Mariana Trench Romance

Phase 3

Technological breakthrough

They overcame one problem after another, came up with a groundbreaking manufacturing method, and developed an Optcal Fiber unit with high water pressure resistance. However, a problem arose during the final test. They overcame days of trials and finally completed the project.

This was an extremely challenging development order, but based on the experience gained with the Dolphin-3K, the company achieved a number of breakthroughs. For example, they overcame one problem after another, such as establishing technology to prevent cable twisting and buckling, and introducing manufacturing know-how for cables with a breaking tension of 50 tons. In addition, because the cable was a single piece measuring 12,000m in length, Fujikura handled manufacturing across three factories. They made the most of their experience and know-how to get through the challenge.

In the development of the primary cable, we established an original tensile strength member processing technology. By utilizing the principle of pressurized steam, we devised a groundbreaking manufacturing method that can reduce the size of the rod system while maintaining ultra-strength. Meanwhile, in the development of the secondary cable, we completed a slot-type pressure-Optcal Fiber unit filled with soft resin to prevent Optcal Fiber from being crushed by water pressure.

After the successful delivery in March 1992, after thorough performance testing, the final test of "KAIKO" was carried out in the Mariana Trench on March 1, 1994, with the press on board. However, the evening news reported that the test was significantly delayed due to the cable, which caused a big uproar within Fujikura. It was a blackout of the optical system of the secondary cable. "KAIKO" was quickly pulled on board and examined, but no abnormality was found and the cause could not be identified. The secondary cable was designed to withstand the water pressure at great depths. Subsequent dismantling and investigation revealed that Optcal Fiber had been compressed in the longitudinal direction, causing microbending, a type of local buckling. The optical loss had increased sharply. To investigate the cause, a high water pressure condition of 1,500 kgf/cm2 was created in a high water pressure test tank, and the cable sample was subjected to continuous bending and twisting, or a combination of these behaviors, and the loss level of Optcal Fiber was measured. The investigation and confirmation test was carried out without sleep for about three months.

As a countermeasure, a method was introduced in which the tensile body layers were filled with jelly so that the movement of the tensile body would not be restricted even under high water pressure. Retests were conducted with this newly designed cable one year after the problem occurred.