Development of a Flexible Pipe for Pipe-in-Pipe Technology
by Toru Kagoura *, Ken’ichi Ishii *, Satoru Abe *2, Tetsuo Inoue *2, Takayoshi Hayashi *3,
Takashi Sakamoto *4, Takashi Mochizuki *4 and Tomohiro Yamada *4

Pipe-in-pipe technology has been implemented, whereby pipes of one continuous
length are laid individually in the existing submarine pipelines belonging topetrochemical complexes in various locations across Japan to construct renovated networks
for fluid transportation. Because the pipelines have a number of sharp bends, development of anew pipe with excellent flexibility and lightweight was required to construct such a network. Thus
Furukawa Electric, Nippon Steel, and Idemitsu Kosan have jointly developed the pipe thereby
succeeded in acquiring a special certification as a flexible pipe for transportation of high-pressureliquefied petroleum gas. The developed pipe has been successfully laid in the existing submarine
pipeline (3.3 km in total length) in the Tokuyama Refinery of Idemitsu Kosan in July 2002.

ABSTRACT1.

INTRODUCTION

Various submarine pipelines for fluid transportation of
petroleum and gas have been laid in petrochemicalcomplexes in different locations across Japan. Someof these pipelines are rather degraded and thus haveto be renovated in near future. We have implemented
experimentally the pipe-in-pipe technology to reconstruct
the degraded pipelines, whereby the existing pipeline
is filled with water, in which a new pipe is laid taking
advantage of liquid buoyancy to construct a renovatednetwork for fluid transportation. To lay anew a pipe in theexisting pipelines of long distance where sharp bends
are involved, it is necessary to apply a pipe with excellent
flexibility and lightweight.
Meanwhile, Furukawa Electric had made practicalapplications of a flexible pipe with a composite structure
comprising metal reinforcing layers and plastic layers foroffshore oil fields 1). Figure 1 shows the basic structureof the flexible pipe in practical use. The flexible pipe has
an established track record overseas in such applications
as a riser pipe which transports oil and natural gas minedfrom the seabed to floating petroleum production facilities
or tankers, or as a “flowline” laid on the seabed 2), 3).
Domestically, however, the pipe has never been appliedin pipelines for transportation of high-pressure liquefied
petroleum gas, nor has it been laid for long distance inexisting pipelines.

Currently the Research Association of Refinery
Integration for Group Operation (RING) is promoting an
R & D program, under the auspices of the Ministry of
Economy, Trade and Industry, to implement “Refining
and Petrochemical Complex Renaissance Concept”
aimed at strengthening the international competitivenessof domestic petrochemical complexes. As related to thisprogram, the pipe-in-pipe technology was experimentally
implemented at the Tokuyama Kombinat centered on the
Idemitsu Kosan Refinery.
This report will briefly describe the design andperformance verification of a lightweight flexible pipe,
which has been carried out jointly by Furukawa Electric,
Nippon Steel, and Idemitsu Kosan.

2.

DESIGN OF FLEXIBLE PIPE

2.1 Governing Standards
Whereas flexible pipes have established a track record
overseas based on the standards stipulated by theOuter sheath

Axial tension
armor

Inner pipe
PressureInterlocked conduit
reinforcement*

Ecology & Energy Lab.
Power Cable Engineering Dept., Power Cables Div.
*3 Tokuyama Refinery, Idemitsu Kosan Co.,Ltd.
*4 Civil Engineering & Marine Construction Dept., Nippon SteelCorporation*2

Figure 1

Basic structure of flexible pipe.

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Development of a Flexible Pipe for Pipe-in-Pipe Technology

Table 1

Design load and allowable stress of flexible pipe.

