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Feature Article

Underwater Repair Method for Ocean Observatory Cables

By Dr. Takashi Yokobiki
Dr. Ryoichi Iwase
Principal Research Scientist
Japan Agency for Marine-Earth Science and Technology
Yokosuka, Japan

Hiroyasu Momma
Marine Technology Adviser
Nippon Marine Enterprises Ltd.
Yokosuka, Japan

Conceptual diagram of the equipment for the underwater cable repair method.
A submarine cabled observation system can provide wideband communications and electric power to deep-sea observatories, making it possible to carry out long-term, real-time monitoring with several kinds of sensors. Five cabled observation systems are presently operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Most of these observation systems use optical submarine telecommunication cables, which are used in intercontinental telecommunication systems. If a cable fails, the system can be repaired by the conventional method used in the submarine telecommunication cable system industry, which requires a cable ship to pull up a damaged cable and replace it with a spare one.

However, an observation system can take a long time to restart after a cable fault occurs because of the lengthy preparations required for the cable ship and spare cable. Unlike telecommunication systems, ocean observatories do not have this equipment at the ready. As a result, the observation system could be halted for a long time.

JAMSTEC has developed an underwater repair method to handle a damaged insulator on a submarine cable using an ROV instead of a cable ship and no spare cable. This method is designed to enable a more timely restart of the cabled system.

Shunt Fault Repair
The typical submarine cable laid on the deep seafloor consists of optical fibers, a tensile strength member, a copper tube as an electric power line and a polyethylene insulator. The cable has a single conductor, and the insulator covering the copper tube directory is the outermost part of the cable. When this cable is laid on hard seafloor, such as rock, the insulator is scraped and occasionally results in a shunt fault, which is when the cable insulation is damaged and the cable short-circuits from coming in contact with seawater. To use the ROV-based repair method, the inside of the copper tube must not be damaged. Therefore, when the insulator is repaired, all system functions will be restored.

The conventional repair method involves a cable ship equipped with cable tanks, sheaves and cable engines pulling up the damaged cable from the deep sea to the ship and replacing it with new cable. This method requires an identical spare cable to the one being replaced or at least one that has passed a qualification test. Submarine telecommunication cable system owners typically store several kinds of spare cable and have a cable ship standing by in case repair is needed. These preparations are part of maintaining the infrastructure in telecommunications, but they are excessive for a cabled observation system, where the cable length is considerably shorter.

JAMSTEC Cabled Observation System
JAMSTEC has been operating a 125-kilometer-long cabled observation system off Cape Muroto since 1997. The system has two ocean-bottom seismometers and two tsunami meter pressure gauges. At the end of the cable, the deep-sea observatory is equipped with a current meter, CTD sensor, TV camera, sub-bottom thermometer and acoustic Doppler current profiler (ADCP). The terrestrial station is powered by a constant current of 437 volts.

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Dr. Takashi Yokobiki is an engineer of the Earthquake and Tsunami Research Project for Disaster Prevention at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). He received his Ph.D. in mechanical engineering at Kyushu University.

Dr. Ryoichi Iwase is a principal research scientist at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). He is in charge of system operation and data management of cabled observatories. His expertise includes multidisciplinary observation and development of cabled observatories in the deep sea.

Hiroyasu Momma had worked on deep-ocean technology with JAMSTEC since 1972, and in 2009, he moved to Nippon Marine Enterprises Ltd. as an adviser for marine technology. He developed JAMSTEC's deep tow system and long-term deep-seafloor observatories. He also served as a leader in marine search operations of lost objects.

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