[Sakamoto] Development of Fatigue-Less Umbilical Cable for Full Ocean Depth 12000m

7/29/2019 [Sakamoto] Development of Fatigue-Less Umbilical Cable for Full Ocean Depth 12000m

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Development of Fatigue-Less Umbilical Cable for Full Ocean Depth 12000m

Katsuya Sakamoto1, Yoshihiro Fujimoto1, Hiroyuki Osawa21Marine Cable Development Section Engineering Dept. Telecommunications Company

The Furukawa Electric Co., LTD

Ichihara, Chiba 290-8555, Japan

+81-436-42-1644 email address [email protected] Underwater Vehicle R and D Group, Marine Technology Research and Development Program

Japan Agency for Marine-Earth and Science Technology (JAMSTEC)

Yokosuka 237-0061, Japan

+81-46-867-9384 email address [email protected]

AbstractWe have developed the fatigue-less umbilical cable for full ocean

depth 12000m. It enables us to use at ease even under the hard

condition such as the highest water pressure in the world ocean by

means of the unique strength members combined with the

structure of completely balanced water pressure.

As the special strength member, we use the new FRP rod. Thecharacteristic of this FRP rod is as follows. It enables us to make

minimum bending radius small compared with the other FRP rod,

for instance, glass FRP and Kevlar FRP. And the tensile strength

of this new FRP rod hardly doesnt deteriorate against repeated

lateral compressive force, repeated bending and repeated twisting

of the cable. We had good results of fatigue estimation of the

cable under the condition of not only atmospheric pressure but

also water pressure more than 120MPa.

Keywords:Umbilical cable; Marine cable; Tether cable; Towedcable; Deep sea; KAIKO; ROV; FRP rod; Water pressure; lateral

compressive force; Repeated bending; Repeated twisting; Fatigue-

less.

1. IntroductionRecently, a scientific research in the ocean gets more and more

important. In a survey of the deep ocean more than 6500m,

remotely operation vehicle (ROV) is often used. In Japan, as the

cable used in the ocean depth 12000m, Japan Agency for

Marine-Earth Science and Technology (JAMSTEC) has already

the umbilical cable of KAIKO system. This system has two

cables, the primary cable and the secondary cable. One is the

tether cable between support vessel and launcher, whose length is

12500m the other is the tether cable between launcher and ROV,

whose length is 250m. These cables have the strength members

composed of Kevlar fibers. Its structure is FRP rod type and

braided net type.

However, it is know that the tensile strength of these cablesdeteriorate gradually due to fatigue given by water pressure,

lateral compressive force from the sheave and so on. In the

primary cable, now, its tensile strength has deteriorated until

about 70% compared with unused cable since it was

manufactured in 2000. Therefore, recently we have inspected its

broken tensile strength once a year. Generally, it is thought that

marine tether or towed cable used under the hard condition such

as deep sea is expendable article. However, in the unique cable

such as primary cable, manufacturing and maintaining it cost a

large amount of money, much time and great care. By reason of

that, a fatigue-less umbilical cable has wanted recently.

Accordingly, we investigated thoroughly the cause of the

deterioration of the tensile strength of these cables. As a result of

that, we found that the deterioration was caused by a damage of

Kevlar fibers in a molecular structure.

In this paper, we have newly developed the fatigue-less umbilical

cable by using the special FRP rod and making the water pressurefor each rod balanced completely. We manufactured two kinds of

trial cables for the secondary cable which is between launcher and

ROV, and made a comparative study of fatigue estimation under

the condition of air pressure and the highest water pressure.

Figure 1. Marine Survey ROV System

2. Cable StructureGeneral requirements for the secondary cable used in the deep sea

are as follows.

- It must be proof against the highest water pressure of

120MPa.

- Its specific gravity must be almost the same with sea water.

- It must be fatigue-less against the motion of the cable underthe condition of water pressure.

Figure 2 shows the cross-sectional structure of fatigue-less optical-

power line composite umbilical cable.

Vehicle

Umbilical Cable

Launcher

Support Vessel

International Wire & Cable Symposium 488 Proceedings of the 57th IWCS


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Figure 2. Cross-Sectional Structure of Fatigue-lessUmbilical Cable

3. Design of Strength Member

3.1 Basic Properties of FRP RodIn the shallow ocean, as the strength member for the umbilical cable

metallic materials are often used. However, in the deep ocean, high

strength fiber is used for the purpose of making the weight in the sea

water light. Generally, Kevlar fiber is well known.

We have developed a new FRP rod, which is jacketed by polyamide

or polyolefin resin, composed of copolymer type Aramid fiber to

make the fatigue-less umbilical cable come true. The new FRP rod

shows an excellent performance in comparison with Kevlar FRP rod.

