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Emerging Subsea Networks

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Lessons Learned from Past Cases for Achieving Disaster Resilient Routing and Economically Viable Maintenance Operation

Yukitoshi Ogasawara, Takashi Kaida, Kenji Goto (Kokusai Cable Ship Co., Ltd.)

Email: [email protected] Kokusai Cable Ship Co., Ltd. 6th Floor, Kawasaki Frontier Bldg., 11-2, Ekimae Hon-cho

Kawasaki-ku, Kawasaki, Kanagawa 210-0007 Japan Abstract: This paper will look into and analyze cable faults occurred near Japan over the past decade and describe effective countermeasures for realizing disaster resilient routing as well as economically viable maintenance operation by identifying the trends of cause geographically. Finally, above lessons will contribute to the development of the submarine cable industry. Key words: Submarine cable, Cable fault, Natural hazard, Human activity and Resilient network 1. BACKGROUND

As explosive growth of the Internet and capacity demand on international communication traffic have been continuing along with information-oriented society, submarine cable systems are significantly important infrastructure where the state-of-the-art transmission technology deeply involves. Also, submarine cabled ocean seismic observatory systems have been deployed for the purpose of reducing damage from natural disasters. However, these submarine cables are always exposed to the risk of being damaged by earthquake/tsunami, fishing and exploitation of marine resources, etc. as those activities and phenomena always interact with submarine cables. 2. FOCASED AREA AND PERIOD

Cable faults occurred at continental shelf

near Japan, Korea and China over the past

decade have been studied based on our

records and statistical data.

3. ANALYSIS OF FAULT DATA

3.1 Fault trend

Figure 1 to 5 show fault histories and trends by factors of water depth, cable type and season, etc. Figure 6 shows geographical locations of faults.

Figure 1 Fault by Water Depth

Figure2 Fault by Cable Type

0

5

10

15

20

25

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

0-100m 101-1,000m

Deeper than 1,000m

0

5

10

15

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30

35

2005200620072008200920102011201220132014

Double Armored Single Armored

Light Weight

Nu

mb

er of faults

Nu

mb

er of faults



Figure 3 Fault by Type

Figure 4 Fault by Burial Status

Figure 5 Fault by month 1992-2014

Figure 6 Distribution of Faults 2012-2014

The following results were derived from those statistical data and map. <Trend of Faults> The number of faults has been increasing year by year, whereas the number of faults in summer has decreased in each of the past 20 years. Typical fault can be categorized as follows;

a. Water Depth: Less than 100 m b. Cable Type: Single Armor c. Fault Type: Short after year 2013,

(Shunt before year of 2013) d. Burial Status: Buried e. Location: East China Sea

According to the data, there is no doubt that fishing is a primary cause of cable fault in this region. The decreasing number of faults especially in summer indicates this fact as it corresponds to the close season for fishing in East China Sea. 3.2 Fault history and trend around the globe Figure 7 shows a categorization data of the cause of cable faults for 10 years around the globe.

0

5

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25

Short Shunt Fiber Open

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30

Buried Not buried

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70

Jan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.

Month

Legend

+:2014

+:2013

+:2012

Pacific Ocean

JAPAN

Sea of Japan

China

East China Sea

Taiwan

Korea

Nu

mb

er of faults

Nu

mb

er of faults

Nu

mb

er of faults



Figure7 Worldwide Cable Faults by Cause 2001 - 2010 [1]

Up until 2007, most of the faults were caused by fishing gears but within the same year the cause has been changed to anchoring. This is a noteworthy feature we should focus on.

4. STUDY FOR NATURAL DISASTER AND HUMAN ACTIVITY

The fiber optic submarine cable system has

adopted modern technology consisting of

cables with armor wires against harmful

aggression, however it is not perfect. 4.1 Natural disaster Natural disasters such as earthquake and tsunami and turbidity current can jeopardize submarine cable. 4.1.1 Earthquake Figure 8 shows observation points of earthquake registered 4 or greater on the Japanese seismic scale between 1998 and 2003. In all, a total of 203,186 earthquakes are marked on the map. Brighter ring surrounds the Pacific Ocean is called the "Ring of Fire".