Load to be considered

Primary load

Internal

Hoop stress: 40 % or 0.5·(1.6- γ ),whichever is smaller, where γ is thepressure

ratio of yield stress to tensile strength
Axial stress: 50 % of yield stress(steady load
under operation)

Allowable stressGas weight 33.3 % of tensile strength or 66.7 %or external of yield stress, whichever is greaterpressure(applicable to stainless steel)

Secondary load
(transient load

Layingduring installation

tension90 % of yield stressand the like)

American Petroleum Institute and the like, they lack indomestic records of practical applications. Consequently,
the pipe had to undergo examinations by the Performance
Evaluation Committee of the High Pressure Gas Safety
Institute of Japan to acquire a special certification. We
made investigations in compliance with the Security
Regulation-Related Standards for Kombinat and the Like(in Japanese, hereafter called Kombinat Standards) in theHigh-Pressure Gas Security Laws, which are applicable tothe existing pipelines.
Ta b l e 1 s u m m a r i z e s t h e l o a d s a n d a l l o w a b l e
stresses specified in the Kombinat Standards thatmust be taken into account in the design of the flexible
pipe. The Kombinat Standards specify the allowablestresses corresponding to specific loading conditions
and materials, and the design should be such that thecalculated stress under each loading condition does notexceed the allowable stress. While the allowable stressincludes suitable safety factors, design standards severer
than the API standards are stipulated with respect tointernal pressures especially, thereby requiring higher
values of design strength than before.
2.2 Study of Pipe Structure
It was feared that, if a pipe with conventional structureis laid in the long-distance pipeline, the laying tension
would increase resulting in an excessive stress overthe breaking strength of the pipe. Thus, it became
an important technological task to reduce the laying
tension in order to apply flexible pipes to the pipe-in-pipe
technology. Various pipe structures were studied together
with preliminary designs, and the result indicated thatapplication of a laying method would be most effective, in
addition to reduction of the in-water weight of the flexible
pipe, whereby water is poured into the existing pipeline
to reduce the laying tension taking advantage of liquidbuoyancy. Moreover, it became clear that the pipe should
be as flexible as possible to pass the bends smoothly, andthat, to this end, the pipe's flexural rigidity and diametershould be as small as possible.

2.2.1 Design Specifications
Design specifications necessitated for structural studies ofthe flexible pipe are shown below. The laying tension has
been estimated, through the study done by Nippon Steel,using a new calculation formula that takes into accountthe tension increments due to pipe’s flexural rigidity andthe contact conditions at bends.
Inner diameter:
5 in
Outer diameter:
7 in approx.
Internal pressure:
2.8 MPa
External pressure:0.12 MPa
Laying route length (pipe length): 3.3 km
Minimum bending radius:
1450 mm
Number of bends:
5
Laying tension:
80 kN (estimated)Tensile strength:
320 kN (four times thelaying tension)
2.2.2 Design of Pipe Structure
In view of the design specifications mentioned above,a single-layered structure was employed, in place ofthe double-layered structure conventionally used, forthe axial tension armor that takes charge of the laying
tension, thereby reducing the weight, flexural rigidity, andouter diameter of the pipe. Moreover, with respect to thesingle-layered armor, a composite structure comprisingsteel reinforcements and plastic spacers of small specificgravity was used, and the structure was optimized tominimize the number of steel reinforcements thereby
making the stress due to the laying tension lower than theallowable stress, and simultaneously ensuring a tensile
strength four times the laying tension.
In terms of the pressure reinforcement layer, given its
structural position sandwiched between the two plastic
layers together with its use under static environments, it
was thought that consideration of wall thickness decrease
due to corrosion or abrasion was not necessary. In
addition, a steel strip of high precision thickness was used
to help optimize the structure to have a tolerance factor of
1.0 over the allowable stress, thus realizing a reduction inboth weight and flexural rigidity.
The measures above mentioned enabled to design
a flexible pipe with an in-water weight of 0.6 kg/m --anextremely lightweight over the in-air weight of 27 kg/m.
Figure 2 shows the structure of the lightweight pipe, andTable 2 the structural specifications and the allowablestresses based on the Kombinat Standards.
2.3 Study of Pipe Strength
Using the optimized lightweight structure, design
strengths of the pressure reinforcement layer and theaxial tension armor layer were studied with respect totheir circumferential and axial stresses due to internalpressure, and it was confirmed that these stresses are
below the allowable stress specified in the Kombinat
Standards. The design strength of the axial tension
armor layer due to laying tension was also confirmed tobe below the allowable stress thus posing no problems.

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Development of a Flexible Pipe for Pipe-in-Pipe Technology

for pressure reinforcement layer of 214 N/mm2. Because
the design is optimized, the tolerance factor over theallowable stress is about 1.0.