Table 1 shows making a comparison of the basic properties between

the new FRP (T-FRP) and Kevlar FRP (K-FRP) rod, which have the

same FRP diameter 2.5mm and the same thickness of 0.35mm. T-

FRP has a unique characteristic that the minimum bending diameter

is very small compared with K-FRP.

Table 1. Basic Properties of 2.5mm FRP Rod

Item K-FRP T-FRP

Tensile strength [MPa] 1558 1676

Youngs modulus [MPa] 68100 47599

Minimum bending diameter [mm] 63 26

3.2 Durability against Water Pressure Cycle

3.2.1 Experimental MethodWe performed the water pressure cycle test of the FRP samples

shown in the Table 1. Test conditions are as follows.

- Maximum water pressure is 119.6MPa.

- About 10 minutes per cycle is kept at the 119.6MPa. It

takes about 30 minutes per cycle.

- The repeated number is 20 and 200 times.

After that, tensile strength ratio for the original FRP, for which

every load wasnt given at all, was estimated.

3.2.2 Results of Water Pressure Cycle Test

The deterioration of the tensile strength for the T-FRP scarcely

occurred, but for the K-FRP, the tensile strength ratio has

gradually deteriorated to about 85% after 20 cycles, and to about

70% after 200 cycles. Close observation of the FRP cross-section

showed that there were cracks in some mono-fibers for the K-FRP

as shown in Figure 3.

Figure 3. Cracks in some mono-fibers in the K-FRP

3.3 Durability against Lateral Compressive Force

3.3.1 Experimental MethodThe test of Durability against the lateral compressive force was

performed as shown in Figure 4. The two kinds of the FRP

samples in the Table 1 were used. Test conditions are as follows.

- Lateral pressure is 1470N per FRP length 22mm. It was

determined by considering the operation of the system.

- Compressive velocity is 10mm per minute.

- Times per cycle are about 7 seconds. The kept times at the

maximum point is nothing.

- Repeated number is 100 and 1000 times.

After that, a deformation of the FRP rod and the tensile strength

ratio were examined.

Figure 4. Lateral Pressure Test Apparatus

3.3.2 Results of Lateral Pressure Test

The test results of the lateral pressure cycle are shown in Figure 5. A

solid line shows the tensile strength ratio for the original FRP, and a

dotted line shows the deformation of the FRP due to the

compressive force. An extent of the deformation is determined by

the following formula, we call it non-circularity ratio.

100+

=

SL

SL

DD

DDR (1)

Where R is the non-circularity ratio, DL is the long diameter, and DS

is the short diameter.

In the K-FRP, large deformation occurred, and the tensile strength

deteriorated to about 79% after 100 times, and to about 54% after

1000 times. However, in the T-FRP, the deformation was small,

and the deterioration of the tensile strength scarcely occurred.

Power Line

Earth Line

Optical Fiber Unit

Polyester Braided-net

Strength Member of FRP Rod

Plate length =

FRP Rod Sample

Repeated Lateral



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0

20

40

60

80

100

0 200 400 600 800 1000

Lateral Pressure Cycle

TensileStrengthRatio

[%]

0

5

10

15

20

Non-circularityRatio

[%]

Tensile Strength Ratio K-FRP Tensile Strength Ratio T-FRP

Non-circularity Rat io K-FRP Non-circularity Rat io T-FRP

Figure 5. Results of Lateral Pressure Cycle Test

4. Trial Cable Design

4.1 Requirements of Trial CablesIn this paper, we describe the development of the secondary cable

between the Launcher and the ROV for full ocean depth 12000m.

We must pay attention to its requirements because it is different

from that of the primary cable between the support vessel and the

Launcher.

In the secondary cable, the cable tension is almost free when the

ROV operates separate from the launcher. However, the motion is

very complicated and hard. For example, under the highest water

pressure, rolling in a reel, paying out the cable, bending, pulling,

and twisting are given repeatedly. Taking all things into

consideration, the most important requirement of the trial cable is

that the damage to the FRP given by the operation of the ROV

must be kept a minimum even if the cable is used in the deepest

sea in the world.

- Stress to each FRP rod given by the water pressure must be

isotropic.

- Water pressure must not prevent the motion of each FRP.

4.2 FRP Rods in Trial CablesWe manufactured two kinds of trial cables in order to show that the

T-FRP rod was superior to the K-FRP rod. Table 2 shows the basic

properties of the FRP rod in the trial cables. These FRP were

designed at the same size to make the deference of the cable

performance clear.

