October 29, 2007
The Japan Agency for Marine-Earth Science and Technology
Chikyu, The Deep-sea Drilling Vessel of the Japan Agency for Marine-Earth Science and Technology, has been conducting operations for the Nankai Trough Seismogenic Zone Expedition (NanTroSEIZE) as part of the Integrated Ocean Drilling Program (IODP)*1. While drilling in the Kumano Basin off the Kii Peninsula on Sunday, October 21, at approximately 2:15 PM, the tools and drill-bit attached to the lower section of the drill pipe disconnected and became trapped in the borehole. After continuous attempts at recovering the disconnected section, JAMSTEC determined to abandon recovery operations on October 29th because of difficulties with the geological formation at the site and condition of the remaining disconnected pipe.
After having drilled approximately 530 m below the seafloor, in water almost 2,500 m deep, sections of the borehole collapsed, trapping the drill string(Fig.1). While trying to clean the borehole and release the drill string, the build-up of torque led to a sudden reverse-rotation that contributed to the loss of the drill string and tools. After 8 days of continuous recovery attempts, including standing-down for one day to avoid a typhoon, JAMSTEC decided to abandon recovery attempts and to plug and seal the borehole. Although some of the tools in the Logging While Drilling (LWD)*2 string use radioactive isotopes (Fig.2), there is no projected environmental impact. The radioactive sources are sealed in a double-walled, pressure-resistance container inside the drillpipe, which will be further sealed in place with concrete.
JAMSTEC will investigate the root causes of the drill pipe loss, and review and discuss preventative measures to prevent future occurrences. Drilling operations will be resumed after necessary measures have been taken. Logging with radioactive isotopes will not be implemented for the remainder of this expedition since spare parts are not available.
The Integrated Ocean Drilling Program (IODP) is an international marine research drilling program dedicated to advancing scientific understanding of Earth by monitoring and sampling subseafloor environments. Through multiple platforms, preeminent scientists explore IODP principal themes: the deep biosphere, environmental change, and solid earth cycles. IODP has started since October 2003 led by Japan and the USA. IODP now has 21 member countries.
LWD is a technique to measure geological formation properties at a site while drilling, by attaching a physical measurement sensor close to the top of the drill pipe.
Through continuous in-situ monitoring of drilling site, geological data can be obtained rather quickly. It is necessary for drilling into complex geological structures, especially in the Nankai Trough, to acquire scientific geological data, and for the following coring operation, to have data on borehole conditions for safety monitoring and risk aversion. Data expected to be obtained by LWD are formation density, porosity, velocity of sound speed, natural gamma ray, resistivity and fluid pressure.
A part of LWD uses americium-beryllium (neutron source) and cesium 137 (gamma-ray source) as sealed radiation sources. There will be no environmental impact since they are sealed in a pressure-resistant container.