Axial tension armor
Outer sheathPressure reinforcement
Inner pipe
Interlocked conduit

2) Axial Tension Armor Layer
Axial stress due to internal pressures is taken charge ofby the axial tension armor layer. The generated stress isgiven by Equation 2.

σ=Figure 2

Table 2

Structure of lightweight flexible pipe.

Function(Material)
Interlocked conduit(Stainless steel)

Reinforcement
against external andlateral pressures

Inner pipe

Allowable stress(N/mm2)
Primary load: 221

Pressure reinforce-

Reinforcement

ment (Cold-rolled

against internal

steel strip)

pressure

Axial tension armor(Zn-galvanized steel
wire)

Reinforcement

Specification
Inner dia.: 127.0 mm

Secondary load: 450 Outer dia.: 134.0 mm
Inner dia.: 135.4 mm

Container for fluid(Nylon 11)

Outer dia.: 148.8 mm
0.5 mmt × 2
Primary load: 214

Inner dia.: 149.8 mm
Outer dia.: 151.8 mm

Primary load: 200

against axial force Secondary load: 360

φ 7 mm × 15
Inner dia.: 153.8 mm
Outer dia.: 167.8 mm

φ 7 mm × 52

Axial tension armor

Spacer(Polyester)

Inner dia.: 153.8 mm
Outer dia.: 167.8 mm

Outer sheathProtection against

Inner dia.: 170.7 mm(Polyethylene)

external damage

Outer dia.: 183.1 mm

In-air weight26.9 kg/m

In-water weight

0.6 kg/m

Minimum bending radius
Flexural rigidity

1450 mm
1.2 × 104 Nm2

Equation 2 gives an axial stress due to internal pressureof 64 N/mm2, which is lower than the allowable stress foraxial tension armor layer of 200 N/mm2, thus assuring asufficient strength.
2.3.2 Study of Pipe Strength against Laying Tension
Axial stress generated by laying tension can be obtained,
since this is taken charge of by the axial tension armor
layer, by replacing numerator A pP in Equation 2 withlaying tension T .
The axial stress due to laying tension thus obtained is
143 N/mm2, which is lower than the allowable stress foraxial tension armor layer of 360 N/mm2 shown in Table 2,thus assuring a sufficient strength.
2.3.3

Study of Pipe Strength against External
Hydrostatic Pressure
Buckling stress of the flexible pipe under hydrostaticpressure against external pressure is taken charge ofby the innermost interlocked conduit, and the bucklingstrength is give by Equation 3.

S=
Below will be described the strength studies with respect
to internal pressure, laying tension, and external pressure(hydrostatic pressure).
2.3.1 Study of Pipe Strength against Internal Pressure
1) Pressure Reinforcement Layer
Hoop stress due to internal pressures is taken charge ofby the pressure reinforcement layer. The generated stressis given by Equation 1.

σ=

P (D -t )2t sinθ(2)where: σ is the axial stress; Ap is the inner crosssectional area of the pipe; P is the internalpressure; N is the number of axial tension
rod; A is the cross-sectional area of one axial
tension rod; and θ is the winding angle.

Structural specification and allowable stress oflightweight flexible pipe.

Major layer

ApP
NA cosθ(1)where: σ is the hoop stress; P is the internalpressure; D is the diameter; t is the thickness;and θ is the winding angle.

3EI
R3

(3)

where: S is the buckling strength against externalpressure; E is the elastic constant of theinterlocked conduit; I is the geometricalmoment of inertia of the interlocked conduit;
and R is the winding radius of the interlocked
conduit.
Equation 3 gives a buckling strength of the interlocked
conduit of 2.39 MPa, which has a safety factor of about 20
over the design external pressure of 0.12 MPa. This is aperfectly problem-free level, since it has been confirmed
from the past data that good agreement can be seen
between the design and measured values of externalpressure buckling.

Equation 1 gives a hoop stress due to internal pressureof 212 N/mm2, which is lower than the allowable stress

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Development of a Flexible Pipe for Pipe-in-Pipe Technology

3.