Table 2. Measured Values of FRP Rod in Trial Cables

K-FRP T-FRPItem

Inner Outer Inner Outer

FRP diameter [mm] 1.22 1.00 1.24 1.01

Sheath diameter [mm] 1.8 1.7 1.8 1.7

Mass [g/m] 3.00 2.51 2.98 2.65

Tensile strength [MPa] 1480 1440 1842 1837

Youngs modulus [MPa] 75550 76875 53500 52400

Minimum bending [mm] 29 26 14 12

4.3

4.4 Initial Performance of Trial CableBoth of trial cables were manufactured using inner sheath core of

the same size, which diameter is 19.3mm. As outer jacket on the

strength member, which are the FRP rods shown in the Table 2.

polyester braided-net was used. Cable diameter is 29mm and the

specific gravity is 1.3 or less.

Table 3 shows the initial mechanical characteristics of these trial

cables. K or T represents that the strength member is the K-FRPor the T-FRP respectively.

Table 3. Initial Mechanical Characteristics of Trial Cables

Item K T

Tensile Broken Strength [kN] 108 114

Elongation at tension of 10[kN] [%] 0.3 0.5

Elongation at Breaking Point [%] 2.8 4.2

5. Fatigue ExperimentsIn order to show the superiority of the trial cable T, four fatigue

experiments were performed under the same condition for the trial

cables K and T. At first, these tests were performed in the water

pressure of 123MPa. For the purpose of these special experiments,

we have developed the fatigue experimental facilities which can

be used under the condition of 123MPa.

Especially, the bending test and the twisting test were also

performed under the condition of the atmospheric pressure for the

purpose of investigating the influence of the water pressure.

5.1 Mechanical Characteristics in 123MPa5.1.1 Experimental Methods

It was supposed that the considerable motion of the cable when the

ROV operates. Table 4 shows the test condition in the water

pressure 123MPa. After that, any numbers of the FRP rods were

picked up, and the tensile strength ratio was measured. Figure 6

shows the experimental apparatus of the S-Bending test. Other testswere also performed in the similar way by changing the S-bending

unit into the other unit.

Table 4. Test Conditions in 123MPa

ItemTension

[N]

Cable Disposition

Other LoadRepeat Times

Pull and

Relax3920 Straight Line 1000

Twisting 3920Straight Line

45 [degree/m]1000

S-Bending 3920S-Shape

R=150[mm]1000

U-Bending 3920U-Shape

R=150[mm]1000



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Figure 6. S-Bending Test in Water Pressure Tank

5.1.2 Test Results in 123MPa

The test results of the fatigue experiments under the condition of the

water pressure 123MPa are shown in the Figure 7. It was confirmed

that the deterioration scarcely occurred in the trial cable T. For the

trial cable K, as expected, the deterioration of the tensile strength is

large in comparison with the trial cable T, but it is small in

comparison with the results shown in the sub-subsection 3.2.2 and

Figure 4.As the reasons, two facts are thought. First, the FRP diameter used

in the trial cables is smaller than that used in the water pressure

cycle test and the lateral pressure cycle test in the section 3. The

difference relates to the stress to the internal part, which is mono-

fiber, of the FRP rod. Secondly, the lateral pressure due to the

interaction between the FRP and the sheave in the tests like U-

bending and S-Bending is different from that of the lateral pressure

cycle test in the section 3. The lateral pressure of the bending tests

in the Table 4 is estimated at about 40%.

86.0

88.0

90.0

92.0

94.0

96.0

98.0

100.0

Pull and

Relax

Tw isting S-bending U -bending

TensileStrengthR

atio[%]

K-FRP

T-FRP

Figure 7. Mechanical Characteristics in 123MPa

5.2 Compared with Atmospheric Pressure5.2.1 Experimental Methods

We manufactured the other trial cable T which was composed of

the other T-FRP shown in the Table 5. The mechanicalcharacteristics of the trial cable T is almost the same as the trial

cable T shown in the Table 3. In order to investigate the influence

of the water pressure to the mechanical characteristics of the cable,

the Twisting tests and the S-Bending tests were performed under all

the same conditions except for the experimental surrounding, which

is in atmospheric pressure or in the water pressure 123MPa. For the

atmospheric pressure condition, the fatigue experimental facilities

shown in the Figure 5 were used out of the water pressure tank.

Table 5. Measured Values of FRP Rod in Trial Cable T

T-FRPItem

Inner Outer

FRP diameter [mm] 1.24 1.00

Sheath diameter [mm] 1.8 1.7

Mass [g/m] 3.02 2.57

Tensile strength [MPa] 1603 1633

Youngs modulus [MPa] 55260 49320

Minimum bending [mm] 13 9

Test conditions of the trial cable T are shown in the Table 6. The

condition of tension 9800N corresponds to the lateral pressure tested

in the section 3.3.1.

