Completion of an Optical Fiber Composite
500-kV DC Submarine OF Cable Projectby Yutaka Nakanishi *, Koichiro Fujii *2, Yasuhiro Goto *3, Noboru Ishii *4, Tadashi Fuku *5,Osamu Satou *5, Seiki Nakamura *5, Takeshi Araya *5, Kengo Kimura *5,Masaharu Kasuya *6, Hisashi Orito *3 and Migaku Takamatsu *3

A construction project of a DC submarine cable, to be known as Anan-Kihoku DC
Trunk Line, has been under way to link the electric power grids between Kansaiand Shikoku. This submarine cable line is designed to have a capacity of 2800 MW on one bipolar circuit of 500 kV DC with return conductor, requiring a cable system with a current capacity of2800 A per line --the greatest submarine DC cable in the world. The cable was developed jointly
by Kansai Electric, Electric Power Development, and four major cable manufacturers in Japan;
and four lengths of the cables, each 50 km long, were manufactured by the cable manufacturers
respectively, spending approximately two years. From April to December of 1998, the cables
were laid and buried, one by one, using a cable laying ship and a laying and burying machine
newly developed. Subsequently, connected with land cables, they satisfactorily underwent DC
high-voltage tests at -700 kV in August 1999. They are scheduled to be in operation in July 2000,
after a series of power grid linking tests. Furukawa Electric has supplied a length to be used as
the main cable of 2nd circuit.

ABSTRACT1.

INTRODUCTION

Anan-Kihoku DC Trunk Line, with a cable section of 48.9
km and an over-head section of 50.9 km --totaling 99.8
km-- has been constructed to link the electric power grids
between Kansai and Shikoku.
In this line, 500-kV DC submarine cables, having alength of 46.5 km each and the greatest capacity in theworld, are used in the cable section. See Figure 1 andTable 1.
The cable was developed jointly by Kansai Electric,Electric Power Development, and four major cable manufacturers in Japan headed by Furukawa Electric; and four
lengths of the cables, each 50 km long, were manufactured by the cable manufacturers respectively, spending
approximately two years. In terms of installation work, thecable was planned to be buried for its entire length, considering that Kii Strait where the cable was installed hadmuch sea traffic and that the cable was susceptible topossible damages due to dragnets of trawl fishery inprevalence in the sea area. In addition, in an effort toimprove the positional accuracy of laying and to exten-

sively reduce construction period, a cable laying ship anda cable laying and burying machine of new development
were employed.
Kobe
Okayama
Osaka
Awajishima
Wakayama

Takamatsu
Tokushima

Kii Strait
YuraAnanKochi

500-kV DC submarine cableFigure 1
Table 1
Transmission capacity
Rated voltageCable route

Outline of the cable system
2800 MWDC ±500 kV(to be operated at ±250 kV in the beginning)*Transmission system

One bipolar circuit with return conductor*2Rated current

2800 A

Number of cables

4 (2 main cables and 2 return cables)

Section length48.9 km (46.5 km undersea, 2.4 km on land)

Maximum route depth

75 m in water

The Kansai Electric Power Co., Inc.
Electric Power Development Co., Inc.
*3 TA Project Team, Power Cables Div.
*4 Power Cable Engineering Dept., Power Cables Div.
*5 Power Cable Manufacturing Dept., Power Cables Div.
*6 1st Quality Assurance Dept., Power Cables Div.

123

This report outlines the manufacture, installation, andDC high-voltage test of the 500-kV DC submarine OF (oilfilled) cable having the greatest capacity in the world.

2.

