Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II)— Development of Mechanical Fitting —by Tomoaki Takeuchi * , Akira Takahashi * , Hidenobu Nakai * , Yoshiyuki Makino * 2 ,Kenichi Ishii * 2 , Kozo Imai * 3 and Tetsuo Inoue * 3

A mechanical end fitting for steel tape-armored polyethylene pipe for gas
transmission has been developed, together with the pipe --a flexible, earthquakeresistant pipe suitable for long-distance installation. The basic structure of the end fitting
adopted a sealing scheme that was applied to conventional pipes including steel pipes for gas
transmission and low-pressure polyethylene pipes, placing particular emphasis on developing a
structure that enables secure clamping of the steel tape reinforcement layer at the end, and onease of assembly in a short time. This report presents the design of the end fitting, along withthe results of various performance evaluation tests.

ABSTRACT1.

INTRODUCTION

The design of a flexible pipe for gas transmission withcomposite structure that enables long-distance, jointless installation along meandering open-cut routes orin underground conduits has been reported in anotherpaper 1). Since the flexible pipe for gas transmissionnecessitates, at its ends, jointing with existing gas conduit
pipes and various piping systems for gas transmission,
the development of end fitting has been carried out
simultaneously with the development of the pipe. The
design and performance of a mechanical end fitting forsteel tape-armored polyethylene pipe (hereinafter called
STAPP) will be presented in this report.

2.

DESIGN OF END FITTING FOR STAPP

2.1 Design Concept
Mechanical fittings have an established track recordin jointing polyethylene pipes for gas transmission
currently in use for low-pressure ranges below 0.1 MPa.
Expansion of application of the polyethylene pipes for gas
transmission to the medium-pressure range of 0.3 MPaor lower is in prospect 2), so that development of mediumpressure mechanical fittings similar to those for lowpressure pipes for gas transmission is under way.
In this development program, the design has beenreviewed for higher pressures to make the fittings
applicable to the range of 1-MPa gas pressure or higher.

Major design improvements aimed at include: first,
upgrading the surface pressure at the sealing portion;and secondly, clamping mechanism for the end portionof the reinforcement steel tape layer. In terms of the latter
objective, given the fact that the pressure resistance ofthe pipe strongly depends on the integrity of the steel
tape layer, a simple and robust clamping mechanism
was developed and was incorporated into the fitting
structure, thus preventing the end portion of the steel
tape reinforcement layer from getting loose.
2.2 Sealing Structure
Figure 1 shows a schematic of the gas sealing structure.
The structure, comprised of a stop ring, a rubber gasket
with rounded top and square bottom and a retainer, hasa mechanism such that by thrusting down the retainerusing the locking bolts, surface pressure is applied onto
the gasket placed in the tapered space between themain body of the fitting and the inner polyethylene pipe,
thereby sealing the structure against the internal gas
pressure. The stop ring in front of the gasket functions asa preventive against cold flow of the gasket.
Since sealing function has to be maintained for a longtime, the initial surface pressure was designed, in view ofStop ringSurface pressure
Main body

Axial force

Internal gas pressure* Tokyo Electric Power Co.
* 2 Ecology & Energy Lab., R&D Div.
* 3 Electric Power Engineering Dept., Power Cables Div.

Rubber gasket
Surface pressure

Retainer

Polyethylene pipe

Figure 1

Schematic of gas sealing structure.

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II) — Development of Mechanical Fitting —Holding ringRubber gasket

Locking ring

Retainer

Steel tape clamp ring

Stop ring

Steel tape clamp ring

Water seal
Stainless steel tape

Main bodyJacketStiffener

Polyethylene pipe

Polyethylene pipe

Stiffener

Figure 2
Table 1

Structure of mechanical fitting for STAPP.

Function and material of the constituting parts of mechanical fitting.
Function

Material

Main bodyParts

Provision of protection, waterproofing, and resistance against axial tension

Cast iron

Retainer

Generation of surface pressure for rubber gasket with the help of locking bolts

Steel or polyoxymethylene

Position setting of locking ring

Steel

Holding ring
Locking ring
Stiffener

Holding of inner pipe

Polyoxymethylene

Protection of inner pipe against collapse due to external force

Steel

Rubber gasket
Stop ring

Sealing of gas

Acrylonitrile-butadiene rubber (NBR)