PERFORMANCE VERIFICATION USING
PROTOTYPE PIPE

A prototype pipe with an optimized lightweight structure
as was designed as stated above was manufactured,and various performance verification tests were carriedout to confirm its safety margin and the design validity as
mentioned below.
3.1 Airtightness Test
3.1.1 Test Method
To confirm airtightness of the flexible pipe, test endfittings were assembled on both ends of a specimen
4 m in length, N 2 gas was filled in, the pressure wasincreased up to the design pressure of 2.8 MPa, and thespecimen was maintained for 10 min to check whether
any abnormalities such as leaks would occur.
3.1.2 Test Result
No abnormalities such as leaks were found, confirmingthat the airtightness performance would present no
problem.
3.2 Hydrostatic Test
3.2.1 Test Method
To confirm withstand pressure of the flexible pipe, asame specimen as for the airtightness test was used,
and water was filled in, then the pressure was increased
up to 1.5 times the design pressure (4.2 MPa), and thespecimen was maintained for 10 min to check whether
any abnormalities such as leaks would occur.
3.2.2 Test Result
No abnormalities such as leaks were found, confirmingthat the withstand pressure performance would present no
problem.
3.3 Burst Pressure Test
3.3.1 Test Method
To confirm internal pressure burst value of the flexible
pipe, a same specimen as for the airtightness andhydrostatic tests was used, water was filled in, and thenthe pressure was increased until the specimen broke
down. Photo 1 shows a view of the test.

3.3.2 Test Result
The burst pressure value was 12.6 MPa --more than fourtimes the normal pressure, confirming that it compares
favorably with the design value. As shown in Photo 2, theburst occurred in the pressure reinforcement, leaving no
breakdown openings in the plastic inner pipe.
3.4 Bending Test
3.4.1 Measurement of Flexural Rigidity
1) Test Method
A hydraulic cylinder and a load cell were attached on bothends of a specimen about 3.5 m in length, on which abending moment was applied using the hydraulic cylinder,
and the bending moment and the bending radius thereby
were measured to obtain flexural rigidity EI . Photo 3
shows a view of the test.
2) Test Result
The measured flexural rigidity was 0.9 × 10 4 Nm 2 ,somewhat lower than the design value of 1.2 × 104 Nm2,
rendering the pipe to be on the safe side in the aspects oflaying work.
3.4.2 Repeated Bending Test
1) Test Method
Assuming a bending history during laying work, a bend
was applied to a specimen about 10 m in length using
a bending gauge 1400 mm in radius, the specimen was
restored to its original position, then a same bend wasapplied in the reverse direction. After this bending process
was repeated five times for 10 bends, the specimen was
disassembled to check for any abnormalities. The bending
radius corresponds to the severest bend in the laying
route, and the number of bending times was given atolerance factor of 2 over the number of bending the pipe
was expected to actually undergo during laying. Photo 4shows a view of the test.

Photo 2

Photo 1Burst pressure test.

Burst point due to internal-pressure test.

Photo 3

Flexural rigidity measurement.

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Development of a Flexible Pipe for Pipe-in-Pipe Technology

Photo 5

Repeated bending test.

Appearance of pressure reinforcement
after repeated bend test.

2) Test Result
Photo 5 shows the result of disassembly. It can beseen that the pipe has no problem in the pressure
reinforcement layer and other structures against repeatedbends of minimum bending radius, thereby demonstrating
its satisfactorily problem-free performance against
bending histories expected during laying.
3.5 Tensile Test
3.5.1 Test Method
To test the tensile characteristics of the flexible pipe,
testing end-fittings were attached on both ends of a
specimen about 4 m in length, a load cell and a hydrauliccylinder were fixed at each end, then a tensile load of 80
kN was applied for 10 min. After the pipe was checked forabnormalities such as fracture, the tension was increased
to check the breakdown tension. Photo 6 shows a view of
the test.
3.5.2 Test Result
It was confirmed that no abnormalities occurred in thetensile test history of 80 kN for 10 min. The breakdown
tension was found to be 321~335 kN, equivalent to thedesign value, confirming that its actual performance wasover four times the laying tension. Photo 7 shows an
appearance of the breakdown point.
3.6 Laying Test
3.6.1 Test Method
To confirm the lateral pressure performance and theexternal damage resistance of the flexible pipe, a laying
end-fitting was attached on both ends of a pipe specimen
about 12 m in length, the specimen was pulled into a miter
bend pipe with a bending radius of 1.5 m, a bending angleof 180°, and a weld bead area that was harsher than the

Photo 6

Photo 7

Tensile test.