CABLE

2.1 Structure of Cable
Since the cable is required to have a current capacity of2800 A per line at 500 kV DC, it was decided to employ anOF cable for its high permissible conductor temperatureand high reliability based on an established track record oflong time. If conventional kraft paper were used for insulation, however, the insulation layer would be of great thickness with poor heat dissipation making the cable size far
exceed the manufacturing capabilities. Accordingly,
polypropylene laminated paper (PPLP) was selected forinsulation for its excellent electrical properties so as toreduce the insulation thickness.
Furthermore, this submarine cable integrates 12 optical

Hollow oil passage
Zinc coated steel tape in spiral (inside diameter: 25 mm)
Conductor (7 sectored)
Insulation paper(kraft paper: 0.5 mm
PPLP: 19.5 mm
Kraft paper: 2.5 mm)

Lead sheath (thickness: 4.6 mm)
Anti-corrosion layer (thickness: 6.0 mm)
Optical fiber unit (outside diameter: 4 mm x 10)
FillerArmoring steel wire (φ8 mm)
Sensing wire for buried location(outside diameter: 4 mm x 10)
Total outside diameter: approx. φ188 mm
Total weight: approx. 100 kg/m

Figure 2

Structure of optical fiber composite submarine OF
cableAnti-corrosion layer(outside diameter: 4 mm)
SUS tube(outside diameter: 0.9 mm)
Jelly
Optical fiber

Figure 3

Structure of optical fiber unit

fiber units for information transmission, temperature sensing, mechanical damage sensing, and communication during maintenance work, in addition to sensing wires (polyethylene insulated copper wire) for the purpose of searching the buried location of the cable. See Figures 2 and 3and Photo 1.
2.2 Manufacturing of Cable
Furukawa Electric began manufacturing the cable in June
1996, and successfully completed the manufacturing inJune 1998 by jointing two lengths of the cable, each about
25 km long, at the factory.
The major manufacturing processes consisted of: manufacturing of conductor, insulation paper wrapping, drying,oil impregnation, lead sheathing, reinforcing by windingstainless steel tape, anti-corrosion layer sheathing, andsteel wire armoring accompanied by optical fiber units
integration.
In the manufacturing process, the cable core consistingof conductor with PPLP insulation was coiled in a large
vessel about 20 m in diameter for drying and oil impregnation. The vessel was evacuated of air to remove gases
and water vapor contained in the insulation layer, while
the cable core was heated to a controlled temperature bycurrent loading on the conductor and steaming on the vessel wall. 72 temperature zones were set up on the vessel
wall and each zone was automatically temperature controlled based on a computer simulation to effect optimum
heating, so that the entire length of such a long cable wasuniformly heated. The entire volume of the vessel was
filled with degasified insulation oil, then it was pressurized
to carry out oil impregnation. During drying and oil impregnation, the equilibrium vapor pressure and static capacitance in the cable were monitored so as to grasp accurately the drying and oil impregnation conditions of themulti-layered coil of the cable core. See Photo 2.
In the lead sheathing process, the cable core was introduced from the oil impregnation vessel to the lead-sheathing machine through a guide pipe filled with insulation oil,and lead sheath was applied. The lead sheath was verified to be flawless for its entire length using an ultrasonicflaw-detection apparatus.
In the armoring process, 12 optical fiber units and polyethylene insulated copper wires for buried location detec-Photo 2
Photo 1

124

Appearance of the submarine cable

Storing the cable core into a large vessel for drying
and oil impregnation

Furukawa Review, No. 19. 2000

tion were stranded under the steel wire armor layer. The
completed cable of about 50 km in length and about 5000
tons in weight was taken up on a large turn table 30 m indiameter and 6000 tons in capacity. In terms of opticalfiber, all fiber units were monitored all the time for theirtransmission loss change, thus confirming that they werefree of any performance change.
2.3 Factory Joint
Factory joint (FJ) was developed to meet the target specifications described below.
(a) To have almost equal electrical performance as thatfor the cable(b) To show no degradation against the mechanicalforces to be encounteredThus, the FJ was designed to have a structure as close
as that of the cable, so that the FJs were provided with thediameter and bending stiffness almost equivalent to that of
the cable. Figure 4 shows the structure of FJ and below
are given the procedures of manufacturing.
(1) Conductor jointing
Seven segments of the conductor were respectively
jointed by TIG (Tungsten Inert Gas) welding. In theconductor welding, the cable portions on both ends ofthe FJ were made to freeze so as to prevent flow outof insulation oil and gas permeation to the cable.
(2) Insulation layer
Insulation building-up was carried out in a dehumidified room, by winding insulation paper of the samequality, thickness, and width as for the cable insulation until the outer diameter became equal to that ofLongitudinal reinforcement
for lead sheath weld