Figure 8 Frequency of the Earthquake Occurrence [2] Japan is located on the western edge of

Pacific plate. Japan and other islands in the

region are geographically exposed to risk

of seismic disaster as they are in marginal

areas of the North American Plate, the

Philippine Sea Plate and the Eurasian

Plate. Japan was hit by a massive earthquake of moment magnitude (Mw) 9.0 on 11th March, 2011, which records the largest in the history of seismic observation in Japan. Figure 9 shows distribution of seismic intensity at 05:46 UTC on the day of “The Great East Japan Earthquake”. The earthquake triggered tsunami that led to the catastrophic damages in the quake-hit areas. Several cable faults have occurred one after another due to this powerful earthquake. The mechanism for these cable faults has been analyzed by earth scientists in the world.

Per

cen

tag

e o

f to

tal f

aults

50 %

20 %

10 %

40 %

0 %

30 %

Anchor: Fishing: Geological: Other:

2001 2005 2009 20074

2003 2010



Figure 9 Distribution of seismic intensity at 05:46 UTC 11 March 2011(Sources: Japan Meteorological Agency)

Figure 10 is a map showing where cable faults occurred by the Great East Japan Earthquake.

Figure 10 Fault locations by the Great East Japan Earthquake

4.1.2 Tsunami Tsunami is generated by submarine

earthquake sometimes. The Great East Japan Earthquake on 11th March, 2011 generated a powerful tsunami which immediately led to a great loss of infrastructure and assets including damages to cable landing stations.

Figure 11 shows maximum height of tsunami observed.

Figure 11 Distribution of maximum height of tsunami viewed from the Pacific Ocean side, issued at 23:00 UTC on 12th March, 2011(Sources: Japan Meteorological Agency) The data traced by later investigations are not included. 4.1.3 Turbidity current It is known that turbidity current, an underwater avalanche, is commonly triggered by landslide after an earthquake. It is caused by the slumping of sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons. Figure 12 shows several submarine cable faults caused by turbidity current after submarine landslide off SW of Taiwan.

Epicenter

+F

Tokyo ●

300 km 100 km

Fukushima Daiichi Nuclear Power Plant

Fault Area

He

igh

t (m)

Observed tsunami height at principal locations

Pacific Ocean

1

2

3

4

5



Figure 12 Epicentres on 26 December 2006 ( ) and submarine cable breaks ( ) off SW of Taiwan [3]

4.2 Human activity Submarine cable systems are exposed to the risk of damage caused by human activities as well such as anchoring, fishing and exploitation for natural resources, etc. 4.2.1 Anchoring The large vessels such as cargo vessels and tank vessels normally anchor themselves at designated anchorage area near port due to port congestion before entering into port or sheltering from adverse weather. It was mainly fishing activities which caused cable faults in the past until 2007 but anchoring has become a major cause of cable faults since then as shown in Figure 7. 4.2.2 Fishing activity Fertile fishing grounds are located off China because shallow water areas of the continental shelf spread to the eastward and big rivers run into East China Sea. Nutrient salts such as nitrogen, phosphorus and silicate are supplied from the river without a break and phytoplankton species

grow there. Animal plankton eat phytoplankton and fish eats animal plankton. Therefore richer fishing grounds are formed by this ecological food chain off China. Photograph 1 is the recovered anchor with the damaged cable tangled around it during cable repair work.

Photograph 1 Recovered fishing anchor

4.2.3 Exploitation of marine resources Exploitation of marine resources have been developed around Japan and East China Sea. The oil and gas industry has been trying to see possibility of commercial use of methane hydrate resources. The methane hydrate resources was confirmed on the seabed in the Japan territorial waters and R&D program has been carried out under the government-supported research. It is something submarine cable industry needs to consider how submarine cable can co-exist with exploitation of marine resources when it comes to cable safety issue.

