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doi:10.2204/iodp.pr.314.2008

Preliminary scientific assessment

Expedition 314 is unique in Deep Sea Drilling Project/ODP/IODP history for two important reasons:

1. It was the first expedition of NanTroSEIZE, the first "Complex Drilling Project" in IODP, and therefore this expedition was explicitly seen as a part of a larger effort and not necessarily a stand-alone science program in its own right.

2. It was the maiden scientific voyage of a new drilling vessel, the Chikyu.

Based on the prospectus, our goal was to drill and log with the full suite of LWD/MWD tools a transect of six primary sites, including two on the incoming plate, two through major active thrust faults, and two that serve as pilots for deeper riser drilling but also address scientific targets in the splay fault thrust sheet and the Kumano forearc basin and underlying prism. The operational plan was structured very differently from previous ODP/IODP expeditions, with approximately one-third of the total 57 days allocated explicitly as undivided contingency time. We also used a logging system completely new to IODP (seismicVISION), as well as several systems that had rarely been used before (sonicVISION and adnVISION).

Overall success

We faced many challenges brought on by mechanical failure, very difficult natural drilling conditions, and our collective inexperience with the organizational structure on board the Chikyu during Expedition 314. Nevertheless, we were able to drill and log successfully to full planned drilling depth (or at least to a depth sufficient to meet our objectives) at most of the planned sites. In particular, we consider the achievement of drilling the two future riser sites (C0001 and C0002) to full planned TD of 1000 and 1400 mbsf, respectively, to be a major achievement. We were able to drill across the potentially unstable frontal thrust zone and ~130 m into the footwall at Site C0006 as planned. We were also able to successfully drill across the main splay fault zone at Site C0004, although this had to be done at a different, shallower depth (by several hundred meters) than originally planned after Site C0003 proved too unstable to successfully drill a borehole to the fault. We established in situ physical properties at sites in the thrust sheets, prism rocks, faults, and basin sediments; documented present-day stresses through borehole fractures and breakouts; and collected key data that will closely tie the 3-D seismic reflection volume to actual physical attributes of the rocks (especially when interpreted along with core data). The breakout stress information in particular directly addresses key objectives central to the NanTroSEIZE mission and is a major result of this expedition.

Challenges

Because of lost time from a variety of events, we were unable to attempt drilling at proposed Sites NT1-01 or NT1-07, the two reference sites on the incoming plate. The large amount of built-in contingency time was consumed by time lost to failure of mechanical systems, difficult drilling conditions including stuck pipe and a BHA with the LWD tools, and simple inexperience with estimating time needed to complete operations. The reference sites were important objectives, but their highest value objectives come through coring, not logging. We deemed these sites least important to obtain LWD logs from, relative to the sites on the active accretionary prism/plate boundary zone. Therefore, while regrettable, the inability to drill and log them during Expedition 314 is not a major setback to the program.

Unquestionably, two major unforeseen events were key in using up the contingency time during the expedition. The first was the failure of the dynamic positioning system as we reached TD at Site C0001. Troubleshooting and fixing this problem used up 7 days of operations time, during which no drilling could be done. The second was the lost BHA incident at Site C0003. Because the BHA included a complete set of the valuable LWD tools and a radioactive neutron source, fishing for the stuck pipe was mandatory, though it proved unsuccessful in the end. This incident cost another 8 days of the expedition. It also had repercussions for the rest of the expedition, as we had no ability to use a radioactive source and had to forego porosity/density data at our final two sites and also had to use significant operational time to drill pilot holes. Both of these downtime incidents were not avoidable in our judgment, given the seriousness of the incidents and the attention paid by the governmental agencies in Japan to loss of the radioactive source for safe and environmentally concerned operations.

The Kuroshio Current also presented a major challenge. Despite optimistic hopes, the current was present at all sites and more often than not ran at 3 kt or greater velocity. This made many different rig operations more difficult, from running drill pipe into the water to operating the ROV. The vortex-induced vibration of the drill string when the current was flowing strongly caused shaking of the top drive and associated rig equipment, necessitating frequent maintenance as bolts were loosened. It also caused substantial downtime as the best mitigation for running into the hole in the current was to move the ship upstream slowly then drift with the current while lowering pipe. The ROV operations were also hampered by the current and, in several cases, damage to the ROV and its umbilical cable were caused by the strong current.

The difficult borehole conditions in this tectonically active faulting environment obviously caused substantial loss of operational time and failure of equipment. At several locations, Site C0005 in particular, pilot hole drilling or hole conditioning used up substantial amounts of time that was then unavailable for LWD operations. Finally, the pioneering nature of this first expedition on the Chikyu meant that we had to work hard to establish basic routines for interaction between the Co-Chief Scientists, Expedition Project Manager, and Operations Superintendent and also for basic operational procedures onboard. While we struggled with these issues in the early weeks of the expedition, we saw considerable improvements as time passed and experience was gained. With more experience on all sides, the organization of drilling and scientific activities on the Chikyu should proceed smoothly in the future.

In summary, considering all the challenges of pioneering a new operation in a very difficult drilling environment, Expedition 314 was remarkably successful and can be considered a very good start to the NanTroSEIZE project.

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