Completion of Submarine Cable Lines Combining
Low Environmental Impact with Low Cost
by Munehiro Furugen *, Yukio Noha *, Hideya Maekado *, Chihiro Yokoyama *2,
Yukio Koshiishi *2, Shigeru Ebashi *2, Kunio Iwasaki *3, Takashi Kijima *3,
Takumi Yamaguchi *4 and Hiroshi Kaizu *4

In a program to provide submarine cable lines having low environmental impact,
Furukawa Electric has designed and commenced construction of the HisamatsuIrabu Line for Okinawa Electric Power (OEPCO) in May, 2000, and of the Niijima-ShikinejimaLine for Tokyo Electric Power (TEPCO) in June of the same year. In the two projects a need for
reduced construction cost was combined with a requirement for methods that did not damage
coral reefs and submerged rocks used for the farming of "ise-ebi" crayfish. This entailed taking
advantage of Furukawa Electric's comprehensive capabilities, from cable manufacture to cablelaying. This total design capacity, covering everything from cable prototyping and design throughcable laying design has made possible the construction of submarine cables that have minimal
environmental impact.

ABSTRACT

1.

22-kV COMPOUND ARMORED SUBMARINE CABLE LINKING MIYAKOJIMAAND IRABUJIMA

1.1 Introduction
In May, 2000 a 22-kV submarine cable was laid in
Okinawa Prefecture, connecting Hisamatsu on the islandof Miyakojima with the island of Irabujima. The cable wasof a newly developed type designated WCLWWA 3×100
mm2 submarine cable (cross-linked polyethylene insulated
cable with double armor and lead jacket for water exclusion), and the compound armor consisted of high-density
polyethylene-sheathed FRP for the first layer and galvanized steel wire for the second layer. This newly developed cable offers improved long-term reliability--something that posed a serious problem in the submarine
cables now in use--by providing the conductors with alead water-exclusion layer, and employs a non-metallic
material--fiberglass-reinforced plastic (FRP) serving-thereby protecting the sheath against electrical corrosion.
The development work on this submarine cable was carried out jointly with OEPCO.

1.2 Problems with Cables Now in Use
1.2.1 Results of a Survey of Installed Cables
At present, OEPCO operates 29 submarine cable lines
with a total length of 143.7 km. The lines lie in the Pacific
Ocean and South China Sea subject to swift-flowing currents, and nearly all the areas where depths are less that
30 meters constitute the habitat of protected corals. Thus
with few exceptions it is impossible to bury the cables.
Since submarine cables are frequently laid on this sortof route, damage to steel armor wire is particularly severe,and under certain circumstances damage could evenextend to the insulation, reducing the service life of con-*

The Okinawa Electric Power Co., Inc.
Tokyo Electric Power Co.
*3 Technology Development Dept., Electric Wire andEquipment Div.
*4 Ocean Project Team, Electric Power Engineering Dept.
*2

Photo 1

Submarine cable laid 28 years ago.

44

ventional submarine cables to the neighborhood of 20
years. Photo 1 shows the condition of a typical cable. This
cable was laid 28 years ago, and there are places where
the steel armor wire has frayed and parted so that onroutes where the cable is lying on the seabed there is achance that insulation faults could occur. And since thefrayed ends of the steel armor wires are needle-sharp,
published studies have long shown that the problem is not
merely that of mechanical friction wear but of electricalcorrosion.
Thus in an effort to improve the long-term reliability ofsubmarine cables, a field survey was conducted to determine the extent of damage to the armor wire and findmeans of upgrading cable structure.
The field survey made use of data from regular inspections conducted every five years by OEPCO. Typicalresults are shown in Table 1.
By means of this survey it was confirmed that damage
to the plastic yarn serving was sustained about 10 years
after the cable was laid, after which corrosion of the steelarmor wire began, and after a further five years breakageof armor wires began to occur. This is attributed to the factthat the functioning of the armor, which has been protected against external damage, deteriorates after about 10
years, making it possible for the cable insulation to bedamaged during the dropping of anchors, or the heavyseas caused by typhoons, etc.
1.2.2 Damage to Steel Armor WireAs was noted above, damage to the steel armor wire is
attributed to electrical corrosion. Reference to the existing
literature 1) suggests the following causes of electrical corrosion:

Table 1

Damage to steel armor wires of cables installed in
Okinawa.
Hisamatsu-

Bisezaki-

Ishigaki-Irabu #2

Ie #2

Taketomi #2

27

19

16

Length

5.0 km

6.8 km

4.6 kmMax. water depth

14 m

92 m

33 m

Tidal current speed

1.5 knots

1.6 knots

0.6 knots

Major damage

Several steel

Steel wires

to steel wire

wires damaged

exposedYears in service

Condition of armor

V
A
C

B

E
B

A
D

Electrical current flows in seawaterfrom B to A; flows into armor between
A and C; flows out from armor
between D and B (electricalcorrosion)

Figure 1

45

B
Electricalcurrent ICI
D

EMF generated by perpendicularcomponent of earth's magnetic field
and tidal flows

Schema of electrical corrosion of steel wire armor
due to tidal flow EMF.