5. CONCLUSION

Following countermeasures should be taken in order to reduce threat from those possible risks to submarine cable systems based on the above mentioned analysis (Refer to Clause No.4). 5.1 For natural disaster a. Distance of landing location. The redundant landing location should be established and the distance between two landing locations should be kept about 500



km in the case we experienced and learned by the Great East Japan Earthquake because simultaneous ruptures at borders of multiple plates caused destructive shaking which extended to over 400 km. In order to mitigate risks associated with this type of earthquake, this sort of countermeasures should be considered. b. Route diversity Route diversity should be considered as continuous services are indispensable and this can be only realized to divert the services to other cables in the network. c. Cable route selection From a topographical standpoint, the seabed canyon or alluvial fan should be avoided as a laying route since turbidity current is often observed in these areas. Shorter route is commonly selected from an economical perspective, however several routes for the existing cable systems run through the similar geographical routes which can be very difficult at the time of cable repair as there are many cable crossings and not enough distance between the cables may not be kept. d. Multiple landing points for system configuration System should be configured with multiple

points which enables for mutual back-up

with terrestrial link. e. Tsunami Landing station is generally located at lower altitude on the coast. However we learned from the Great East Japan Earthquake that it should be built high above sea level. Consequences of disaster in the past and construction cost to build the station at a higher location should be considered at the same time. Betterment of a cable station building such as watertight construction and reinforcement

architecture against powerful tsunami could be alternative countermeasures. f. Bottom slack for laying cable Optimized bottom cable slack should be kept during installation and repair works, especially at steep slope or rocky area where cable can be worn out or damaged quite easily. 5.2 Human activity a. Laying route Preferably cable route should avoid areas where fishing, anchoring end exploitation of marine resources are highly active. Illegal fishing activities are observed very often at EEZ boundary or some other fishing zones. It should be avoided as those areas require political settlement. b. Cable protection Cable should be protected by articulated pipes or by burial after cable is laid. Burial depth is determined based on the result of various activities which caused cable faults and 3 m burial is one of effective countermeasures. c. Cable maintenance system Having dedicated maintenance ship stand-by for each cable system. Ideally, more than two ships should be arranged to ensure the case of multiple cable faults at the same time. d. Be informative Share the cable route information with relevant organizations and fishery unions, etc. e. Protection cable by government [4] Governments of Australia and New Zealand have set out "the cable protection zone" to protect cable as important component of national communications infrastructure.



The submarine cable systems are also patrolled by governmental maritime safety agency within their EEZs.

f. Electronic Chart Display and Information System, ECDIS

Almost all vessels except small ones are required to equip ECDIS under the law. ECDIS can display submarine cable routes on the screen as well. It can be very useful to avoid cable damage by anchoring if the system have an alarm function when a vessel gets closer to a submarine cable route.

6. POSTSCRIPT

Upon completion of cable laying,

submarine cable system faces to severe

environmental conditions such as high

water pressure, corrosion, natural disaster

and human activities which will continue

until the end of system life, generally up to

25 years. Submarine cable system has been

playing a significant role all over the world

to support our daily life, however most

people are not conscious of its existence

and contributions. If serious damage would

occur to a submarine cable network, it will

impact communications such as the

Internet, electronic commerce, financial

dealing and money remittance in a

tremendous way. Recent engineer who

involved in submarine cable system should

make strong plan for the effective

countermeasure against those risks and

threats to the safety of submarine cable

network as well as terrorism. We learned the lessons through repairs of those damages/faults in cable systems in the region to be more careful in route studying and engineering, designing redundant cable configuration and so forth in order to realize resilient submarine network. Also, it is suggested that certain investment should be allowed to minimize

a chance of losing money for repairs and circuit restoration. Optimized and effective countermeasures may contribute the development of the submarine cable industry as well.

7. REFERENCES

[1] Power Point Presentation and Papers presented at the 2011 Workshop National University of Singapore Session-1-Ian-Douglas-Stephen-Holden-GMSL [2] http://blogs.smithsonianmag.com/smartnews/files/2012/07/07_02_2012_earthquake-history1.jpg [3] Records of submarine natural hazards off SW Taiwan Chih-Chieh Su, Jing-Yi Tseng, Ho-Han Hsu, et al Geological Society, London, Special Publications 2012; v.361; pp.41-60 doi:10.1144/SP361.5 [4] Guide - Declaring a submarine cable

protection zone NOVEMBER 2015 Australian Communications and Media

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