers, and two cloth tape layers areprovided above and underneath the steel tape layers toserve as binding and bedding, respectively.

2) The thickness of polyethylene pipe should be suchthat pipe alone can bear the internal pressure.
2.2.1. Steel Tape Thickness
The thickness of the steel tape was calculated using
Equation (1), which has been applied conventionally tothe design of steel tape reinforcement for water pipes
and oil pipes, and subsequently the adaptability of theequation was reconfirmed by experiments as describedlater in Section 3.1. The numerical values used in
the equation such as the internal pressure P and theallowable stress of steel tape σ have been appropriately
selected based on the Interpretation Examples.
t

1.5PD(2σ ·Ff ·sin2 φ P)

(1)

where:D is the outside diameter of the polyethylene
pipe, Ff is the reinforcement efficiency of the steeltape, and φ is the steel tape wrapping angle.
2.2.2. Polyethylene Pipe Wall Thickness
The wall thickness of the polyethylene pipe wascalculated using Equation (2) in accordance with ISO and
JIS.
MOP

1 Polyethylene inner pipe

2 MRS
c (SDR--1)(2)

2 Bedding layer (cloth tape)where:MOP is the maximum operating pressure,
MRS is the minimum required strength ofpolyethylene, c is the overall safety factor, and SDR
is the ratio of outside diameter/wall thickness.

3 Steel tape reinforcement layer
4 Binding tape layer (cloth tape)
5 Plastic jacket

Figure 1

Cross section of STAPP.

Since no general standard for STAPP exists currently,
it was decided that the design should be carried out
adopting the Interpretation Examples of the TechnicalStandard for Gas Facilities of Japan 3) (hereinafter called
Interpretation Example) as the most important guidelines,
whereby the fundamental idea for designing the internalpressure resistance was defined as:1) The thickness of steel tape should be such that thesteel tape can bear the internal pressure, taking thesafety factor into consideration.
Table 2Main layers (Material)

With respect to the wall thickness of the polyethylene
pipe, it was decided to ensure a safety factor of not lessthan 1.0, whereby the pipe was expected to perform its
function properly even in case of possible destruction of
the steel reinforcement due to external damage. Basedon the stipulated value in ISO 13761 and the results oflong-term creep test using an all-polyethylene pipe, we
settled on a safety factor of 1.4.
Table 2 shows the structure of a prototype pipe
manufactured by way of trial for performance evaluation
tests.

Structure of prototyped STAPP.
Function

Allowable stress (MPa)Specification

Air-tight sealing of inner fluid

10

Inside diameter: 156 mm
Wall thickness: 12 mm

Reinforcement layer for internal pressure(SUS 304 steel tape)

Reinforcement for internal pressure

232

0.35 mmt x 2Outer sheath (polyvinyl chloride)

Protection against external damage

Polyethylene pipe (PE 100)

Weight in air

Thickness: 5 mm
Outside diameter: 195 mm
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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —500

CAHARACTERISTICS OF STAPP

450

Various characteristics evaluation tests were carried out
using the prototype pipe 4).
3.1 Stress in Steel Tape due to Internal Pressure
It is thought that when internal pressure acts on the
STAPP, the pressure transmits to the steel tape throughthe polyethylene pipe, so that the most part of thepressure is borne by the steel tape. To confirm this, thestrain in the steel tape due to the internal pressure wasmeasured, and thereby the stress in the steel tape wasanalyzed.
3.1.1. Sample and Test Procedure
Both ends of a 3-m sample of STAPP were sealed using
two flanges, and the flanges were connected by axial
force reinforcements to make the sample free of the axial
force that is caused by the internal pressure. See Figure
2.
The plastic jacket was removed at two locations
separated in the axial direction to expose the steel tape,and strain gages were attached parallel to the tape axis,
on each of the outer and inner layers of the steel tape, onfour locations equally separated circumferentially. For the
inner steel tape, small-sized strain gages were attachedto the exposed areas between the windings of the outer
steel tape.
Plastic jacket

FlangeSteel tape

Strain gage

Figure 2

Reinforcement for axial force

Pressurizing pump connected here

Sample for internal pressure test.