Insulator steps

the cable.
(3) Lead sheathing
Single layer lead sheath was employed to make theouter diameter as close to that of the cable as possible and ultrasonic inspection was applied to confirm
the soundness of sheath welds.
(4) Reinforcement layer
Reinforcement was applied in the longitudinal andlateral directions so as to relieve the stress on thelead sheath welds.
(5) Anti-corrosive layer
PE tape molding was used to finish the FJ, making
the difference between the diameters of cable and FJ
as small as possible.

3.

CABLE LAYING WORK

3.1 Outline of Cable Laying Work3.1.1 Laying and Burying Method in the Strait
In the cable laying work in the strait, the continuous layingand burying method was employed as shown in Figure 5,
in which the cable was continuously laid and buried as it
was paid off. The position of the buried cable was simultaneously monitored throughout the laying process, using
GPS (Global Positioning System) and sonar.
It was estimated to take about two weeks to lay thecable across the strait, if a conventional burying machineof water jet type were used to bury the cable to a depth of2~3 meters. Accordingly, the authors jointly developed awater jet assisted plow type burying machine, planning tocross the strait in two days including nights. See Photo 3and Table 2.

PE mold

Single layerJoint of segmented
lead sheath
conductor
Back-winding
Reinforcement layerof insulator

Figure 4

Structure of factory joint

Turn table

Photo 3
Table 2

Guide pipe and cable

Cable laying ship

Water jet assisted plow type burying machine

Outline of specifications for water jet assisted plow
type burying machine
Item

Excavating method

Specifications
Water jet assisted plow type excavation
Number of plows: 6

Water jet assisted plow typehigh speed burying machine

Figure 5Continuous laying and burying method

Excavation depth

0~3.3 m (adjustable)

Dimensions

Length: 13.5 m, Width: 8.2 m, Height: 7.1 m

Weight

105 tons in air; 31~87 tons in water125

A cable laying ship named Giulio Verne, which belongs
to Pirelli in Italy, was employed. The ship was equippedwith a turntable that enabled loading and laying of such along and heavy cable of 50 km in length and 5000 tons inweight and was capable of towing the newly developedburying machine.
3.1.2 Laying and Burying Method in Tachibana Bay
On the Tokushima side, it was impossible for the cablelaying ship to approach a shallow sea extending 5 km inTachibana Bay. The cable was transshipped, therefore,
from the cable laying ship to a barge at the entrance of thebay as shown in Figure 6, and was subsequently laid andburied continuously to a depth of 2~3 meters using theburying machine.
Tachibana Bay is dotted with five rocky spots in theseabed. A rock trencher shown in Photo 4 and Table 3
was used to trench the rocky spots prior to cable laying.
Subsequently, each length of the cable was laid in thetrench about 1 km in length using the burying machine
towed by the barge.
During the cable laying, in order to lead the burying
machine along the excavated trench, the position of thebarge and the burying machine was precisely measured
by laser light survey and the location of the trench aheadof the burying machine was monitored by sonar.
3.1.3 Laying and Burying Method near the Seacoast
Near the landing point off the seacoast of Wakayama, it
was impracticable to obtain a sea depth sufficient forcable laying by the cable laying ship. Therefore, post-lay
burying method using the burying machine was employed,and at both ends of the sector, the cable was buried manually by divers.
On the coast of Tokushima, manual burying was also
carried out at the transshipment point and at the landing
point where the burying machine was not usable. After thecable laying, sand was supplemented to the trench in therocky spots and the place of manual burying, and theseabed was restored by leveling.
The cable laying and burying work described above
gave satisfactory results, i.e., the cable burying depth satisfied the specification as shown in Figure 7; and the cablelaying accuracy with respect to the planned route was ±5m and ±1 m for the cable laying ship and the barge,respectively.