Table 6. Test Conditions of Trial Cable T

ItemExperimental

Surrounding

Tension

[N]

Cable

Disposition

Other Load

Repeat

Times

Twisting

Atmosphere

and 120MPa

3920

9800*

Straight Line

45 [degree/m] 1000

S-

Bending

Atmosphere

and 120MPa

3920

9800*

S-Shape

R=150[mm]1000

*Tension 9800N is the only condition of the water pressure 120MPa.

5.2.2 Test Results

For all the conditions in the Table 6, the tensile strength ratio was

99% or more. These results show that in our cable structure, the

water pressure doesnt influence to the motion of the cable even if

the cable is used in the deepest sea in the world.

6. Other Performances of Trial Cable T6.1 Transmission Characteristics of Optical Fiber

For the trial cable T, a transmission loss change was monitored at

the wavelength 1550nm in the middle of the fatigue tests in the

section 5.2.1. For all the conditions in the Table 6, the transmission

loss change of the cable was 0.02dB or less.

6.2 Extended Durability for Bending

6.2.1 Experimental Methods

We have succeeded in the development of the fatigue-less umbilical

cable for full ocean depth 12000m. However, it is the results

verified under the condition of repeat times 1000. Therefore, it is

necessary that we should show more extended durability.

In the atmospheric pressure, S-Bending test of the trial cable T

shown in Figure 8 were performed under the following conditions,

the sheave diameter is 400mm, tension is almost free, and the repeat

number is 320000 times. In the middle of the test, the transmission

loss change was monitored at the wavelength 1550nm. After that,

the tensile strength ratio was measured. The cable deposition of this

test is different from one shown in Figure 6. Bose of the cable

terminations are fixed at two sheaves respectively, and the sheaves

rotate.

Water Pressure

Fatigue Experimental Facilities

Trial Cable

S-Bending Unit

Tension Equipment



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Figure 8. S-Bending Apparatus for Extended Durability

6.2.2 S-Bending Test Results of Extended Durability

We had excellent results that the tensile strength ratio is 96% and

over, and the transmission loss change of optical fiber is 0.1dB or

less. It was showed that the deterioration due to repeated bending

scarcely occurred.

7. ConclusionsWe have obtained the fatigue-less secondary umbilical cable by

studying a mechanism of the deterioration of the Kevlar fiber. This

ideal umbilical cable can be used in the deepest sea in the world,

and has the excellent characteristics of that the tensile strength

scarcely deteriorates against the motion of the cable under the hard

condition. By using this new FRP rod, it is expected that the fatigue-

less primary umbilical cable can also be realized in the near future.

8. AcknowledgmentsFor writing this paper, we give the special thanks to Advanced

Underwater Vehicle R and D Group, Marine Technology Researchand Development Program, Japan Agency for Marine-Earth Science

and Technology, Aramid Product Development Section, High

Performance Fibers Research and Development Department,

TEIJIN TECHNO PRODUCTS LIMITED, and UBE-NITTO

KASEI CO., LTD. And also, we greatly appreciate the united

efforts of a number of people.

9. Pictures of AuthorsKatsuya Sakamoto


6 Yawatakaigan-dori, Ichihara,

Chiba 290-8555, Japan

[email protected]

He was born in Oita, in 1968. He received the M.Sc. degree in

theoretical physics from Shimane University, Japan in 1993. He

jointed The Furukawa Electric Co., Ltd. in 1993 and has been

engaged in research, development and production engineering of

marine cable technology. He is now a senior engineer of marine

cable development section, engineering dept., Telecommunications

Company.

Yoshihiro Fujimoto


6 Yawatakaigan-dori, Ichihara,

Chiba 290-8555, Japan

[email protected]

He was born in Chiba, in 1959. He received the B.E. degree in

Metal engineering from the Chiba Institute of Technology, Japan

in 1983. He jointed The Furukawa Electric Co., Ltd. in 1992 and

has been engaged in research, development production

engineering of marine cable technology. He is now a manager of

engineering dept., Telecommunications Company.

Hiroyuki Osawa, Dr. Eng.

Japan Agency for Marine-Earth

Science and Technology

2-15 Natsushima-cyo, Yokosuka

237-0061, Japan

[email protected]

He was born in Tokyo, in 1963. He received the Ph.D Ocean

Engineering, Nihon University JAPAN in 1996. He joined Japan

Marine Science and Technology Center in 1996 and has been

engaged in research, marine technology. He is now group leader

advance marine technology research program, marine technology

center.

Trial Cable

Sheave


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[Sakamoto] Development of Fatigue-Less Umbilical Cable for Full Ocean Depth 12000m