November 2, 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 Experiment (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, the tools and drill-bit attached to the lower section of the drill pipe disconnected and became trapped in the borehole (released on October 22nd). Drilling had to be stopped during attempted recovery of the disconnected section. JAMSTEC determined to abandon recovery operation on October 29th (released on October 29th) and on October 30th, the borehole with the disconnected section of the drill pipe was plugged and sealed by cement. All causes and countermeasures related to this incident have been reviewed and discussed. JAMSTEC decided to resume the Logging While Drilling (LWD)*2 operation at 6 pm on November 2nd, at drill site NT2-01 (Fig.1) in the Kumano Basin, off Kii Peninsula (33°13.2'N, 136°43.3'E, 2,524meters depth of water).
After this experience, JAMSTEC organized a committee to examine drilling operation safety, "Drilling Operation Investigative Committee" (chairperson: Prof. Nobuo Morita, Waseda University) for reviewing and discussing the causes and measures to be taken in the future to prevent or mitigate this type of incident.
Based on the committee's examination, JAMSTEC has judged, at the present, that when the drill bit reached the fault zone, sections of the borehole collapsed, trapping the drill string and 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.
Drilling sites chosen for this project have complex subsurface structure because of the faults and diverse geological formations. Additionally drilling equipment is continually being vibrated and torqued by strong Kuroshio Current (at times higher than 4 knots). Considering these factors, the committee recommended future countermeasures as follows :
At approximately 1,300meters southeast from NT2-01 drilling site (2,524 meters water depth), aiming to drill in forefront of Megasplay fault, JAMSTEC will conduct pilot hole drilling following recommendations above. And after safety evaluation, LWD drilling is going to be commenced at 6 pm on November 2nd. Logging with radioactive isotopes will not be implemented for the remainder of this expedition.
Drilling sites will be selected taking into account drilling and scientific requirements and remaining days, and the Expedition 314 will be continued until November 16th as scheduled.
"Chikyu" departed the port of Shingu on September 21st, 2007. As of today, completed operations include a pilot hole at Site NT2-03 (drilling depth 1,000 meters below the seafloor, or mbsf) on September 28th, LWD hole at Site NT2-03 (drilling depth 976mbsf) on October 7th and LWD hole at Site NT3-01 (drilling depth; 1,401.5mbsf) on October 18th. Drilling at Site NT2-01 was commenced on October 21st. During drilling operation on October 21st, the lower part of the drill pipe disconnected, and after continuous attempts at recovering the disconnected section, JAMSTEC determined to abandon recovery operations on October 29th.
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 near the drill bit.
Through continuous in-situ monitoring during drilling, 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 mitigation. 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.
Fig.1 Drilling Site Map