In the test five cycles of pressurization, depressurizationand hold were applied, and during each cycle thechanges in the strain in the steel tape were measured in
0.1-MPa steps up to the maximum pressure of 3 MPa,
thereby the stress corresponding to these strain changes
were calculated.

Hoop stress (MPa)

3.

400

Outer layer of steel tape
Inner layer of steel tape
Calculated value

350
300
250
200
150
100
50
0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Internal pressure (MPa)Figure 3

Relationship between internal pressure and steel
tape stress.

sectional packing factor, and φ is the steel tapewrapping angle.
The results show that, as can be clearly seen fromthe Figure, the measured steel tape stress is in good
agreement with the calculated value, thus confirming thatthe internal pressure is almost entirely borne by the steel
tape.
3.2 Internal Pressure Burst Characteristics
3.2.1. Test Method
In order to confirm the internal pressure burst strengthof STAPP, the test sample was filled with water and waspressurized until burst.
3.2.2. Test Results
The internal pressure burst strength was measured tobe 9.6 MPa, twice as high as the designed minimum
burst strength of 4 MPa. The designed strength, having
been set to four times or more the working pressure (i.e.
1 MPa), was calculated based on an allowable stress ofsteel tape that is one fourth the minimum tensile strengthof steel material. Thus, the measured strength confirmsthat the design requirement is met, and that the actual
pipe strength has been upgraded due to the margin
in strength of the steel material actually used over the
specified minimum value.
Meanwhile, the two stainless steel tapes completely
broke at the burst portion, while the polyethylene pipe
showed only swells without having breakage openings.

3.1.2. Test Results
Figure 3 shows the relationship between the internalpressure and steel tape stress measured in the fifthpressurization cycle, where the stress hysteresis overpressurization and depressurization became stabilized.
Calculation of the steel tape stress σ was done using
Equation (3) which represents the hoop stress of a thinwalled circular cylinder.

σ = PD / 2t · Fp · sin2φ(3)

Photo 1

Internal pressure burst test.

3.3 Bending Characteristics
Bending deformation test was carried out to verify theflexibility of the STAPP.

where:P is the internal pressure, D is the outside
diameter, t is the thickness, Fp is the cross-

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —3.3.1. Sample and Test Procedure
Two types of samples were used, i.e. a 3-m piece of
STAPP and its inner pipe of polyethylene. As shownin Figure 4 and Photo 2, bending load was applied on
the sample and the relationship between the bending
angle and bending moment was measured. The tests
were carried out under the 1-MPa pressurized andunpressurized conditions, and until either air leak orbuckling occurred.

Bending angle

The following could be confirmed from the results of thebending deformation tests.
• The bending rigidity of the STAPP is about 24 kNm2,
demonstrating that the pipe is twice as flexible as an
all-polyethylene pipe with a thicker wall for the sameworking pressure having a bending rigidity of 56
kNm2.
• The bending angle limit of the STAPP is not lessthan 80° with sufficient resistance against buckling,
showing a bendability twice as large as that of a steel
pipe of the same dimensions, when their bending
angle limits are compared.

nd

ing

rad
ius

ρ

3.4 Axial Compression Characteristics
Axial compression test was carried out as part ofearthquake resistance evaluation for the STAPP.
3.4.1. Sample and Test Procedure
STAPP and its inner polyethylene pipe, both 0.5 m and
1.0 m in length, were used in the test, where compression
load was applied in the axial direction until buckling
occurred, and the relationship between the displacement
and axial compression force was measured. The
tests were carried out under the 1-MPa pressurized
and unpressurized conditions. Figure 6 and Photo 3
show a schematic and a scene of buckling of the axial
compression test, respectively.

Be

Load Pθ

Figure 4

Bending test method.

