Sea Technology

DEC 2015

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www.sea-technology.com December 2015 / st 43 T he Japan Agency for Marine-Earth Science and Tech- nology (JAMSTEC) in Yokosuka, Japan, is developing a dense ocean foor network system for earthquakes and tsu- namis (DONET) for disaster prevention and mitigation. An automated cable-laying system is being successfully used to install a thin optical-fber submarine cable that connects an observatory with a science node. The automated cable- laying system is operated by an ROV and can automatically payout the optical-fber submarine cable on the deep sea- foor at a rate that keeps pace with the ground speed. More- over, the tautness of the cable can be adjusted. Thus far, approximately 185.36 km of cables have been laid on the seafoor, and 21 observatories—including the borehole observatory C0002—have been connected by the automated cable-laying system. Cable-laying operations are usually performed by a cable ship. However, the cable for connecting the observatory with the science node, or the extension cable, must be installed by an ROV. This is be- cause it is thin, light, and prone to breaking, making it very diffcult to be laid on the deep seafoor by a cable ship. In contrast, an ROV can lay the cable at a short distance from the seafoor and deploy the cable along the planned route accurately and safely. Thin Optical-Fiber Submarine Cable The extension cable was customized for DONET and manufactured by the Ocean Cable & Commu- nications (OCC) Corp. in Yokohama, Japan. It has two single-mode optical fbers for communication and eight copper conductors for the power supply. The outside diameter is 6.15 ± 0.15 mm, the weight is less than 55 kg per km in air and less than 23 kg per km in water, the breaking strength is more than 980 N, and the depth rating is 5,000 m. The distance between the observatory and the science node is 6 to 9.5 km, and the initial amount of cable on the cable bobbin is 10.5 km. The full weight of the cable bobbin with the 10.5-km cable is approximately 660 and 227 kg in air and water, respectively. The cable was manufac- tured to be as thin and light as possible for loading the 10.5-km cable to the ROV and falling to the seafoor at a suitable speed. DONET DONET comprises landing stations, repeaters, branch- ing units, terminal units, science nodes, and observatories. The science nodes and the observatories, including the ex- tension cables, are deployed by an ROV, and the others— except the landing stations—are deployed with backbone cables by a cable ship. DONET1, which includes 20 ob- servatories, was installed at Kumanonada, Japan, July 2011 to monitor the seismogenic zone around the Nankai Trough, and its seis- mic and tsunami data are currently provid- ed to the Japan Me- teorological Agency and the National Automated Cable- Laying System ROV-Based Installation of DONET2 Ocean Floor Observatory By Dr. Jin-Kyu Choi • Dr. Takashi Yokobiki • Dr. Katsuyoshi Kawaguchi The automated cable-lay- ing system automatically installs a thin optical-fber submarine cable on the deep seafoor at a rate that keeps pace with the ROV's ground speed.

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