(1) EMF generated by tidal flows: This is due to the
EMF generated by the earth's magnetic field and tidal
flows and electrical current flows into and out of thearmor wire near points where there are changes inflow velocity.
(2) Concentration cell effect: When the cable passes
through areas of differing seabed formation or temperature, a current flows between two points on thearmor wire that have differing potentials(3) Differential aeration cell effect: This is an effectby which portions with an insufficient supply of oxygenact as an anode, so that parts of the armor wire adjacent or in contact are comparatively prone to corrosion.
(4) Leakage current from DC railway lines on land:
Leakage current from electrified railway lines flows
out of and into submarine cables.
(5) Ground current: Stray current flows out of andinto the armor wire causing electrical corrosion.
Of these, it is considered that the main cause of theelectrical corrosion of the armor wire of submarine cables
is EMF generated by the earth's magnetic field and tidal
flows. Note that abrasion against stones on the seabedand physical abrasion due to the severe motion of gravelin seawater caused by wave action are thought to be factors in the localized occurrence of natural corrosion due todamage to the plastic yarn serving, electrical corrosion
and localized cell action, these are probably necessary
conditions for damage due to electrical corrosion.
To summarize what has been stated above, the causeof damage to the steel armor wire surveyed was a combination of electrical corrosion, natural corrosion and physical abrasion.
Designing Submarine Cables with a View to Long-Term
Reliability
Based on the results of the survey of cables now in service, we considered how to design a submarine cable suited to the conditions in which it would be laid.
Following are the items that were considered to assure
the long-term reliability of submarine cables, together withthe conclusions arrived at.
(1) Power cable (insulation): To attain long-term reliability on the seabed, we decided on a lead-jacket
water-exclusion structure that would make possible acompletely waterproof structure for the power cable.
Naturally the interfaces between the core and theinsulation shield and the lead jacket were filled with awatertight compound, forming a watertight structurethat would, insofar as possible, prevent the entry of
seawater into the power conductor, even if it was parted by damage due to the dropping of anchors or othercause.
(2) Armor: electrical corrosion has been identified as
the cause of damage to the armor, which should
therefore be made of a material in which electricalcorrosion does not occur. This makes it necessary to
use insulation made of a non-metallic material to completely eliminate electrical corrosion. The performance
demanded of the armor, however, requires a material

Furukawa Review, No. 21

2002

Table 2

Cable construction.

Item

Communi-Power cablecation line

100
Conductor cross section (mm2)(watertight over

3P×2

total length)
Conductor OD (mm)

12.0

1.8

Insulation thickness (mm)

7.0

0.8

Insulation OD (mm)

26.0

—

1

—

1.6

—

—

2.0

Semiconductive tape thickness (mm)0.5—Core OD (mm)

32.3

—

Insulation shield thickness(approx., mm)
Lead jacket for water-exclusion
layer thickness (mm)Sheath thickness (mm)Braiding OD (mm)70

Bedding thickness (mm)2.0High-density PE clad FRP OD (mm)6.5

Bedding thickness (mm)2.0Galvanized steel wire armor OD (mm)8.0

Plastic yarn serving anti-corrosion
layer thickness (mm)Figure 2

Cable cross-section.

5

Photo 2Finished OD (approx., mm)

119Approx. weight (kg/km)

30390

that has high strength, and suffers little damage fromseawater over the long term, and when account istaken of the record of long-term service established in
the field, the choice is naturally limited.
The results of this study suggested FRP as a material
that had a long record of service in seawater. FRP is amaterial that satisfies these conditions, and it has a recordof use as the armor material of Furukawa Electric's submarine cables 2) for some 18 years. FRP can, however,
have a wide range of constituents, each with different
properties, and it goes without saying that before it can be
put to use its properties must be subjected to the fullest
evaluation.
1.3

Watertight conductor
Conductor shield
Cross-linked polyethylene insulation
Insulation shield
Lead jacket
Polyethylene yarn serving anti-corrosion layer
Plastic filler
Binder tape with corporate name and date
Plastic yarn bedding
FRP coated with high-density polyethylene
Plastic yarn bedding
1Galvanized steel wire armor
Outer plastic yarn bedding

Construction of the Newly-Developed Submarine
Cable, and Test Results
Table 2 shows the construction of the 22 kV 3×100 mm2
WCLWWA (1st layer FRP, 2nd layer steel wire armor).
There is no particular problem in using FRP wire in the
manufacturing process, and experience has shown that it
can be handled in the same way as steel wire. For FRP,
however, splicing technology is crucial in that, dependingon the formation of the splice, the drop in tensile strengthof the FRP wire may be greater than desired. The splicing
method developed for use in this work involves crimpingof a stainless steel sleeve, and is extremely convenient.