Appearance of breakdown point due to tensile test.

Photo 8

Pipe laying test.

6
Horizontal
Vertical or lateral

4

OD deformation (%)

Photo 4

2
0
-2
-4
-6
0

50

100

150

200

Tension (kN)

Figure 3

Relationship between tension and outer diameter
deformation in laying test.

existing pipelines, and then tension was applied between
the two end-fittings to provide a lateral pressure history
at pipeline bends. Subsequently, under a counteracting
tension, the specimen was pulled in using a winch toconfirm whether there was buckling in the pipe specimen
and to check for external damage. Photo 8 shows a viewof the test.

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Development of a Flexible Pipe for Pipe-in-Pipe Technology

4.

Photo 9Appearance of outer sheath after laying test.

Various tests for performance verification were carried out
using a prototype pipe of optimized lightweight structure,and it was demonstrated that there were no problems
with respect to the airtightness characteristics, hydrostaticpressure characteristics, burst pressure characteristics,
laying tension resistance characteristics, bending
characteristics, and pull-in characteristics for laying.
Based on these achievements, the flexible pipe developed
here has passed the examination by the Performance
Evaluation Committee comprised of The High Pressure
Gas Safety Institute of Japan and the experts, and thereby
has acquired a special certification as a flexible pipe fortransportation of high-pressure liquefied petroleum gas.

5.
Photo 10 Appearance of pressure reinforcement layer
after laying test.

Photo 11 Appearance of interlocked conduit after laying test.

3.6.2 Test Result
Figure 3 shows the relationship between the tension andpipe deformations (horizontal and vertical) at the center ofthe bend. The relationship is seen to be linear up to about
100~120 kN, posing no problems such as buckling up toa laying tension of 80 kN or so. What is more, a pipe pullin history with a laying tension in excess of 80 kN was
given to the specimen, and the result indicated that noabnormalities such as buckling were observed and thatthe external damage due to weld beads was insignificant
as shown in Photo 9 thus not reaching a problematic level.
As a result of disassembling investigation, it has been
confirmed that every reinforcement layer and the inner
pipe had no structural abnormalities, achieving sufficientperformance in terms of pull-in history. Photo 10 and
11 show the internal pressure reinforcement layer andthe interlocked conduit of the disassembled specimen,
respectively.

SUMMARY OF PERFORMANCEVERIFICATION TESTSIN CONCLUSION

We have developed, in collaboration with Nippon Steeland Idemitsu Kosan, a lightweight flexible pipe suitable
for laying, by means of floating extension method, in longdistance pipelines with severe bends. The pipe developed
here was first applied to, after its manufacturing wascompleted in one continuous length in January 2002, thereconstruction work of the existing submarine pipeline in
the Tokuyama Refinery of Idemitsu Kosan in July 2002,
and the pipe laying work was accomplished in safety.
When the verification test for gas transportation that iscurrently under way is completed, this will be domesticallythe first application of a flexible pipe for transportation ofhigh-pressure liquefied gas.
In closing the authors will be most gratified if the flexible
pipe developed here can contribute to reconstruction
works of petrochemical complexes. We would also like toexpress our thanks to those who kindly cooperated in thisdevelopment.

REFERENCES
1) Makino et al.; Development of Flexible Pipes for Deep Water
Applications, Furukawa Review, No.13, 1994, p.85
2) Y. Makino, K. Ishii, T. Fuku; Development of Light-Weight
Flexible Pipe, OTC7362, 1993
3) K. Ishii, Y. Makino, T. Kagoura; Flexible Riser Technology for
Deep Water Applications, OTC7726, 1996
4) T. Mochizuki et al.; Experimental and Analytical Study for
the Installation of Flexible Pipe in the Existing Submarine
Pipeline by Pipe-In-Pipe Technology, ISOPE, 2003

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