Sector where thebarge was used(about 5 km)

3.2 Work Schedule
Furukawa's cable was laid third according to the timetableshown in Table 4. The cable laying ship was made toberth in the base (Kobe harbor) after cable loading until
the beginning of laying preparation so as to avoid the risksof laying work during the typhoon season. Throughout thisperiod, however, we regularly checked and monitored thecables.

4.

COMMISSIONING TESTThe submarine cables laid during the period of April toOctober in 1998 were subsequently jointed with the land
cables; and in August 1999, a DC high-voltage test of -700
kV x 15 min (CIGRE recommendations) was carried out,
confirming no abnormalities in the insulation performance.

5.

CONCLUSION

In this world-class construction project of an optical fiber
composite 500-kV DC submarine cable system, long
lengths of the cable were manufactured spending about
two years, laid during the period of April to October in
1998, jointed with the land cables, and satisfactorily
passed a DC high-voltage test of -700 kV in August 1999,
confirming no abnormalities in the insulation performance
existed. The cable system is scheduled to be in service inJuly 2000, after undergoing the tests of electric power
grids linkage.

Photo 4

Table 3

Sector where the laying ship was used(about 42 km)

Outline of specifications for rock trencher

Item
Yura-town,
Wakayama

Rock trencher

Excavating method

Specifications
Drum cutter for seabed leveling
Chain cutter for trenching

Point oftransshipment

Submarine cable route

Anan-town,
Tokushima

Figure 6

126

Cable route across Kii Strait

Propulsion

Self-propulsion

Excavating depth

0~2.2 m (adjustable)

Excavating width0.65 m

Dimensions

Length: 11.5 m, Width: 5.5 m, Height: 3.4 m

Weight54 tons in air; 14~38 tons in water

Furukawa Review, No. 19. 2000

Tokushima duct
entrance

Transshipment
point

Prefectureboundary

Wakayama duct
entrance

3.5

Burying depth (m)

3.0
2.5
2.0
1.5
1.0
0.5
Laying sector for barge

Laying sector for cable laying ship

0.0
0

5,000

10,00015,00020,00025,00030,00035,00040,00045,00050,000

Cable length (m)Figure 7

Table 4

Cable laying depth record of the Furukawa's work

Record of laying work of Furukawa's cableContent of work

DateStart

July 3

Cable loading on board
Preparation for laying(Transshiment)

EndJuly 13

Start

October 3

End

October 5

Start

October 7

Laying by cable laying ship including landing
EndOctober 12

Start

October 6

Laying by barge including landingEnd

October 24

Start

October 25

EndOctober 28

Start

October 25

End

October 28

Post-lay burying on the Tokushima side

Post-lay burying on the Wakayama side

REFERENCES

1) Inoue et al.: Development of a 500-kV DC submarine OF cableand its accessories, Furukawa Electric Review No.100,
September 1997. (in Japanese)2) Fujimori et al.: Development of 500-kV DC PPLP-insulated OF
submarine cable, Trans. IEEJ, B, Vol.116, No.9, 1996. (inJapanese)
3) Inoue et al.: Installation of 500-kV DC PPLP-insulated oil-filledsubmarine cable, Jicable (1999)
4) Nakanishi et al.: Installation of submarine cable for Kii Strait,Proc. Energy Sec. IEEJ, No.329, 1999. (in Japanese)5) Koyama et al.: Development of submarine cable installation
method for rocky seabed of Tachibana Bay, Proc. Energy Sec.
IEEJ, No.330, 1999. (in Japanese)
6) Uno et al.: Practical use of continuous laying and burying
machine for submarine cable, Proc. Energy Sec. IEEJ, No.331,
1999. (in Japanese)
7) Inoue et al.: Development of a 500-kV DC submarine cable andits installation, Electric Review, October 1998. (in Japanese)
Manuscript received on December 13, 1999.

It is our hope that many new technologies that havebeen verified and put into practical use during the courseof this project contribute a great deal to the developmentof power transmission technology.
The successful accomplishment of this project is certainly brought about by the cooperation and guidance ofmany people concerned. In closing, we wish to renew ourdeep gratitude to them.

127