Cable landing on Miyakojima.

1.4 Environment-Friendly Cable Laying Method
The cable developed in this work was laid between
Hisamatsu on Miyakojima and the island of Irabujima--adistance of about 6,500 m. The method used to lay thecable was characterized by environment-friendly and protective measures designed to prevent, insofar as possible,
any damage to the coral growing along the entire route.
The cable laying process started from landing on the
Miyakojima end, and then proceeded toward Irabujima.
The landing point on the Miyakojima end was a pylon, andon the Irabujima end a cable house. Laying was carriedout by an anchoring technique that made possible precise
navigation of the cable-laying barge. In this method, as
can be seen in Figure 4, an anchoring wire was first
deployed and then taken up by a winch mounted on thecable-laying barge, moving it forward and enabling thecable to be laid precisely along the pre-determined route.
The water along this route was comparatively shallow,
raising fears that as typhoons approached, wave action
would shift the cable over the seabed. This posed thedanger of damage not only to the cable but also to thecoral reefs in the vicinity, and made it necessary to lay thecable as directly as possible on the seabed to avoid, asmuch as possible, the bridging over declivities of theseabed that could result.
The cable laying process was guided over the entire
length of the route by divers, so that laying in the gaps or
natural troughs between the coral reefs was precise andbridging did not occur. This was a level of precision without parallel anywhere in the world, and it is thought thatthe elimination of cable bridging will be an effective means

46

-0.3

Cable laying speed(4 m/min)

Pilot wire
(18 mm)Shallower than 5 m

6,377 m5,725 mFigure 3

Route

-5 m

Dry at low tide

Port of Hirara
Channel

2,700 m-5 m

5/3

2,400 mFlat coral reefs and sand (exclusive of shore and land)

D.L0 mNets for "mozuku"(seaweed)
Dry atPilot wire (18 mm)
low tide
Nets for "mozuku"
No. 1(seaweed)turning point

Dry at low tide

Shallower than 5 m-5 m

Route of submarine cable from Hisamatsu on Miyakojima to Irabujima.

3,325 mShallower
than 5 m-0.4Work boat
at 1640-m point

5/4

Dry at low tideWork boat

Dry at low tide

Rock reef with extremely uneven profile

No. 2 turning
point

5/5
Work boat moored at anchor
Shallower than 5 m

652 m

-9.5

-2

Rock reefs

Port of Nagayama

Irabujima

1.3 knots
1.2 knots

47

Furukawa Review, No. 21

2002

0m

Hisamatsu
fishery port

Channel closed

Miyakojima

NGuard boat

Guard boat
TugboatTidal flow

Boat passing-by

Diver boat
Forward anchor(Position of barge moored atanchor when laying starts)

Supportboat

Supportboat

Guard boat

Guard boat
Forward direction
Guard boat

Guard boat

Laid cable
Diver (inspecting cable)

Figure 4

Cable laying by an anchoring method with diver support.

of extending cable life. The use of compound armoring forthe cable and the consequent reduction in stiffness isadvantageous in keeping the cable directly on the seabed.
1.5 Conclusion
In this work, with the objective of extending cable service
life, a 22-kV submarine cable was developed having acompound armor (first armor layer: high-density polyethylene-sheathed FRP + galvanized steel wire), and it waslaid using environment-friendly practices.
As a measure to counter electrical corrosion--a major
problem for submarine cables--nonmetallic (FRP) wires
were used in a portion of the armor, making it possible toimprove the long-term reliability of the cable. The adoption
for the armor of a compound structure using steel wire
increased its mass when under water, thus eliminating themechanical damage caused to coral cable by cable movement due to wave action along routes where the cable isnot buried, and also damage to the cable itself.

2.

Forward anchor

6.6-kV SUBMARINE CABLE LINKINGNIIJIMA AND SHIKINEJIMA

2.1 Introduction
In June, 2000 a 6.6-kV WCLWA 3 × 60 mm2 steel wire
armored submarine cable (with single-layer armor, withcross-linked polyethylene insulation and lead jacket forwater exclusion) was laid between the islands of Niijima
and Shikinejima, which are part of Tokyo. The project wasundertaken to satisfy increasing power demand on
Shikinejima. Based on studies of cables now in service,
Furukawa Electric was able to set forth clearly the cable
construction, route, cable laying process and the functions
required of the protective construction, along with proposed savings in overall costs, and we achieved a reduction in total construction cost.