Guide ring

Photo 2

Bending test.

3.3.2. Test Results
Figure 5 shows the relationship between the bending
curvature and bending moment of each sample, obtainedby the bending deformation tests.

Figure 6

Schematic of axial compression test.

Bending moment (kNm)

14
12
10
8
6
4
2
0

0.0

0.5

1.0

1.5

2.0

2.5

Curvature (1/m)

Figure 5

STAPP, 1 MPa

PE pipe, 1 MPa

STAPP, 0 MPa

PE pipe, 0 MPa

Photo 3

Axial compression test where buckling occurred.

Relationship between curvature and bending
moment.

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Figure 8

250

200

Shearing force (kN)

Axial load (kN)

200
150

100
50

150

100

50
0

20

40

60

80

100

120

140

160

Displacement (mm)Figure 7

Setup for shearing deformation test.

3.5.2. Test Results
Figure 9 and Photo 4 show the results and a scene of theshearing test, respectively, from which the following could
be confirmed.

2500SampleHydraulicpress

3.4.2. Test Results
Figure 7 shows the relationship between the axial
displacement and axial compression force measured
using the 0.5-m long samples. The results confirm thefollowing.
• The STAPP withstood buckling until the axial
displacement reached a large deformation range(i.e. 7~10 %), demonstrating a large buckling
deformation limit in comparison to a steel pipe of thesame dimensions.
• The buckling resistance of the STAPP is larger thanthat of the all-polyethylene pipe by 30 %, showing
some improvement in reinforcement due to the steel
tape and the plastic jacket.
• The strain at buckling of the STAPP increased at
1-MPa pressurization than under unpressurized
condition by about 15 %, indicating a slight
improvement due to the steel tape.

Pressurization

Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —STAPP, 1 MPa

PE pipe, 1 MPa

STAPP, 0 MPa

PE pipe, 0 MPa

003.5.1. Sample and Test Procedure
STAPP and its inner polyethylene pipe, each 2 mlong, were used in the test. As shown in Figure 8, thesample was fixed at both ends, and displacement wasgiven at the center portion to yield three-point-bending
deformation, whereby shearing loads were applied
on a shearing span of 200 mm in length to obtain therelationship between the displacement and load. The
measurement continued under the 1-MPa pressurized
and unpressurized conditions until the contained water
began to leak.

200

300

400

500

600

Displacement (mm)Relationship between axial deformation and axial
compression force.

3.5 Shearing Characteristics
Shearing deformation test was carried out to evaluate theearthquake resistance of the STAPP.

100

STAPP, 1 MPa

PE pipe, 1 MPa

STAPP, 0 MPa

PE pipe, 0 MPa

Figure 9

Relationship between shear deformation andshearing load.

Photo 4

Shearing test where breakdown occurred.

• The shearing elasticity of the STAPP is about twicethat of the all-polyethylene pipe, demonstrating
reinforcement improvements due to the steel tapeand the plastic jacket. With respect to the presenceof internal pressure, the difference was not very
remarkable in terms of shearing elasticity.
• The crack-opening displacement due to shearing
for the STAPP is equivalent to that for the allpolyethylene pipe when unpressurized, but raised by

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —about 40 % when pressurized, showing the effects
such that the steel tape protects the polyethylene
pipe thus relaxing localized concentration of thestrain.

3.7 Impact Resistance Test
Rapid crack propagation test was carried out in
conformity to ISO 1347 (S4 test) to confirm the impact
resistance of the STAPP.

3.6 Tensile Characteristics
Tensile breakdown test was carried out to evaluate theearthquake resistance of the STAPP.

3.7.1. Sample and Test Procedure
A part of both the plastic jacket and the steel tape wasremoved from the sample to expose the polyethylene
pipe, and a blade bit the exposed area to evaluate thebehavior of crack propagation.
Then, polyethylene pipe was exposed at the end of a
STAPP, and a crack was generated to see whether the
steel tape can suppress crack propagation.