Protection against the cold flow of rubber gasket

Cotton cloth + NBR

Holding of steel tape ends

Steel

Anticorrosion layer

Acrylate resin

Steel tape clamp ring
JacketSealing surface pressure (MPa)providing a surface pressure that is sufficiently high for
upgrading gas transmission pressure, to secure a service
life of 50 years.
Figure 2 shows the basic structure of the mechanicalfitting for STAPP, and Table 1 shows the function andmaterial of the constituting parts.
Figure 3 shows the test results of surface pressure
retention characteristics for the gas-sealing rubber
gasket. It can be seen from the Figure that the surfacepressure estimated to remain after 50 years is 5.4 MPa,
thus achieving the retention of a long-term surfacepressure, i.e. required surface pressure multiplied by thesafety factor equaling to 1.0 x 4 = 4.0 MPa.
y = 8.7502x-0.0371

10

1

1

10

100

1000

10000

2.3 Steel Tape Clamping Structure
Since stress is generated in the steel tape of a STAPP
when an internal pressure is applied to the pipe, thetapes have to be clamped at the joints to counteract thetensile stress accumulating at the steel tape end.
In the steel tape clamping structure shown in Figure 4,
a steel tape locking ring (Photo 1) and a steel tape clamp
ring (Photo 2) were introduced in order to clamp the steel
tape end both circumferentially and axially. The former
is a half-opened fitting with a slit for steel tapes, and thelatter is a flange fitting with two halved collars.
It was decided to clamp the steel tape end using thesteel tape clamp ring, whereby the steel tapes are folded
after passing through the slit on the locking ring, thesteel tape clamp ring with halved collars is set around thefolded steel tapes, and the bolts are tightened to clamp
the collars circumferentially, thus fastening by friction thelocking ring with the steel tapes.
The design of the steel tape clamp structure wascarried out in accordance with the procedures set forth in
Figure 5.

100000 1000000

Elapsed time (hr)Figure 3

Relaxation curve of sealing surface pressure.

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II) — Development of Mechanical Fitting —Steel tape clamp ring
Steel tape

Steel tape anchoring force

Steel tape folded after
passing through the slit

Figure 4

Photo 1

Steel tape clamp ring

Steel tape clamp structure.

Steel tape locking ring.

Photo 22.3.3 Steel Tape Constraining ForceSteel tape constraining force Fr refers to the frictional
fastening force between the steel tape locking ring andthe steel tapes, which is generated by fastening theclamp ring circumferentially using the locking bolts, andis calculated by Equation (2)Calculation of required steel tape anchoring force (F)Setting of safety factor (C)Calculation of steel tape constraining force (Fr)F

Figure 5

C

Fr

Fr

Procedures for designing steel tape clamp structure.

P(D t)
2t · Ff · sin2 φw tµ· N · P2

ϕ sinϕsin(ϕ /2)

(2)

where: µ is the friction coefficient between thesteel tape and the steel tape locking ring, N is thenumber of bolt, P is the bolt fastening force, andφ is the contact angle between the fastening cleat
and the steel tape locking ring.

2.3.1 Steel Tape Anchoring ForceThe steel tape anchoring force at the end F isrepresented by Equation (1).
F

Steel tape clamp ring.

(1)

where: P is the maximum working internalpressure, D is the outside diameter of steel tape
reinforcement layer, w is the steel tape width, t is
the thickness of steel tape, Ff is the reinforcement
efficiency of steel tape, and φ is the steel tapewrapping angle.
2.3.2 Safety Factor
The safety factor was set as 4 over the tensile strength,
the same as for the steel tape.

3.

VERIFICATION OF STEEL TAPESTRESS

Since steel tape clamping is the most important
technique in the design of the mechanical fitting for
STAPP, the stress generated at the steel tape end wasmeasured experimentally.
3.1 Experimental Method
The end portion of a STAPP sample of about 1.2 m inlength was removed of its jacket over a length of about
150 mm to expose the steel tape ends, while constraining
the steel tape ends using a metallic belt. Strain gages

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II) — Development of Mechanical Fitting —were attached on the steel tapes parallel to the tape axis
to measure the hoop stress as shown in Figure 6 andPhoto 3. As for the steel tapes underneath the jacket,
windows were made in the jacket so as to attach strain
gages on the center areas of the steel tape on the upper
layer. Thus 21 measurement points were prepared in total
including one point for the steel tape end, and 20 points
for the steel tapes underneath the jacket.
No.2

No.1

Constraining belt
(1·2 strain gage)

No.4

No.3

No.5

150

Figure 6

No.6-18

No.7-19

No.20

No.21

Constraining belt
(21·21 strain gage)

900

loosening of the steel tape is suppressed due to thefrictional force from the jacket.
3.3 Verification by FEM Analysis
The finite element method (FEM) was used to analyze thestress on the steel tape end, where the steel tapes weremodeled using spiral shell elements to simulate the stress
distribution on the steel tapes under the conditions ofconstrained steel tape end, and as of released steel tapeend.
3.3.1 Constrained Steel Tape End
The result of analysis is shown in Figure 8. The steel tapestress of 120 MPa with a uniform distribution is in good
agreement with the experimental result.