2.2 General Description of ProjectThis project involved the laying of a submarine cable overa distance of approximately 4,242 m between Niijima andShikinejima. On the cable route there are mounds of sandin mid-channel, but in the areas close to the two shores
there are scattered rocks--not a desirable location for thelaying of a submarine cable. Accordingly cables now inservice have two-layer steel-wire armor, and in addition
are provided with a protective pipe 3).
2.3 Design of Cable Laying Work
This expansion project was subject to the following
requirement:
1) Lower construction cost,2) High line reliability,
3) Minimization of environmental impact.
With specific reference to 3), the areas of submerged
rocks near the landing shore were used for the farming of"ise-ebi" (crayfish), leading to strong representations
being made by those in the local fishery industry.
Generally speaking, cables being laid over submerge
rocks use the same specification for two-layer steel-wirearmor adopted for cables now in service. When there isalso a danger of damage due to boulders, etc. it is common to assure cable safety by the use of a cast iron protective pipe, or by crushing and removing the rock.
In the present case, however, it was decided, based on
the study of cables now in service, that we would not bebound by the safety-first design concept grounded in pastexperience, but would select a cable with single-layer
steel-wire armor.
Preparatory studies were carried out with great thoroughness, and in an effort to select a route where thecable would be naturally buried by tide-induced movementof the sand, we chose a meandering route that avoided
submerged rocks, thereby cutting down the number ofprotective steel pipes.

48139°20'

Cape Nebu

NiijimaExisting route

Jinaishima

Niijimalighthouse

34°20'

Shikinejima

Figure 5139°20'Additional route

Route of submarine cable from Niijima to
Shikinejima.

Table 3

Cable construction.

Item

Photo 3

Power cable

Conductor cross section (mm 2 )60

Conductor OD (mm)9.3

Insulation thickness (mm)5.0Insulation OD (mm)19.3

Insulation shield thickness (approx., mm)

0.6

Lead jacket for water exclusion layer
thickness (mm)—Semiconductive tape thickness (mm)0.25Core OD (mm)—Braiding OD (mm)58

Bedding thickness (mm)2.0Galvanized steel wire armor OD (mm)8.0

thickness (mm)Finished OD (approx., mm)
Approx. weight (kg/km)

3.5
81
17590

To assure that the cable was laid precisely along the
proposed route, an anchoring technique like that used on
the Miyakojima-Irabujima route was adopted, with guidance by divers provided throughout. This route guidance
also led to a reevaluation of the structure of the armor,
and the resulting adoption of single-layer steel-wire armor,
was extremely effective, decreasing the rigidity of thecable.
As a result of these measures, it was possible to reduce
the use of protective pipe and the crushing and removal ofsubmerged rock to the minimum levels possible, realizinga saving in total construction cost of approximately 5%.
The reduction in the crushing of submerged rock also
made it possible to reduce the impact on marine life tominimum levels.

49

Watertight conductor
Conductor shield
Insulation
Insulation shield
Semiconductive compound
Lead jacket
Binder tape
FillerBinder tape
Plastic yarn bedding
Galvanized steel wire armor
Outer plastic yarn bedding(anti-corrosion layer)

1.5

Sheath thickness (mm)Plastic yarn serving anti-corrosion layer

Cable landing on Niijima.

Figure 6

Cable cross-section.

2.4 Conclusion
In the project discussed here an optimal cable design wasdeveloped based on the experience of cables now in service, and reliability was achieved by the cable-laying
design. In the future Furukawa Electric intends to makefull use of its comprehensive capability in the design andlaying of cables on a variety of submarine routes, grounded on a track record of laying more than 1.5 million kilometers of cable and the experience gained in the projectdescribed here, in proposing cable-laying techniques thatare both cost effective and low in environment impact.

REFERENCES
1) Ezoe et al.: On electrochemical corrosion of submarine cables,
Journal of IEEJ, June 1956 (in Japanese)
2) Yasuzato et al.: Development of FRP cross-winding, double
armored submarine cable, Furukawa Electric Review, No. 75,
March 1985 (in Japanese)
3) Nishimura et al.: Development of optical fiber composite, electric
power submarine cable, Furukawa Electric Review, No. 79,
December 1986 (in Japanese)

Manuscript received on October 22, 2001.

Furukawa Review, No. 21

2002