3.6.1. Sample and Test Procedure
The test samples, either hydraulically pressurized to
1-MPa or unpressurized, were to be tensile tested until
breakdown. All-polyethylene pipe was also tested for thesake of comparison.
3.6.2. Test Results
Figure 10 shows the relationship between the tensile
elongation and stress in the tensile breakdown tests,
and Photo 5 a scene thereof. The following could be
confirmed from the test results.
• The tensile breakdown elongation and the yield strainof the STAPP are 60 % or more and about 20 %,respectively, demonstrating that the pipe can follow
large elongations and contractions.
• The tensile behavior was approximately the samefor different samples, i.e. with or without steel tape
reinforcement, or pressurized or unpressurized,
confirming that the behavior largely depends on thetensile characteristics of polyethylene pipe.
35

3.7.2. Test Results
Figure 11 shows the measured results of crack
propagation limiting temperatures of polyethylene pipe,and Photo 6 a scene of the test. Major test results include
the following.
• The polyethylene pipe showed no crack propagation
whatever at normal temperatures, and it is not until inthe low-temperature range of below –40°C that crack
propagation was observed.
• In the test using the exposed end of a STAPP, thecrack generated on the polyethylene pipe at –40°C,
2 MPa in internal pressure, propagated in the axial
direction until it reached the region covered with thesteel tape and plastic jacket. No abnormalities suchas crack or peeling were found in the steel tape.
Photo 7 shows the appearance of a sample after the
test.
1200

25

Length of crack (mm)Tensile stress (MPa)

30

20
15
10
5
0

0

100

200

300

400

500

600

Elongation (mm)
STAPP, 1 MPa

PE pipe, 1 MPa

STAPP, 0 MPa

PE pipe, 0 MPa

1000
Crack propagates (--40˚C)
800
600
400

0, --20, --30, --35˚C

00˚C

Crack propagation suppressed

200

0

0.5

1

1.5

2

2.5

3

3.5

Pressure (MPa)Figure 11 S4 characteristics of polyethylene pipe.

Figure 10 Relationship between tensile elongation and load.

In an ordinary S4 test for polyethylene pipes, cracks
propagate exhibiting a decaying wavy pattern. In
the case of steel tape-armored pipe, on the contrary,cracks propagate linearly probably because the impact
is impeded by the steel tape to disperse, and thus
propagate through the steel tape into the plastic jacket.
However, this does not seem to pose practical problems
because cracks do not propagate unless below –40°C.

Photo 5Tensile test where breakdown occurred.

3.8 Inflammability Characteristics
3.8.1. Sample and Test Procedure
Inflammability verification test of the STAPP was carriedout in conformity to IEEE 383. A pipe sample about 2-m

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —3.9 Gas Permeation Characteristics
3.9.1. Sample and Test Procedure
Gas permeation characteristics verification test was
carried out for the STAPP using the setup shown in
Figure 12. A gas receiving cylinder of stainless steel was
installed to accommodate the pipe sample airtightly,
and the pipe sample was pressurized by a methanegas supply to 1 MPa constantly. A gas chromatographdetector was connected to the receiving cylinder, carrying
out long-term measurement of the built-up gas containedin the cylinder. Measurement was also done for an allpolyethylene pipe of the same dimensions for the sake ofcomparison.
Gas chromatographPhoto 6

S4 test of all-PE pipe.
Methane gas
supplyGas receiving cylinder

SampleFigure 12 Setup for gas permeation test.

STAPP sample after S4 test.

long was fixed vertically onto the platform, and a burner
was placed in a position 600 mm from the floor surface
and 75 mm from the sample surface. The burner was
lit for 20 min continuously, and subsequently the burnt
length on the sample and the lasting time of residue
flame were measured.
3.8.2. Test Results
It was confirmed that the pipe amply satisfies thequalifying standard of IEEE 383, with the results of 610
mm in burnt length, and 2 min 33 sec in the lasting timeof residue flame. Photos 8 and 9 show the tests being
carried out.