Setup for stress measurement on steel tape ends.
Fitting side

Jacketed portion

Photo 3Stress measurement for steel tape ends.
Fitting side

The sample was pressurized with an internal pressureof 2 MPa, and the constraining belts on the both steel
tape ends were undone to measure the stress changes
over time.
3.2 Experimental Results
Figure 7 shows the stress on the steel tape after theconstraint on the steel tape ends was released.

Exposed portion

Figure 8

Distance from end in mm

Stress distributions on the steel tape withconstrained end.

300
400

Steel tape stress (MPa)

200

600

800

250
200
150
100
50
0

3.3.2 Released Steel Tape End
Figure 9 shows the result of stress analysis when thesteel tape end is released. It can be seen that the region
where steel tape stress decreases corresponds to a small
section of about one turn of spiral wrapping from the end,
showing good agreement with the experimental result.

Steel tape
end
0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21

Number of wrapping from the tape end (turn)Figure 7

Stress on steel tape after the constraint on the steel
tape ends was released.

The stress on the steel tape ends decreases due toloosening as the steel tape ends are released, together
with its counterpart underneath the jacket, but it has been
confirmed that the influence of the steel tape releasespreads only as far as the first turn of wrapping fromthe end of the remaining jacket, and beyond there, theFigure 9Stress distributions on the steel tape with released
end.

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II) — Development of Mechanical Fitting —4.

PERFORMANCE OF END FITTING

The end fitting for the STAPP has been tested incompliance with the tests for mechanical fittings specified
in ISO 10838 (Mechanical fittings for polyethylene piping
systems for the supply of gaseous fuels) and in Technical
Data for Polyethylene Pipe for Gas Transmission 3). In thetests, the test pressure has been set so as to correspondto an internal pressure of 1 MPa.
Table 2 shows a summary of a series of tests carriedout for the end fitting for STAPP with 1-MPa rating. Since
there are no problems in any of the tests, it is believedthat the end fitting developed here is amply suitable foruse in practical applications with 1-MPa rating.

5.

Photo 4

Tensile test.

Photo 5

Bending test.

EASE OF INSTALLATION

It has been confirmed that the end fitting developed
here can be assembled simply by fastening the parts,and thus has superior ease of installation at the site. For
reference's sake, the assembly of an end fitting is finishedby one worker in about one hour.

6.

CONCLUSIONAs described above, a mechanical end fitting for steel
tape-armored polyethylene pipe applicable to a working
pressure of 1 MPa or higher has been developed, and its
design methodology was upgraded and its performancewas evaluated. Hereafter, it is hoped that the end fitting
will be widely used for flexible, earthquake resistant, longdistance installation piping systems for gas transmission.
Table 2
Test item

Summary of tests for end fitting.

Content

ResultAppearance

Checking of any defects of practical importance

No abnormalities

Internal pressure
resistance

Checking of internal pressure resistanceNo leak up to 9.6 MPa

Air tightness

Checking of any leak or deformation

1.5 MPa x 24 hr, no abnormalities

Internal pressure creep at
Checking of any leak or deformation under specified conditions
elevated temperatures

5-MPa circumferential stress x 1000 hr x 80°C,
no abnormalities

Bending fatigue

Checking of fatigue resistance against vibrations corresponding to±2.6 cm x 2.3x106, no abnormalities
50 yearsTensile test at RT

Checking of any leak or deformation under specified conditions(Photo 4)

12 MPa x 1 hr, no abnormalities

Tensile test at HT

Checking of any leak or deformation under specified conditions

3.54 MPa x 500 hr, no abnormalities

Bending

Checking of any leak or deformation under specified conditions(Photo 5)

Cantilever 35 cm in length, no abnormalities

Heat cycle

Checking of heat cycle resistance against seasonal changes
corresponding to 50 years–5~40°C x 50 cycles, no abnormalities

Sealing surface pressure

Checking of retention of specified surface pressure after 50 years(Photo 6)

Figure 3. Estimated surface pressure after 50 years:
5.4 Mpa

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Steel Tape-Armored Polyethylene Pipe for Gas Transmission (II) — Development of Mechanical Fitting —Photo 6

Test for the relaxation of sealing surface pressure.

REFERENCES
1) Takeuchi et al.: “Steel Tape-Armored Polyethylene Pipe for Gas
Transmission (I) --Development of Pipe--,” Furukawa Review, 26,
17 (2004).
2) Agency for Natural Resources and Energy: “Interpretation
Examples of the Technical Standard for Gas Facilities, First
Revision,” 2003. (in Japanese)3) The Japan Gas Association: “Technical Data for Polyethylene Pipefor Gas Transmission,” 2001. (in Japanese)

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