CDEX Web Magazine 地球発見


Adapting to the Kuroshio Current

 In mid-November, CHIKYU completed the retrieval and installation of the temporary observatories at Site C0010, after which CHIKYU made a 24-hour port call in Shimizu, leaving on November 27 for Site C0002. Finally, work to install the long-term observatory began. "The instrument package is installed about 800 m below the seafloor (mbsf), the deepest point in the borehole which is about 1,000 mbsf. To do this, the package is attached to the end of a series of steel pipes called tubing; therefore, the package is suspended from the wellhead down through the borehole casing pipes into the seafloor. The inner diameter of the borehole casing is approximately 21 cm, and the tubing is about 9 cm in diameter. Electrical power and data cables, and hydraulic tubing are all run down along the outside of the 3.5 inch tubing. These cables and tubing are very delicate; any contact with the borehole wall has the potential to sever or damage them, therefore great care was taken with attaching and running the entire system down into the water and the borehole. Additionally, the accretionary prism strata are unstable, and safely installing the monitoring system into the well is demanding work by itself", says Dr. Eiichiro Araki, one Co-Chief Scientist of IODP Expedition 332. Furthermore, the Kuroshio Current, a strong western boundary current that flows through the research area, is another obstacle in the way of Long-Term Borehole Monitoring System operations, hampering Dr. Araki and the scientists.

 In March 2010, tests were conducted to examine ways to reduce the harmful effects caused by Vortex-Induced-Vibration (VIV) that the Kuroshio Current has on scientific equipment and tools, discovered during operations during IODP Expedition 319. During that expedition, it was discovered that the vibration caused by the Kuroshio Current was strong enough to destroy scientific instruments attached to 3.5 inch tubing at the end of the drill string, necessitating a redesign of the tools and instruments before deployment during Expedition 332.

Long-term Observatory Schematic

Long-term Observatory Schematic

 "Until that point, our focus had been on refining the design of the instruments so as to collect the best data, but this proved that there were, in fact, major obstacles before that. We had a little over one year, during which something had to be done", Dr. Araki recalled. Dr. Araki, together with CDEX's Technical Development Group and the IODP Observatory team discussed the situation and methods to address the VIV problem. In March 2010, half a year to go before the actual expedition, a trial experiment was conducted on board CHIKYU to test various methods for reducing the vivration. The ultimate solution was somewhat unexpected; attaching ropes to the drill pipe. "Detailed analysis is necessary as to why vibration is reduced when the rope is placed along the pipe. Still I think that the rope may interfere, making the vortex generation difficult", says Dr. Araki.

 Furthermore, various other plans were developed to help reduce possible damage to the observatory during installation. These included working with the manufacturer of the well head and running tool to redesign and strengthen them, and also work with the CDEX and JAMSTEC engineers to redesign the observatory sensor carrier. All the components were tested and checked for resistance to vibration in preparation for the expedition start date of 25 October 2010.