Gas permeation volume (mm3)Photo 73.9.2. Test Results
Figure 13 shows the results of methane gas permeation
test for the STAPP and the all-polyethylene pipe.
70000
60000
50000
40000
30000
20000
10000
0

0

10

20

30

40

50

60

70

80

90

100

Time (day)All-PE pipe

STAPP

Figure 13 Results of methane gas permeation test.

From the test results, methane gas permeation rate perunit length was calculated as shown in Table 3. The gas
permeation rate for the STAPP was about 3 % that of the
all-polyethylene pipe, demonstrating a significant effect ofgas shielding due to the steel tape armor.
Table 3Rate of methane gas permeation.
Rate of methane permeation(ml/day/m)

SamplePhoto 8

Inflammability test,
burning sample.

Photo 9Sample right after
the burner was put
out.

Steel tape-armored polyethylene pipe

0.88

Polyethylene pipe

27.2

3.10 Long-Term Durability
Long-term durability test with a constant pressurizing
period of 10000 hr was carried out to confirm the

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (I) — Development of Pipe —durability for internal pressure of the STAPP. It was
confirmed that, as shown in Table 4, the pipe withstood
without problems the long-term pressurization having anapplied pressure more than twice the design pressure.
Table 4

Results of long term pressure test.

Test condition

4.

ResultTemperature

Pressure

Time

20°C

2.0 MPa

10000 hr

No problem

80°C

2.9 MPa

10000 hr

No problem

80°C

3.5 MPa

10000 hr

No problem

80°C

4.4 MPa

10000 hr

No problem3) Agency for Natural Resources and Energy: “Interpretation
Examples of the Technical Standard for Gas Facilities, First
Revision,” 2003. (in Japanese)
4) Makino et al.: “Characteristics Evaluation of Steel Tape-Armored
Polyethylene Pipe,” Seikei Kako (Periodical), 03, 69 (2003). (inJapanese)5) Ishii et al.: “Design and Installation of Water-Intake Pipe of Air-Lift
Type,” Techno Ocean 2000, 447. (in Japanese)
6) Kagoura et al.: “Development of a Flexible Pipe for Pipe-in-Pipe
Technology,” Furukawa Review, 24, 69 (2003).

CONCLUSION

A medium- to high-pressure, flexible pipe for gas
transmission has been developed that is suitable for longpiece transportation as well as long-distance installation.
A composite structure pipe, such that stainless steel
tapes for internal pressure reinforcement are provided
over a thin-walled polyethylene pipe, was designed andprototyped, and the following conclusions were obtained
through various performance evaluation tests.
1) A prototype pipe with an inside diameter of 156
mm for internal pressure of 1 MPa was used tomeasure the stress on the stainless steel tape underinternal pressurization, and it was confirmed thatthe measured value agreed well with the theoreticaldesign value. The flexibility of the pipe suitable forreel winding was also confirmed through bending
characteristic test.
2) Internal pressure burst test, long-term internalpressure durability test, inflammability test and gas
permeation test were carried out thus confirming thatthe pipe satisfies various performance requirements
needed for medium- to high-pressure gastransmission.
3) P r o t o t y p e p i p e w a s e v a l u a t e d i n t e r m s o faxial compression characteristics, shearing
characteristics, tensile characteristics and impact
resistance, and it was confirmed that the steel tapearmored polyethylene pipe has superior earthquake
resistance.
Hereafter, for practical implementation of medium- tohigh-pressure gas transmission line, we intend to confirm
the conformity with related regulations and to develop
installation methods of high efficiency and high reliability.

REFERENCES
1) The Japan Gas Association: “Technical Data for Polyethylene Pipefor Gas Transmission,” 2001. (in Japanese)
2) Takeuchi et al.: “Steel Tape-Armored Polyethylene Pipe for
Gas Transmission (II) --Development of Mechanical Fitting--,”Furukawa Review, 26, 25 (2004).

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